R308 VHF Receiver

See Part 2 of the refurbishment

 

 Above, anxious to discover whether I'd wasted my £32, I've pulled the chassis partly from the case. It had only a single securing screw.

 

 Much to my surprise the interior of the receiver is in excellent condition. I'd been concerned that, as I hadn't seen anything other than external crusty views before I'd acquired it, the set may have been a real problem to restore. As per usual the (12 volt) vibrator unit is missing from the power supply but that isn't a worry as I'll use it on a mains supply. The valves as you can see from that red one look brand new as does that screening can.... rather than being over 80 years old.

Those yellow and black cables carry power and the odd signal so that either the RF or IF/AF module can be detached from the chassis for any maintenance requiring them to be powered up. You can see the reason (inaccessibility) if you look at the underside views (particularly the RF unit).

 

Another view, showing the RF front end chassis. This uses three VR136 valves (= EF54) plus a pair of AW2 voltage stabilizers similar to the type 7475 fitted in the R206 power supply. In this view you can see that the front panel has received significant stress in its life which has distorted the top edge of the metalwork. A few years back I devised a simple way of fixing this sort of thing ** and if I can remember what it was I'll tackle that as soon as possible.. There's a crack in the plastic dial which is probably associated with the panel damage but as this has a flat profile and unlettered a new piece can be simply made.

** I placed a large furniture cramp (edge on) along the front, resting this on the bushes for the front lid (not supplied) securing screws then tightened it to remain in position. I then placed a small machine vise at the bend and tightened this against the cramp until the panel was straight.

When I removed that securing screw so I could sort out the bent panel I found a suggestion of "bike-shed" design. As you can see, the screw is fitted with a bush about an inch and a quarter long. This is so the screw can't be tightened too much resulting in a bent panel. The bush presses on a tapped bracket on the inside of the case and to prevent losing the bush it's secured by a small key punched through a hole in the screw. This rather extravagant piece of engineering may have been a penalty for using the R107 case?

 

 Bike-shed design.

HDRS also had a similar Ldesign feature in its sledgehammer.

 This addition of an S-Meter by the first radio ham owner (a really good idea) is basically the only modification that's apparent but strangely all its wiring has been removed as if the last owner had decided to return it to its original condition but stopped perhaps awaiting a replacement panel etc. The original area was used for headphone storage and had a metal compartment behind a hinged lid (note the space behind the front panel.. was this space left deliberately for what might have been another "bike-shed" idea by a committee?

SRDE would have had MoD representatives (or their 1942 equivalent) present at handover meetings to KB. SRDE would have concentrated on really complicated technical aspects but MoD would have commented on basically irrelevant points and being chair of any meetings would have minuted stuff like the design of securing screws and making space for the storage of headphones. In the event the screws were great but the receiver was a bit iffy. Some 50 years later when I was managing HDRS MoD guys were no different. See the sledgehammer.

Why was that octal socket fitted on that added panel? Maybe the receiver was pressed into service as an IF for an external home-brew front end because, having FM, it would make a decent 2m or 70cm receiver? At least in the 1950s to the 1960s.

A puzzle is the countersunk hole drilled in the identification plate at the top right of front panel.. maybe this was used to mount a single-screw-mount BNC connector?

 

I thought a perusal of old Practical Wireless magazines might provide a clue to suggested modifications to the R308? But no.. I didn't really find anything other than a few "for sale" adverts by dealers.

It's also not prolific on the Net, where I can see only a few examples... Serial Numbers "7", "68" and "185". Mine is "158" so all are in the in the initial KB tranch of 200 (see below).

 

 

 The follow-up to the R308, the R216 (19-157MHz)
 

Now, a little on the background to the R308 receiver.

The R308 was designed as a range extension companion for the much more common R107 and uses many identical parts, to no-doubt save money but more importantly to save development time, for example the outer case plus various miscellaneous components, such as knobs, switches, headphone and power sockets etc.

The way it was born is interesting. Design was almost completed by SRDE in 1942 when a contract was made with Kolster Brandes for its rapid development leading to the manufacture of 200 examples. The end result wasn't up to much so my guess is that first batch of 200 was the total quantity built before a replacement (to be called the R508) was decided on. The R508 may have been KB's baby but the replacement ended up as the "R216" manufactured by Ekco. More about this set can be seen by clicking the picture on the left.

Although KB had been making radios since the 1920s, SRDE must have decided they weren't sufficiently well versed in VHF equipment so switched to Ekco. One important aspect in the replacement was the use of a turret tuner rather than an outdated yaxley switch for range selection. Turret tuners were used in the WW2 receivers such as the R206 and the DST100 and offered much better repeatability of performance and repair & maintenance of the tuned circuits. For example, one of the bad features of the R308 (and the R1132) was in the operation of the VHF local oscillator when it would cease to oscillate due to absorption of the signal due to resonance in neighbouring coils and wiring.

 The key tuning parameters for the R308 are as follows..

Ist IF = 9.72MHz; 2nd IF = 2.1MHz

Range 1 = 20 to 30MHz, Range 2 = 30 to 48MHz

Range 3 = 48 to 75MHz, Range 4 = 75 to 112MHz

Range 5 = 112 to 145MHz

 I'm expecting to find a few shortcomings with this old receiver and, if I do, should I try and improve the design or just accept it for what it is?

The front panel has been poorly re-painted (fortunately the legends are engraved) so if I can remove all the control knobs (a common "major" problem) I can repaint it.

Below is a provisional circuit diagram stitched together from six pages. Click to see full size.

 

Components tables

 Condensers

  

 Resistors

 C1

 A

 B

 25uF

C12 

 A

- 

 2nF

 C23

 A

 C

 50pF

R1

 A

 C

 7R

R12

 A 

 -

 700R

 R23

 A

 -

 22K

 R34

 A

 C

 100K

 C2

 A

 E

 8uF

 C13

 A

 -

2nF

 C24

  A

 D

50pF

R2

 A

 -

 22R

 R13

 A

 -

 820R

 R24

 A

 -

 27K

 R35

 A

 C

 220K

 C3

 A

 B

 2uF

 C14

 A

 B

 500pF

 C25

  A

 C

 5-47pF

R3

 A

 -

 22R

 R14

 A

 B

 1K

 R25

 A

 -

 27K

 R36

A

 -

 220K

 C4

 A

 -

 2uF

 C15

 A

 D

 400pF

 C26

  A

 E

 3-30pF

R4

 A

 -

 42R

 R15

 A

 -

 2.5K

 R26

 A

- 

 27K

 R37

 A

 B

 330K

 C5

 A

 F

 500nF

 C16
  A

 C

 300pF

 C27

  A

 B

 12.5pF

R5

 A

 -

 120R

 R16

 A

 -

 3K

 R27

  -

- 

 R38

 A

 B

 470K

 C6

 A

 E

 100nF

 C17

 A

 -

 280pF

 C28

  A

 B

 10pF

R6

 A

 -

 150R

 R17

 A

 -

 3K

 R28

 A

 C

 27K

 R39

 A

 -

 470K

 C7

 A

 -

 50nF

 C18

 A

 -

 230pF

 C29

  A

 -

 3-10pF

R7

 A

 B

 220R

 R18

 A

 D

 4.7K

 R29

 A

 B

 47K

 VR1

 A

 B

 5K

 C8

 A

 AL

 10nF

 C19

 A

 -

 200pF

 C30

  A

 -

 6-8pF

R8

 A

 -

 220R

 R19

 A

 -

 4.7K

 R30

 A

 C

 68K

 VR2

 A

 -

 2K

 C9

 A

 -

 10nF

 C20
  A D   200pF    C31   A  E  2-8pF  

 R9

 A

 -

 270R

 R20

 A

 -

 10K

 R31

 A

 -

 68K

 VR3

 A

 -

 1M

 C10

 A

 -

 5nF

 C21

 A

 H

 150pF

 C32

  A

 -

 2-4pF

 R10

 A

 -

 470R

 R21

 A

 -

 10K

 R32

 A

 F

 100K

 -

 -

 -

 C11

 A

 B

 4nF

 C22

 A

 G

 100pF

 -

  -

 -

 R11

-  

 -

 -

 R22

 A

 B

 10K

 R33

 A

 B

 100K

 -

 -

 -

 Looking at the table above one can see the influence of two designers. The resistor values are linked to their numbers with higher numbers being higher value resistors. Different resistors having the same value and wattage are given codes A upwards. Condensers though are back to front with lower numbers having higher capacitances. Type C8 is exceedingly common being the sets standard decoupler.

To help identify the resistors, hence measuring the voltages, I've made the tables below which also indicate the main associated condenser.

Also shown is an underside view showing the positions of the valves (listing follows).

From a working receiver I'll add the voltages or notes later.

Resistor connections (with associated parts)

 R1

 A

 7R

V1D

 G1
 C8M

 R1

 B

 7R

V5A

 G1
 C8R

 R1

 C

 7R

V5B

 G1
 C21M

 R2

  A

 22R

V5B

 A
 C24D

 R3

  A

 22R

V8A

 A
 C10A

 R4

  A

 42R

HEATERS

-
 C8AA

 R5

  A

 120R

V1A

 K
 C14B C17A

 R6

  A

 150R

V1C

 K
 C6A

 R7

  A

 220R

V4A

 K
 C8K

 R7

 B

 220R

 V1D

 K
 C8P C6C

 R8

  A

 220R

V1A

 A
 C8C

  R9

  A

 270R

V5A

 K
 C8S

 R10

  A

 470R

V4A

 AT
 C22C

 R11

A 

 -

JACKS

-

-

R12

 A 

 700R

 V8A

 K
 C1B

 R13

  A

 820R

 JACKS

-

-

 R14

  A

 1K

 V1C

 A
 C8F

 R14

 B

 1K

 V1D

 A
 C8O

 R15

  A

 2.5K

 JACKS

-

-

 R16

  A

 3K

 V7A

 K
 C1A

 R17

  A

 3K

 V8A

 G2
 C3B

 R18

  A

 4.7K

V1A 

 A
 C4A

 R18

 B

 4.7K

 V2A

 A
 C19A C19B C15C

 R18

 C

 4.7K

 V4A

 A
 C8L

 R18

 D

 4.7K

 V3A

 A
 C8U

 R19

  A

 4.7K

 V4A

 AT
 C8M

 R20

  A

 10K

 V3A V3B
 STAB C2A

 R21

  A

 10K

 V2A

 K
 C22A

 R22

  A

 10K

 V1D

 G2
 C8N

 R22

 B

 10K

 V7A

 K
 C4A

 R23

 A

 22K

 V1B

 G2
 C12A

 R24

A

27K

V6A

A1 A2
 C20C C20D

 R25

A

27K

V7A

A
 C3A

 R26

A

27K

V1C

G2
 C8E

 R27

A

27K

V1A

G2
  C14A

 R28

  A

 27K

V4A

 G4
 C8J

 R28

 B

 27K

 V4A

 G4
 C8J

 R28

 C

 27K

 V4A

 AT
 C22C

 R29

  A

 47K

V1B

 G1
 C30A

 R29

 B

 47K

 V4B

 AT
 C8X

 R30

  A

 68K

V4A

 GT
 C8K

 R30

 B

 68K

 V4B

 GT
 C22E

 R30

 C

 68K

 V6A

 K1
 C23C

 R31

  A

 68K

V5B

 A
 C4D C8Y

 R32

  A

 100K

V1A

 G1
 C8A

R32

 B

 100K

 V1C

 G1
 C8A

 R32

 C

 100K

 V4A

 G1
 C8G

 R32

 D

 100K

 V1D

 G1
 C8M

 R32

 E

 100K

 V5A

 G1
 C8R

 R32

 F

 100K

 V7A

 G1
 C22F C22G

 R33

  A

 100K

V5A

 G2
 C8T

 R33

 B

100K 

 V4B

 AT
 C8X

 R34

  A

 100K

 V3A V3B

-

-

 R34

 B

 100K

 V4B

 G4
 C8V

 R34

 C

 100K

 V5B

 G3
 C8W

 R35

  A

 220K

 V5B

 G1
 C27B

 R35

 B

 220K

 V6A

 K2
 C23C

 R35

 C

 220K

 V7A

 A
 C3A

 R36

  A

 220K

 V5B

 G2
 C4D C8Y

 R37

  A

 330K

 V7A

 D1 D2
 C6E

 R37

 B

 330K

 V7A

 D1 D2
 C6E

 R38

  A

 470K

 V6A

 K1
 C23C

 R38

 B

 470K

 V6A

 K2
 C23C

 R39

 A

 470K

 V8A

 G1
 C13A ***

 VR1

 A

 5K

 JACKS

-

-

 VR1

 B

 5K

 V5B

 K
 C8W

 VR2

 A

 2K

 V5A

 K
 C8S

 VR3

 A

 1M

 V7A

 G1
  C4A

Valves

DESIGNATION

 TYPE

 COMMERCIAL

 FUNCTION

 V1A

 VR136

 EF54

 RF AMPLIFIER

 V1B

 VR136

 EF54

 1ST OSCILLATOR

 V1C

 VR136

 EF54

 1ST IF AMPLIFIER, 9.72MHz

 V1D

 VR136

 EF54

 2ND IF AMPLIFIER, 2.1MHz

 V2A

 VR92

 EA52

 1ST MIXER, 9.72MHz

 V3A

 AW2

 7475

 VOLTAGE STABILIZER

 V3B

 AW2

 7475

 VOLTAGE STABILIZER

 V4A

 ARTH2

 ECH35

 2ND OSCILLATOR + 2ND MIXER

 V4A

 ARTH2

 ECH35

 BFO

 V5A

 ARP34

 EF39

 3RD IF AMPLIFIER, 2.1MHz

 V5B

 ARP34

 EF39

 DISCRIMINATOR, 2.1MHz

 V6A

 ARDD5

 EB34

 AM+FM DETECTORS

 V7A

 6Q7G

 DH63

 AUDIO AMPLIFIER + AVC

 V8A

 VT52

 EL32

 AUDIO OUTPUT

 

 The front panel paintwork is in very poor condition, being very flaky and can be readily brushed off.

Also, the whole front was covered in what seems to be dried mud. I was cleaning some of this away when I noticed this Headphone/Line jack socket escutcheon was fitted upside down.

This suggests the front panel has at some time been detached and painted (with an unsuitable paint type or without properly cleaning the metalwork). At first I thought the escutcheon was fitted accidently upside down, but, it's possible the owner wanted to re-position the smaller hole to suit an alternative jack plug without a thick shoulder?

 

 

 

Below a view of the underside of the receiver and the three individual modules.

Click any picture to see full size.
  

 

Here are three separate views of the underside... click any to see full size

 

 

 

 Here's a view of the RF (aerial) coils through the aperture at the bottom rear of the RF module.

 

Because of the layout of the coils and their close proximity I can see a potential problem in absorption of RF by adjacent coils unless a shorting ring is present on the wavechange switch. This might result in flat spots in one or more tuning ranges and, if the same design is used in the oscillator section, either a similar flat spot due to reduced oscillator amplitude or even the oscillator dropping out..

 

 

 Above the coils in the previous picture you can just make out the mixer diode. Here's a better view of the VR92 (=EA50). There's some tarnishing on its pins so it may be necessary to remove the valve and clean them.

Is that elastoplast valve remover original?

Reminds me of an acceptance meeting regarding equipment to be fitted in a satellite ground station in Germany...

"Vot is dat... string?"

"It's not just ordnary string it's NATO codified string!"

 Where to start on tackling this receiver? As with nearly all my collection I'd like to see if it can be made to work. The biggest drawback of course are the aged components, particularly the old paper condensers which will leak. In this case, instead of just removing each in turn and stuffing with modern components (my preferred choice is to use high voltage chips), I intend to connect a high voltage from a bench power supply and gradually increase the voltage from zero to 250 (or 320 volts for the main rail). This done I can measure any leakage and at least replace any condensers that are getting warm. To this end I'll use a thermal camera.

 Here's the first results. I didn't have my thermal camera to hand so basically relied on smoke (wisps appeared when the drain was 82mA).

I located a convenient point on the 250 volt rail which happened to be the top of a large orange resistor (R17A).. the feed to the audio output valve V8A screen grid. Cranking the variable HT bench supply to 250 volts resulted in a current drain of 55mA. This gradually increased and at 82mA I switched off the supply. R17A was pretty hot as was the metal-cased condenser shown here C3B (decoupling the screen grid to ground). I clipped its ground lead and repeated the test. Oddly the result was exactly the same with a draw of 82mA before I switched off. Again C3B was hot, so clearly its metal case (clipped to the chassis) was connected to the negative terminal. This is marked with the wavy black ring (right). The condenser is marked as you can see here, being made in April 1944.

Checking with a meter proved the outer case was indeed shorted to the negative terminal but the DC resistance to the positive terminal was a few megohms. The latter condition is typical of old condensers and doesn't really hint at breakdown when HT is applied. The way that current gradually increases suggests a slight short which reduces in resistance as the condenser gets warm and the short is compressed in-line with internal expansion of the innards.

Clipping the positive lead and repeating the test resulted in a constant current of 24mA. Waiting for a few minutes proved this was unvarying other than a couple of mA up and down. One of the voltage stabiliser valves V3B (AW2 = 7475) was illuminated so a little of the drain will be due to this. If you look at the schematic you'll also note several resistors configured as potentiometers which will also draw current. The remainder might be leakage at the metal rectifiers and leaky decoupling condensers.

I reckon it'll be safe to now apply mains power and see what happens.

 

 

 Applying power wasn't straightforward. The on/off switch was lazy and continuity between the pair of mains pins to the transformer primary was intermittent and high resistance. I checked the fuses and found these were is a fairly poor corroded condition. I decided to bypass the switch, fuses and 2-pin mains connector and applied power directly to the mains transformer primary, using the +10 and 230 volt pins (these were already wired). The pair of dial lamps lit and nothing was heard from the loudspeaker after a couple of minutes so I plugged in a pair of headphones. The centre socket wasn't very loud so I plugged into the RH socket and heard a very loud 100Hz hum. Trying the various knobs and switches (significantly the wavechange switch) showed the receiver was probably in a generally working state, but the loud hum meant that the HT was likely to be unsmoothed, or maybe the metal rectifiers were bad, so I turned off the set.

 

 These excerpts from the schematic show the power supply circuitry. C2 x 4 are 8uF and C8 x 2 10nF.

Sure enough all four 8uF condensers measured "open circuit" on my capacitor tester so I checked with my universal tester and initially it was confused. C2E was 75nF then a varying 459Kohm, C2D was just a few nF then give or take 44Kohm, C2C was 3nF and C2B only 14pF.

The HT rectifiers are Selenium types. W1A seems to be a single full-wave bridge. This needs to supply only a small current for the voltage stabilisers and the first local oscillator V1B. W2A comprises a pair of much larger rectifiers as it has to supply all the valves bar V1B.

 

 

On the right is the area causing a problem with Slydlock fuses shown as F1A, F1B and F1C. The mains switch is quite a large robust affair and a squirt of switch cleaner will bring it back to normal.

The dotted area looks a bit confusing but represents a short cable terminated in an octal plug which can be inserted into one of two positions.. either mains or battery operation.
  

I cut the positive connections to the four bad condensers and loosely soldered temporary replacements, soldered a pair of mains connections directly to the 240 volts terminals on the mains transformer, plugged in a pair of headphones, switched on my signal generator to 80MHz AM at 10dBm and turned on power. The dial lamps came on and, after only 30 seconds, I heard a healthy hissing sound and a weak audio signal. By chance I happened to be switched to Range 3 and tuning the knob a few degrees the 80Mhz signal came in very loud and clear. Whatever its weaknesses the R308 seems to be extremely reliable.

 

 

 Following are some experimental pictures of a couple of resistors preceded by a picture of the area under review.

 

 

 This picture is interesting because it shows R25A which is part of the anode feed to V7A, a 6Q7 whose anode current should be no more than 1mA. the 27K resistor should be dissipating about 27mW but from its temperature it's running at a lot more than this.

The schematic reveals the answer. There's a 2uF decoupling condenser which must be very leaky.

The proof is the tiny adjacent resistor which is 220K (R35C also part of the 6Q7 anode feed) and this would be incrediby hot if it were drawing the same current as R25A.

 This is R7A which is the 3K screen grid resistor for the audio output valve a VT52 (=EL32). G2 of this valve should be drawing around 4 or 5mA. This equates to about 75mW disipation in 3Kohms so its temperature looks a bit high.

I already disconnected its 2uF decoupling condenser which was short circuit. My guess is that either the resistor has changed its value or the EL32 is not performing correctly.
  

 When I looked at the picture of the hot resistors I realised that their temperatures and physical sizes didn't really mean much to me, for example... what temperature does a half watt resistor run at when the voltage across it draws enough current to sink half a watt? The temperature of course will be dependent on ambient, cooling airflow and heat sinking etc but take a resistor in air with no airflow.. what happens?

I set up a simple experiment, choosing a 200 ohm carbon resistor made sometime between 1938 and 1946.

This measured about 12mm or 1/2 inch long and 4.5mm or 3/16 inch diameter which I assume is rated at half a watt.

I waited about one minute before measuring each temperature. Figures are approximate.

From these results the 27K is definitely too hot and the 3K probably about right.

 VOLTAGE

 CURRENT

 WATTAGE

 TEMPERATURE

 NOTES

 10V

 50mA

 0.5W

 49C

-

 12V

 62mA

 0.75W

 58C

-

 15V

 68mA

 1W

 74C

-

 17V

 89mA

 1.5W

 91C

 slight smell

 20V

 100mA

 2W

 116V

 hot smell

 24V

 125mA

 3W

 165C

 very hot smell

 28V

 143mA

 4W

 179C

 burning smell, discolours

 Now a picture of the EA50 mixer diode followed by the 250 volt HT rectifiers. The diode has a piece of elastoplast stuck over the body.

 

 

 Whilst looking over the R308, having discovered its reservoir and smoothing condensers were all bad, I wondered if the same rules applied to selenium rectifiers (two of its three pictured above) as valve rectifiers. In particular the maximum value of the reservoir condenser. Digging out a 1961 valve data manual I was surprised to read the term, not "maximum", but "minimum" value for the reservoir capacitance, unlike vacuum tubes which is the reverse. The R308 designers chose 8uF for both reservoir and smoothing, but perhaps this was a practical decision based on physical size or even the price of larger ones?

Bearing in mind selenium rectifiers have a relatively high resistance, then any capacitor surge would be quite low. I'd decided to look for new capacitors and restricted my search to 450VAC motor start types, being easy to mount (with their fixing stud), but I'm now thinking I can use normal electrolytic types as I have plenty salvaged from written-off lift equipments.

Those two rectifiers shown above have an inked code "280/LU530C" written on their ends. Each has three connections so will each be half a bridge (two diodes in series). Maybe 280V @ 530mA ?

 I removed the suspicious 2uF condenser decoupling that 27Kohm resistor that's getting very warm and tested it. It's marked 2uF at 350/425 volts but both my capacitor testers said it measured 5uF with less than 0.5 ohms ESR, but my general component tester weirdly said it was an 11 volt zener diode one way round and an 11pF capacitor the other. Maybe the case is shorting to the positive terminal, but no... neither terminal was shorting. Still puzzling I connected it to my bench HT supply and found it broke down at 100 volts and went dead short. Presumably this was only temporary with the 27Kohm current limiter as it had previously recovered.

 

 

 Next job is to fit new Slydlock fuses.. due here today (11 Sept 2025), then new reservoir and smoothing capacitors. The fuses arrived and I discovered that in order to remove the old ones I had to remove the power supply. Surprisingly I found that this aspect of the receiver design was really good. The power supply could be detached after removing just four 0BA screws and its cabling was long enough to balance the chassis out of the way.

 The three fuses are located bottom left of the front panel adjacent to the 4-pin plug for 12V battery power.

These fuses are... top to bottom 12VDC, live mains, neutral mains.

The mains plug fits on those two large pins (typical in 1944)

 

 This is the view revealed once the power supply chassis has been lifted away (the lowest fuse has already been replaced).

Each Slydlock fuses is held in place by a 4BA screws and nut.

A design weakness can be seen in that the connections to the fuses are made with large nuts and washers which are extremely close the the chassis.

The fuses turned out to be different to the ones I'd bought. Those very long threaded posts are an integral part of the old fuses and cannot fitted to my new ones.

This meant I had to modify the replacements so they'd fit.
  

 Connections to the existing heavily tarnished fuses are made by long threaded posts which screw into the innards of the fuses. The new fuses are different. I had to remove the two brass fittings from them and drill and tap these to accept those posts. In fact I dispensed with the old posts and used shorter M4 bolts suitably modified to fit. Are the originals 4BA?

I spent ages working out the best approach but in the end each fuse was modified within 10 minutes. I drilled through the existing tapped hole (=M4) right through the fitting and threaded the rear hole by tapping through the top as shown. This is to ensure the top and bottom holes line up and are not angled. The rear of the fuse has indentations which are drilled out to accommodate the threaded posts.

For the fuse to work mechanically the modification must be very accurate otherwise the slide and lock doesn't work smoothly.

 

 

 This picture shows a modified fuse. The plastic posts were cut from the old fuse and serve two purposes. One is a safety feature as the threaded posts go through the front panel, but essentially that nut screwed tightly prevents the post from rotating when connections are made to them.

This exercise was really annoying but as the original fuses were in poor shape and in fact open circuit there wasn't an option if the receiver is to be kept original.

 

Below, all three Slydlock fuses installed.

 

 
 

 The next task was to fit reservoir and smoothing capacitors.

I selected a pair of 220uF x 450V reservoir capacitors and a pair of 470uF x 450V smoothing capacitors but I'd not noted which yellow sleeved wire was which except to mark them with stripes 1 to 4.

 

One choke is fitted at the end of the chassis and the other inside the power supply chassis but which was which? L11A serves the 320V supply and L12A the 250V supply. I worked out that the latter is physically larger and underneath the chassis but I turned on the mains power and measured the voltages at the yellow wires. This enabled me to identify which was which. The voltages on the 4 wires emerging from under the chassis read 257V,183,195 and 20V. The highest (257V) is the 320V supply at the rectifier output whilst the 20V is a leaky figure from the regulators from the 250V rail. 195V is the input to L12A and 183V the 250V rail.

 

Having established the identity of the wires I connected the four capacitors and checked the sets voltages. The 320 volt supply settled at 313V and the main supply dropped to 238V after the valve heaters had warmed up.

 

The PSU (opposite) will be tidied up soon. Although the chassis can easily be detached from the main receiver chassis it's not too easy to access.
  

 Now that the receiver is powered correctly, and with steady HT voltages, I can investigate its operation. The first job was to identify all the control knobs.

 

 

 Now that the power supply is producing correct voltages I began a bench test (using wires soldered directly to the mains transformer), initially with that signal generator peeking out above the receiver and headphones, but after about 30 seconds the Wavetek told me it wasn't happy having failed its self test. Next was the Rohde & Schwarz (on the right of the receiver) which turned on, passed its self test and prepared to give me a test signal.

A really annoying feature of this signal generator is its AM settings as the default is a stupid value like 60KHz. Anyway I set it to 25MHz, minus 10dBm and 1KHz AM. The R308 responded but with the dial showing a slightly high frequency. Resetting the signal generator I found all the R308s five ranges were working OK. One of our strongest FM stations is Classic FM at around 100MHz so I set the R & S to 100MHz FM and tuned this in. I then removed the signal generator and connected an aerial. Tuning a little to one side I heard Classic FM at good strength but somewhat distorted. I tried various knob twiddling and the results were pretty much the same but with the limiter knob working in FM. Audio volume was perfectly adequate (in fact some controls noisy enough to be quite painful on my ears) but when I checked the loudspeaker it seemed completely dead. The speaker is marked 140ohm and measures around 200ohms but is completely silent. That's the next thing to sort out. I recall the speaker on my last R107 (physically it looks exactly the same) was also duff.

 

This is the audio output stage with a 6Q7 driver and EL32 output stage. As you can see it uses a common transformer for phones and speaker and a line output also (derived from the phones output) as shown below.

 

Of note (left) are three components viz. C3A (currently removed but probably not important with the new 470uF smoothing capacitor), C1B which is likely to be bad, and the commonly bad coupling condenser C13A. I'll wap that first, then renew C1B.

Checking the voltages across R3A and R17A will define the results.

 

Why the designers went to the expense of providing L15A and L15B (and C10A, C11A and C11B) is very odd. It reminds me of an exercise during the design of an air defence system at Plessey back in the early 1980s. A small group of three engineers were recruited and given the job of overseeing the design of the interface between the air traffic controllers, fighter interceptors and missile people. They spent an inordinate amount of time and money coming up with special VOGAD amplifiers and hugely expensive transformers and filters.

The day of the big demonstration of their efforts using specially purchased test equipment that used artificial speech. A queue quickly formed.

"This will show you the difference between a basic amplifier and our super duper filters". Sure enough one of the two tests produced beautifully clear speech and the other was muffled. Unfortunately all the neutral listeners chose the basic circuit. The reason was that the headphones and microphones used in the system were already designed for ideal audio characteristics so adding another layer of 300-3000Hz audio filtering didn't work too well.. it sounded muffled. 

 I was surprised to dscover that C13A had very little leakage. When C13A was disconnected the EL32 anode rose from 152 to 157 volts. The joint anode screen current therefore dropped from about 19.6mA to 18.6mA.

Notwithstanding, I'll remove C13A and test it but fit a modern equivalent.

The dissipation of the 3K resistor feeding the anode and screen is about 1.2W and hardly changed. A check across the cathode resistor R12A supported the figures above (12.72V over 700 ohms = 18mA). During testing I noticed reception of Classic FM sounded almost identical in AM, CW or FM so some work is needed to resolve this.

 

 It's now 9 months later in May 2026 and before proceeding any further I'll tidy up the mains power supply. As I mentioned earlier the PSU chassis seems to be mighty difficult to work on so my plan is to leave the bad smoothing and reservoir condensers physically in place and mount a new metal panel over the top of these which will accommodate four new capacitors.

I checked on the rating of the metal rectifiers and there's no problem using modern high value capacitors.. unlike their valve equivalents which must have relatively low value capacitors to avoid ruining the rectifier valve.

   

 I'd tested the receiver using the layout on the left but it wasn't particularly good so I decided to fit the parts more securely. This turned out to be trickier than I'd hoped. One problem was the screws securing the transformer bakelite top plate were too short to fix the new metal mounting plate. The solution was to drill and tap the top cover (it's about 3mm thick) and secure the new plate with three 6BA screws. Because of the mains wiring under the top cover the positioning was critical. I now need to make sure that upending the chassis doesn't result in the new parts bearing the weight.

I'd powered the receiver directly from the mains transformer terminals on the top of the transformer because of problematic Slydlock fuses and now that these had been replaced I decided to repeat the test using the two-pin mains plug. I have a couple of special mains leads for this connector but first checked the continuity between the mains plug and the transformer. It was all rather puzzling. Initially I was using croc-clip leads and discovered these were both faulty because the copper wire wasn't crimped. I soldered the wires and was still puzzled. The transformer primary resistance was 22.6 ohms but the resistance at the mains plug was about 3.6 ohms more than this. Several connections shown in the diagram below could account for this difference but I guess the mains whetting ciurrent will deal with any resistive connections. The culprits might be the pins at PL1C (the plug & socket for selecting mains or battery) or S3A tarnishing. The fuses F1B/F1C are new so should be blameless. To monitor HT current I'll fit a resistor in the HT feed at the output of L11A. A 100 ohm 2W should provide 10 volts at 100mA.

 

 Despite my concerns, after powering the receiver I found it was receiving BBC Radio 2 with no aerial. I added 18 inches of wire at the centre aerial terminal and tuned in Classic FM at 100Mc/s on the dial. All waveband settings produced a mush in headphones so everything appears to be fine. Then I checked and realised the tuning dial wasn't set to 100Mc/s but to "100" on the logging scale. The wavechange knob was pointing to Range 5 but its grub screw was slack and it was actually set to Range 4. The frequency was a little under 92Mc/s which is BBC Radio 3. Clearest reception was correctly in the FM setting.

HT measured 310 volts at 10mA (I found one volt across the monitor resistor) and 230 volts. The choke for this latter supply is under the chassis making it awkward to add a monitor resistor. The reservoir condensers had 346 volts and 231 volts across their terminals.

 

 Below are views of the chassis showing the two posts I added so the chassis can be upended safely.

My previous project, that signal generator/test set, can be seen ready for testing the R308.

 

 

 Here's an interesting question.. can one detach the power supply unit easily? The answer is yes but it will involve unscrewing a few parts from the front panel, probably after unscrewing four mounting bolts so the power supply can be moved back a few inches. That's if everything must be kept intact, otherwise wires can be snipped.

The main supporting chassis appears to be demountable from the front panel quite easily.

In my pictures you can see a couple of sections of dexion equivalent which I added so the receiver can be turned upside down. From the limited documentation available I think one of the main criteria in the receiver requirement spec was to make it easily serviceable, maybe by substituting a sub-chassis.

 In order to continue testing this receiver I now need to investigate the dead loudspeaker. Firstly the Speaker On/Off switch was dicey but I also found the speaker connections were open circuit. After removing parts of the speaker I identified that the coil had failed.. possibly due to corrosion at the wire junction on the speaker cone and was not repairable. The speaker is marked 15 ohms and is a relatively tight fit in the space provided. After searching for a suitable junk box replacement I found a 4 ohm speaker having a suitable rating (the magnet size in the new one was a bit larger) and a quick test proved it sounded very good. The only problem was its larger physical size (about half an inch wider and taller). Apart from being too big to fit into the space the mounting holes in the front panel are about 71mm apart and I didn't want to drill the panel to accommodate the new spacing of 82mm. After puzzling over the prospect of squeezing the new one into a small space I decided to mount it at an angle using two (horizontal) strips of steel. The picture below shows the mounting method where it fits over the rear of the speaker on/off switch. I also swapped the unsightly original perforated cover with nicer piece of metal. It worked very well and I'll wire it up properly and fit correct 4BA screws.

The receiver seems to work tolerably well but I'll check alignment and tidy up the tuning of the IF strip and check the discriminator etc to optimise FM tuning. I'm not sure if the BFO is working or whether it's just miles off frequency.

   

 I need to test a few critical components.. in no particular order before RF work.

C13A grid feed to the EL32.. leakage would make its control grid positive and increase the standing anode current

Oddly, I'd replaced this condenser but hadn't soldered it in plce so it was very lucky it'd made good cobntact otherwise I'd have had no audio during the previous tests. I also found I'd removed C3A, the bad 2uF but I guess this may not have had much effect with the new PSU components.

Screen resistors ... ten of these. High values will reduce gain.

I checked the following fitted around the underside of the IF unit. R17A 3K read 3.1K, R32E 100K read 120K,

R39A 470K read 558K, R25K 27K read 34K, R35C 220K read 313K, R31A 68K read 92K, R16A 3K read 3K,

R32K 100K read111K, R35B 220K read 259K, R38A 470K read 537K, R38B 470K read 539K, R12A 700 read 700.

AGC decouplers... three condensers C8A, C6B and C6E. Any leak at all will result in overloading and distortion

 

Electrolytics.. ten: four PSU are already cut plus the screen decoupler of the EL32. Untested C1A, C1B, C2A, C3A and C4A

C4A 2uF measured as near perfect and read 2100nF

 Having worked out thar the receiver was ready for an RF check I decided to initially use my miniature tester my Tiny SA.

I immediately discovered some very strange things. Firstly the set wouldn't respond to its IF of 9.72MHz. By tuning the Tiny SA I found the IF strip was tuned to about 10.3MHz with two humps indicating the stages were off-tune but about 600KHz off. As the receiver was working pretty well it would seem that the last owner (or more likely the original owner because the S-Meter circuit had been stripped out) had either assumed the IF was maybe 10.7MHz (the standard commercial IF) or somehow the entire IF had drifted 600KHz which is highly unlikely. I decided to log the various stations in Range 4 and see how they correspond to reality.

They were all a bit out in respect of the dial numbering but not too much.. it's just the dial has really accurate markings making it seem that stations are way off when they're not that bad. Is the problem due to the local oscillators or bad IF tuning? I initially tackled the conundrum by measuring the local oscillator with my TinySA (results below).

 RANGE

 FREQ- LOW

 HARMONIC

 dBm

 FREQ-HIGH

 HARMONIC

 dBm

 NOTE

 Local Osc

 1

 28.53

 1

 -47

 163.06 (54.4)

 3

 -37

Range 18.5-49MHz 

 Higher

 2

 158.02 (39.05)

 4

 -39

 57.7

 1

 -35

 Range 30-49 MHz

 Higher

 3

 114.9

 1

 -38

 85.41

 1

 -31

 Range 47-76 MHz

 Higher

 4

 127.9

 1

 -26

 102.9

 1

 -30

 Range 74-113 MHz

 Lower

 5

 85.94

 1

 -49

 136.8

 4

 -17.7

 Range NA-146 MHz

 Lower

 Now an analysis of these rather odd results. My test meter indicated the strongest signal present at the aerial terminal and the primary frequency was sometimes weaker by a few db than the plentiful harmonics. The local oscillator should differ from the signal frequency by the receivers 1st IF which is supposed to be 9.72MHz.To reach the 2nd IF amplifier the 9.72MHz signal is mixed withan oscillator at V4A triode section runnng at 7.62MHz producing the 2nd IF of 2.1MHz. The measured IF though looked like 10.3MHz.

Take for example Range 4 whose end dial readings are 74MHz to 113MHz. The local oscillator at these frequencies should register 74-9.72 (=64.28MHz) and 113-9.72 (=103.28MHz) or 74+9.72 (=83.72MHz) and 113+9.72 (=122.72MHz). A receiver will be designed to have its local oscillator either higher or lower depending on a number of factors and its not always easy to detrermine which. Unless the correct method is known alignment can be difficult, particularly in respect of the dial calibration. With a specific tuning capacitor the dial markings reflect the value of a "padder" capacitor whose value is very accurately chosen when the RF design is tackled.

If we reverse engineer the RF design we see there are no padders for Ranges 4 and 5 which suggests 5s oscillator is lower and 4s oscillator could be higher or lower than the tuned frequency. Range 1 uses a 230pF padder and Ranges 2 and 3 both use 200pF.

Range 1 must use a higher frequency local oscillator in order to get enough frequency swing. Ranges 2 and 3 are probably also higher.

My next step has to be a better analysis of the local oscillator so my Rigol spectrum analyser will be pressed into service. That will more accurately define the strange IF frequency and also check on the 2nd IF and whether the local oscillator is higher or lower than the dial on each of the five ranges.

Results below. The dBm figures are the measured level of the local oscillator at the aerial terminal. I measured the 1st IF at 10.17MHz instead of the design figure of 9.72MHz.

 Range

 Dial

 Oscillator

 dBm

 Dial

 Oscillator

 dBm

 Oscillator

 Note

 1

 30

 39.7

 -75

 20

 29.7

 -54

 Higher

 Min dial unmarked but 28.3MHz osc

 2

 48

 57.7

 -45

 30

 39.7

 -56

 Higher

 2nd harmonic 79.3MHz -40dBm

 3

 75

 85.3

 -38

 48

 58

 -45

 Higher

 2nd harmonic 170MHz -30dBm

 4

 112

 102.7

 -24.5

 75

 65.7

 -48

 Lower

 100MHz on dial is 90.3MHz -44dBm

 5

 145

 136

 -21

 112

 100.67

 -51

 Lower

 Min dial unmarked but 85.3 MHz osc

 The figures above show a couple of problems. Firstly the indicated frequencies on the dial are generally wrong and secondly of course the IF tuning is bad.

For example, take the reading of 100MHz in Range 4 with a local oscillator of 90.3MHz. The IF is 10.17MHz so the actual response at an indicated 100MHz is 90.3MHz + 10.17MHz = 100.47MHz. Not too bad except the IF of 9.72MHz which is 420KHz lower will result in a response when the dial is set for 100MHz to be 100.47-0.42=100.05 which suggests the local oscillator setting is fine. It's just the IF tuning which is bad. Maybe someone twiddled the tuning without reference to a 9.72MHz oscillator?

The next table shows an interesting correlation. The correct IF is indicated in most range settings with slight errors perhaps due to setting the dial pointer or drift due to ageing? Because the IF is off tune by 420KHz all dial responses are wrong by this amount. The answer is to realign the 1st IF amplifier. As far as the 2nd IF is concerned.. this is governed by a 7.62MHz crystal (ie. 9.72MHz - 2.1MHz = 7.62MHz). Oddly I couldn't see any 7.62MHz signal at the aerial terminal.

 Range

 Dial MHz

 Oscillator MHz

 Difference MHz

 Dial MHz

 Oscillator MHz

 Difference MHz

 1

 30

 39.7

 9.7

 20

 29.7

 9.7

 2

 48

 57.7

 9.7

 30

 39.7

 9.7

 3

 75

 85.3

 10.3

 48

 58

 10

 4

 112

 102.7

9.3

 75

 65.7

 9.3

 5

 145

 136

 9.0

 112

 100.67

 11.33

 Next, I tested the R308 with my recently repaired R & S signal generator and an audio wattmeter connected across the loudspeaker.

The IF turned out to be between 10.353MHz and 10.379 depending on the settings for AM, FM and bandwidth. The 2nd IF tuned between 2.138MHz and 2.142MHz.

Spot readings at roughly the same place on the dial were as follows..

25MHz = 24.507MHz; 39MHz = 38.525MHz; 62MHz = 62.03MHz; 94MHz = 95.06MHz and 125MHz = 123.37MHz

I'm continuing on a second page as I'll be attempting to align the revceiver correctly. I have a suspicion that the last owner reset the highest wasveband to receive the whole of the UK 2m amateur band and in doing so caused errors between the dial and the incoming frequencies.

 Goto Page two of the R308 restoration

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