Overhaul of a Marconi Model 219

May 2014

Trader Sheet 675

 An AC mains receiver dating from 1936, the set was basically working but needed some attention and a check to see if it was safe to use.

As the radio had obviously been worked on recently, and had a new mains cable and modern 13Amp plug, it was reasonable to plug it in and find out if it was worth proceeding.

With the back removed I could see that all the valves and the dial lamp lit up. Plugging in a long wire aerial proved it received several stations on long and medium waves but some were distorted and there was a slightly gurgly sound on speech. Also, as the aerial was really long, the set wasn't especially sensitive, so clearly work was required to restore good performance.

The model 219 is a solid wood-cased receiver with a chassis common to a number of models made around 1936, including the 239 and 249 radiograms, plus several HMV badged sets, the 381, 445, 622, 545, and several Columbia badged sets, the 381, 622 and 632. A similar chassis without the tuning indicator and "Q" circuit was used in the Marconi 209, HMV 425, 446 and 546.

This is the chassis

and the front view of the model 219

 As you can see above the set has five control knobs and there's a toggle switch (S14) on the side of the set which turns on the mains supply.

Tuning is the centre knob and wavechange the upper right knob. Volume is on the left, but what about the other two?

On the bottom left is a 3-position tone control which sets the mellowness and provides a clearer setting for voice, but what is the control knob at the bottom right?

The answer is that it's a special switch (S13) for enabling or disabling a clever method of muting the receiver whilst it's being tuned. A feature of the set is called "Mechanical Q Noise suppression". The idea is to mute the loudspeaker when turning the tuning knob whilst watching the signal strength meter on the left of the dial. To aid this feature, the wavechange switch has muting contacts which temporarily short the loudspeaker whilst setting the waveband because these old radios can very often have a loud scrunching noise when changing the waveband. The method of muting tuning noises relies on a friction operated switch coupled to the tuning mechanism. Whether this still works after 78 years remains to be discovered, as does the signal strength meter which is a moving iron meter connected in the anode circuits of the RF and IF stages. The latter did not illuminate in the initial air test, but it may be merely a failed lamp? The slightly puzzling fact is the meter readout is an illuminated section of dial which is said to get brighter as a station is tuned in. How does a moving iron meter manage this? Some dismantling is in order to find out....

One of the first tasks is to quickly test resistors and to replace C25 the audio coupling capacitor. This part can place the output valve into a positively biased condition if even slightly leaky. From distortion heard on strong stations I also need to check the automatic volume (AVC) components.

After confirming the various voltages on screen grids and cathodes are OK I can attempt to check the alignment of the set. Before I tackle this, just for interest I'll check its sensitivity and dial readings using a signal generator. Pretty straightforward as there's no short waveband and the Trader Service sheet is excellent showing positions of the various resistors and capacitors.

First some more pictures of the set... the label on the inside of the case which lists the valves.

MX40: RF/Mixer heptode

VMP4G: variable mu IF pentode amplifier (a 7-pin Mazda AC/VP1 has been fitted)

D41 Detector and AVC rectifier, double diode

MH4: Audio amplifier triode

MPT4: Audio output pentode

U12: Full-wave HT rectifier

 A rather crude potentiometer, R35 fitted to the rear of the chassis, used for "Hum Control" which sets the centre tap for the valve heater transformer winding. This is a throwback to the days when a directly heated cathode was used for the output valve and possibly unecessary? The contacts either end of the pot should be around 2 volts (with respect to the chassis) as the valves used have 4 volt heaters.

 Needs investigating for reasons of safety? The wiring (below) is the set of the connections to the loudspeaker, including a couple of hundred volts or so DC so not pleasant if they were to be inadvertently touched. The wiring is on the rear of the chassis but normally covered by the rear panel. This has been arranged so that the speaker can be disconnected allowing the chassis to be more easily be removed, but as I'm doing a complete overhaul I'm removing the speaker as well as the chassis mainly because (a) the speaker includes the smoothing choke whose disconnection would prevent HT getting to the valves and (b) the output transformer is part of the speaker assembly and to run the set without this would degrade the audio output valve. The reason is that, starved of anode current, the screen would consume lots of current. In this instance, because the screen resistor is 5kohm it wouldn't be too bad, but certainly without this resistor (many sets do not use a screen resistor) the screen would glow red hot and often this would damage it or result in gases being released which would make the valve "soft".

 

The Loudspeaker is used to mute the set

A particularly interesting aspect of this set is the muting system used for noiseless tuning. Above, I mentioned the smoothing choke. A clever way of reducing costs was to dispense with the large permanent magnet in a moving coil loudspeaker and instead use the HT current flowing in a large coil to provide the magnetic field. The coil can also serve as the HT smoothing choke thus saving about 5% of the manufacturing costs. This type of speaker is referred to a "mains energised" speaker. a slightly misleading term as it's DC rather than AC that produces the magetic field.

The designers of this set needed a large value negative bias supply for turning off the audio as part of their muting feature. The bias voltage could have been supplied from the mains transformer, but this would have requied a new design of transformer plus a rectifier. Instead, the designers thought laterally. The bias supply is generated by connecting the HT choke in the HT negative feed to the chassis between the centre tap of the full wave HT winding and ground. This results in a voltage less than that of the chassis at the transformer centre tap. Because the HT consumption of the set is reasonably constant, mainly due to the constant current drawn by the Class A output valve you get a reasonably constant negative bias voltage. In the circuit diagram the choke is L20 and has a resistance of 2.8kohm. The HT consumption is around 50mA, chiefly 30mA from the MPT4 so you can expect a bias voltage of around 140 volts. The HT wrt chassis is about 230 volts. The AC RMS voltage at the rectifier is indicated as 358 volts and when rectified will produce a peak DC voltage of around 500. This is less under load (because of circuit resistance) but will account for the HT supply plus bias of 370 volts.

The designers also used the bias voltage for controlling the output valve. Often this type of valve would be self-biased by using a fixed resistor and electrolytic capacitor in the cathode connection. For example, a 470 ohm resistor would have resulted in a bias voltage for the MPT4 of 14 volts. In this set however, the cathode connects to the chassis resulting in an extra 14 volts across the valve (an extra 0.4 watts). Bias is obtained by feeding the grid to a potentiometer comprising R32 and R33 which places 14 volts on the end of the grid leak. Pretty neat, but the idea is now almost lost in the mists of time you might say. In case you've forgotten, the bias voltage on the output valve will determine it's anode current and set the "class" of operation.

As an aside, the output valve is an MPT4. I first encountered an MPT4 in a neighbours set when I was called in to fix it. With the set turned on I grasped the output valve and attempted to pull it out. I hadn't seen a catkin valve before so did not anticipate that I was holding its anode. This point was brought home to me when I received a nasty shock from the HT voltage. I can remember the situation in clear detail to this day, sixty years later. The MPT4 in this model 219 is a standard glass valve however. I suspect that replacing a valve in a set from the mid 1930s was not altogether straightforward. The MPT4 can be a 5-pin or a 7-pin valve. So can the AC/VP1 IF amplifier.

If you read the text in the service notes you may be slightly puzzled. The text refers to "V3" being "paralysed" by the bias. I prefer "V4 is cut-off" by the bias. In their excitement to explain the operation the technical author wrongly named the valve. Friction-driven from the main tuning mechanism is a changeover switch which when operated in either direction places a huge negative voltage to the audio amplifier control grid. Once rotation of the tuning knob has almost ceased a muting delay which works out at 2 seconds is set by the 0.1uF capacitor C21. The idea is the user retunes the set to an alternative station whilst watching the signal strength meter and stops tuning when the signal indication is strongest. A bit hit and miss these days, but in the 1930s listeners would know exactly where their favourite stations were located on the dial. If the feature proved really annoying it can be disabled by turning off the lower right switch S13. Rather than use the term "turn off this annoying feature" Marconi use the term "when it is desired to receive uncalibrated transmissions" (which has the echo of pirate radio).

This takes us to the listeners' choice in 1936.

The dial below shows European stations in metres, cheaply printed on thin plastic and probably a lot darker in colour than when first made.

Many of these stations still occupy the same place on the dial in 2014, nearly 80 years later.

 Rear view. Of interest is the aerial trimmer AT on the extreme left. Most users would employ a wire in a sideways L shape, the horizontal section fixed between the house and a pole at the end of the back garden. Regulations covered the aerial size. An earth wire was usually specified in the user handbook (socket E) as well as a lightning protection knife switch.

You'll see the marking MA OFF between A and E. This is interesting as in those days it was common practice to use the mains supply as an aerial. One of the two mains connections (note there's no mains earth connection provided) was connected via a small capacitor to a flying lead and a black plug. The plug could be inserted into A to provide a long wire aerial (long=hundreds of miles) or MA OFF if one preferred the garden aerial. I recall back in the late 1940s my father bought a mains aerial which wasn't great. Even then the mains was starting to get noisy. Nowadays it's absolutely horrendous. A few people used their telephone wires but this was rare because not many people had a telephone. Although a phone line worked really well it was prone to loud crackles when anyone within 20 miles dialled a number.

The set featured connection for a gramophone to P.U. and E, selected by the wavechange switch.

Two sockets connected via the speaker harness to the audio output transformer were provided for connecting a permanent magnet extension speaker (EXT L.S.) and hum elimination was the H.C. control. The pink wire was a previous owner's aerial wire, its short length suggesting much greater sensitivity than achieved during initial testing.

 Model & serial number. No mention of paying royalties to Mr.Marconi as he owned Marconiphone! Also the "illegal" mains lead.

What responsibilty does one have I wonder? The original mains lead was a two conductor type and would typically have been inserted into a ceiling light socket fitted with a 2-way adaptor. There was never a provision for a mains earth and a connection to "Earth" via the rear socket was entirely optional. A modern PAT test would result in a resounding FAIL rendering use of the set in an environment other than in a private home rather a risky proposition. Even then, once a request has been to "make it safe" what's the position?

Certainly some words of warning are necessary. At least the 219 is an AC set with a mains transformer not a dodgy AC/DC set with a live chassis.

I've replaced the mains lead with a three conductor type, connecting mains earth to the chassis. The 13-amp plug is fitted with a fuse and all metal that could come into contact with a user will be at chassis potential. The back will be properly secured but where is the warning notice about its removal at one's peril?

"Mains energised" loudspeaker showing edge of HT-energising coil.

 Next, I removed the chassis from the case. Because the loudspeaker is "hardwired" to the chassis this had to come out also. This step requires four large scews to be removed with the case on its side, holding the chassis in place while the last screw is removed, then removing no less than eight screws holding the speaker assembly to the case. The assembly comprises the loudspeaker, the output transformer which drives it and the wooden mounting panel. There is no speaker material to bother about as the set uses gold-coloured metal pressed in an open basketweave pattern instead of cloth. This is a feature of Marconi sets of this vintage.

 With the chassis removed there is access to the underchassis components. With this model there is no inspection panel so there is no option but to remove everything. I noticed the set had been serviced, but certainly not recently. Maybe 50 years ago... although a new mains cable and wiring were evident from quite recently.

Here's a decent-sized underchassis layout drawing

 There are a dozen or so capacitors that look too old to be good. These are nearly all the type as shown below. Two or three empty metal tubes indicate some have already been replaced.

 Old replacements; the TCC low voltage electrolytic capacitor will almost certainly be in poor shape but the 8uF high voltage capacitor might be OK.

Surprisingly both were still really good.

A third replacement is a metal-cased Radiospares dating from the 50's.

 

 

The tone control switch.

 Not original.. 2-conductor mains lead connection at the top, and toggle switch connection at the bottom. The on/off switch is a single pole toggle switch mounted on the side of the case. This type of set using a mains transformer really needs a ground connection to be made via a 3-core mains cable, not a two core. The original cable had only two cores, and if this was so the receiver should fail any modern safety tests. Old mains cables used rubber insulation which hardens over the years then gets stiff resulting in the rubber breaking away from the wire and eventually causing shorts, hence the modern replacement.

I'll have to replace the mains cable and add an earth connection.

 The wavechange switch is not the usual Yaxley design. Rotary Yaxley switches have a self-cleaning action, but this old style switch will probably need cleaning to ensure good RF performance.

   

 I checked the capacitors by disconnecting one end and measuring their DC resistance. To my surprise all measured in the tens of megohms and the ropey looking replacement electrolytic measured 37uF with an ESR of only 0.8 ohm, so I left all these intact. Despite intending to fit a new audio coupling capacitor come what may, I forgot!

IF Alignment

 Turning next to improving the receivers sensitivity I made a mistake (I should have studied the circuit diagram). Setting the signal generator to 125KHz, the IF, I adjusted the two screws sticking out of the centre rear can, thinking these were standard dust iron cores. Whilst twiddling there was a crackling noise and the receiver went completely deaf.

I had no option but to remove the IF can and investigate what I imagined to be a broken core. Sometimes a core will disintegrate and come adrift from its screw, but I was surprised to find a pair of trimmer capacitors. These were positioned so that unscrewing them resulted in the head of the screw scraping on the coil. The upper coil was now open circuit evidenced by a tiny patch of bright copper. As the break wasn't too far from the end of the winding, I elected to detach the pair of coils, remove some turns up to the break, apply solder then rewind them back. This was a nasty exercise as the cotton covered wire was only a few thou thick, however I perservered and soon had the coil repaired and reassembled. To avoid a future problem I packed the space behind the trimmers with a piece of paper.

Following the service instructions I tuned the IF coils. Because of the very low IF this is done by injecting first 125KHz, then 123Hz and finally 127KHz, adjusting the four trimmers (in two cans) for maximum output as instructed. Failure to stagger-tune like this will result in instability as the gain achieved at 125KHz is such that you get positive feedback and oscillation.

Medium & Long Wave alignment... abortive attempt

 Next, I attempted to align the medium waveband. The dial settings didn't correspond to input frequencies and at about 520 metres the set went deaf. Either it's a short in the oscillator tuning condenser or a low voltage within the oscillator circuitry. As I had a similar problem with my Racal RA17, which turned out to be a worn out EF91, the problem might be poor emission in the heptode frequency changer. This valve has lost its metallising and may be original? Hopefully I can resolve this without replacing the MX40 which could be a tricky problem. It's probably a resistor gone high?

In fact the problem was a bent plate in the tuning capacitor, easily remedied by careful straightening.

Component checking

 Well, after 20 minutes of checking resistors and finding these pretty close as 78 year old resistors go I found a decoupling capacitor C11, which decouples the mixer oscillator HT to have a 500kohm leak. I fitted a new capacitor and found very little difference. At this point, better late than never, I checked the voltages. Much to my surprise I found the bias voltage was minus 240 volts and the HT to be 160 volts. The total voltage is OK (400v) but the bias supply was miles too high. Unplugging the output valve, which wasn't particularly hot (well it wouldn't be with 160 volts on the anode) the bias dropped to minus 140 volts. This is pretty high and reflects a current at the HT centre tap of 46mA or 82mA with the output valve in place. The latter is taking therefore 36mA which is correct. So what's drawing 46mA?

V1 should be taking 5.6mA, V2 5.9mA and V4 2.1mA totalling 13.6mA. This leaves 32.4mA. There's a potentiometer comprising R10 and R11 which can account for maybe a further 2mA perhaps, leaving more than 30mA. Heatwise this will be 4.5 watts so I don't think anything will be obvious. Maybe I should check screen and anode voltages to see if there's a capacitor or two leaking HT? Somewhere maybe 5kohm or less is present?

I disconnected the main HT electrolytic and this seems OK although the reservoir capacitor lead has been cut and a replacement fitted. I then checked the resistors in the area of the bias circuitry. R33 which looks 1950'ish is marked 470kohm and measured 547kohm so is OK, R32 should be 50kohm but measured 93kohm and R31 which is coded 360kohm perhaps, measures 3.4Mohm. I'm not too worried at present as all these are concerned with cutting off the triode during tuning and have no bearing on the excessive HT drain.

What might be important is a 1950s 33kohm which appears to be fitted in place of R24. This should be 35kohm but measured 900kohm and R25 a second 35kohm which measures 55kohm. This pair are in the feed from the HT line to the anode of the AF amplifier and almost certainly resulting in clipping and distortion on audio as the values would result in a very low triode anode voltage. Still not explaining the low overall HT however. I tried shunting the speaker energising coil with 10kohm. This increased the HT slightly and introduced some hum but little else. I then decided to check the resistance of the mains energising coil and much to my surprise it measured 5.6kohm, double the value shown in the service manual. This of course explains the strange HT readings as the current through 5.6kohm to give 160 volts (from above) gives 28mA and the likely current without the output valve is circa 16mA leaving only 9mA for leakage.. much more realistic. Shunting the energising coil with 10kohm increased the HT to around 200 volts, decreased the bias but introduced a hum and distortion on audio.

The speaker looks original and nothing appears to be wrong with it so what's the explanation... why should its resistance measure 5.6 Kohm instead of 2.8 Kohm? If it was 2.8kohm the HT would be correct and the bias half what I'm reading.

Strange distortion

 There's some weird audio distortion apparent. Could this be the result of excessive bias at the output valve grid? With a bias voltage of 240 volts, the bias voltage is double what the designers intended, putting the output valve more into Class C than Class A?

After puzzling over the problem I reckon what happened was the set failed around 1953 and the repairer discovered the smoothing capacitor C32 had failed short-circuit. This may have damaged the rectifier but the result would have been to place the whole of the HT across the choke L20. The peak value of the HT would be 358 times root 2 or 506 volts across L20 resulting in a peak current of 180mA or a peak dissipation of 91 watts. This would have destroyed the coil and the speaker would have needed replacement. Three new capacitors were fitted at some time in the past and, as the speaker was unusual, a replacement might have been taken from a scrap set (or perhaps ordered incorrectly from the manufacturer, or indeed correctly ordered but incorrectly supplied). Testing the repaired set would have given little clue to the error and, unless voltages were measured, there would be nothing to indicate anything was amiss. The HT should have been about 230 volts and the bias about 130 volts. In fact the HT is 160 volts and the bias 240 volts.

The low HT would do little to degrade the set except to prevent full audio output being attained (see below) but as this is subjective it's unlikely the owner would have noticed. The very high bias voltage would not have been apparent except to slightly increase the muting period and to increase the output valve bias. In that respect, any leakage in the audio coupling capacitor (and there's often some) would balance some of the additional bias.

Referring to the figures for the MTP4 valve, the audio output from the set would have been about 2 watts when new using rough figures (27.5% of the input power 30mA x 240v=7.2w) reducing to around 1.3 watts with the wrong speaker (27.5% of the input power 30mA x 160v=4.8w).

However, with the high resistive value of the HT smoothing choke, there is powerful negative feedback which masks a problem...read on.

Tracking down the weird distortion.. finally changing the audio coupling capacitor

 Lift repairs slackened off today so I decided to look again at this Marconi radio. Armed with a couple of valve data books, the underchassis layout drawing, and the circuit diagram I set about working out why the set was distorting. It was a sort of warbly distortion with some hum. First I tried to measure the output valve grid voltage.

There's a network of resistors feeding minus 240 volts, ground and the anode of the audio amplifier valve to the grid of the MPT4 output valve. This is a little unusual having a B5 base, no top cap but a side connection for the screen grid. The anode is fed through the complicated loudspeaker circuitry and the screen via a resistor which looks a trifle unhappy. I'd worked out that the MPT4 should have had around 20 volts negative bias and this was the level at the potentiometer feeding the grid circuit, however the voltage at the grid stopper sat at positive 10 volts. The penny dropped. Although I always advocate changing the audio coupling capacitor I'd neglected to do this because the set appeared to work properly and the output valve was not hot (usually a leaky coupling capacitor results in a high anode current and a really hot valve). I cut the capacitor wires and fitted a convenient new capacitor in its place. Switching the receiver back on I found the distortion had vanished and the grid bias was back to its proper level unaffected by leakage from the audio amplifier anode. Checking the discarded 0.1uF coupling capacitor I found it to read a couple of megohms. Not low enough to cause trouble elsewhere in the circuit, but connected to the output valve grid, enough to forward bias the valve.

Much to my surprise I found the HT voltage was now almost normal; over 200 volts. With the MPT4 drawing excessive current the loudspeaker choke had dropped the HT sufficiently to avoid a red hot output valve. It was cool enough to suggest it was correctly biased but only because HT voltage had dropped by 50%. Because the valve was heavily forward biased the clever muting circuit which operates via a clutch when the tuning knob is turned wasn't working either.

The HT choke was producing lots of negative feedback resulting in a set of voltages allowing the set to work in such a way as to mask the real problems.

Once the audio coupling capacitor had been replaced the set was transformed. Performance improved, the AVC assumed its rightful characteristics, and the strange distortion on strong stations disappeared. I can now align the remainder of the circuits and took a look at other problems starting with the dial assembly.

The dial had a plastic scale which had been placed on a gold-sprayed metal backing plate. The glass carried markings for the waveband indicator and the signal strength meter. Although the former was working properly (a mechanical arrangement) there was nothing showing up at the signal strength aperture so I proceeded to dismantle the parts to discover the reason.

The Tuning Indicator

 When the set was made in the five years preceding WW2 there was loads of competition from manufacturers and there were many gimmicks to push particular models. Some, like Murphy for example chose to produce sets with particular eye catching cabinet styling, other makers offered weird and wonderful tuning aids such as the new "magic eye" valve or dials lit with flashing lamps. This set was sold in 1936 and was slightly early in design for the new magic eye and its designers had come up with an illuminated signal strength indicator.. read on.

I found a very strange moving iron meter held within a black sprayed metal enclosure. A flimsy flattened aluminium pointer was resting in a slot in a moveable disk. I connected the terminals to a low voltage power supply and discovered the pointer moved when the current went up to 10mA or so. Sometimes the pointer stuck and while I was examining it the thing fell out. With a magnifying glass I found a tiny rectangular block of clear plastic at the bottom of the slot where the pointer had been. From marks on the pointer I would guess someone had fiddled with it before. After puzzling over the thing I decided to place the pointer directly over the plastic block and then apply a tiny amont of superglue to hold it in place. I suppose this option was not available to the previous repairer in the 1950s? Having done this the needle moved without jamming up as it had been liable to do previously. Next I thought I'd test it. It didn't work and then I noticed it couldn't work because there was a part missing. In order to illuminate the scale, light from the dial lamp, fitted behind the meter, passes through the clear plastic block in a direction parallel to the dial assembly and needs to be reflected through 90 degrees so it can illuminate the scale on the set's plastic dial. To do this a mirror is required (like the one in a periscope). There's a slot into which the mirror must have fitted, but it was missing. I'll need to make a reflecting surface 15mm long by 5mm wide...

As very few useful things are thrown away in our house I asked my wife to look for a suitable mirror which I could cut down. After only 20 minutes a small mirror was discovered as part of a lipstick case. It was exactly the right width and after scoring with a glass cutter was trimmed to 15mm. I fitted this into the metal enclosure with the small tabs provided and, peering through the slot for lamp illumination I found I'd made a tiny periscope. I refitting the restored device to the rear of the dial and fixed the lamp in place. I connected a low voltage power supply to the terminals and switched on the radio. Increasing and decreasing the voltage to the device produced an illuminated bar on the dial.

After the test I rewired the meter into place. The method of operation is as follows.. The meter measures the current consumed by the IF amplifier valve and the flattened pointer moves downwards blocking the light passing through the plastic block under the needle. As the current drawn by the IF amplifier valve is reduced by the negative AVC voltage resulting from a strong station the pointer moves upwards allowing light coming through the plastic block to pass below the flattened pointer, hit the mirror, and illuminate the dial. As the station signal strength increases the pointer moves further upwards and the illuminated bar increases in size. For all this to work successfully the AVC circuit has to be in good order and the IF amplifier has to work exactly as the designers had planned in 1935/1936.

In practice the indicator works after a fashion: not as good as originally intended but good enough to be useful. There is some residual illumination when a station isn't tuned in, caused partly by huge present-day medium and long wave noise levels. Long gone are the days when you could hear a pin drop whilst tuning medum waves. Would you believe you could hear local US radio broadcasts in the UK by tuning to the spaces between European broadcasts? Other reasons why the indicator isn't quite right are caused by the HT and bias voltages being miles adrift because of the wrong loudspeaker energising coil and a different IF amplifier valve to the original being fitted.

 The picture above shows the tuning indicator in position (on the left) after the dial had been removed. On the right is the circular plate fitted immediately behind the dial with a shaped slot (at 4 o'clock) through which light passes from a lamp to illuminate the station markings. Another gimmick, in place of an ordinary pointer.

 View of the moving iron meter with the back of its case detached. I suspect the pointer might have been originally straight?

View looking into energising coil. The plastic block acting as a light-guide is at the base of the pointer inside the section.

Above.. the reassembled meter with the new periscope mirror in place. The mirror is only 5mm x 15mm.

 

 

 

 These pictures show the three labels stuck to the glass dial. The centre of the station name display is positioned over the gold-painted circular metal plate with the aperture for the illuminated pointer.

"NATIONAL" is the marking for the long wave station at Droitwich, currently BBC Radio 4 Long Wave. This had been originally on 1600 metres but in 1934 moved to 1500 metres or 200KHz. When new long and medium wave frequency allocations were instituted to overcome interference from increasing numbers of stations, this agreement was based on precise channel spacing, forcing the BBC to eventually retune Droitwich to 198KHz in 1988.

Now to realignment

 The set has fairly standard tuned circuits except that there's a three-gang tuning capacitor having an extra tuned circuit for bandpass operation.

This model is a superhet and it's oscillator works at a frequency of 125KHz higher than the incoming signal. This is evidenced by the oscillator tuning capacitor being smaller than the RF tuners. As the set covers around 200 to 550 metres (1500 to 545 KHz) for medium waves and 1000 to 2000 metres (300 to 150KHz) long wave the oscillator has to track 1625 to 670KHz and 450 to 275KHz respectively. Because of potential image problems there's a special tuned circuit in the input to reduce the effects. There's also a preset trimmer at the rear of the chassis in case the user's favourite station needs tweaking.

Alignment is straightforward once you've figured out where the trimmers etc are located. During alignment I'd noticed the set wasn't as sensitive as I thought it should be. I'd also noticed the signal strength meter wasn't showing much movement. Enough to indicate precise tuning and that's all. I found I had to use a test signal of 100uV to produce a decent response although with a long wire aerial sensitivity wasn't an issue. As a normal listener would most likely be using a random wire aerial of not too great a length I decided to track down the cause of the sets deafness.

Finding the reason for low sensitivity

 As I hadn't tested the valves this was the first objective. Using my AVO tester on the frequency changer, MX40 and the IF amplifier, AC/VP1 (a decent substitute for the original VMP4G), proved both were in excellent shape. Just to be sure I also checked the MH4 audio amplifier which also turned out to be 100%.

Next I rechecked all the capacitors and resistors but found nothing amiss.

I tested the RF input circuits which are unusually complicated but all were fine, however by trial and error I found a poor capacitor in the RF area. Cutting this and adding a modern capacitor improved sensitivity a little. Next, I removed the screening can from the previously repaired IF coil because I'd found the upper coil wasn't peaking correctly. I removed some turns from the repaired section and got the coil to peak nicely at 125KHz. By now the set was responding to test signals of around 10uV, but was still a bit listless with a short aerial.

During checking of voltages I found the bias voltage on the IF amplifier was a bit high at minus 3.7 volts. Bearing in mind the abnormally high voltage of the negative bias supply due to the wrong loudspeaker energising coil plus the fact that the IF valve is self biased with a 500 ohm cathode resistor (which was nearer 600 ohms) it crossed my mind that the valve may be operating close to cut-off. This would be compounded by the AVC voltage which increases the negative bias when stations are tuned in.

I couldn't do anything about the loudspeaker, but I could short out the cathode bias resistor. I tried this and was rewarded by a huge increase in background noise and sensitivity was such that the 10uV test signal went from just detectable to really loud. Success. Tuning across both wavebands proved the set was performing really well and as an added bonus the signal stregth meter was working properly. It went from a third to full illumination instead of just flickering on strong stations.

In Summary

 An interesting old radio with several unique features. It's a pity the loudspeaker wasn't changed correctly back in the 1950s as the HT voltage and the bias voltage are miles out and make the performance substandard.

Providing a decent aerial is used the owner should find the set is satisfactory and its mellowness of tone and the smell of hot valves will make it a lot different to modern radios. It's a great pity that the UK longwave transmission is currently unreliable and threated with closure. Still there's a decent Irish station and several French transmissions still on longwaves. Medium wave broadcasts seem to be rather limited in content, but after dark the band should be jam packed. Interference can be abominable and will require a suitably positioned aerial. At my workshop medium waves are plagued by noises including my network camera which puts out an awful woodpecker noise and gets switched off when listening to broadcasts. It used to be fluorescent light interference that was worst but this is nothing compared with interference from switch-mode power supplies for modern lamps.

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