Repair of Hi-Fi Equipment
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A number of items arrived
at the Radio Museum today for repair (July 2026)
These are as follows:
Two Leak Stereo 20 Amplifiers
Two Leak Stereo Pre-Amplifiers
Teac Open Reel Tape Deck
These had differing reported
faults and from a cursory inspection they needed new components
to restore them to working condition.
Below are pictures as they were
received plus a view of the undersides of the amplifiers.
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You can see in the picture,
above the circuit diagram (click this for an larger drawing),
several points of interest. In the centre appears to be evidence
either of a design error where the engineer under-rated the resistors
or perhaps there are failed components elsewhere. Looking at
the capacitors there must be several suspects for leakage or
loss of capacitance. The main smoothing and reservoir capacitors
(top left and right) will need checking as if these are in poor
condition there'll be hum on the audio.
Comparing this view with the
second amplifier you can see that all four resistors down the
centre of the board are supposed to be 270 ohms, however those
burnt ones both read about 7Kohms. Why are the resistors burnt
and very high in value? Well, anyone with knowledge of valve
circuitry will tell you that the capacitors providing audio from
the preceeding amplifier stage might be leaky. Any leak will
feed some of the HT line to the output valves control grid via
the anode load resistor of the ECC83. The presence of any undesired
positive voltage will raise the anode current of the EL84. This
in turn will increase the current through its cathode resistor
and result in excessive anode dissipation.
The cathode resistors are 270
ohm and the design voltage at the cathode is 10 volts. Hence
the resistor will dissipate (10x10)/470 or about a quarter of
a watt. The anode current of the EL84 should be about 20mA but
if the grid voltage moved more positive the anode current would
rise and the cathode resistor would soon fail.
Here's something that will really
upset the purist. Those large 0.25uF capacitors are marked with
the outer foil connection at the banded end. You'll note that
in one pair (in both amplifiers) the foil ends are connected
back-to-front.
If you look to the right of
those large capacitors.... where are the two capacitors which
are present on the second amplifiers circuit board? They're the
cathode decoupling capacitors which should be like those two
green 50uF capacitors. Of course, being in close proximity to
the hot resistors they would have failed and presumably someone
has clipped them off?
Those four 470 ohm resistors
seen in the second amplifier are marked 5 watts (a bit of an
overkill for a dissipation of a quarter of a watt !!) but clearly
someone has solved the resistor overheating problem but failed
to correct the reason for this. Did the last user see a problem?
Unlikely, as the audio would be reasonable but the output valves
would be running red hot.
In fact if one looks at the
driver stages there are additional leaky capacitors. These will
apply an undesired positive voltage to the control grids of V2
but what would be the result as far as a user is concerned? The
driver valve would be running too much anode current and if the
audio input was high, but ws still within spec, would clip the
output resulting in distortion... however this scenario would
require a very loud audio output and in the average listening
environment would be most unlikely. This means the amplifier
fault would remain undetected.
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The second amplifier whose
circuit board is shown above (said to have been fine since purchased
secondhand) carries a date on the chassis of 1963. I measured
the value of all the resistors and found lots in really poor
condition. As a matter of interest why are the EL84 cathode resistors
(R14/R15) 5 watt? My guess is that many years ago the amplifier
was overhauled by someone with limited knowledge of component
failings. The chief reason for excessive anode current is a leaky
capacitor carrying a positive voltage to the control grid of
an output valve so I disconnected the four large 0.25uF capacitors
(C4R/L and R6R/L)and tested them. I used a bench supply providing
a maximum of only 31 volts and found that three capacitors leaked
between 12 and 15uA meaning that they would have a resistance
leak at 31 volts of about 2Mohms.Bearing in mind the ECC83 anodes
are sitting at over 200 volts this is really bad as up to 40
volts could end up on the EL84 grids. In fact this isn't the
full story because two of the four 470kohm ECC83 grid leaks (R12/R13)
measured 660kohm and the other two were open circuit.... meaning
that the whole anode voltage of the ECC83 is available at the
control grid of each of two EL84s.
Anyway cutting to the chase
the picture below shows the components that I replaced... then
a view of the repaired circuit board 63 years after it was made.
Next I'll need to apply mains
and see if the new components restore operation... hopefully
the valves are in fair order.
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Before applying mains
power I removed all the valves except the GZ34 HT rectifier and
carried out a few basic checks and soon found the heater supply
to the left channel valves was peculiar. After several tests
I discovered there must be a dry solder joint (in the green circuit)
under the circuit board but as the board is mounted in the wiring
it wasn't a good option to look at the wiring, underneath it,
carrying the dry joint.
The electro-mechanical design
of the amplifier is intended to impart a look of symmetry and
because of this one can work out the likely way the heater wiring
is done. Using a little logic, the most likely place for the
dry joint is close to the mains transformer where the heater
supplies to the left and right channels split (the point of maximum
current). This means that supplying a secondary feed from the
heater winding to the nearest valve in each channel will overcome
the problem. The heater winding is centre-tapped to ground leaving
a green circuit and a grey circuit. I added two pairs of wires
from the 6.3 volt winding to the heater pins of the nearest EL84s.
I then checked that all the valve holders carried these connections.
The next step was to check the
HT circuit. This was satisfactory so applied 240 volt mains.
The HT rose slowly from 383 volts and eventually measured 387
volts. The heater circuit measured 6.8 volts AC.
I noticed a grommet in the end
of the chassis and below this was a shorting wire in the transformer
primary marked "switch" . As there's no ON/OFF switch
in the amplifier I removed the grommet and added a toggle switch
in the hole. This will make testing a lot easier.
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I refitted all the valves,
connected a loudspeaker in the left channel and powered the amplifier.
The thing warmed up and there was a low background hum, probably
because the input was open circuit.
The following table lists the
voltages measured at the valve pins. The raw HT was 329 volts
(325 volts), smoothed HT was 305 volts (310 volts)and the HT
at the driver was 285 volts (295 volts) with the input valve
HT at 118 volts (120-195 volts).The DC voltage across the 100
ohm HT smoothing resistor was 14.4 volts indicative of an HT
current of 144mA. All figures in brackets are those taken from
the circuit diagram. The circuit diagram indicates that the HT
current is 150mA so my figure of 144mA is pretty good. Discrepancies
in voltage reading at the input stages may be due to R20 and
R19 being slightly high.
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VALVE |
PIN 1 |
PIN 2 |
PIN 3 |
PIN 4 |
PIN 5 |
PIN 6 |
PIN 7 |
PIN 8 |
PIN 9 |
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ECC83-C |
100 (135/160) |
0 |
1.0 |
H |
H |
101 (135/160) |
0 |
1.0 |
H |
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ECC83-L |
210 (215) |
22 |
31 (37) |
H |
H |
208 (210) |
21 |
31 (37) |
H |
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EL84-L |
NC |
0 |
11 (10) |
H |
H |
NC |
305 |
NC |
307 |
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EL84-L |
NC |
0 |
10.5 (10) |
H |
H |
NC |
305 |
NC |
307 |
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ECC83-R |
213 (215) |
19 |
33 (37) |
H |
H |
217 |
19 |
33 (37) |
H |
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EL84-R |
NC |
0 |
9.7 (10) |
H |
H |
NC |
304 |
NC |
306 |
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EL84-R |
NC |
0 |
9.2 (10) |
H |
H |
NC |
304 |
NC |
306 |
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I was puzzled by the discrepancy
in the common ECC83 voltages so I looked at R20 and R19. These
cannot be read in-situ because of the large electrolytics. I
unsoldered them and found R20 was 5.1Kohm and R19 was a whopping
245Kohm. I fitted a 4.3Kohm and a 43Kohm in their place and sure
enough the ECC83 anode voltages were a lot higher.
Checking the current there was
60.8 volts across 4.3Kohm which works out at 1.4mA or about 0.7mA
for each anode. Across the 4.3Kohm I measured 19.5 volts or 4.5mA
which means that the left channel driver is sinking about 1.5mA
per anode. The total HT current is now 148mA.
Each of the EL84s is drawing
from 34mA (=10.37 watts) to 41mA (=12.4 watts) although this
includes the screen grid.
I'll need to commission one
of the Pre-Amps to see the final output power.
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VALVE |
PIN 1 |
PIN 2 |
PIN 3 |
PIN 4 |
PIN 5 |
PIN 6 |
PIN 7 |
PIN 8 |
PIN 9 |
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ECC83-C |
185 (135/160) |
0 |
1.8 |
H |
H |
187 (135/160) |
0 |
1.8 |
H |
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ECC83-L |
210 (215) |
20 |
39 (37) |
H |
H |
208 (210) |
20 |
39 (37) |
H |
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EL84-L |
NC |
0 |
11 (10) |
H |
H |
NC |
305 |
NC |
307 |
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EL84-L |
NC |
0 |
10.5 (10) |
H |
H |
NC |
305 |
NC |
307 |
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Having analysed the condition
of the parts and got it working one might wonder what sort of
audio output power is this amplifier capable? Well, it was designed
many years ago when output figures were true and not faked. The
key parameter is distortion and the Leak Stereo 20 can
deliver (per channel) 10 watts RMS at 0.1% distortion at 1000Hz.
Frequency response is from 20Hz to 20KHz. Bump up the input and
one can get 11 watts at the expense of some distortion. |
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Now, I'll tackle the first
of the two "Leak Point One Stereo" pre-amplifiers carrying
the stamp "Lund Brothers October 1958" |
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Below are views
of the pre-amplifier |
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Above you can see the provision for
adjusting three of the inputs to make them match in volume, presumably
to match the volume for the Tape input which doesn't have a trimmer
pot. The octal plug connects via a cable to the Leak amplifier.
This connection carries audio which is slightly odd because one
would have expected a twin screened phono lead to have been specified.
Looking at the markings above the phono outputs are marked "RECORD"
and these are fixed and do not include the volume control. I
guess these could be used to connect to the amplifier phono inputs,
although the octal connections with their drawbacks are still
in place.
Below.. the top view showing
the four EF86 valves. The preamp appears to have been modified
long ago when the volume control twin gang potentiometer was
replaced. The original had a single gang switch which was wired
to those twisted yellow wires. That feature was designed to be
used to switch the mains supply in the associated power amplifier,
however in both the Leak amplifiers that arrived with the preamp
that particular feature wasn't wired up.
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The black cylindical
Hunts capacitors all leaked around 40uA (= about 750Kohm) with
my 31 volt power supply connected.
These capacitors are oil/paper
types no better than others of the same composition.
The yellow Plessey electrolytics
mostly measured 75uF.
I fitted fourteen 100nF x 500V
chips and four 47uF x 63V electrolytics. that green capacitor
in the picture below was marked 8uF and measured 10uF with zero
ohms ESR so I'll leave it in place.
The view below shows the circuit
board carrying replacement capacitors.
I checked the resistors and
found most were high but within 20% except the 10K which were
up to 15K.
I decided to power up the preamp
and compare the voltages against those in the circuit diagram
below and for reasonable balance between the left and right circuits.
If there's any serious imbalance I'll check and replace resistors
if necessary.
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Click to see the circuit
diagram full size
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