Leak Stereo 30. One channel faulty.

[Goodizzy]

I purchased a Leak Stereo 30 some years ago and partially serviced it myself. I then put it up on the shelf and forgot about it until after I moved.

I say partially serviced becaused I was able to pass signal through both channels but there were still a few problems. The amp would motorboat if the volume was turned up, and there was pink noise on both channels. In order to alleviate the motorboating, I replaced the filter caps.

.. I also chose to recap all the 50+ year old electrolytics while I was in there. This was a poor decision at the time, as I was still somewhat new to soldering any type of PC board. This seemed to result in some cold solder joints and the DC fuse blowing.

I returned to this amp a few days ago and reflowed all the suspect solder joints which alleviated the fuse issue. However, I no longer have sound on the right channel. I have the amp hooked up to some older, throw away Isophone satellite speakers, and all I can hear from the right speaker is a very low hum.

I had previously cleaned all the switches, but as its been sitting awhile, I hit them again with cleaner. So I took some voltages at minimum volume on the transistors and this is what I got:

(All voltages negative, with the order being: emitter, collector, base

#. Left. Right.

T1. (0.1, 4.6, 0.2). (0.1, 4.7, 0.2)
T2. (4.5, 12.3, 4.6). (4.6, 11.5, 4.7)
T3. (4.2, 13.9, 4.3). (4.2, 13.1, 4.3)

T4. (1.4, 3.8, 1.4). (0.1, 0.1, 0.1)

Its when I got to T4 that it seems something went wrong. T4 is an OC44 and germanium so I assume something has gone bad? Most of my limited experience is with point to point valve amps. I measured the rest of the transistors and all had voltage discrepancies. The rail voltages at C30 and C2 measured fine and the rail voltage at C38/C39 was -40.2V, so a little under -42V but close. (I have the amp voltage selector on 130V for US)

The remaining voltages:

T5. (3.5, 22.9, 3.8). (0, 39.2, 0.1)
T6. (23.1, 39.6, 23.2). (0, 39.2, 39.5)
T7. (23.3, 0, 23.2). (0, 0, 39.8)
T8. (23.4, 40.2, 23.5). (0, 40.2, 0)
T9. (0, 23.4, 0). (0, 0, 0)

The output transistors T8 and T9 are AD140s, like in the schematic. Apparently they vary with units? Also of note, I suppose, is that I used a cheap digital multimeter to measure the voltages.

The left channel is noisy regardless of volume, which according to the brief troubleshooting section in the service manual may be indicative of a noisy T4. (Excessive noise with volume control at minimum)

What is my next move here? Pull the screen lead and measure to the other legs for leakage? I have read about the dreaded AF118s- could they also be contributing? I can't read any of the output transistor cans without bending the leads- which I'm hesitant to do!

Thanks for anyone that can point me in the right direction.


Edit:

I forgot to mention that I tried to set both V1L/R and V2L/R. I was able to set V1R on both channels and it held after fifteen minutes or so. The same for V2R in the left channel.. However, the right channel would not set. This is also in the service manual as (Test point voltage cannot be adjusted to -23V). The fault listed in the manual indicated failure of T4 or T5.

[steve1010uk]

Have you checked C 31 ?

Steve

[Goodizzy]

Yes- C31L measured -14.5v, C31R was a little under at -13.3v.

[steve1010uk]

With -13v on C31 it seems there may be a short to chassis between the collector of VT4 or the base of VT5, a resistance check between this point and chassis may throw some light on the problem,
if low resistance then removing the collector of VT4 or the base of VT5 one at a time may narrow down the short which is possibly due to a short in one of the transistors VT4 VT5 .


You can compare resistance checks between channels

Steve

[Goodizzy]

T4 measures around 10k in both channels. T5 measures around 9k in both channels. Measured with my analog RS multimeter. I didn't bother lifting the legs...

[steve1010uk]

I can only suggest carefully checking things around VT4 and VT5, there's a 22k resistor ( R46 ) between C32 and collector of VT4 with around 13v at one side but nothing at the other so something is drawing current through this resistor.
VT4 could be conducting heavy due to too high voltage at it's base but this is not supported by your original voltage measurements VT4 could be leaky or VT5 leaky between base / emitter

Does adjusting P6 have any effect ? i wonder if the AF118 suffers from the dreaded "Tin Whisker " fault, whats the resistance between VT5 base and chassis, try the meter leads both ways around does the resistance change ?

Steve

[Goodizzy]

Adjusting P6 has no effect on the V2R voltage. I measured with the leads reversed and still the same.

I will try to keep going though I imagine it might be time to pull a leg on T4/T5 and measure for definite...