Pye Cambridge 1225 Stereogram

[EF80TVVALVE]

Hi, I have picked up one of these today. After powering it up and cleaning the pots I have noticed that the right channel is lacking as much volume as the left. The balance control has to be close to full right in order to balance the audio. Does anyone know what I should start looking for? It is a 1970s transistor unit, it is also doing the fault across all of the settings (both radio and gram). Attached is a photo of the cirguit diagram for the amplifier, sometimes the left channel drops out dead but shaking the radio/amp chassis knocks it back in with a thud through the speaker, is this all likely to be caused by cold solders or is it likely that some of the transistors are due for replacement?

[paulsherwin]

The first place to look is the switchgear. I assume you've cleaned the contacts, but have another go. It can be difficult to wet all the contacts sometimes.

The second thing to check is the balance control.

Does the amp use Lockfit transistors? It's worth changing these for modern equivalents anyway (BC148=BC548, BC158=BC558 etc).

[EF80TVVALVE]

Hi Paul,

Thanks for the advice, I have cleaned all of the switching but I shall give it another go. I'll take a look at the balance control also.

Cheers
Bren

[Edward Huggins]

This model was the last Radiogram/Stereogram that Pye ever made in 1979 and 80. The failure of one channel is very common with these. It could well be a dry joint or cracked PC board.

[EF80TVVALVE]

There's no lockfits fitted to the amp or radio either

[Nuvistor]

Check the electrolytic audio coupling capacitors, if they are low capacity the volume will be reduced.
Frank

[EF80TVVALVE]

Hi Frank,

Thanks for the help, I think they could be the problem, the sounds is distorted, sometimes worse sometimes it is almost fine but still lacking in volume. The left side also drops out and returns and a tap on the cartridge usually brings it back but it also does this on the radio too. I have redone a few dry joints but the fault is still there. The transistors used in the amp are ED1402E (2× per channel), ED1701E (1× per channel), 2SA715D (1× per channel) 2SC1162D (1× per channel). The amp is basically a PCB with a mirrored image of the first half printed on the second so both of the channels have a seperate amplifier on the same board, I haven't seen anything like this before although no doubt others have. Am I right in thinking the audio coupling caps are after the first transistor in the circuit, I have photographed that part to make it easier to see.

Cheers
Bren

[Nuvistor]

There is one in the base of the first transistor as well.

Use some standard fault finding techniques, don't just change them. Either use a oscilloscope to trace the signal or signal generator to inject a signal at each stage, also do some voltage checks. You can compare the good channel to the bad one.

It will be easy to tell using those techniques if there is signal on one side of the capacitor and not the other.
There is also the possibility of a faulty cartridge in the record player as well as a faulty amp.
Frank

[Grubhead]

The thing to remember with stereo amps is that they are two mono amps put together. So you have at least one good channel. So you can compare the bad channel to the good. I find the buzz test useful. Take an insulated tool and touch the channel points. If it buzzers in one place it should buzz in the same spot on the bad channel. You can work forward from the input with this test. Quiet handy if you don't have a signal injector or no scope.
Any movement related sudden bursts back into life, are always faulty connections or bad soldering.