Pre3 has lost its right hand channel

[NickW]

Hi folks,
I need some help! I came to switch on my Pre3 last night only to find it has lost its right hand channel. It sounds like it is completely dead, except that every now and then there is a very low-level and dull-sounding "plop" sound from the right speaker. Then silence again for at least another 15 seconds or more.
I'm failing to find what is wrong and so am looking for ideas. So far, I've tried:
1. Swapping over the valves and changing both valves. No change.
2. Swapping input leads to another set of input connectors. No change.
3. Swapping over the output leads. The problem moves to the left hand channel. So it is definitely something inside the Pre3 connected with the right channel.
4. I measured all the heater voltages. All within spec at 6.5V sitting on top of ~ 39V.
5. I measured both the cathode voltages. ~ 4.5V (pin 8) and ~ 9V (pin 3) as per spec.

I'm stuck as to what to try next? Any thoughts anyone?

Thanks, NickW

[bob orbell]

The thing I would do Nick, is go over all points on the board with a DMM set to ohms, one end to the earth post and try to do left then right at the same point to see if there is a big diffrence in the reading, do also the input and output, and volume pot, with it switched off of coarse, you will find the meter will change rapidly when going through a capacitor, but get a reading and move to the other chanel which should be the same, if you find a reading that is short or open curcuit, you may have found the problem. You can also try a wiggle test with it on and playing, gently wiggle all the wires to see if it is a loose joint, be very carefull, there is 250volts DC inside, and that bites. BOB

[pre65]

A well insulated "wiggling stick" is a handy tool to have.

A length of PTFE rod is ideal.

[NickW]

Thanks for the tips so far. I'm going take another look this evening and dig deeper. I'll see if I can get any joy from resistance measurements and DC voltages. It's hard to see what is wrong given the cathode voltages look okay, the heaters are working, and the valves are glowing as expected. It's not even a very complicated circuit. It's worked for years now, and just a bid odd that I've lost a channel very suddenly!?
More tomorrow....
Nick

[NickW]

Hi guys, help !!!!!
I've now measured every resistor, capacitor and voltage possible. All are within spec, EXCEPT I modified the amp to have a shunt volume control, and...
The left (working) channel does as expected sweeping up from 0 to 47k ohms. However, the right hand channel only sweeps up to 22.2K Hmm??

I then hooked up a sine wave generator (set 1V pk-pk, 1 Khz) to the inputs so that I can trace the signal through the amp. However, and this may be the source of the problem...

On the left (working) channel I can see the sine wave rise to 0.5V pk-pk on the input to the PCB as I turn the volume control up to full. The amp has to be on of course for the signal to get through the relay switching board (hint!).

On the right hand channel, nothing comes through the volume control and on to the PCB. And if I look at the output of the sine wave generator there is no signal either. So something is shorting the i/p to 0V. There is no change if I move the i/p between CD, tuner, aux. So I'm now beginning so suspect either a failed vol control or something on the relay switching board. However, given that the vol is configured as a shunt vol control, there is always that series resistor in the signal line. So even if the vol control was a dead short (which it isn't because it sweeps up to 22.2K) then the series resistor should prevent the sine wave being shorted to ground at the input of the amp. So... It must be a fault on the relay switching board, but where? I don't have a schematic of the relay board; does anyone know where I can find one?

Any thoughts as to what has failed or is going wrong?

Many thanks, Nick

[NickW]

Hi guys,
please see my posting a minute ago for background....

But, blimey, I am a dummy!! It is obvious if the i/p to the faulty right channel is shorted before the in-line shunt vol resistor, then you would expect the resistance of the vol control to sweep up from 0 to 22.2k ish ohms because the in-line 47K is now connected across the pot. So it has to be a short circuit to ground BEFORE the in-line resistor. I can't see how the internal signal cabling could have suddently developed a short so my money is on the relay switching board. Thoughts?

Many thanks, Nick

[bob orbell]

Nick, could it be the bare wires conected to the phono socket earth touching the signal track, or the signal cable from the relay board. You may have to unsolder the cables at the volume pot. and check with a meter. BOB

[NickW]

Hi all,

I've found it! It turned out to be a tiny fleck of loose solder that had somehow managed to float from somewhere and land between two tracks on the relay PCB that are the Right channel o/p and ground. Where it came from, and why now after so many years of perfect operation is still a complete mystery. Still, it's all fixed now.

Cheers, Nick

[Dik Dolan]

Well done Nick, bet that's a relief=)

[bob orbell]

Well done Nick, give yourself a pat on the back, sit down and listern to some nice music with a glass of beer or whisky, and think of all those poor buggers out there with cheap stack systems. BOB