Proton 1100 Preamp - Extremely low output

[Chris __R]

Hi all, I recently picked up a Proton 1100 preamp from an estate sale - wondering if anyone might be able to give me some direction to troubleshoot. I don't have a lot of experience troubleshooting, but I've been learning on the fly trying to fix my Luxman M-120A (another thread).
I need to turn the volume all the way up to hear anything from either channel. If I switch to high output, same thing, but I don't have to turn it up as much.
This is for all inputs.
I have cleaned all switches and pots.
The bass and treble adjustments do make a noticeable difference.
Not sure if I should start with looking at transistors or if there is another obvious thing I should look at first.
Thanks!
Chris

 

[davideck]

There's a relay for mute delay. Can you hear it click on after power up?

 

[Chris __R]

Yes, it does click a few seconds after power up. I suppose there could be another problem within the relay itself.

 

[davideck]

If you hear the click, then I'd expect at least one channel to work. I'd say the relay is fine. Since the problem affects both channels, I'd first check all the voltages with a meter or scope.

I stumbled onto the schematic here;

https://audiocircuit.dk/downloads/proton/Proton-1100-pre-sm.pdf

 

[Chris __R]

If you hear the click, then I'd expect at least one channel to work. I'd say the relay is fine. Since the problem affects both channels, I'd first check all the voltages with a meter or scope.

I stumbled onto the schematic here;

https://audiocircuit.dk/downloads/proton/Proton-1100-pre-sm.pdf

Thanks! That's a much clearer version than the one I downloaded from HFE. It doesn't appear that expected voltages are labelled along the schematic - unless I am misreading. Any suggestion as to how to compare the voltage readings?

 

[davideck]

Start with "C" +30V and "D" -30V

Many of the voltages are labelled somewhere on the schematic.

 

[Chris __R]

Start with "C" +30V and "D" -30V

Many of the voltages are labelled somewhere on the schematic.

Found them. I had been focusing on the right channel - voltages are noted on the left...
Forgive me but I'm a bit of a newbie: when you say start with C +30V and D -30V, what does that mean in practice?

 

[avionic]

Forgive me but I'm a bit of a newbie: when you say start with C +30V and D -30V, what does that mean in practice?

He is requesting that you verify those two power supply voltages are present at point C & D.

Using ground reference. Use your multimeter with the black lead connected to the turntable ground binding post. With the red lead. Probe points C & D and record the voltages.

Set your multimeter to read DC volts.

 

[avionic]

n focusing on the right channel - voltages are noted on the left..

Should still be close to the same voltages on the right channel at the same electrical point.

 

[Chris __R]

He is requesting that you verify those two power supply voltages are present at point C & D.

Using ground reference. Use your multimeter with the black lead connected to the turntable ground binding post. With the red lead. Probe points C & D and record the voltages.

Set your multimeter to read DC volts.

Thank you. That makes a lot of sense now.

 

[Chris __R]

Well, I took the measurements and there are two matching that are way off.
Then, (shame on me for not doing this sooner) I opened the bottom of the unit to check out the underside of the boards.
Ugh - I had suspected that the prior owner had replaced some caps but what a mess. See pics.....
Not sure what the extra resistor is about - and there are clearly PCB points with voids.