Ok. Thanks for the photos. Those will do for now.

It looks like you have some work ahead of you. You will have to remove the top board in order to get access to the bottom board. This means firstly removing the control knobs and then the retaining nuts that hold all the jacks and controls on to the front panel, also the two screws holding the BALANCED LINE OUT.

Before you go ahead and do that, turn the MASTER VOLUME to 0 and describe as best as you can the noise you are hearing.

 

It sounds like when a radio station stops broadcasting in a storm, just static; with the occasional lightening making some crackling noise
as you listen to your radio. That happens a little bit also. Nothing major but once in awhile some crackling and pops if you try an plug in to play the amp with a guitar.

 

Here is the plan of attack.

When you turn MASTER VOLUME all the way to 0, the sound from the speakers should be dead quiet.
Looking at the circuit schematic below, if you were to unplug the connector J15, five red wires from the bottom board, there should be no signal to the main amp. You ought to get the same effect if you short either lead of R75 to GND.

Here are possible culprits (in order of suspicion):

1. Q4, 2N5460, P-channel FET
2. C48 47μV/35V
3. D19 (1N914)
4. IC6
5. C46 10μF/16V
6. ±15V supply to IC6

I would do the following first:

Unplug the 8-wire connector (red wires) that connect between the top and bottom boards.
Confirm that the noise is still there.

After gaining access to the bottom board, disconnect the top board and put it aside.
Unsolder Q4 or lift the D-pin of Q4 from its solder pad. (Make sure you make a note of the orientation of Q4 before removing.)

(Or you can unsolder one lead of C48 instead of tackling Q4 as above.)

Check if noise goes away.

 

I will report a little as I do them. Leaving tomorrow for a few days not sure when I will get back to this maybe some on the weekend but not sure. So far.... unplugged the eight from the top and it made no change to the noise. Got the upper disassembled and located Q4. I will work on the lower.

 

I will report a little as I do them. Leaving tomorrow for a few days not sure when I will get back to this maybe some on the weekend but not sure. So far.... unplugged the eight from the top and it made no change to the noise. Got the upper disassembled and located Q4. I will work on the lower.

Before you tackle Q4, there is one quick simple test you can do.

Find the connection between C46 and R75.
Solder a jumper wire between this junction and GND.

If the noise is still there, then the noise is coming from Q4 circuitry,
If the noise goes away, then the noise is coming from IC6.

 

Too late I unsoldered it and put it in my little tester I got off ebay here are it's readings 1=D, 2=S, 3=G Reading I (eye) = 1.6ma, VGS = 0.12v. Of course I do not know what it should read but I do verify it is a 2N 5460 M maybe motorola 351.

Reassembled and plugged in with very low noise hum. This is without the Q4.

 

I don't have one and not sure my local radio shack has one either. However, I'm trying to figure out if this is shot or not.

 

I don't have one and not sure my local radio shack has one either. However, I'm trying to figure out if this is shot or not.

You cannot test Q4 with a simple test meter.

Plug in your guitar or any music source and describe the quality of the sound, noise wise, without Q4.

If the noise is still there we have to keep looking for the source of the noise.

 

Has mid-range hum not awesome/loud hum and might be a bit treble rich with the guitar. So if this is not it, should I resolder it back in? The noise did decrease without Q4 but that was when nothing was plugged in.

 

Something interesting - all the IC's on the top and bottom pre-amp boards check in the 15volt range (touching the 8th leg on the chips) but 2 IC's on the lower board near Q4 read 16v almost 17v. I wonder if that is because I am running it without Q4 soldered back or if this is just an indication of where the problem lies?

 

Something interesting - all the IC's on the top and bottom pre-amp boards check in the 15volt range (touching the 8th leg on the chips) but 2 IC's on the lower board near Q4 read 16v almost 17v. I wonder if that is because I am running it without Q4 soldered back or if this is just an indication of where the problem lies?

No. Don't solder Q4 back in as yet until we find the source of the noise.
Q4 has nothing to do with the sound quality. Its function is to delay the signal to the main amp only during the power ON startup period.

btw, you do not have to reassemble the top board. You should be able to run all future tests with the top board removed.

Can you post a photo of the area around IC6? The next step would be to replace IC6.

I will wait until you return before proceeding with more tests.

 

Yes we are going to leave pretty early but I will post a pic. When I got this amp the guy had busted off one of the sliders and the traces were messed up also, so with a little wire I replaced the missing one. I hope the noise is not coming from here. In fact, I am almost sure it isn't because I did try the amp before without the ribbon cable attached to the lower board but I thought you might want to see it. These sliders do effect the volume though.

 

The MASTER VOLUME is the final control before the main power amp. Once this is set to level 0, all sound from all prior stages are eliminated.
Any noise or hum would be coming from circuitry after the MASTER VOLUME control. We need to focus on this single aspect of the circuit.

I have to examine what capacitors you have replaced. We need to consider more carefully the power supply to opamp IC6.

 

Note that there appears to be some labeling errors in the circuit documentation.

 

Also there is some confusion on the labeling in the low voltage power supply.
One has to trace the PCB to confirm the actual power supply connections.

The RED circled labels appear to be incorrect. The GREEN markups are what I would expect.

In any case, C52 and C53, 100μ/35V and C54 and C55, 47μF/35V would be candidates for replacement.

Two zener diodes both labeled as D22 are also candidates for replacement.

 

Something interesting - all the IC's on the top and bottom pre-amp boards check in the 15volt range (touching the 8th leg on the chips) but 2 IC's on the lower board near Q4 read 16v almost 17v. I wonder if that is because I am running it without Q4 soldered back or if this is just an indication of where the problem lies?

This needs thorough investigation.

We want to make certain that the +15V and -15V supplies are actually connected to the output side of R82 and R83, i.e. filtered and zener regulated as in the marked up schematic above.

I know this is extra trouble for you. I need to see the PCB traces around that 5-pin connector and IC6. Can you remove the lower board and take photos of the top side and the bottom side.

Also if you can have some light shining through the board from the back side while you take a photo of the top side I should be able to trace the PCB layout.

If you are using a fixed-focus camera, try backing away to get a better focussed photo.

 

Not real sure about these but let me know if you need better, I will try again.

 

Again

 

Sorry to be a pain. But we're looking at the wrong board.

I would like to see the lower input board. I only need the half or quarter portion from the 5-pin connector end.
It doesn't have to be strong back light. Just enough so that I see the PCB tracks.
Also take normal pics of the top side and bottom side of the board in order to identify components, resistor color codes etc.
and the bottom side to make sure we're looking at the correct track.

 

This is the portion I need to see. It is on the lower board.