The next place to check for signal is at the VOLUME control, terminals 1, 2 and 3.

ok. getting a signal on terminals 2 and 3 . no signal on terminal 1

 

Measure the DC voltage at this point.

 

Measure the DC voltage at this point.

View attachment 307187

The 4th. chip doesn't seem to be sitting in its socket properly! There's also a red wire at the lower left which is rather bare and could be making (or could later possibly make!) contact with the resistor lead below it?

 

The 4th. chip doesn't seem to be sitting in its socket properly! There's also a red wire at the lower left which is rather bare and could be making (or could later possibly make!) contact with the resistor lead below it?

will check that . thankyou

 

Measure the DC voltage at this point.

View attachment 307187

no reading there.

 

will check that . thankyou

How the hell did you spot that Sarah ! ???? Ive got signal on all op amps , varying degrees of volume but all singing !!! all I did was make sure they were snug in their sockets . slight movement in one .

 

There is (was) one brand of IC sockets that was very prone to corrosion and bad connections in general. Maybe the company that produced this amplifier bought a bunch of those sockets. Knowing about that problem allowed me to do some very fast fixes back in that era. Others may recall that, the company known by it's two initials at the time. They still make good ICs, but no longer make sockets.

 

There is (was) one brand of IC sockets that was very prone to corrosion and bad connections in general. Maybe the company that produced this amplifier bought a bunch of those sockets. Knowing about that problem allowed me to do some very fast fixes back in that era. Others may recall that, the company known by it's two initials at the time. They still make good ICs, but no longer make sockets.

The original board did not use IC sockets. I asked the TS to install sockets.

 

We are waiting for reports on the overall operational status of the amplifier.
If no joy, TS should go back and check Test Point #1 as in post #128.

 

We are waiting for reports on the overall operational status of the amplifier.
If no joy, TS should go back and check Test Point #1 as in post #128.

View attachment 307204

even though all op- amps are working the sound is still crappy through the amp speaker . distorted , hissy . just like the audio I uploaded . test point on that cap is good ,signal coming through .

 

Here, we will assume that the signal is getting processed properly by the op-amps.
Now we have to trouble-shoot the main power amp.

 

Here, we will assume that the signal is getting processed properly by the op-amps.
Now we have to trouble-shoot the main power amp.

View attachment 307222

ok. if yr up for it .

would the speaker be suspect at all .? should I remove it and inspect it more closely ?

 

How the hell did you spot that Sarah ! ???? Ive got signal on all op amps , varying degrees of volume but all singing !!! all I did was make sure they were snug in their sockets . slight movement in one .

Just seen so many similar examples over the years, and ones where a pin has been bent under and missed the socket altogether!

 

Just seen so many similar examples over the years, and ones where a pin has been bent under and missed the socket altogether!

well bravo there . thanks eagle eyes

 

We will begin this trouble-shooting process by measuring DC voltages.
Firstly, disconnect the loudspeaker.
With no signal input, measure the DC voltages at the two power transistors, at the nut and bolt (C) and the two pins, B and E, all with respect to GND.

 

We will begin this trouble-shooting process by measuring DC voltages.
Firstly, disconnect the loudspeaker.
With no signal input, measure the DC voltages at the two power transistors, at the nut and bolt (C) and the two pins, B and E, all with respect to GND.


View attachment 307265

ok. as pic above : left: nut - 40.3 right : nut - -00.1
E - -00.1 E- -40.3
B- -00.1 B- -40.3


those are both the new transistors by the way.

hmmm the layout changes when I post it . I had them in two rows . hope it makes sense

 

Something is wrong with the positive power rail.
Go back and measure the voltages on the large smoothing capacitors. You should be reading +40V and -40V with respect to GND.

 

Something is wrong with the positive power rail.
Go back and measure the voltages on the large smoothing capacitors. You should be reading +40V and -40V with respect to GND.

View attachment 307272

L: 40.8 R: -40.8

 

I think the 2M2906 (metal case) driver transistor is actually a 2N3906 (as shown) so it matches the 2N3904 driver transistor.
Then both little transistors are overloaded and produce distortion and smoke at high output levels and if the 2N3055 output transistors have normal but low gain.

 

L: 40.8 R: -40.8

Did the 2N3055 transistors come with transistor mounting hardware?
Did you use the insulating washers to mount the transistors to the heatsink?
Remove any insulating washers. The case of the transistors must be electrically bonded to the mounting nuts and bolts, i.e. the metal heatsinks are live, one at +40V and the other at speaker output voltage.