Trace Elliot Bass amp repair

[SLEEAudio]

I replaced the cap because it looked like it was damaged, but as soon as I fitted the new one it did not discharge at all. Like you said, as if there is no load at all.

Some readings.

V+ 76.6
V- -77.4

TR1
C = 35.9
B = 0.4
E -0.4

TR2
C = -66
B = -1
E = -0.4

TR3
C = -1.7
B = 75
E = 76

TR4
C = -2
B = -60
E = -57

And yes, output transistors are seeing the full 70 volts.

Hope you can help,

Cheers

[Jazz P Bass]

Those voltages on the differential input look screwy.
If TR4 base reads -60Vdc, then how can TR1 Collector read 35.9 Vdc.
It is the same point.
I would check that the trace is intact.

[SLEEAudio]

I do apologise for that, I was reading TR5 instead of TR4.

Collector of TR3 reads -0.4v and TR4 -0.7

[Jazz P Bass]

So what are the readings on TR1 & TR2.
R4 feeds both of their emitters.
It has +70Vdc on one side. What does the other side read?
The collectors are both tied to the-70 Vdc rail through separate 4.7K resistors.
What reading do you get on the collectors.
The base of TR1 is the input. This point should idle at zero volts.
Do you have a Vac signal there.
The base of TR3 is the feedback from the output.
This point should idle at zero volts.

[SLEEAudio]

R4 has 76.6V on one side, and is dropping nearly all of it - feeding the emitters of TR1 and TR2 with just 1V.

The collectors are both reading about -66V

The base of TR1 idles at about 400mA, and the base of TR2 idles at about 2V. No Vac reading there.

The base of TR3 is idling at about 76V. So would this suggest that TR3 has gone open? It has passed a continuity test.

Thanks for your help

Charlie

[Jazz P Bass]

[QUOTE=SLEEAudio;204153]R4 has 76.6V on one side, and is dropping nearly all of it - feeding the emitters of TR1 and TR2 with just 1V.

The collectors are both reading about -66V

The base of TR1 idles at about 400mA, and the base of TR2 idles at about 2V. No Vac reading there.

The base of TR3 is idling at about 76V. So would this suggest that TR3 has gone open? It has passed a continuity test.]

The base of TR3 is attached to the collector by a diode.
0.6 volt drop is good.
All the voltages look good to me, except TR2 base having 2 volts.
This may be because you do not have a load on the output.
Is the signal fed into the amp still stopping at TR1?
Is the signal on the other side of C1 (preamp side)?
I would check that TR1 base is configured properly.
Is the base 56K from ground.
Is there a 4.7K connection to C1 to TR1 base.
Is C1 healthy.

[SLEEAudio]

The signal stops at TR1, the signal on the base of TR1 doesnt look healthy, much like it has some modulation on it.

C1 is fine, signal is good on both sides.

And yes, 56K from TR1 base to ground, and 4.7K from C1 to TR1 base.

Is there anything else it could be?

Thanks for your help

[Jazz P Bass]

Replace TR1 & TR2.

[SLEEAudio]

Replaced both TR1 and TR2. The signal still doesnt pass out of the emmitters.

Also, the negative rail still acts as though it has no load, and doesnt discharge.

Any bright ideas? Im pretty stumped now

Cheers

Charlie

[Jazz P Bass]

Why would you expect a signal at TR1/2 emitter?
They are tied to the plus voltage rail through R4.
Follow me here.
The signal path is:
Input to base of TR1.
Output from TR1 collector drives TR4 base.
TR4 collector turns on negative rail MosFet J50.
Tr2 is in the feedback path.
It's collector drives TR5 base
TR5 collector turns on TR3 base.
TR3 collector turns on positive rail MosFet K135.

[SLEEAudio]

I do apologise, I did mean the collector.

The signal stops at the base of TR1. No signal at any other transistors.

The only other thing that sticks out is the positive supply isnt as smooth as the negative.... not that will have anything to do with the signal making it through TR1...

Thanks for your help Jazz, sorry for the confusion

[SLEEAudio]

I had to edit the last post because before there was signal clearly present at both emitters of TR1 and TR2, but now there isnt.

Maybe there is an intermittent faulty part somewhere?