Legend A30 Output section problem

[waspclothes]

Hey guys,
I recently inherited a Legend A30 with some problems in the output section. It looks like repairs were attempted, some capacitors replaced etc. I've tried my best to troubleshoot the output section, but to no success since the whole situation is complicated because of the previous attempted repairs.

The output looks correct on the scope when there is no speaker load attached. As soon as a load is attached (speaker or dummy), the output quickly drops to be nearly silent. The only thing that comes out of the amplifier is highly gated sounding, and on the scope appears to be just the very tops of the positive peaks (but inverted) of the signal going into the power section.

Attached is the relevant part of the schematic (full schematic here: http://music-electronics-forum.com/t29662/ ) and I've also shown a bunch of test points and the results of the scope measurements I made at each of those test points, both with the load attached and with the load disconnected. I also wrote up all of the transistors in this section and what they've been replaced by. The schematic is hard to read, so there's a question-mark next to a few. Also note that Q12 (D41ES ?) was replaced with an A706 and the pin different pinout looks to be correctly compensated for.



I hope the diagram helps, let me know if there's anything you'd like me to test.

Thanks again,

[Wakculloch]

Concentrate on getting DC conditions right. Q12 will only turn on when C is at 0.5 to 0.6V.
This may be a symptom rather than the problem.

[Jazz P Bass]

Which schematic matches the amplifier?
One has TIP33/ NPN output transistors.
The other one shows 2N3055's. (NPN)

At any rate, when the amp works without a load & then collapses with a load attached, that is a good sign that the drivers are not cooperating.(Q15 & 12 (misprint?))

To remove the limiting circuit (Q16 & 9), you could lift the diodes that are connecting the collectors to the drivers. (CR smudge & CR15).

As posted above, the Vdc conditions must be brought in line.

With the limiter out of the equation, you are down to a pretty basic circuit.

The LTP (Q12 (misprint?) & 11) drives Q10.
Q10 alternatively drives Q15 & 12.
Which drive the outputs.

If you do not have a scope, you could monitor the base of Q10 with a meter set to read Vac & a signal running through the amp.
See if it drops out when a load is connected.

If not, then move up to the driver bases & try the same test.

[waspclothes]

Concentrate on getting DC conditions right. Q12 will only turn on when C is at 0.5 to 0.6V.
This may be a symptom rather than the problem.

Thanks Wakculloch,
I actually went back and retested this and it goes from .6V with the output unloaded, down to .5V loaded. I've corrected the diagram as well.

[waspclothes]

Which schematic matches the amplifier?
One has TIP33/ NPN output transistors.
The other one shows 2N3055's. (NPN)

Hi there, thanks for the reply. The one with TIP33A output transistors matches this amp

At any rate, when the amp works without a load & then collapses with a load attached, that is a good sign that the drivers are not cooperating.(Q15 & 12 (misprint?))
To remove the limiting circuit (Q16 & 9), you could lift the diodes that are connecting the collectors to the drivers. (CR smudge & CR15).

As posted above, the Vdc conditions must be brought in line.

With the limiter out of the equation, you are down to a pretty basic circuit.

The LTP (Q12 (misprint?) & 11) drives Q10.
Q10 alternatively drives Q15 & 12.
Which drive the outputs.

If you do not have a scope, you could monitor the base of Q10 with a meter set to read Vac & a signal running through the amp.
See if it drops out when a load is connected.

If not, then move up to the driver bases & try the same test.

Thank you ,please see the attachment for measurements from the scope on each of the transistors, with the load attached and without. I've also updated it to show that Vdc at Test Point C goes from +.6V with speaker output unloaded to +.5V when loaded. I will try to lift the diodes to the limiting resistor as you said and see what that does. I can also desolder the output transistors and test them.

[52 Bill]

Thank you ,please see the attachment for measurements from the scope on each of the transistors, with the load attached and without. I've also updated it to show that Vdc at Test Point C goes from +.6V with speaker output unloaded to +.5V when loaded.

Either you are not measuring the dc voltages correctly or there are major problems here, as there are no voltages listed at many of the transistor bases, like TPs J, K, M.

[J M Fahey]

You are missing the upper half so maybe Q14 or Q15 or R66 are open; or the protection circuit is on all the time (Cr20, Cr15, Q16, Q9 shorted) or there's a crack on the track(s) feeding +26V to Q14 or Q15 .

That said, the "universal SS repair rule" is that any NPN Silicon transistor needs around +600mV BE plus > 1 or 2 V CE to pass any current at all, same but reversed polarity for PNP .
Do not measure relative to ground but always relative to actual emitter.

As said above, I suspect open Q15 or Q14 .

Also recheck output TIP33 pinout and measure said voltages at the actual transistor legs (avoid slipping a probe and shorting them) ; given the amp vintage those transistors are probably not PCB mounted but through wires, so the possibility of error always exists.

You might even have replaced transistors following what you found ... which might have been garbled at an earlier repair attempt.

But the "universal rule" should sort that out.

[waspclothes]

Either you are not measuring the dc voltages correctly or there are major problems here, as there are no voltages listed at many of the transistor bases, like TPs J, K, M.

Thanks for the reply Bill. I measured them by looking at the scope, looking at the voltage peak-to-peak and the DC offset. But truth be told, with some of the waveforms (like on TP J, K, M, etc.) I don't know where in the waveform I measured the DC offset from. Should I measure it from the flat-line part of the waveform, or the middle of the peak-to-peak amplitude? Or better yet, should I just use a multimeter for the DC measurements?

I can measure again and re-post.

[52 Bill]

Or better yet, should I just use a multimeter for the DC measurements?

I can measure again and re-post.

This will probably be easier for you and more accurate for the low level voltages.

[Jazz P Bass]

To add, do not send a signal through as yet when testing the static dc conditions.

[jvm]

Not to hijack this thread but if I could ask a favor of you, waspclothes. I have a A30 with a shorted PT secondary. Could I trouble you to measure the no load AC voltage at the PT secondaries? I have 12VAC filament on yellow wires, 36 VAC on blue wires, only 2-3 VAC on red wires and the tranny overheats very quickly. I probably have a salvage PT around that would work but I'd like to know what my seconday voltage should be. I would surely appreciate it.

[waspclothes]

Not to hijack this thread but if I could ask a favor of you, waspclothes. I have a A30 with a shorted PT secondary. Could I trouble you to measure the no load AC voltage at the PT secondaries? I have 12VAC filament on yellow wires, 36 VAC on blue wires, only 2-3 VAC on red wires and the tranny overheats very quickly. I probably have a salvage PT around that would work but I'd like to know what my seconday voltage should be. I would surely appreciate it.

Hi JVM, that winding reads 435VAC, hope that helps!

[jvm]

Thanks. I appreciate you taking the time to do that for me.

[waspclothes]

I'm happy to say that the amp is now working again! (sounds pretty decent too!)

I desoldered every transistor and tested it with the multimeter. Turns out Q16, the MPSA20 was toast. I replaced it with a 2N3904 and the amp fired up great. Nothing seems to be getting too hot and it sounds good.

Thanks for the help guys, glad this amp can keep goin'

[52 Bill]

I'm happy to say that the amp is now working again! (sounds pretty decent too!)

Glad to hear that it's working again.