Acoustic 270 voltage on output

[pontiacpete]

Hi, this amp came in with no sound, with voltage on the output. Output transistors were MJ15022. Found two or 3 shorted so I put in
8 new 2N3055s. Turned on with the light bulb limiter and there was no voltage on the output, so I attached a speaker and got a lot of noise but it was passing signal. It could be heard under all the noise. Flipped the limiter off and 4 of the 2N3055 shorted.(3 on one side and 1 on the other) Replaced these and also found a diode shorted, CR409. Replaced with a 1N4007.
Now on the limiter I'm still seeing voltage on the output with the limiter. Pulled all the 2N3055 and do not find any are shorted with the simple diode check.
Checked all resistors.
Re-soldered the whole board with the outputs.
Checked all transistors and caps so far in circuit finding no shorted or open components. Went ahead an replaced Q410 and Q405
Any suggestions where to go from here?

Thanks

[TigerAmps]

Hi - You replaced Q410 and Q405 (the ones with the heat sinks attached); What did you replace them with and how?
The heat sink is connected to the collector and the heat sink can (in some amps) acts as a jumper connecting two traces on the board together.
If you replaced the transistor in the heat sink (still using the heat sinks), check to make sure there is a good connection between the transistor and the heat sink.
Chop stick testing also help me find issues in some Acoustic repairs as well. I found most issues on the power board and cracked traces due to vibration which
required a small jumper.

The amps look simple but can be a pain because certain drop in replacement parts are no longer available.

[pontiacpete]

I replaced them with 40410 and 40409 with the heatsinks.

Thanks
I'll continue looking for connection issues.

[pontiacpete]

Here I am again, confused. The 2N3055s from Q412- Q414 and Q407-Q409 measure as shorted between base and emitter. Finally realize that this is expected being that there's a 1ohm resistor across the base and emitter.
Found an open 1ohm resistor opened up on the Q411-14 side, R426.
Pulled 2N3055 to check for shorts out of circuit, all look good.
But still with the limiter I am getting a rising voltage on the output, goes up to about 21vdc.
I'm wondering at this point should i flip the limiter off? Hate to lose any more 2N3055.
Any thoughts?

EDIT: also just noticing the 45v in the rectangle between L401 and C504. Could C504 be shorted or leaking?

[Jazz P Bass]

The output is coupled to the speaker through a capacitor.

Until (unless) you have a load attached, you will see a voltage on the cap.

[pontiacpete]

Ok , hooked it up to a dummy load and no voltage.
Hooked it to a speaker and smoke. ouch!

[The Dude]

Have you checked the output coupling cap itself?

Edit: In case anyone wants to play along (Thanks nickb)


http://bmamps.com/Schematics/acousti...ice_Manual.pdf

[pontiacpete]

It's not shorted. I did try swapping in a 3300uf

Now I have to go through the 2N3055s again, I'm sure to have a several shorted

EDIT: thanks for posting the schematic, thought I did that already

[52 Bill]

This amp has a single positive power supply, so the midpoint of the amp will be at about 1/2 the supply voltage, or 45 volts dc. This is why the speaker must be connected through a blocking capacitor.

The eight power transistors are used in two different ways. The first two are drivers and the other six are the actual outputs. The six outputs are paralleled with no isolation resistors. I would think that for testing, you can use just two outputs until the amp is working and stable.

I would keep the limiter on until you are certain that the amp is stable.

The voltages in the rectangles are dc voltages, the triangles are ac signal voltages. How do your dc voltages compare to the schematic?

[olddawg]

If you use a variac and and amp meter, you will save blowing up a lot of output transistors. Years ago there was a tech in a shop I worked at that we affectionately called "Oonk". That was because of the sound most of his amps made while troubleshooting with a lightbulb limiter. Mr Oonk always had a pile of legs up TO3s.

[pontiacpete]

Thanks for the help here.
I set up an ammeter across the main fuse, put it on a variac and removed Q408, Q409, Q413, and Q414. Here are some voltages

With 117vac the amp is drawing .43A. This is what I have:
Across the R504 there is 23vdc, no voltage on the 4ohm dummy load
At emitters of Q401-2 there's about 6vdc
And both of the bases have about 6vdc
24vdc on the collector of Q404
23vdc on it's emitter
24vdc on the base of Q410
500mVdc on the base of Q412

[pontiacpete]

I think I left out one of the voltages, probably the reason why for the lack of interest in this thread

Q411 B - 1.1v

It looks like there should be 720mV there when all 3 outputs are in place. Since I still have only one this seems to be off.
Any ideas?

[pontiacpete]

Here are some more voltages, I'm not sure what they should be but maybe this will help.

on the 2n3055s that are in now

Q406 C - 50vdc
B - 25v
E - 25v

Q407 C - 50v
B - 26v
E - 25v

Q411 C - 24.5v
B - 1.1v
E - .5v

Q412 C - 24.9v
B - .5v
E - 0

Does anyone know what voltages I should expect on these?

[pontiacpete]

I guess I can figure some of this out, since the power supply is putting out 87v and the mid point between the sets of output transistors at the inductor is 45v
then at about half power the collectors are correct to read 49v on top and 25v on the bottom.

BTW, the dashes are not negative signs - got to leave those out from now on.

The emitter on Q412 looks good because this is ground. It's the emitter on Q 411 that is at 500mV and not 160mV as the schematic says, is confusing me.
Wondering if that is high because 2 of the outputs are not in place.

The base and emitter voltages on the top set of outputs is odd. I have some more head scratching to do.

[pontiacpete]

How can there be 43.7v (according to the schematic) at the collector of Q404 when it's connected to a CR408 a 4.3v zener?
Am I reading this wrong or whats goin on?