Acoustic 450 With Low Output

[allsignalmustpass]

I'm working on a 450 with low output and I have a question about the schematic.

My particular unit is making 11 Watts clean before the sine wave at the output starts to look like a triangle. The issue appears to be in the power section since when I drive the power amp via the "access" jack I have the same result. I've replaced every electrolytic in the entire unit except maybe one in the graphic EQ.

I've started going through and measuring voltages and I'm curious where the 27v is coming from at the bottom of Q401 (the differential PNP pair at the input of the power amp. My voltages so far are as follows:

Q401
E: 24.3v
B: 23.4v
C: 3.2v

Q402
E: 24.2v
B: 23.6v
C: 0v

Q403
E: .86v
B: .32v
C: 44.8v

I've pulled Q401 and Q403 out of the circuit and tested them with a Peak tester and both are fine. Obviously something weird is going on with 403 as well.

I've attached the relevant chunk of the circuit from a previous post. A complete schematic is here in this post but for some reason I couldn't get it resized to upload.

https://music-electronics-forum.com/.../page2?t=20743


Thanks!

[allsignalmustpass]

Looks like a misprint on the schematic. I thought to check the near-identical Acoustic 470 schematic and it has 22v at the collector of the diff pair which makes way more sense. Now I have to figure out why I only have 3v.

[Enzo]

How about if we read it as 2.7v instead of 27v? Then the R41/411 voltage divider makes the base of Q403 make more sense.

Besides, the whole point of all that is to get the collector of Q403 to sit at about half V+, which it seems to be doing.

Does your output look the same with no load as it does loaded? Or does it look good unloaded but can't drive the load?

[allsignalmustpass]

Thanks Enzo! I have way more signal with no load and it looks different. Rather than a triangle I've got something that looks more like a sine wave but asymmetrical with squared-off peaks. I'll measure the voltages on the output devices shortly.

[Enzo]

Squared off peaks sounds like clipping.

Just a thought, unsolder one end and lift CR410, CR411 - associated with Q412, Q413. ANy help?

[allsignalmustpass]

No luck lifting the diodes. What kind if B-E drop should I be seeing on the 2N3055? I've got 2 transistors with .6 - .7v across the B-E junction and 4 that don't really have any drop or where the drop is negligible.

[Enzo]

Is it a drop or a reverse voltage?

[allsignalmustpass]

Here are the voltages on the output devices. I'll see if I can get the rest of the voltages today:

Q407
C: 92.9v
B: 46.8v
E: 46.0v

Q409
C: 93.0v
B: 46.2v
E: 45.7v

Q411
C: 93.0v
B: 46.3v
E: 45.8v

Q406
C: 46.0v
B: .68v
E: 166mV

Q408
C: 46.0v
B: 164mV
E: .9 mV

Q410
C: 46.0v
B: 165 mV
E: 1.0 mV

[allsignalmustpass]

Here are the rest of the voltages:

Q401
E: 24.3v
B: 23.4v
C: 3.2v

Q402
E: 24.2v
B: 23.6v
C: 0v

Q403
E: .86v
B: .32v
C: 44.8v

Q404
E: .69v
B: 44.2v
C: 44.8v

Q405
E: 45.5v
B: 46.1v
C: 90.2v

Q412
E: 44.7v
B: 44.7v
C: ramps from .1 to 1.5v?

Q413
E: 44.7v
B: 44.7v
C: 45.8v

Q414
E: 39.5v
B: 40.1v
C: 75.9v

I removed and tested all of the output transistors with my Peak tester and they all tested OK. The tester wanted to call Q8 a silicone transistor so i double checked it with my bench meter. Obviously Q412 looks weird. Not sure if that's a fluke with my Fluke!

[TomCarlos]

ASMP - I too am working on an Acoustic 450 Amp. Here is my thread - Acoustic Control 450 Base Amp - Check Up / Tune Up Low Output

Can you do me a favor? Can you please check R423 and tell me the value of resistor that is in the amp? I found a 10 ohm but the schematic calls for a 1 ohm. It does not look like anyone changed that resistor, so I am wondering the where and how anyone might have stuffed the wrong resistor in that spot.

I am hoping to finish replacing a couple components over the weekend and trying to fire up the amp once again (keeping my fingers crossed).

Thanks, Tom