Ampeg B2

[Euthymia]

On the bench again we have our very favorite type of repair: an Ampeg-by-Crate bass head.

Symptom: instrument signal is very faint and very distorted (the distortion actually sounds kinda musical, kinda hard rockin').

Gets a clean sine all the way to the Preamp Out jack.

Running a 1K 1V p-p sine into the Power Amp In jack, it's fine until IC4 pin 7 (test point 6 on the schematic) where it is a 30V square wave.

Here is the schematic of the section I'm talking about:

http://www.euthymia.org/image/42704h0.pdf

Any ideas? I've replaced IC4.

[Ray Ivers]

It could be a shorted output or bad output transistor (the "greatest hit" of SS amp failures). An output short would remove the NFB from the "OUTPUT - see sheet 2" node, causing stage IC4b to have a lot more gain and probably hard-clip.

Ray

[Euthymia]

Thanks, Ray.

I probed all the MOSFET's in-circuit and they measured the same; pulled one and it was fine, so I concluded that they were probably okay. Usually they fail to a dead short.

So that is a NFB loop going back to IC4. I'll check that to make sure the signal is making it. I'll also check to see if something's shorted across the output.

[Enzo]

FOrget NFB, what about DC levels through the amp. What is at TP 7,8,9,10? The two input transistors of the power amp on page 2 need +/-16VDC, is it getting there? Are the output devices getting their power rails? How about C13 and C32? Are they charged up?

All the NFB does is report back to the input. There is no output to report, so the op amp amplifies the input to the max trying to compensate for the lost signal at the output. The output of the op amp is not being amplified by the power stage. You are not blowing fuses, so the MOSFETs are probably OK.

[Euthymia]

....what about DC levels through the amp. What is at TP 7,8,9,10? The two input transistors of the power amp on page 2 need +/-16VDC, is it getting there?

TP7 and 8 are at 5.5 and -5.5V. The chart calls for +5.5 at 8, which I assume is a typo?

The other sides of R32 and R33 measure 16V and -16V.

TP9 and TP10 show zero volts with no signal. The chart calls for a 5.5V offset.

The MOSFET's are all getting their rails.

The + side of C13 measures 62V and the - side of C32 measures -61V.

R52 and C16 measure good.

[Euthymia]

Found the problem.

Ray had the right idea with it possibly being a shorted output.

The gol-durn "startup pop" relay that has messed me up so many times on CrAmpegs was the problem.

The contacts are shorted together: I pulled it and measured zero ohms between the poles (and the wiper of course).

My favorite replacement source for these is derelict computer monitors, but I think I'm gonna stock up....

Thanks for the help, gentlemen!