Ampeg B2 head scratcher

[gbono]

Ampeg B2 which came in with damaged output MOSFETs - replaced Q10-12 along with fuse resistors R36/38/40. Also resoldered R69 since there was no -16V supply. The issue now is the power amp is not working though preamp is working correctly.

With the preamp disconnected - 1Vpp 1KHz signal connected to PIN 2 of J9 (PINs 3 and 4 are shorted) - no load on output. Issue is that IC4B is saturated and sits at rail voltage. Checked all input connections and feedback components around IC4B with no issues. I went so far as to solder in a socket and try several TL072's but no luck. What am I missing?

[MarkusBass]

Have you checked other transistors/resistors in the power amp? When the output MOSFETs fail, there is much more failed components than MOSFETs and fuse resistors.
There is a global feedback loop in the amp. If the output stage is not working correctly, the input stage (IC4) also works incorrectly.

Mark

[R.G.]

+1 to MarkusBass. There is likely to be some issue with other parts.

It's remarkably effective to measure the DC voltages on all the active parts if you can without something else burning out. This often tells you what else is burned out.

[J M Fahey]

Issue is that IC4B is saturated and sits at rail voltage.

Minor question: which rail?

Major question: where does speaker out sit at that moment?

Ic4 is probably trying to do the impossible job of centering a still damaged FET power pack.

Hopeful question: can you disconnect it? (whatever's on page 3) .
Not sure you can but if you could, I'm reasonably certain Ic4B output will self center, seeing that it has a local DC NFB path (R17/R26) meaning I don't think the problem lies there (might be wrong, of course) .

You should measure voltages all over the place, trying to find odd/impossible values, specially odd/impossible BE junctions.

[gbono]

I believe the OP Amp is railed at -16V since when I DC coupled the scope I think the DC value was below ground (negative). Mmmm does that mean that the N channel FET side of the "power pack" is still not happy?

Going to recheck the operation of everything active on the power amp section again.

BTW is IC4B acting as a "external" (voltage) gain stage?

[J M Fahey]

I believe the OP Amp is railed at -16V since when I DC coupled the scope I think the DC value was below ground (negative). Mmmm does that mean that the N channel FET side of the "power pack" is still not happy?

IC4 is wired as an inverter, so I guess the positive side of the power pack might be bad and it's trying to center it.
Anyway, the point is that IC4 is probably fine so we'd rather concentrate on the output block.

BTW is IC4B acting as a "external" (voltage) gain stage?

Not external, it's part of the power amp, which to be clear is everything to the right of C36 on page 2 up to the speaker out on page 3

Q2 and Q17 are grounded base amplifiers used as level shifters to couple Op Amp output (which can only swing +/-15V or less) to Vas Q4 and Q13 respectively which are the ones which provide the big gain and voltage swing.

Output MOS only follow that large +60 V to -60 V rail to rail signal.

So besides the power MOS, these 4 transistors just mentioned are also suspect, you might also have some burnt or open resistor or some open/cracked track or soldering.

That's why often shotgunning is not enough,you may replace parts and yet the system does not work.

I suggest measuring voltages and if an unusual one is found, think why.

Example: one NPN transistor has , say, 4Vbe ... impossible.
Or -15Vbe ..... why? Is it fine and some other transistor or IC is desperately trying to turn it OFF?
Or a track has +9V on one end and +4V somewhere else : grab the loupe and start searching for the crack.

[gbono]

Thanks JM - you should teach this in a college - of course there is no money to be made.


If the voltage at TP8 and 7 are not equal (say 5.5 on one side and 5V the other of R77) would that point towards which side of the circuit is mal-functioning?

[Jazz P Bass]

I would replace all of the MPSA06/ MPSA56 transistors.
2, 3, 5, 6, 8, 17.
I don't know what it is about those transistors, in those positions but they do not like getting wacked.

[gbono]

Okay went back to my notes and the amp had previously been "repaired". Getting out the eye loop I found a trace that was heavily damaged by a gouge in the PCB. The broken trace was between R23 and Q17 (common base level shifter). Repaired trace and I now get 46 Vpp at the output (TP12) with no load and 1 Vpp into power amp circuit.

My only issue now is that I cannot adjust bias per the instruction on page 3. With no signal and no load I get very strange voltages across the output transistor source resistors:

Voltage across R47 = Vr47=2mV
Vr48=1.2mV
Vr50=1.2mV

Vr37=.7V
Vr39=2.3V
Vr41=2.1V

No change to bias with API trimmer.

Q5 voltages Vc=2.3V
Vb=.1mV
Ve=.6V
???????

Transistor Q5 checks out okay with DMM and Q6 voltages look normal. What's up with the bias control??

I also noticed that the output at the VAS transistors was not equal Q4=38Vpp and Q13=49Vpp.

I really have tried not to shotgun components on this board since the traces delaminate very easily.

[J M Fahey]

Thanks JM - you should teach this in a college - of course there is no money to be made.


If the voltage at TP8 and 7 are not equal (say 5.5 on one side and 5V the other of R77) would that point towards which side of the circuit is mal-functioning?

Weird, first time I see a typo/mistake in an Ampeg/Crate schematic (the best in the Industry )
In a normal amp TP7 should be roughly +5V and TP8 -5V (or thereabouts), the circuit is symmetric.

Measure Vbe in Q2 and Q17, I would expect around +0.65 and -0.65V respectively, also some +/-50Vce (or some 10V less than actual rail voltage to be more precise) .

As I unbearably insist, voltage measurement is a powerful diagnostic tool, doubly so when working without schematic (often the case in Argentina )

As`suggested earlier, hunt for "weird/impossible" voltages and then look around that point and imagine what might be the cause.

EDIT: too sleepy now to properly answer your post #9 ,check again later