Ampeg B200R Bass Amp

[Jazz P Bass]

#1.
Disconnect the speaker until you have this thing sorted out.

#2.
Did you verify that the gate diodes sre good (D8,9,10 & 20)?

You have to get a firmer grip on how this output section operates.
U3B is drives Q7 & Q8 by putting a sine wave on both emitters when it is tasked with having to amplify an instrument signal.
Q7 acts on the upper half of the sine wave & Q8 operates on the lower half.
By touching the emitter, you are basically telling the transistor to turn on.

[robgrif]

thanks for the info .
all those gate diodes test good . all the outputs were changed as were pre-drivers and were good. all resistors test good correct ohmns readings. the preamp board is not installed nor is the tube section just the power amp board is hooked up. the board under magnified inspection shows no breaks cracks or burns. no components look burnt. just do not know where to look next. is it possible u3 is somehow involved?I apologize for my lack of experience in troubleshooting or being able to find the problem .I appreciate all the advice so far and will keep trying to save this amp.
to blow those drivers i am thinking something must be shorted somewhere allowing excessive current to short the mosfets but so far i have not found the culprit(s)
perhaps q6 and q7 require input from u3 to behave correctly

[Jazz P Bass]

I would suggest that you connect the preamp.

U3 output pin sits at zero volts with no signal.
As a signal comes through, the output pin will duplicate the sine wave & drive Q6 & 7.

Remove the speaker & ditch the limiter.

[The Dude]

I am not too sure that this applies to the circuit.
The emitters are opposite polarity, so they blend to the voltages shown.

I would question why, with a 60 watt limiter, the 50 volt rails are sitting at 18 volts.

It may be time to ditch the limiter, as it may be upsetting the apple cart.

I would install a 1 amp fuse in the mains fuse holder & have at it.

Jazz,

I'm not sure what you mean here? For instance, we have Q7, which is an NPN transistor. According to the OP, it has .06V on the emitter and 3.16V on the base. I don't see how this is possible unless the EB junction of the transistor is open? The same goes for Q6 only opposite polarity since it is a PNP. Could you enlighten me? It wouldn't be the first time I was wrong or missed something, but the only scenario I see is that the transistors are bad.

[Jazz P Bass]

I think that the OP needs to ditch the limiter.

[g1]

ok thanks here are the values that I got
Q7 E=+.06 v C=+18v B=+3.16v
Q6 E=-.06 v C=-18v B=-2.6v

It would appear from your measurements that Q6 and Q7 are open.

If those measurements are correct (and the e,b, and c are not mixed up), then Q6 and Q7 must be bad.

[robgrif]

thanks for all input lots of thought thanks
anyway i think JAZZ p Bass is on to something about the limiter upsetting things. put new mosfets in 60 watt limiter glowing bright .Installed preamp board and tube module.
light limiter went dim. put in 100 watt light bulb bulb went right down to faint glow.a bit later removed limiter and amp stayed on. tripped over myself and thoughtlessly hooked up speaker thinking this was it fixed at last. hooked up to input with signal nothing coming through and boom out go the lights. all four mosfets went down this time.
plus fuse.wtf happened here ,. seemed so close to success. anyway inspection right after 10 w 330 power supply resistor very hot. the other one not and as i pulled on it one leg of cold resistor came out of board. so my question is if this solder joint was disconnected at power supply resistor would that unbalance things enough to short rails to a ground some how turning on all four mosfets? as to the other discussion about q6 and q7 i am not sure those measurements are correct for base value. may have taken measurement from wrong place on board hard to get to pins under heat sink took measurement at closest resistor so could be wrong however double checked each one with meter when board was out and they both tested fine forward and reverse. so used up all my parts will order more and try again . thanks again for your help here. also just before the mosfets blew the amp light went out it is connected to each side of rails .

[Jazz P Bass]

The 330 ohm/ 10 watt resistor is used on the + or - 16 volt supplies.

The proper function of the bias/ driver circuit relies on both supplies being present.

[robgrif]

ok thanks will return as soon as new parts arrive
happy thanksgiving to all those celebrating

[robgrif]

Okay hi all parts arrived put in 4 new mosfets. The amp is working on limiter right now. Bulb is dim. Measuring 1.0 volts dc on speaker output. Preamp output working.touched. Small 8 ohmns speaker to speaker output. Light dim no hum just hiss of amp.Coming through.Put guitar signal through.it came through fine.on small speaker. limiter bulb glowing brighter and dim with signal.
briefly hooked up ampeg cabinet speaker and limiter bulb went much brighter all controls at 0.disconnected immediately.speaker measures about 6.5 ohmns .retested with small 8 ohmn speaker bulb dim .what is happening here .is there still some problem with this amp. would it be ok to remove limiter .or is there some way to test non destructively.to
verify operation. thanks for any suggestions.

[Jazz P Bass]

I would suggest that you remove the speaker load.
If there is a problem, that will only provide a path to ground for the output.

Install the smallest value fuse that you have, remove the limiter & test the output for any Vdc.

IF it is reading in the mv range, then you can install a speaker & try it with the proper fuse.

Note: I cannot understand why a 6.8 ohm load, compared to an 8 ohm load, is pulling severely more current.
Unless the 6.8 ohm speaker or the wiring is somehow shorted.

[g1]

The speaker measures 6.5 ohms, but how about connecting cable,etc. Maybe a short that is only showing up with signal applied.
Try another cab and cable.

[robgrif]

I would suggest that you remove the speaker load.
If there is a problem, that will only provide a path to ground for the output.

Install the smallest value fuse that you have, remove the limiter & test the output for any Vdc.

IF it is reading in the mv range, then you can install a speaker & try it with the proper fuse.

Note: I cannot understand why a 6.8 ohm load, compared to an 8 ohm load, is pulling severely more current.
Unless the 6.8 ohm speaker or the wiring is somehow shorted.

ok removed limiter 1 amp fuse no speaker 3.5 volts dc on output terminals this does not change with bias adjustment fuse blew while I was adjusting bias. installed new one amp fuse . still 3.5 volts dc on speaker output. any thoughts on possible cause or source of the dc ?

[The Dude]

A couple of things:

First, I'm still hung up on your post 23. The voltages were wrong on Q6 and Q7. Have you fixed this? At the risk of sounding harsh, you need to figure out the problem. The shotgun approach is counterproductive. You could be installing new problems.

Second, the bias control won't change your DC offset problem. It adjusts idle current of the output stage. If you get DC on the output, you have a problem. No amount of adjusting will matter. There is no reason to mess with the bias control until the amp is fixed.

So, let's back up. What are the current voltages on Q6 and Q7. Measure base to emitter voltages on both and report. Also report polarity of these voltages. In other words, for example, I got voltage "x" with my negative probe on the emitter and positive on the base. You said you checked those transistors. HOW did you check them? Only for shorts? Diode check function? Did you check for valid junctions? Unless your readings were wrong, you had a problem there. I see 3 possible scenarios. 1) The transistors are bad. 2) You replaced them and the basing diagram or cross reference is not correct. 3) Readings are incorrect.

We know we had/have a problem there. Let's figure it out instead of snipping wires, randomly replacing parts, etc.

[robgrif]

A couple of things:

First, I'm still hung up on your post 23. The voltages were wrong on Q6 and Q7. Have you fixed this? At the risk of sounding harsh, you need to figure out the problem. The shotgun approach is counterproductive. You could be installing new problems.

Second, the bias control won't change your DC offset problem. It adjusts idle current of the output stage. If you get DC on the output, you have a problem. No amount of adjusting will matter. There is no reason to mess with the bias control until the amp is fixed.

So, let's back up. What are the current voltages on Q6 and Q7. Measure base to emitter voltages on both and report. Also report polarity of these voltages. In other words, for example, I got voltage "x" with my negative probe on the emitter and positive on the base. You said you checked those transistors. HOW did you check them? Only for shorts? Diode check function? Did you check for valid junctions? Unless your readings were wrong, you had a problem there. I see 3 possible scenarios. 1) The transistors are bad. 2) You replaced them and the basing diagram or cross reference is not correct. 3) Readings are incorrect.

We know we had/have a problem there. Let's figure it out instead of snipping wires, randomly replacing parts, etc.

ok thanks probe + on emitter - on base q6=+.6 v probe +on emitter - on base q7 = -.45 v