Ampeg B200R Bass Amp

[robgrif]

Hi All
I have an Ampeg B200r Bass Amp with a problem
Stopped working, speaker started humming seems to be drawiing excessive current MOV blew out and fuse was sparking but did not blow fuse The correct fuse is installed. preamp board and tube circuit are disconnected
only ouput board is hooked up now.
With a light bulb limiter attached amp draws a lot of current am using 25 watt bulb and it glows quite brightly
and there is 2.5 volts dc at speaker terminals no speaker attached
q4 was testing bad took it out other mosfets seem ok. tested still drawing excessive current
removing red transformer supply wires from board dims limiter bulb almost completely.
there is voltage output from rectifier to circuit.however values are low as result of limiter function.
any ideas?Should I remove any other mosfets or is the problem somewhere else. could supply capacitors be faulty?
Thanks in advance for any suggestions and advice.

[52 Bill]

I don't see a MOV in the schematic, where is it and what does blew out mean?

If there is 2.5 vdc on the speaker out, you still have a problem with the power amp. Besides the outputs, what transistors have you tested?

[robgrif]

b200r

I don't see a MOV in the schematic, where is it and what does blew out mean?

If there is 2.5 vdc on the speaker out, you still have a problem with the power amp. Besides the outputs, what transistors have you tested?

thanks for reply
schematic does not show MOV it is on black supply line before fuse a piece of the device broke off when problem occured. also transformer has same wires as figure for export 240 volt model not sure why but amp was working fine.
q6 ,q7, seem ok 47 ohmn resistors ok . correction to post removed q5 (irfp140n) not q4 sorry
also dc is negative at speaker terminal. would that indicate source is negative rail
. cement resistors ok . U3 pin 7 about 0.8volts
2 volts a gates . running this with one mosfet removed (q5) is that ok?

[Jazz P Bass]

I would remove all four of the output transistors to see if the lamp limiter settles down.
Typically, when one of the IRF's go, the other three take a hit too.

[The Dude]

If you haven't, disconnect any speaker until you get it fixed. The toasted MOV makes me think power surge. If that is the case, it's quite possible filter caps suffered an overvoltage condition. Check any rectifier diodes/bridge rectifier. Also, the first thing I would do would be to measure at the speaker leads for AC voltage. This will tell you if you have a power supply filtering issue/bad filter caps. If one of your rails is low because of poor filtering, you will often get a DC offset problem. The thing to note is whether the hum is more AC or more DC.

[robgrif]

Ampeg B200R Bass Amp

I would remove all four of the output transistors to see if the lamp limiter settles down.
Typically, when one of the IRF's go, the other three take a hit too.

ok thanks for that . just going slow removed q8 . limiter 60 watt bulb barely glowing
should i continue to remove other two mosfets
would this be all that is wrong or could there be underlying condition which would blow mosfets upon reinsertion
thanks for any advice

[robgrif]

If you haven't, disconnect any speaker until you get it fixed. The toasted MOV makes me think power surge. If that is the case, it's quite possible filter caps suffered an overvoltage condition. Check any rectifier diodes/bridge rectifier. Also, the first thing I would do would be to measure at the speaker leads for AC voltage. This will tell you if you have a power supply filtering issue/bad filter caps. If one of your rails is low because of poor filtering, you will often get a DC offset problem. The thing to note is whether the hum is more AC or more DC.

thanks speaker not connected
no ac at speaker terminals
btw acdc rocks

[The Dude]

As with any direct coupled amp, give everything a once over. Any open resistor, shorted driver, etc. can upset the apple cart. Here's a schematic if you don't have one.

ampeg-b200r.zip

[robgrif]

b200r

As with any direct coupled amp, give everything a once over. Any open resistor, shorted driver, etc. can upset the apple cart. Here's a schematic if you don't have one.

[ATTACH]31571[/ATTACH]

thanks very much for advice and schematics.
this amp failed about one year ago with same problem i think but ithought the cause was the incorrect factory board wiring of crossover
is it possible the bias control is incorrectly set?causing some sort of imbalance?
there is a note on schematic for setting bias but i am not sure of the test point r72-r75 would that be from point to point between resistors or one resistor at a time.

[J M Fahey]

1) that "MOV" is very probably (as in 99% probably) a series NTC inrush current limiter, which might be in the schematic or not, just an afterthought.

Be sure to replace it with the same one or equivalent, friends around here might suggest one.

2) you might have more damaged MosFets and extra problems.

Too sleepy now, can anybody turn the power amp schematic .pdf into a readable .gif or something and post it here, so we are all looking at the same image at the same time?
Thanks.

Back to sleep, bye bye little angels

[robgrif]

here is jpeg of power amp
and power supply
thanks

[Jazz P Bass]

But the part that failed is not on the schematic, correct?

As to the bias adjust: what is indicated on the schematic is that they want you to measure the current through all four 0.33/ 10 watt resistors.(R72, 73, 74 & 75)
Yes, measure directly across each resistor.
Then, average them & use that value to set the bias to 15-20mv DC.
When setting the bias, use the resistor with the reading that is closest to the average.

[robgrif]

the part is an in rush current limiter that failed . identified by J.M.Fahey (thanks) it is an sl22 1000r .still passes ac to amp
q5 and q8 removed 60 watt light bulb limiter showing low idle current. checking other components now . q4 qnd q11 reading same on meter.not sure if that proves anything. debating whether to replace them or leave them in . any reliable way to test them in circuit? perhaps run amp on one set /side only .is that possible or safe?and see if function is ok .

[Jazz P Bass]

My take on the repair:
Replace all of the output mosfets.
Replace the 47 ohm gate resistors.
Replace the driver transistors if you feel like it (Q6 & Q7)as they most probably got stressed.
Double check the gate diodes.

Note: DO NOT try to run the amp with only one side functioning.
You will not learn anything in doing so.
The output section is a literal balancing act, rail to rail.
Everything must work for it to work properly.

[robgrif]

Okay replaced all those bits however amp still pulling high current on light bulb limiter. Not sure what else could be drawing current power amp is only board hooked with nothing attached. The last set of pulled parts all checked good. Any thoughts on possible failed part which would cause this. The only bad parts were the mosfets q5 and q8
pulled at first. A little puzzled . Also there appears to be 4.5 volts ac on speaker measured one way reverse leads read. 1 volts on meter A/C range not sure if this. Is really A/C or not