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    Ampeg B200R Bass Amp

    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.

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    Last edited by robgrif; 11-21-2014 at 07:09 PM. Reason: mistake in title

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    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?

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    b200r

    Quote Originally Posted by 52 Bill View Post
    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?

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    Supporting Member Jazz P Bass's Avatar
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    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.

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    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.

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    Quote Originally Posted by The Dude View Post
    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

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    Supporting Member The Dude's Avatar
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    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

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    Last edited by The Dude; 11-22-2014 at 01:39 AM.
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    b200r

    Quote Originally Posted by The Dude View Post
    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
    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.

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    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

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    here is jpeg of power amp Click image for larger version. 

Name:	ampeg B200R power amp sch.jpeg 
Views:	1868 
Size:	191.5 KB 
ID:	31574
    and power supplyClick image for larger version. 

Name:	ampeg B200R power supply.jpeg 
Views:	1814 
Size:	206.6 KB 
ID:	31575
    thanks

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    Supporting Member Jazz P Bass's Avatar
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    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.

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    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 .

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    Supporting Member Jazz P Bass's Avatar
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    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.

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    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

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    Last edited by robgrif; 11-25-2014 at 12:49 AM.

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    Supporting Member Jazz P Bass's Avatar
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    How much current is flowing through each 0.33/10 watt resistor.

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    Old Timer J M Fahey's Avatar
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    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
    No, it's DC .

    But cheap multimeters get confused measuring DC, they think it's roughly 2.5X higher AC.

    The telltale sign is that it measures 4.5V one way , 0.1V the other ... AC would be symmetrical so you actually have some 2V DC there .
    Confirm by remeasuring on a DC scale, also measure both rail voltages.

    Your amp will eat lots of current, even if using all new parts, if overbiased, but first answer the previous questions, then we go on.

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    20.7 @source q8 20.8 source @q11

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    Last edited by robgrif; 11-25-2014 at 01:55 AM.

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    There is 49.5 milivolts through them I tried to adjust it down but it did not go lower the bias pot measures 500 ohmns and responds to adjustment down to zero.

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    Last edited by robgrif; 11-25-2014 at 08:04 PM.

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    Sorry wrong meter scale. Reading is 49.5 milivolts dc . On. 33 ohmns resistors

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    Ampeg B200R Bass Amp

    Quote Originally Posted by Jazz P Bass View Post
    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

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    ok replaced power supply caps one was causing more current to flow when touched.still drawing excess current. would it be possible for U3 to be causing this problem.?just guessing now.

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    How about taking a few static (no signal) voltage measurements.
    The drivers & the bias transistor.(EBC)
    U3 output pin should sit at zero volts.

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    Quote Originally Posted by Jazz P Bass View Post
    How about taking a few static (no signal) voltage measurements.
    The drivers & the bias transistor.(EBC)
    U3 output pin should sit at zero volts.
    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
    Q3=E 2.6v C=3.16v B=1.5 v
    U3 ouput = 0v
    light bulb limiter 60 watt bulb

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    It would appear from your measurements that Q6 and Q7 are open. Also check R41, R43, R58, and R67.

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    [QUOTE=The Dude;367281]It would appear from your measurements that Q6 and Q7 are open. Also check R41, R43, R58, and R67.[
    ok checked those resistors all good

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    Quote Originally Posted by The Dude View Post
    It would appear from your measurements that Q6 and Q7 are open. Also check R41, R43, R58, and R67.
    thanks
    checked the resistors they are all correct values and good
    as for q6 and q7 not sure what is happening there.

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    Supporting Member The Dude's Avatar
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    If the voltage is the correct polarity from b to e on a transistor, the only way it can be more than .7 or so volts on a standard transistor is if the transistor is open/bad/defective.

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    Supporting Member Jazz P Bass's Avatar
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    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.

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    Quote Originally Posted by Jazz P Bass View Post
    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.
    ok thanks something strange is happening.when the emitter of q7 is touched with + probe from meter light limiter dims amp starts to hum and dc on speaker goes to -47 volts. i have two meters this behaviour only happens with one of the meters and does not require other test lead to be hooked to anything . why would this happen? is meter acting as some sort of capacitor or ???
    shall i go for fuse install anyway

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    Supporting Member Jazz P Bass's Avatar
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    #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.

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    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

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  32. #32
    Supporting Member Jazz P Bass's Avatar
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    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.

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    Quote Originally Posted by Jazz P Bass View Post
    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.
    ok installed fuse it blew also both mosfets q5 and q8 blew as well oops fuse was 2amp my bad
    any way this is the same problem originally so whatever it is remains a mystery i will purchase two new mosfets and install

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  34. #34
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    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.

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    Supporting Member Jazz P Bass's Avatar
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    I think that the OP needs to ditch the limiter.

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