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Thread: Marshall 5520 BIAS and smoke

  1. #1
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    Marshall 5520 BIAS and smoke

    Hello at all,

    Here i have a Marshall bass 200 combo (5520).
    This amp have dirty sound when Play hard

    I have joint schematic power amp

    The problem is kind of weird, with no load & no signal, BIAS values trought resistors are for :

    - R12 : 7.3mV
    - R13 : 7.2mV
    - R29 : 5.9mV
    - R30 : 5.7mV

    Amp run with 2 pairs of MJ3001 and MJ2501, but in fact transistor is Inverted Comparing with schematic ...

    But when i plug my dummy load 8R, the BIAS values trought resistors are for :

    - R12 : 5mV
    - R13 : 73mV ( resistor hot)
    - R29 : 1.3V ( resistor very hot)
    - R30 : 0.14mV

    Once signal is applied and amp work R29 smoke and output signal is deformed ...

    BIAS problem ? need recal? NFB problem ? why with juste load plug BIAS value tought resitors move a lot ?

    Sorry for my ad English i'm French.

    Thanks in advance for your help
    Attached Files Attached Files

  2. #2
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    Quote Originally Posted by Feufeu View Post
    But when i plug my dummy load 8R, the BIAS values trought resistors are for :

    - R12 : 5mV
    - R13 : 73mV ( resistor hot)
    - R29 : 1.3V ( resistor very hot)
    - R30 : 0.14mV
    Higher bias values when the load is connected point to the output(s) not sitting at 0V. Measure the output offset voltages to check they are close to zero. Measure from the junction of R12,13 to ground and the junction of R29,30 to ground.

  3. #3
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    Quote Originally Posted by Dave H View Post
    Higher bias values when the load is connected point to the output(s) not sitting at 0V. Measure the output offset voltages to check they are close to zero. Measure from the junction of R12,13 to ground and the junction of R29,30 to ground.
    Thanks for your reply,

    Okay, between R12,13,29,30 side "center tap" to ground have 10K for each resistor. 4 resistors values are good same mesures : around 0.49R with my meter.

    when you say" Measure the output offset voltages to check they are close to zero " you want Base voltage ?

    thanks.
    Last edited by Feufeu; 01-11-2017 at 01:07 PM.

  4. #4
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    I didn't want a resistance measurement. With the dummy load disconnected measure the voltage between ground and the amplifier outputs. The junction of R12/R13 is the output of the left amplifier. The junction of R29/R30 is the output of the right amplifier. It's not easy trying to communicate in a foreign language. Believe me it would be a lot worse If I was trying to do this in French

  5. #5
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    no problem , it's good for me try speak English ...

    Ha okay, have -1.70VDC on speaker output

    and between Ground and speaker + have 85mV DC

  6. #6
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    Quote Originally Posted by Feufeu View Post
    have -1.70VDC on speaker output

    and between Ground and speaker + have 85mV DC
    Is speaker + the TR11,12 output or the TR13,14 output? It's not marked on the schematic.
    An offset of 85mV DC is probably OK. What is the offset voltage between the speaker - and ground?

  7. #7
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    Quote Originally Posted by Dave H View Post
    Is speaker + the TR11,12 output or the TR13,14 output? It's not marked on the schematic.
    An offset of 85mV DC is probably OK. What is the offset voltage between the speaker - and ground?
    Speaker + si after couple resistor & choke .

    between ground and negative speaker polarity have : 1,85Vdc !

  8. #8
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    Quote Originally Posted by Feufeu View Post
    Speaker + si after couple resistor & choke .

    between ground and negative speaker polarity have : 1,85Vdc !
    Looking at the schematic the output of the left amplifier (TR11,12) is at 85mV dc wrt ground and the output of the right amplifier (TR13,14) is 1.85V dc wrt ground. Is that correct?

    It's a bridge output stage with the speaker connected between the outputs of the two amplifiers. The right amplifier is a unity gain inverter driven from the output of the left amplifier and it's DC coupled so the right amplifier should be sitting at -85mV dc wrt ground not +1.85V. I'd guess there's a fault in the right hand amplifier. Check for shorted or leaky transistors in the right hand amplifier. I'd start with TR13.

  9. #9
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    [QUOTE=Dave H;444680]Looking at the schematic the output of the left amplifier (TR11,12) is at 85mV dc wrt ground and the output of the right amplifier (TR13,14) is 1.85V dc wrt ground. Is that correct?

    Yes exatly.

    I had done the reverse TR11 & TR13 but the preblem remains the same ...

    I see what : " Check for shorted or leaky transistors in the right hand amplifier. I'd start with TR13. "

  10. #10
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    Quote Originally Posted by Feufeu View Post
    I had done the reverse TR11 & TR13 but the preblem remains the same ...
    I'm not sure what you mean by that.

    You could check out the left amplifier by itself by connecting the dummy load between the left output (right side of R22) and ground.

  11. #11
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    [QUOTE=Dave H;444680]
    I'd guess there's a fault in the right hand amplifier. Check for shorted or leaky transistors in the right hand amplifier. I'd start with TR13.

    Have compare transistor's values in diode mode between TR13 & TR11 it's same values...
    Have swap TR13 & TR11 alaway same problem !, R29 is hot ! (TR13 & 11 are MJ2501 on my amp, not 3001 like on schematic.)
    Have measured shorted or leaky with ground, no problem.

    You say : "You could check out the left amplifier by itself by connecting the dummy load between the left output (right side of R22) and ground." :

    Thanks, I do with a signal? I must get what, measure the offset?
    Can you explain to me ?

    Thanks .

  12. #12
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    Quote Originally Posted by Feufeu View Post
    (TR13 & 11 are MJ2501 on my amp, not 3001 like on schematic.)
    That's confusing. Are TR12 & 14 MJ3001 then? Is it just a schematic error?

    Quote Originally Posted by Feufeu View Post
    You say : "You could check out the left amplifier by itself by connecting the dummy load between the left output (right side of R22) and ground." :

    Thanks, I do with a signal? I must get what, measure the offset?
    The offset was measured above at 85mV wasn't it?
    Yes, Do it with a signal. The left amplifier will only produce 1/4 power (50W?) when used single ended but it will check it's working.

  13. #13
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    Quote Originally Posted by Dave H View Post
    That's confusing. Are TR12 & 14 MJ3001 then? Is it just a schematic error?



    The offset was measured above at 85mV wasn't it?
    Yes, Do it with a signal. The left amplifier will only produce 1/4 power (50W?) when used single ended but it will check it's working.
    yes 85mV and 1.85V (for negative speaker and ground)

    I think is schematic error ... amp have never was repair ...

    I have plug dummy load between R22 & Ground chassis, amp working , and R29 is not hot.

    I have compare signal between R22 / dummy load / ground and normaly output jack whith dummy load plug, and, have no amplitude signal difference ...
    amp developed same power ...

    Thanks you very much for your help

  14. #14
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    Quote Originally Posted by Feufeu View Post
    I think is schematic error ... amp have never was repair ...

    I have plug dummy load between R22 & Ground chassis, amp working , and R29 is not hot.

    I have compare signal between R22 / dummy load / ground and normaly output jack whith dummy load plug, and, have no amplitude signal difference ...
    amp developed same power ...
    Should the schematic be as marked up in red below?

    5520pwr.png

    It looks like the left amp is working but the right amp isn't.

    From post #1

    Quote Originally Posted by Feufeu View Post
    The problem is kind of weird, with no load & no signal, BIAS values trought resistors are for :

    - R12 : 7.3mV
    - R13 : 7.2mV
    - R29 : 5.9mV
    - R30 : 5.7mV

    Amp run with 2 pairs of MJ3001 and MJ2501, but in fact transistor is Inverted Comparing with schematic ...

    But when i plug my dummy load 8R, the BIAS values trought resistors are for :

    - R12 : 5mV
    - R13 : 73mV ( resistor hot)
    - R29 : 1.3V ( resistor very hot)
    - R30 : 0.14mV
    Thinking out loud here -
    The numbers above don't add up. 1.3V across R29 is 3.9A (but only when the dummy load is connected). Where is all that current going? There's only 0.22A through R13. Speaker -ve is at 1.85V, dummy load is 8ohms, 1.85/8 = 0.23A which I think is the current through the dummy load and R13, that leaves 3.67A through R29 which I think must be going to ground. Does the speaker -ve (jack sleeve) become grounded (perhaps through the scope) when the dummy load is connected to the normal jack? There should be no ground connection to the speaker -ve in normal operation.
    Last edited by Dave H; 01-14-2017 at 10:47 PM. Reason: ,

  15. #15
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    [QUOTE=Dave H;444866]Should the schematic be as marked up in red below?

    SORRY HAVE NO ERRORS, I was wrong

    It is cliff jack, not leaky ground possible. I have mesure between groud and negative speaker (-VE) have 10K Ohms and positive speaker and ground have also 10K Ohms.

    EDIT:
    Ok i found the problem, if i plug the ground of my probe , i have 1VDC on R29. and this burning when amp working.
    If i disconnected ground probe , no problems , have 0.78mV on R29 ...
    Ground prob make shot circuit with amp topology put negative speaker at the ground by the amplifier and oscilloscope earth Connections ... but it's only R29 was very hot ?

    Now bias mesure with load and no probe ground:

    R12 : 5mV
    R13 : 79,5mV
    R29 : 77,5mV
    R30 : 2,5mV

    For you it's normaly R13 (TR12) and R29 (TR13) have more curent consumption ?

    Thanks and sorry again for bad wiring ...
    Last edited by Feufeu; 01-16-2017 at 04:18 PM.

  16. #16
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    Quote Originally Posted by Feufeu View Post
    Ok i found the problem, if i plug the ground of my probe , i have 1VDC on R29. and this burning when amp working.
    If i disconnected ground probe , no problems , have 0.78mV on R29 ...
    Ground prob make shot circuit with amp topology put negative speaker at the ground by the amplifier and oscilloscope earth Connections ... but it's only R29 was very hot ?

    Now bias mesure with load and no probe ground:

    R12 : 5mV
    R13 : 79,5mV
    R29 : 77,5mV
    R30 : 2,5mV

    For you it's normaly R13 (TR12) and R29 (TR13) have more curent consumption ?
    Does the right amplifier still have a +1.85V offset wrt ground? The scope probe ground connection was shorting the +1.85V to ground and the short circuit current was cooking R29.

    It's not normal for R13 and R29 to have more current. It must be because of the 1.85V offset on the right amplifier. 1.85/8 = 0.23A through R13 and R29 so the R13, R29 voltage is 0.23A x 0.33 = 76mV. You need to fix the voltage offset on the right hand amplifier.

    If you have two scope probes with no load connected check the waveforms on the speaker +ve and -ve. They should be the same amplitude but 180deg out of phase. I suspect the right amplifier (speaker -ve) isn't working.
    Last edited by Dave H; 01-16-2017 at 11:32 PM.

  17. #17
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    Quote Originally Posted by Dave H View Post
    Does the right amplifier still have a +1.85V offset wrt ground? The scope probe ground connection was shorting the +1.85V to ground and the short circuit current was cooking R29.

    If you have two scope probes with no load connected check the waveforms on the speaker +ve and -ve. They should be the same amplitude but 180deg out of phase. I suspect the right amplifier (speaker -ve) isn't working.
    Voltage offset stay the same.

    Okay, have same amplitude signal on ve- and ve + , with 180 out of phase ,
    with my scope can rotary phase, I have superimposed 2 signals, and they are identical

    i have also do this test with dummy load and have also same amplitude signal, amplifier seems to work properly...

    - How to fix voltage offset on this amp ? is possible ? and what is TR10 transistor ?
    - D1 and D2 are BIAS setting diode ?

    I have again this amplifier just for 2 days, after i will have to return it to my friend ...

    Again thanks for your help Dave H

  18. #18
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    Quote Originally Posted by Feufeu View Post
    - How to fix voltage offset on this amp ? is possible ? and what is TR10 transistor ?
    - D1 and D2 are BIAS setting diode ?
    D1,2 are to protect TR4,6 from reverse voltage. TR4,6 are part of the output current limit protection circuit. They monitor the voltage across R12,13 to sense the current and hold off TR11,12 if the current is too high. The bias isn't set by diodes it's set by transistors TR10 and TR5.

    So the amplifiers seem to be working except for the 1.85V offset on the right amp. Is that right? I'm not sure why it has an offset. I'd have a look at the long tailed pair TR7,8. Measure the base and emitter voltages of TR7,8 and the tail voltage at the bottom (on schematic) of R35. It should be -9.1V (ZD2 voltage).
    Last edited by Dave H; 01-18-2017 at 08:12 PM.

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    Quote Originally Posted by Dave H View Post
    So the amplifiers seem to be working except for the 1.85V offset on the right amp. Is that right? I'm not sure why it has an offset. I'd have a look at the long tailed pair TR7,8. Measure the base and emitter voltages of TR7,8 and the tail voltage at the bottom (on schematic) of R35. It should be -9.1V (ZD2 voltage).
    Thanks for the explanation

    Okay;

    For TR7 : Base -167mV / Emitter -73V
    For TR8 : Base -165mV / Emitter -73V
    R35 bottom (ZD2) -8.9V

    if you wanna for compare with TR1,2

    For TR1 : Base -98mV / Emitter -0.66V
    For TR2 : Base -91mV / Emitter -0.66V
    R14 bottom -0.66V ZD1) -9.1V

    I don't know if values have to be the same on two long tailed pair ?

    -> on my amp R31 as a 100K resitor, R9 is a 10K resistor just for say ...

    maybe DZ2 problem , or need remplace TR7,8 ?

    Thanks
    Last edited by Feufeu; 01-19-2017 at 10:11 AM.

  20. #20
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    Forecast a little windy (as Chuck would say)

    Looking at TR1,2 first, I think the voltages are OK. The transistor bases will be slightly -ve because of the base current voltage drop by R1,9. This makes C5 reverse polarity. I've seen this mistake before. C5 should have its -ve terminal connected to TR2 base. R9 is the feedback resistor. It works with R8 to set the AC gain of the left amp. AC gain is (R9+R8)/R8. C5 blocks DC making the amp unity gain at DC. Any leakage through C5 (because of reverse polarity) will increase the DC gain and amplifier output offset voltage. The left amp has low offset voltage doesn't it? But I think I would still replace C5 and reverse its polarity.

    Wait, I missed R14 bottom -0.66V ZD1) -9.1V
    I don't understand. It should be R14 top -0.66V, R14 bottom -9.1V.

    TR7,8 voltages don't look right. It can't work like that. The emitter voltages can't be -73V can they? Are you sure it's not -0.73V? -8.9V is OK for ZD2. The LTP should work OK with -8.9V. Yes, I think I would replace TR7,8.
    R31 and R28 set the right amp gain to -1 (unity gain inverter). They have be the same value to do that. R23 is 10k so I would use 2 x 10k (or better 2 x 20k) for R31,28. It will balance the LTP better and should reduce the output offset. While you are in there check the values of the other LTP components to be sure nothing is missed.

    Good luck.

  21. #21
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    Oups, Yes in fact R14 top it is -0,66V and -9,1v on bottom
    Yes it is -0,73V for voltage Emitter. sorry for mistake.

    Il try this if i can . Amp going tomorrow ...

    Thanks again for your help Dave

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