Hello at all,
Here i have a Marshall bass 200 combo (5520).
This amp have à 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
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 ?
Last edited by Feufeu; 01-11-2017 at 12:07 PM.
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
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
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.
[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?
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. "
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 ?
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.
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
It looks like the left amp is working but the right amp isn't.
From post #1
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 09:47 PM. Reason: ,
[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.
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 à 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 03:18 PM.
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 10:32 PM.
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
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 07:12 PM.
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 ?
Last edited by Feufeu; 01-19-2017 at 09:11 AM.
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.
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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