Marshall 5520 BIAS and smoke

[Feufeu]

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

[Dave H]

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.

[Feufeu]

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.

[Dave H]

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

[Feufeu]

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

[Dave H]

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?

[Feufeu]

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 !

[Dave H]

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.

[Feufeu]

[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. "

[Dave H]

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.

[Feufeu]

[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 .

[Dave H]

(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?

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.

[Feufeu]

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

[Dave H]

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?



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

From post #1

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.

[Feufeu]

[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 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 ...