Marshall 2098 lead 100 repair

[Luisflores]

Hi,i'm new in the forum. Just got an old marshall lead 100 ( jmp 2098) than was in an horrible condition. It got al the electrolytic capacitors damaged, 2 output transistors blown up, output transformer disconnected and almost every transistor in the pcb was damaged. I managed to change everything that was damaged (that was almost everything), but the amp doesn't sound well. I get a distorted sound in the output and it doesn't let me adjust the bias current. Every information that can be given to me will be appreciated thanks

[vintagekiki]

When you plug guitar into Return jack, do you get clean or distorted sound?
Bias current is measured by connecting DC mA meter (range 200 mA) in series with emitter resisistor 0.33.
Bias current adjust with 1k in BC184 circuit.
If bias current doesn't adjust check transistor BC184 and passive components in BC184 circuit.

https://drtube.com/schematics/marshall/2200-pwr.gif

https://drtube.com/schematics/marshall/2200-pre.gif

[Mick Bailey]

Are the the rail voltages correct?

You can more easily measure bias by measuring the voltage across the emitter resistor and use Ohm's law to give you the current V/R=I

Just a note that the output autotransformer has to be connected to a speaker load in these amps, otherwise a large flyback voltage is produced causing extensive damage. SS amps are usually fine with no load, but not this model. As well as the output transistors, did you check the power amp diodes?

[Helmholtz]

Just a note that the output autotransformer has to be connected to a speaker load in these amps, otherwise a large flyback voltage is produced causing extensive damage.

Being curious, under what circumstances did you see the large flyback voltage as this would require a sudden decrease/disruption of inductor current?
Or is it rather an instability of this amp causing oscillation with an inductive load?

[vintagekiki]

You can more easily measure bias by measuring the voltage across the emitter resistor and use Ohm's law to give you the current V/R=I

Just a note that the output autotransformer ... ...

I'm very curious how to measure 10mA at 0.33 Ω.
Ohm's law says it is 3.3 mV. With analog Vmeter can hard to measure, but DVM has walking on display +/- 1 digit.

Autotransformer enables that amplifier has approx same power regardless of speaker load impedance.
If you suspect on flyback voltage, repair can also be done without autotransformer.

[Helmholtz]

If you suspect on flyback voltage, repair can also be done without autotransformer.

Good point!

[Dave H]

the amp doesn't sound well. I get a distorted sound in the output and it doesn't let me adjust the bias current.

The bias current won't adjust but what does it measure? To set the bias current I'd disconnect the output transformer and speaker otherwise a small offset voltage on the output could result in more current flowing than the 10mA bias current.

[Luisflores]

Nope, if i connect the mixer output straight into an amp i get a clean sound but if i do that i can't get a drive sound out of it

[Luisflores]

Im measuring the bias into the collector of the mj2501 (that's what the schematic says) and it goes from like 800ma when turn on and 30 ma when i let like 30 minutes on

[Luisflores]

And to measure the bias i disconnected the OT and let no load on it. Even like 2 days ago i was checking the amp with an imput of a guitar and i turned the bass pot full clockwise, the mj3001 opened from base and emitter

[Enzo]

Why did we break into the collector lead of the MJ2501 instead of just checking voltage across one of the 0.33 ohm resistors?

Look at the DC base voltages of the MJ2501 and MJ3001. Are they the same? As in they seem shorted together? And in fact I am concerned the BC184 bias transistor might be. If it is shorted, you get maximum crossover distortion.

[Luisflores]

I've just checked what you ask. The base voltages are similar but not the same in the transistors, they doesn't seem shorted. The same for the bc184k. What let me concerned about this amps is that i've seen like 3 of them and every single pcb was different to each other.

[Luisflores]

In the night i will post some images of the amp and measure again the voltages in the output transistors to let you know. Tell me also everything you want or need to know about the amp to see with what can i help tou. Thanks

[Mick Bailey]

Being curious, under what circumstances did you see the large flyback voltage as this would require a sudden decrease/disruption of inductor current?
Or is it rather an instability of this amp causing oscillation with an inductive load?

I think it's perhaps instability, but I just looked back through my notes and saw that the 220n cap in the output network can go open and the inductor solder joints crack so maybe the circuit is OK if everything else is good.

[Helmholtz]

I think it's perhaps instability, but I just looked back through my notes and saw that the 220n cap in the output network can go open and the inductor solder joints crack so maybe the circuit is OK if everything else is good.

One thing is for sure: An output/speaker fuse would be a very bad idea when there is an output (auto-)transformer.