Marshall Lead and Bass SS JMP 2195 issues

[henter2017]

Hi Folks!

Im working on a 2195 Solid state Darligton driven Lead and Bass amplifier.
It was burning up fuses but found out a bunch of burnt resistors from the Darlignton's power amp part of the amplifier, also the diode bridge was gone, wich has now been replaced, also D3 and TR10 were gone, also replaced now (used a turned around 2N5088 while waiting for the correct BC182 for it), now the amp is turing on but R28 has catched fire!), wondered if the MJ3001 darlignton was working fine, so took it off and tested it but it did not showed any resistance while shorting the collector and base while measuring from base to emiter which apprently should mean is fried...)

I have ordered a pair of MJ3001 and MJ2501 to install, since this is the first time I work on a darlignton powered amplifier and have no idea about biasing the transistors, could you give me some ideas, there is a pot RV1 which I guess is used for this purpose...

Thanks in advance!

http://elektrotanya.com/PREVIEWS/634...2195.pdf_1.png

[J M Fahey]

1) please do not:

not showed any resistance

, every measurement shows something (unless meter is turned off), I guess you meant "it showed zero ohms" or a *very low* value or "a short" ; in any case "a number" is best.

2) yes, they re very probably shorted.

Replace them properly, clean the heatsink first, use new mica and thermal paste, just in case sheck metal case is not shorted to heatsink, not very usual but sometimes a metal burr or shaving or whatever gets crushed by the transistor (thatīs why I suggested cleaning the heatsink) and punches insuation.

Also biild and use a lamp bulb limiter until you are certain the amp is fine, also do not plug speakers or any load yet.

Turn it on, check that lamp limiter blinks and then almost goes off (might glow red or dull orange) and measure voltages, speacially at speaker out.

[henter2015]

Thanks a lot!
I have received now a new pair or MJ3001 and MJ2501 and these are testing good in comparation with the old ones, the old ones tested 0ohm in all positions and did not change when sorting to base.
I will be installing this new darligntons as soon as the other parts arrive and give it another test with a lamp in series as suggested!

[henter2017]

Hi Folks, I have installed the new pair of MJ3001/MJ2501 on the amp as well as TR10 to the correct BC182, I still need to swap R31 (as you can see is burned) but is not shorted and still measuring the correct value 470K, so used it meanwhile for a test.

I used a bulb limiter with a 40w lamp, and when the amp is on the bulb goes ON but with low light, when I plug a load to it it goes totally bright, something else must be wrong, Im probably missing something but I cant see what!

I have recorded a small video showing some voltages...

https://www.youtube.com/watch?v=ZN-EL9vLY28

[henter2017]

the burned PCB was something that was fixed before, not by me, is all wired up from the back of the PCB

[Enzo]

So with no load, but powered up, see if there is DC voltage across the speaker wires. Plugging a speaker onto that output with DC present will draw a lot of current - and thus light the bulb.

[henter2017]

Hi Enzo, thanks for helping out, I can measure 1V on the speaker output but it drops slowly while measuring...
Also I have just noticed that the transistor that I replaced TR10 is overheating like crazy, im pretty sure it will explode if it wasnt for the current limiter bulb..

[henter2017]

bump! anybody could give me a hand? Im a bit desperate with this one! thanks!

[J M Fahey]

Ok, Iīll bite.

Th video does not tell much, the camera moves too fast and you donīt speak, so no clue on what you are measuring.

1) please do and answer exactly whatīs asked for so as to nt waste time
2) weīll first simplify it as much as possible, and once the core is working, address minor matters.
Iīm not so sure about it since the PCB was burnt and repaired, but **absolute worst case you can mount another 100W amplifier board there, feed and drive it from original PCB and preamp** so the amp is not lost; you have ample space inside that chassis.
This only as Plan B, so donīt desperate.
3) start by lifting one end of diodes D3 and D4 , you just suck one solder pad and lift diode leg a couple mm away from the PCB, this momentarily puts protection circuit away
4) then with a short piece of wire, you short from top of R24 to bottom of R25 so we put biasing away.
5) no speaker connected, of course, now we turn amp on through the bulb limiter, post +V and -V rails for now, and speaker out rail voltage, all relative to ground.
Thanks.

[henter2017]

Mil Gracias Juan Manuel!

5) no speaker connected, of course, now we turn amp on through the bulb limiter, post +V and -V rails for now, and speaker out rail voltage, all relative to ground.
Thanks.

+40 Rail = 37V
-40V Rail = -37V

Speaker out tip: 28.49V
Ground = 0V

The limiter flashed bright when pluged but then return to a very very low light while ON.

[henter2017]

cheeky bump!

[Jazz P Bass]

What happened?

Post #7: output has 1 volt
Post #10: output 28.49 volts (is that + or -/ it matters)

Is TR 10 still getting hot?
TR10 & TR11 are for current limiting the output.
You do not need them in circuit for it to 'work'.
So lift D3 & D4 to take them out of the equation.

If the output is +Vdc, I would look at all three voltages of TR7.
It is a PNP transistor, so the Emitter should be at +40Vdc.
The Base, a little less.
The Collector should have a low Vdc on it.
(the lower the Base voltage, the harder the transistor will turn on, making the Collector higher.)

[g1]

What happened?

Post #7: output has 1 volt
Post #10: output 28.49 volts (is that + or -/ it matters)

Is TR 10 still getting hot?
TR10 & TR11 are for current limiting the output.
You do not need them in circuit for it to 'work'.
So lift D3 & D4 to take them out of the equation.

I think the output went to 28.49 because he lifted D3 & D4 (suggested in post #9).

[J M Fahey]

Please take these voltage measurements and post them here.

In those showing just the red probe, the black one should go to ground; those red and blue show where the red and black probes should go respectively; I expect values less than 2V in these so use that scale.
Now if any value is more than 2V , set scale to 20 or 200V DC as needed to get a reading.

The ones measured relative to ground may need the 20 or 200V DC scales.

[henter2017]

Thanks!

I coudnt measure a couple of them as the resistor are right on the top of the markings but I got the rest

This is the voltages with no load and the diodes D3 AND D4 lifted.

2: 41V
3: 17.39V
4: 23.75V
5: 18.19V
6: 41.8V
7: -7.38V
8: -40.5V
9: -58mV
10:41.1V
11: -8.13V
12: 9.80V
13: 64.4V
15: 64.9V
16: 41.1V
17: 24.38V
18: 41.1V