Marshall Guitar Amp Transistors overheating - help

Well that is what I SAID, and in the context of MARSHALL, its quite possible.

Ive seen power amps blow from having transistors unable to handle the rail to rail voltage they saw off load before.

First job in one company was to work that out and select transistors that COULD.

((((( not evasive Phil, I just refered to them as BDV's as you did in your question - and to be brief. BDV64 & 65 are discontinued at RS & Farnell so I fitted BDV64a & 65a. Given that the failure is exactly the same as when I carried out the FIRST repair using pucker BDV64 &

65's I assume suffix 'a' is compatible or adequate. Farnell p/nos 1208581 & 1208579 )))))

((((( thanks Phil )))))

IIRC and I'm pretty sure I do, the letter suffix indicates the working voltage, A being the lowest. That's certainly the case with some BDXs I used. Then they deleted the BDXxxD and we had to have devices selected from BDXxxCs. Not my choice of device I'm glad to say, so I could say "told you so" !

Graham

Wouldn't put it past the Indian factory that makes them these days (I've seen the line) to achieve that. Usually by substitution of inferior components.

Graham

Just watching BBC 4 right now. Its the story of les Paul and the 1st electric...

Kevin Aylward

Which is why the switches are (still?) upside-down, and the inputs are on the opposite side.

You likely have leaky drivers. Never just replace output devices without checking upstream to see if other components were damaged when the outputs shorted. It happens.....

"Neil" "Phil Allison" "

((((( not evasive Phil, I just refered to them as BDV's as you did in your question - and to be brief. BDV64 & 65 are discontinued at RS & Farnell so I fitted BDV64a & 65a. Given that the failure is exactly the same as when I carried out the FIRST repair using pucker BDV64 &

65's I assume suffix 'a' is compatible or adequate. Farnell p/nos 1208581 & 1208579 )))))

** Just one more try:

Did YOU buy the BDVs from Farnell or off eBay ???????

I ask because there are so many **fake** devices offered on eBay.

Never buy semis from eBay.

..... Phil

"Neil"

** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked, the amp will oscillate at supersonic frequency - probably intermittently.

..... Phil

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Graham

Umm....WTF are you talking about? You never heard of drivers taken out as a result of output device shorting? Happens all the time.

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM recently. Last I checked they were still offered in spite of me telling them they were junk and sending them back. Bean counters at work.....

Taken out =/= leaky.

Graham

No ! CROOKS at work.

Graham

Report them to the genuine manufacturer.

Graham

200C

Not my post but okay....

Sorry its been a while since my last message!

I've just popped R29 out of circuit to test. Allowing for the error in my DVM (doesn't go to Zero) the resisitor value is fine (as are the other 0.33 ohm resistors).

All solder joints seem oaky and theres no sign of cracks or burning.

An earlier message suggested the MJF122 darlington so I popped this out too and there are no short circuits. With the dvm set to diode function on the ohm scale and the neg probe on the centre pin and pos probe on left pin the reading is 0.73 & 0.64 with the pos probe on the right pin.

Measure the bias currents into the 'base' leads... somehow, somewhere, there's too much, usually a leaky, perforated driver. __ Steve .

Appologies for the delay getting back to the thread - I thought I posted at the weekend but it doesnt appear to have uploaded.

Anyway, I removed transistor MJF122 as someone suggests it might be faulty. With a DVM set to diode function on the ohm scale and the +ve probe on pin 2 there's no reading on pins 1 & 3. With the -ve probe on pin 2 pin 1 reads 0.74 & pin 3 reads 0.66. With the -ve probe on pin 1 and the +ve probe on pin 3 there is no reading, and reversed the reading is 1.03

I popped out the 0.33 ohm resitor and the value is fine, as are the rest of them and the solder joints too

I have an oscilloscope - can anybody advise where and what signals I should be looking for please?

Thanks in anticipation.