Marshall 8008 Rack mount Blowing transistors

[blindboybenton]

Hi All

Ive been asked to have a look at a Marshall 8008 rack mount. The customer had a Ch B Blow up. So he replaced Both channels output transitors
with the correct ones and at the end of his band practice. Ch B blow again. Ive had a little meter about can't seem to see anything wrong. Lifted some components I'm unsure about putting another set in the amp. for them to blow maybe days later.

http://www.amparchives.com/Amp%20Arc...2x80w_8008.pdf

Does anyone have any experience with these amps.

BBB

[The Dude]

There are many possibilities here ranging from other defective components to the customer using the same shorted speaker cable he used before he blew the amp the first time.

I'd advise monitoring the voltage drop (which will give you idle current) across the emitter resistors of the blown side (after transistors are replaced) and comparing to the known good side. If there's a huge discrepancy between the two, it's obvious you have other problems (resistors that have gone towards open, blown caps, leaky driver transistors, etc.). I'd make sure his cables/cabinets are without problems, too.

[Enzo]

What are you replacing the bad ones WITH? More of those BDV things? I recommend just using TIP142 and TIP147 in their place. I also think it was a bad idea to replace the output transistors in the good reliable channel. It was working, don't "fix" it.

If TR18 and TR4 died, we replace them, but we also check the resistors that conect to them. When those die, I always replace the drivers too, that would be TR17 and TR9. And check all the resistors connected to them as well. When transistors short out and die, they often burn or damage the resistors around them. And in that case, I'd also replace bias transistor TR13.

And on the schematic, look over by the channel B output jack, see the 10 ohm 5 watt R64? That resistor and cap are your stability network, they prevent oscillation. Make sure that resistor is not open. The cap rarely dies.

[blindboybenton]

Many thanks Enzo

I dont know why the customer changed both channels outputs. I was looking over the schematic this morning . Ill change the outputs for the TIP142/147, reading on net last night about using the tip instead of the BDV. Think i will change the drivers aswell and then test and compare voltages from both sides. Will test for a few days and hope for the best.. Customer is an electronics engineer, so if he wants my knowledge and yours he will have to pay.. Your share of the cash is in the post

[blindboybenton]

There are many possibilities here ranging from other defective components to the customer using the same shorted speaker cable he used before he blew the amp the first time.

I'd advise monitoring the voltage drop (which will give you idle current) across the emitter resistors of the blown side (after transistors are replaced) and comparing to the known good side. If there's a huge discrepancy between the two, it's obvious you have other problems (resistors that have gone towards open, blown caps, leaky driver transistors, etc.). I'd make sure his cables/cabinets are without problems, too.

Many thanks Dude

Im going to ask the customer to check or show me his cab's and cables

BBB

[J M Fahey]

And suggest he uses a Lamp Bulb Limiter first until he's happy with voltages, mainly offset and bias.

[blindboybenton]

Hi All

Well the story of this amp blowing outputs up is finisned

I replaced them for the TIP142/147 and found a couple of possible Dj's on the TIP129. Turned it on all looks good. been running now for 2 days
His fitting of the outputs lasted about 5 hrs

BBB

[Gtr_tech]

If the BDV parts are out of production, its quite possible he got hosed with asian fakes. They are everywhere now since they figger'd out that they can sell a ton of subpar junk parts labelled with a hard to find number.

With SS meltdowns, I like to check everything right back to the diff amp. Certain designs (Yamaha home stereo amplifiers especially) will blow everything in the output path.

[Enzo]

Even Marshall used TIP142/147 in some production runs of that series of amp models. Those are readily available from reliable sources like Mouser.

[blindboybenton]

Yes enzo. I read that they were good. But I placed a order with a UK firm called cpc who are owner by farnell.. who I think operate in the us as Newark? The last part I was ordering was the bdv Xx and they didn't stock them. So I went for the tip instead. Got a few spare ones too. Well it's 1am here. Time for bed.