B200S behaving badly - blown fuses

[MikeB]

Yes please.

[MikeB]

So I found a guy in the UK selling a BC538.  His ebay account says he is away until the 14th which was yesterday but when i sent him a message the out-of-office reply said the 19th.  I have been trying to find a compatible transistor but I am struggling because I don't know which of the specs are critical, which need to be exactly the same, which need to be at least as much and which have to be in the same ball park.  Is there another one that will do the job so that I don't have to wait until the 19th to even start to get it shipped from the other side of the planet?

[MarshallJMP]

How about these?

http://www.ebay.com.au/itm/5pairs-or-10PCS-Transistor-MOTOROLA-ON-TO-92-MPS8099-MPS8599-MPS8099G-MPS8599G/401304959496?epid=738048176&hash=item5d6fa3ba08:g:RG4AAOSwc-tY6Fra

[MikeB]

Thanks.  Don't know why that never came up in any of my searches.  :dunno:
Can't wait to get this thing back together.
I have learnt a lot about transistors.

[Iperfungus]

Uhm...I've to check fuses on my B200s then: maybe B channel isn't dead!!!  :banana-dance:

[MikeB]

OK. So here is the latest.
My packet of MPS8099 and 8599 transistors arrived about a week ago.  I bought heaps because I don't want to have to go through that again.  Timing was rubbish , because I haven't had an opportunity to put it all back together until today.
Everything went back in ok and when I switched it on there were no pyrotechnics.
I then played through it and the same channel was dead.  All the fuses were intact but I was getting no output.  I tried alternating the cables to eliminate dodgy cables and that was when I noticed that there was in fact some signal in that channel, just very very quiet.  I had to have the volume on that channel all the way up and I could only just hear it.  The other channel that didn't blow is behaving completely normally.
So, not all bad news, the sound was fine, just very weak.  Any ideas of where to look next?
I hope everyone had a nice Christmas.
Cheers,
Mike.

[MarshallJMP]

Did you replace all the transistors?

[MikeB]

Yep.  Five in all.
I remember testing the four big ones as they came in, but was so impatient by the time the little one arrived that I just put it straight in.  Do you think that could be it?

[MarshallJMP]

Hard to tell, but since you have one side that works I would suggest measuring some voltage and compare both channel and see if something is not the same.

[MikeB]

Cool. I'm in the process of teaching myself about electronics, so that I can understand more of what is going on and this will be an ideal project for me.

[MarshallJMP]

Well this way you can check all the dc voltages, maybe you can find something since this is a dc coupled amp.

[MikeB]

I'm on it.
Step 1: Find out what "DC coupled"  means...

[MarshallJMP]

Here's some more info

https://en.wikipedia.org/wiki/Direct-coupled_amplifier

http://www.andiha.no/articles/audio/dcamp.htm