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Beomaster 5000 failed amplifier section

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Jeffrey P Brooks
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Jeffrey P Brooks posted on Wed, Nov 24 2021 6:10 AM

I have been struggling trying to get one working Beomaster 5000 working out of 3 non-working receivers.

Two of them had failures in the PC board in the same trace leaving the rectifier.  Must have been a design flaw in the PC board.

After repairing these, and finding what I thought were 4 good BF857 (TR108/208 and TR109/209) and four good Darlingtons (BDV64B and BDV65B) I had a spectacular failure which blew the lid off of the BDV65B on the right channel along with a nice puff of smoke.

I removed all 4 Darlingtons and am focusing on getting all the DC voltages correct before trying some new parts.  I put some test wires in the base of each of these transistors.

Currently moving left to right, I'm measuring -40.6V, -40.8V, -40.8V and -40.8V on the base of the 4 Darlington transistors.

I also measured the collectors of the 4 BF857 and come up with 40.6V, -40.8V, 40.7V and -40.9V.

The only measurements that make sense are the 40V measurements on two of the BF857s as they have collectors wired directly to the 40V power rail.  The others should measure -1.1V (same as the base of the BDV64Bs).

So the base voltage for all 4 darlingtons is about -40V when it should be around -1.1V.  In other words, way off.

Does anyone have a suggestion of where to look for this?  Both channels seem to behave the same way.

Jeff

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Dillen
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Dillen replied on Wed, Nov 24 2021 6:30 AM

You cannot measure voltages without the components installed.
When you powered it up, did you monitor the idle current(s)?

Martin

Jeffrey P Brooks
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Yes, I did adjust the idle trimmers when I started out.  And more recently, I replaced them both because they were original.  The fact that the right channel BDV65B just exploded last time I had it all put together has me nervous about trying it again so I'm going slow.  Can a bad transistor result in this kind of excitement or is it more likely a voltage condition going into a formerly good transistor?

These darlington transistors are a bit strange since it looks like they have a resistor path between the base and emitter so I'm not 100% confident have have a set of 4 good ones to put in!  

Dillen
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Dillen replied on Fri, Nov 26 2021 6:23 AM

Don't get me wrong, but repairing amplifiers like this can some times give even trained and skilled tech guys a good run for their money.
It's not a task for a beginner, and from what you tell us, the terms you use and your working methods, I would say you are not the one to do this repair.
And I say that bearing the greatest respect for DIY'ers.

It could be bad darlingtons, it could be wrong idle current setting(s), it could be a heavy DC introduced somewhere in the amplfiier.
You will need some instrumentation - and not least a good amount of experience to do a repair like this.

Martin

sonavor
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Another thing to consider...
The circuit boards on the Beomaster 5000 (type 2323) are rather delicate. Too much heat when de-soldering can easily cause traces to lift and break. You mentioned using spare parts from other Beomaster 5000 units so it sounds like you purchased several used condition units.  You won't know the history of those boards regarding what other people have tried on those boards before you got them. I would go over all of the traces with a magnifier looking for lifted traces, solder bridges and previous repair attempts. 

Keep in mind that when the receiver was new it was perfect from the factory.  Years later it has failed either due to aging parts (components out of tolerance), some catastrophic event or a combination of both.

-sonavor

chartz
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chartz replied on Fri, Nov 26 2021 3:08 PM
Sorry but your measurements aren’t very useful. What knowledge do you have of electronics? How far can you go?

I don’t really see the point of swapping boards around. Each of them could be fixed!

As John says, ageing is the real issue and none of these boards will be up to specs or even working properly – that’s impossible!

Plus what was done on them – failed repair attempts? – is indeed a source of further problems.

Jacques

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