Dumb BeoMaster 8000 Repair Follies

So, after it wheezed into death, I thought I would give a go at repairing my BeoMaster 8000. I knew all along it would make more sense to send it to Rudy for this, but I just had to do it myself. This unit had shipping damage when I got it, so I did not want to invest money into fixing it.

I ordered up the parts to re-cap the power supply board, which had been cracked, and which I had diagnosed with a dead capacitor. After recapping the power supply board, the unit would power on, but the relays would not activate. It was then that I noticed electrolyte had burst from one of the capacitors on the RH output amplifier board. 

Last night I re-capped both output amps and replaced the trimmers. I got it all trimmed up and it was working beautifully. Then, in my infinite wisdom, I screwed the heatsinks back on and thought I would turn it back on one more time before going to bed.

That was when I realized that I had accidentally left the input panel in such a place that it was shorting out 5-TR207, and my joy rapidly transitioned into anguish. After that, I can turn the unit on, and the right channel will briefly produce sound until it goes silent. The electronics and controls all work as they should, but the outputs do not.

I pulled the heatsink on the LH side off and checked the resistance on the six transistors, and none seem to be particularly low or high, but I suppose I have no idea what I am looking for, so maybe I have missed something. 

Does anyone have any ideas as to what my best DIY path forward is from here? My guess would be to locate a suitable replacement for TR207, but I figured I would reach out to the community for advice before I proceeded.

Many thanks,

Tom

EDIT: Forgot to add a photo of the shorted component. You can see the burn mark on the top right.

 

That machine also gave me headache many months ago... Don't give up. It may be not that bad after all. My guess is the machine stops because there's something wrong in the left output and it goes into protection. And maybe that something wrong is only this bf859. If i remember correctly it's the driver transistor so it's directly in the audio path.

Guillaume

Thanks, Guillaume. I am hoping that it is something simple. 

A quick update: If I disconnect the yellow/green/red wire that feeds the LH board #5 ,the unit will power up and seemingly works as if nothing were wrong. I have sound out of both speakers, and if I didn't know any better, I would tell you that there was nothing wrong. I expected to find that the system worked with sound out of only the RH speaker, but that's not what I have found.

This has served to remind me of how little I understand about how this system works.

Are you talking about the connector P38? One of the pin of this connector actually trigger the fault switch, so maybe it's not a good idea to leave it disconnected to bypass the protection. Find the fault. Do you still have the 18mV between both emittor resistors and 0V at the speaker output?

Hi Tom,

The next step is to check whether 207 and the output transistors are o.k. or not. In absence of a component tester, you can give it a shot with a multimeter in resistance setting. Just measure the resistance between the C and E leads, and see if you get something significant. If it is close to zero, the transistor is toast (http://beolover.blogspot.com/2013/09/beomaster-8000-output-transistor.html). This test is not a 100% conclusive, but often works. If it is not conclusive, unsolder the transistor and check the BC and BE junctions with the diode tester.

It would be best if you could run the output isolated with bench supplies limiting the current. See here for details: http://beolover.blogspot.com/2011/09/output-stages-testrecap.html.

This would immediately tell if everything is o.k. after you exchange parts, and where the current is going.

Good luck!

Rudy

 

 

Thanks for the input, guys. I do not see a P38, but the wire I disconnected was P8 on the LH ouput board #5. I have no intention of leaving it disconnected, but I was feeling reckless and went for it.

Rudy: I saw your transistor testing blog entry and checked all six LH transistors, and all seemed to have a normal resistance. 

I'm thinking that I'll order up a BF859 and swap it out to see if things improve. They're not very expensive, and available locally, believe it or not. I do wonder why the Manual calls for a BF857 or BF849 when the unit installed is a BF859. Perhaps a running change made to later production units, or maybe it has been replaced before?

As for bench power supplies, I don't have 'em. Maybe I can find someone locally to lend them or just send the whole lot to you a when I REALLY fry it?

trymes:

I do wonder why the Manual calls for a BF857 or BF849 when the unit installed is a BF859. Perhaps a running change made to later production units, or maybe it has been replaced before?

BF859 has the same properties of BF857, but with better maximum voltage spec. 

Well, it's back together with a new BF859, though that might have been superfluous, as there was a burned trace in the board. Now the unit is back up and functioning. I have trimmed the offset voltage and the quiescent voltage as best I can, though they both seem to be less than rock steady as the components warm and cool. The new BF859 is quite hot, but so is the one on the other output, so I suppose that's normal.

The one new fault I can see is that the LH speaker will come on at approx a 2.0 level of volume at 0.4 or lower on the volume dial. There are some other things is would eventually like to work on, but nothing that wasn't there all along.

Thoughts?