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Hi, I tried to repair Beomaster 8000 and i found problem. I replaced 100 ohm potentiometer R226 to multiturn, and replace 6 power transistors IC201-206(TIP142-147). I can set now 18mV for iddle current, however I see on output oscillations ~450kHz (without load, with 8 ohm resistor it start even worse). I guess I am not first one but I didn't find answer on forum. Thanks in advance for any response.
Hi
First check all grounding and voltage lines.
Do you have the oscillations on bothe sides left and right?
You wrote that you changed 6 TIP's... in one side or random in both sides?
Had a similar problem with a Beomaster 6000, sorted out that it was not a good idea to take darlingtons with higher C-E voltage TIP142 =100 volt , TIP141 80 volt), measured that they have a bigger hfe so the amp started to oscillate.
It's hard to do anything against oscillation in a power amp.
I know it's a lot of work with changing all of them but maybe you try original TIP141 and 146 first on the right side.
Good luck and let us know when you got further
Kind regards
Christian
Thanks Christian for your remarks. Maybe I will add short story. I bought this with damaged right channel. There were distortions - after put 1kHz signal to amplifier, on output I saw that lower part of sinus is completely different than upper part. At the same time left channel was clean. I disassembled unit and found broken transistor in current limiter block TR210. I replaced them and started device. Then right channel exploded. I realized that during replacing TR210, potentiometer R226 lost conductivity. After power up all 6 power transistors in right channel due to cross conduction immediately were damaged (short circuited CE). I replaced all 6 transistors with TIP142-147. Earlier this amp was repaired because there were located transistors BDV64B-BDV65B (for 100V), so I think TIP142-147 should be good too. However, you are right, could be that this batch of transistors have completely different hfe. After new transistors were placed on heatsink I switched on amplifier and set iddle current - 18mV between TP200-TP20. It was OK, but after adding 8 ohm resistor instead of speakers, I saw that this current is not stable. Then I check output with oscilloscope and I realize there are oscillations. Left channel is at the same time silent and OK. I see the PCB is in bad condition, overheated with black stains under components. I afraid, that during further investigations, bending PCB up and down, another tracks can be broken. So I try to collect some ideas just to touch PCB less. I tried to play with frequency compensation in power amp but no luck. Next idea is to add ferrite beads on wire attached to base of power transistors.
Please feel free to add any remarks, thoughts how to follow with this issue.
Best regards
Andrzej
Sounds like it was "BANG" and Olufsen.
I know these kind of toasted boards, with lose traces lost solderjoints and so on.
Maybe this could help:
https://archivedforum2.beoworld.org/forums/t/40659.aspx?PageIndex=2
Member Mohawk has build new poweramp PCB's, for sale on the big auction site ;-))
I'm not in touch with him and I don't get any directors fee ;-)
Regards
I agree with Christian. I would contact Fredrik and order a couple of his new output amplifier boards for the Beomaster.
-sonavor
Christian, you made me laugh yesterday with "It was "Bang" and Olufsen".
Probably you are right and new PCB will be the fastest way to fix this amplifier.
However, I don't want to give up and I'll try to repair, new PCB will be the last chance :)