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Beomaster 5500 cd,tape and phone doesn’t work

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Pumbaadk
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Pumbaadk posted on Sat, Apr 27 2019 12:20 AM

 

 

I have bought a cheap 5500, with a few issues, most of them are solved. 

So far most of the caps has benne changes together with relays, bias pots. 

But the amplifier is behaving very strange, it has been adjust to 10 mv in idle, if I have it set to radio everything works fine, if I switch to cd tape of phone, it still works like it should. But as soon as I turn vol up or down, the bias current goes up and I have 300mv in idle. If I go back to radio the bias will go down. 

I have already add 100nF between B-E on 2TR8, and by measure with a scope  I can see that the power supply has I sinus around 300khz. This scenarie only occurs  in tape Phone and CD and change the vol.

Any one have a idea what I should look for ?

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Dillen
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Dillen replied on Sat, Apr 27 2019 10:51 AM

Measuring idle current when the amplifier is not idling, makes no sense.
Idle current is measured when the amplifier is cold (livingroom temp.), volume at zero, no speakers attached. (If the DC offset is far off, some
idle current would run through the speakers, and the reading and setting would be wrong).
Adjust to correct setting, keep it under observation for a couple of minutes, adjust if it drifts a little (if it drifts a lot power off and diagnose the problem).
After 5 minutes you're done. 

Put a scope to the output and look for oscillation.
If you fitted low-ESR capacitors in the output stages the risk of self-oscillation is great and I suggest fitting standard capacitors instead.
Did you replace the small capacitors in the low-voltage regulator circuits? If not, I suggest you do.

Martin

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