I have a 9090db that is in protect. I found one fuse blown F01. If I take the driver board out, it comes out of protect. Outputs all test OK. I took out and tested all the trannies for the side that blew the fuse. All test ok out of circuit. Tested all the diodes in circuit, all ok. Tested all the fusistors, all ok, replaced what was needed.

Also, When I go to test the Bias across either f04 or f05, I get no readings....

I have worked on many of these, but these dang driver boards drive me nuts....

Could be one of the trimmers pots. Test all four.
Peter

I did find a blown fuse on the relay boars too. Replaced it, but still the same results.

Ok I was able to get it almost up and running. I Was able to adjust both DC offsets. And the left bias, but now the right bias is off the charts. It does come out of protect.

What parts might be suspect?

I guess I'll have to test all those trannies OOC.

Go over the bias circuit, which is located in the base circuits of the NPN and PNP class-AB driver transistors. Check every part in this circuit. If nothing is found to be wrong in the bias section, I suspect a leaky transistor or transistors. Check the class-A driver transistors, which, as you know, connect directly to the bias circuit for leakage. Also, check the class-AB drivers and the output transistors for leakage.

Best,

Fred

I am gonna test them all out of circuit.

Of note, when I power on, the bias is in the normal range. After a few seconds it jumps off the charts.

Not sure if that tells the experts anything.

Does a variac pulse?

I am gonna test them all out of circuit.

Of note, when I power on, the bias is in the normal range. After a few seconds it jumps off the charts.

Not sure if that tells the experts anything.


Check all the transistors in the feedback circuit. There is no feedback until the unit comes out of protection, You will probably notice that it is when the relay clicks that the bias jumps. Bad feedback can cause bad bias.

I didn't see any mention of electrolytic capacitors. That set is just about old enough for them to be leaky. Have you checked/replaced any on the driver board? Electrolytics are cheap and often bad by now. Just a thought...

Check all the transistors in the feedback circuit. There is no feedback until the unit comes out of protection, You will probably notice that it is when the relay clicks that the bias jumps. Bad feedback can cause bad bias.

This is exactly what it is doing, but I checked all the transistors ,a nd they test ok, out of circuit...:scratch2:

Do you have an oscilloscope? If so, put it on the output of the bad channel, before the relay. It may be oscillating, which would make it draw lots of current.

Do you have an oscilloscope? If so, put it on the output of the bad channel, before the relay. It may be oscillating, which would make it draw lots of current.

I do have one. Don't know if it works. Even if it does, I don't know how to use it....

How else could I test this?

Of note on the channel. While tinkering with one of the fuses (F04 or F05) powered off. I touched one end of the fuse to the outer heatsink on the thermal bias transistor. It threw quite a spark. This may have caused this bias problem. Thing is, I tested all transistors out of circuit, and all tesed ok.

I'll bring over a scope that works. I'm even happier to see if we can figger out more functions on it together. I've got a lot of test equipment I wanna learn how to use....:yes:

I got a function generator now too. Will that be of use?

Man this thing is driving me nuts....:screwy:

Here's another thing to look at. On the 9090 and 9090dB the speaker relay shorts the speaker terminals (NOT the output of the amp) to ground when it is off. Sometimes the contacts weld together and cause the output of the amp to short to ground. Pull the driver board and check at the speaker terminals with an ohm meter, make sure it isn't shorted after the relay clicks on.

I think you are on to something...

I took the driver board out. and powered on. and the suspect channel is shorted at the terminal.

It looks as though the middle contact is stuck upon visual inspection.

That got it!!! Had to do some repair work to the relay.

Thanx

Congratulations! :banana:

A big Thanks to Dr Audio for the tip !!!!

You are the man...:thmbsp::thmbsp::thmbsp:

Ta-daaaaa! Now you should replace the relay, repairing it is a false economy, the contacts are shot; the gold plating is burnt off and they will oxidize and you'll be dropping channels. Also you need to cut the trace on the relay board that connects the relay contacts to ground. It is unnecessary and causes problems as you saw. When the relay is off, the speakers are disconnected from the amp, so shorting them to ground is just plain silly. Here's a thread showing how to cut the trace:
http://www.audiokarma.org/forums/showthread.php?t=117552