Need help with CR2020 blowing B+ amp fuse and out put transistors

Did you do the SB mods?

There's a silk screen error on the PS board as I recall that will get you in trouble with a backwards cap install if you're not careful.

Don't recall the symptom for the error, sorry.

Had the replacement for TR608 in backward. No more shorts but not able to adjust idel current. checked VR601 (variable resistor) along with Tr604,05,06 and 07. all appear to be good. I hear the relay click twice but no sound yet.:tears:

Two clicks is not a good thing.Check your PM's.

have only one click again however I still can adjust idle current. It is at 0mv. The yellow and brown wires to ground are at 2.3mv. I've checked T702 and T706, TR607 and TR608 all are ok. Checked all Caps.

I still can adjust idle current. It is at 0mv.

Click to expand...

Can or can not ?

ronjon489 said:

have only one click again however I still can adjust idle current. It is at 0mv. The yellow and brown wires to ground are at 2.3mv. I've checked T702 and T706, TR607 and TR608 all are ok. Checked all Caps.

Click to expand...

What test points are you using for the idle current(bias) adjustment. TP1 and TPO ?

The yellow and brown wires to ground are at 2.3mv.

Click to expand...

That would be your DC offset. Yellow and brown wires are the amplifier out's, 2.3mv is excellent. A good thing!

DC offset is totally different than idle current. There is no adjustment for DC offset.

using TP1 to TP0 for idle current measurement. can't adjust idle current it stay's at 0mv

I'm a bit confused as I read the folloing thread on bias "using E and TPO measure the dc offset after the protect circuit relay has tripped. If you still have less than 100mv of dc offset then you have a problem in the protect circuit itself or a power supply problem ." If E is the same as ground then accodung to this thread I have a problem because I have only 2.3 mv rather than the minium of 100mv. Please explain

If E is the same as ground then accodung to this thread I have a problem because I have only 2.3 mv rather than the minium of 100mv. Please explain

Click to expand...

DC offset or DC Balance
Ideally..you want 0.000 volts at the amplifier out.Zero volts doesn't happen realistically. (TP0 - E )..Less than 100mv !! 2.3 millivolts is 0.0023 volts which is excellent! Don't worry about the DC offset.(TP0 and E)!!!

If your relay clicks once - the protect circuit is working as it should.

The idle current adjustment between TP1 and TP0 is the issue at hand.

Look at the schematic. TP1 and TP0 is measuring the voltage across one of the output transistor emitter resistors R631 a .47 ohm 5 watt wirewound .

What your trying to do when you adjust the trimmer is to adjust the quiessent (idle)current flow through R631 to approx 21 milliamps
The adjustment procedure has you adjust it to 10mv
Using ohms law..
10mv/.47ohms = 21 ma

I am running into a similar issue. Both fuses are blowing on one amplifier board. Not sure where to begin with the troubleshooting on this particular issue. Dealing with a pretty hacked up CR-2020 over here.