Alpine 3545 blows all fuses after 5 seconds of power

[bmore5077]

Does anyone know how close the transistor pairs should be. Above when I tested leg 3 of the npn and pnp transistors there was 3 pairs and they read 1.0, 1.1 and 1.3mv. Is that close enough or about how close should they be. Thanks

 

[Perry Babin]

If you went back through, did those numbers remain the same for all 3?

 

[bmore5077]

After 20 min the bad side. 1st pair went from 1.1 to 1.7 second pair went from 1.3 to 1.9mv. Thanks

 

[Perry Babin]

After the temperature stabilizes, if they're about the same across all 2/3 pairs, I think it's OK. For tests to see if current sharing is good for all transistors, it's better to test with a sine wave at higher power into a load. If you use all matching parts from the same batch and the emitter resistors are within tolerance, it's likely OK. It's not something to really worry about. The initial tests where the bias went to 7.whatever mv was what concerned me initially. All appears OK for now.

 

[bmore5077]

Thanks again. So I guess I'll let it warm up at the 1 mv you had me set it at and then set it to 4mv from the testpoint as the service manual states. Thanks

 

[Perry Babin]

4mv is too high, in my opinion. I'd leave it around 1mv.

 

[bmore5077]

Ok sounds good. Thanks

 

[bmore5077]

Ok so I replaced the non and pnp transistors on the one channel and adjusted both to 1mv idle current but the one repaired channel still plays much lower than the other. Not sure what's going on. Thanks

 

[bmore5077]

Actually it starts off louder than it cuts it down. Maybe thermal but not sure

 

[Perry Babin]

That sounds like a preamp problem. Check the signal level for both channels where the preamp signal goes into the main board.

 

[bmore5077]

Ok I'll look at the schematics and see if I can figure out where to check it. Thanks

 

[bmore5077]

Hello I don't see preamp on the schematics. I do see muting circuit....would that do it. Thanks

 

[Perry Babin]

It could but the muting circuit for both channels is driven by the same signal. It's possible for a muting transistor to fail but rare.

Page 7. RCA PC board. Follow the lines with gray shading.

 

[bmore5077]

Ok I'll see if I can find where to take the readings from but it looks like that board is covered so I can't see the back. I'm sorry I have to bother you with so many questions but I'm trying to learn. I did some electronics mainly circuit card repair when I was in the marines but it's been a long time ago. Thanks again

 

[bmore5077]

I'm assuming I could read the voltage for both channels from both the ic 201 and ic 202 if I can get to them but it looks like I can't. So maybe read from one of the pins at the driver ic 501,,502. Thanks

 

[Perry Babin]

Don't check at the driver. One slip could cause catastrophic failure.

Do you have a scope?

 

[bmore5077]

No i dont. Sorry

 

[Perry Babin]

The voltage is going to be low. The input to the RCAs is going to have to be detectable with the meter. Expect only about 200mv AC through the circuit.

 

[bmore5077]

Anyplace on the main board to read the signal. I can't get to the rca board... It's totally covered and I know you said not to read at driver. I appreciate any advice. Thanks

 

[Perry Babin]

read off of the header pins first.