Pioneer A88-x with Bad Channel

[tomharrison1]

Hi Mooly. So the reading is simply 000 when set on Diode measurement setting with positive and negative leads of meter connected either way. I'll check it again w new meter and then reconnect them so I can check voltage across R17 and R18.

 

[tomharrison1]

Diode test w new meter inconclusive. Same number both directions. As you said. Can't measure. So I'm seeing 14.5 across R17, 15.7 across R18.

 

[tomharrison1]

Hmmm. Just rechecked Q1 and Q2. I'm seeing 300mV where I should see 0 and 5.85 where I should see 6. Both showing same positive voltages. What the heck?

 

[tomharrison1]

And...I'm seeing 7mV and 4mV DC offset at speakers where I was seeing over 100mV on the right channel after it went out. Hooked up speakers and its singing again on both channels. Turned it off after 10 seconds. I may still have an over heating issue somewhere. I suppose the good news is no components have completely failed.

 

[tomharrison1]

My new DMM has Capacitor tester. Should I pull and test some of the original electrolytic caps on the Voltage Amp Assy? Or would an overheating issue likely be one of the transistors on that board?

 

[llwhtt]

R115/116 still hot?

Craig

 

[tomharrison1]

Not seeing any smoke if that's what you mean. I didn't leave it on long enough to check any other voltages. When right channel went out followed by left 10 seconds later, I had played Aux for about 5 minutes and just switched to Phono to test it. Music cutout after about 2 minutes.

 

[tomharrison1]

Here's a pic of R113/115 and the new caps I put in. Definite sign of heat at base of 115.

 

[Welcome]

My new DMM has Capacitor tester. Should I pull and test some of the original electrolytic caps on the Voltage Amp Assy? Or would an overheating issue likely be one of the transistors on that board?

No. Ignore the caps, don't remove anything. You don't want to introduce new problems.

Measure idling current. Put meter on an emitter resistor and tell us the value (in mV). Do this for both channels.

 

[tomharrison1]

Ok. But forgive my ignorance. I haven't gotten the terms and locations of components down just yet. Which R numbers should I measure?

 

[tomharrison1]

Also. Am I checking both ends of the resistor, or voltage across them?

 

[llwhtt]

R99 thru R104, they are dual resistors, do they have test points on top? Measure end to end for now, should be 48mv.

Craig

 

[tomharrison1]

Ok.

R99: -47.9.
R101: 44.9
R103: -48.5

R100: 45.3
R102: -47.9
R104: 45.6

 

[tomharrison1]

And those are all mV.

 

[llwhtt]

Those look OK, that's good because I see no way to adjust the idle current.

Craig

 

[tomharrison1]

From what I have read IC1 and IC2 perform auto-biasing.

 

[llwhtt]

If you notice pins 10 and 12 have the same combined 48mv. So IC1/2 are probably monitoring these voltages and adjusting accordingly. I wonder what else IC1/2 are doing?

Craig

 

[tomharrison1]

So the only other area I see overheating is on Power Supply Assy next to protection relays. R15 and R22. See pic.

 

[llwhtt]

That all looks normal, the big 2.2K resistor is dissipating nearly 1 Watt at all times when the amp is on.

Craig

 

[tomharrison1]

So... Next steps? Turn it on and leave it on to see if it overheats again? Then what?