DH 220 Lost a channel FIXED!!!!

[RicharD]

My David Hafler DH220 lost a channel.  I wasn't rockin it so it's not a blown output, but I suspect I might have hit an input with a HV transient, whereas I drive it with a home brew tube preamp.  Anyway... the DC rails are fine.  I did give this amp a cap job about a year ago.  So far I've replaced D1 - D8 & Q1 - Q6.  It's starting to get old blindly replacing every component.  Here is a link to the entire manual:
http://www.hafler.com/techsupport/pdf/DH-220_amp_man.pdf

Here is a gif with the amp schematic and the voltage chart.  Most of the voltages measured were close enough but I've listed the bad measurements in red.  Any help would be greatly appreciated.





Thanks!
-Richard

[PRR]

Not a simple amp.

The manual has you measuring transistor legs to ground. But what really matters here is currents, mostly set by voltages across resistors to the power rails. It may be more illuminating to measure the small voltage to the rail, not the large (and poorly defined) voltage to ground.

Use a floating (battery-power) meter and read across R8. It should be on the order of 1.2V (three diode drops minus an emitter drop). Likewise at R9.

Also check the base-emitter voltage of each transistor near the supply rail. Should be 0.6v-0.7V zone. Watch polarity: 0.6V the wrong way is wrong.

The current in R9 flows up and splits to R13 and R14. If you do V/R math it should add up. When happy, R13 R14 flow identical currents; if one is cut-off that's not right (but maybe an effect not a cause).

We could trace through Q3 Q4 Q7, but the zero-bias on the output gates would make me jump to R21  R30, Q8 and Q11, see what current they are flowing.

Who soldered this? Check joints.

[RicharD]

Voltage across R8 is 1.17VDC.  Voltage across R9 is 0.  Voltage across R13, 14, 21, & 30 are all also 0.

This unit appears to be factory wired.  It sure looks too clean to be a hand soldered job.  I've had this amp in service for close to 2 decades.  It started sounding crappy a year ago and I gave it a cap job.  I turned it on the other day and 1 side was dead.  No bangs, pops, blown fuses, or smoke.

[RicharD]

I think the problem might be Q9 whereas it's 0V on the base, collector, and emitter.  I'm gonna replace C6 while I'm right there too.  Mad dash for transistors.

[PRR]

> problem might be Q9 whereas it's 0V on the base, collector, and emitter.

Effect. Not cause. Q11 is not flowing current through Q9.

Why is R9 zero? Do you see how it is supposed to bias-up?

[RicharD]

I wish I could say yes but.... no.  This circuit has my goose cooked.

It looks like Q1 sets the bias of Q5 & 6, and Q2 sets the bias of Q3 & 4.  Q2 loos suspect to me as does D4, 5, & 6, but all of those have been replaced.  Perhaps one of the diodes was out and cooked the new Q2 before I replaced the diodes.  I have the partz on hand to replace all 4 of those again.

I'm lost.

[RicharD]

>Do you see how it is supposed to bias-up?

Aha!  Now I do!  P1 balances the bias of the 2 halves.  R6 was wide open.  No path for my little electron friends.

Thanks!  You Rule!

-Richard