HRDlx Volume Loss

[kbblues]

Hello,
The volume loss has affected both channels (though at wide open the distorted channel sounds a little bit louder than the clean channel). Overall I would estimate the volume at max is approximately equal to previous volume settings < 2. I changed all tubes, checked bias and gave it the chop stick treatment. Any help would be appreciated.
Thanks,
KB

[mageerc]

Since the volume loss is in all channels I would suspect that the problem might lie in the phase inverter circuit. The plate load resistors on the phase inverter have a history of going open circuit and that would cause a big loss in volume as only 1 power tube will be driven. Check R57 and R58 around V3. R57 is an 82K, R58 is 100K... if you have voltage on the upstream side and little or nothing on the downstream side, that resistor is toast. Use 1/2 watt or better resistors to replace.

[kbblues]

I checked the plate load resistors in the PI. R 57 tested at 80 k ohms and 383 vdc and 245 vdc. R58 tested at 100k ohms and 382 vdc and 236 vdc. I also checked to make sure the output tubes were seated properly. I will check all voltages at V1, 2 and 3 and report back.

[mageerc]

OK... now you might try re-flowing the solder on the power tube sockets...

[kbblues]

I will try that later today as well as check the voltages.

[kbblues]

The problem was an open resistor at R16. I will replace it later today with a 1 watt MO. thanks for the help mageerc.

[mageerc]

Another plate load resistor! I'd go ahead and replace all the plate load resistors while I was in there with 1 watts... glad you got it resolved!

[cdw2000]

Interesting! I wouldn't expect a problem with a load resistor on one of the preamp tubes. 1/2 watt is pretty much the standard size for these on all amps, so 1 watt might be overkill. This might just have been a part with a latent defect.

I thought I might have read somewhere else on this forum that in addition to this amp's other reliabililty problems, there also seemed to be a rash of bad carbon film resistors that folks had trouble with. (Not to be confused with the larger power resistors that folks have had to raise off the circuit board for better cooling, less board scorch.)

Anyone else recall general problems with smaller CF resistors on these amps?

[mageerc]

There was a service bulletin some years ago from Fender saying that they discovered some substandard resistors from a supplier and that the plate loads were failing...

[cdw2000]

That would explain it then...