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HK330C - Bias/Idle Current Too Low After Main Amp Recap

RyanJHippy

RyanJHippy

New Member
Hey all,

I recently started re-capping my Harmon Kardon 330c after seeing a swelled cap on the power supply board that made me nervous, figured I'd just knock it all out. I've been following @dlucy thread on his recapping experience with this unit, and so far I've been able to get most of the components on his recap+upgrade list, but not all.

I've taken the approach of doing one board then testing to make sure it still works properly to hopefully aid troubleshooting should something go wrong. After doing the main amp board, I went and tested and the bias pots could only push the current on the two resistors to 4mV, as opposed to the 10mV +/-1mV spec. I did the power supply board first, replacing the following:

C3,C4 with UVR1H682MRD (dlucy's recommended was UKW1H682MRD, but it was unavailable)
C7,C8,C13 with UKA1V102MHD (rec was UPW1V102MHD, but it was unavailable)
C12 with UPW1V220MDD
D1 with BR62
D4,D5,D601,D602 with UF4003-E3
D6 with 1N5243BTR
Q1 with BD1396STU

The bias didn't change after these replacements on the power supply and I was able to get a solid 10mV.

Then I did the phono board, though I can't imagine that'd effect the bias, I don't ever use Phono.

C101,C102,C119,C120 with UKL1V220MEDANA
C113,C114 with MKS2C041001FKI00
Q101,Q102 with KSC1845FTA

Then I did the main amp board, which is where the problem lies presumably.

C405,C406 with UKL1V220MEDANA
R433,R434 with CPF222R000FKE14 (Rec was CPF2-F-22 but it was seemingly discontinued)
VR401,VR402 with 3329S-1-202LF
D407,D408 with UF4003-E3
Q421,Q422 with KSC1845FTA

I think it's worth noting that, in the @dlucy thread, he had to attach the wiper lead on the new 3-lead variable resistors to one of the legs, since the original parts only had 2 leads. I found that the 3rd hole is just soldered shut, so I stuck all 3 legs of the new VR through. This was after I initially did the solder-lead-to-outside-leg method, thinking it was the cause for the low idle current/lack of range.

I appreciate any help anyone is able to give!
 
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I did some learning about potentiometers and, more specifically, preset resistors/trimpots. I connected leads 1 and 2 to the board in the same location the original pots were in, bending the 3rd leg out of the way connected to nothing. This worked perfectly besides the leads being kind of short on these specific pots. Thanks @cademan!
 
In general it's not good practice to not use the third lead of trimmer potentiometers. If the wiper makes a bad contact and only two legs are connected, the resistance of the trimmer pot will be infinite and that may lead to catastrophic failure. If the wiper is connected to the third lead (as dlucy suggested) the resistance of the trimmer pot will never be larger than the nominal resistance of the potentiometer if the wiper makes a bad contact.
 
ecluser said:
In general it's not good practice to not use the third lead of trimmer potentiometers. If the wiper makes a bad contact and only two legs are connected, the resistance of the trimmer pot will be infinite and that may lead to catastrophic failure. If the wiper is connected to the third lead (as dlucy suggested) the resistance of the trimmer pot will never be larger than the nominal resistance of the potentiometer if the wiper makes a bad contact.
Click to expand...
Thanks for that explanation. I can't figure out why it wouldn't have worked properly when I first tried dlucy's method. Should there be a resistance difference between having the 3rd lead disconnected versus connecting it to the wiper lead?
 
After doing some TinkerCAD experimentation to confirm my understanding of potentiometers, I wrapped lead 3 around 2 (wiper) and soldered it. Put it back in and it works just fine, though this time with the added safety of the third lead as a failsafe. Thanks @ecluser
 
Where is the problem?
RyanJHippy said:
After doing the main amp board, I went and tested and the bias pots could only push the current on the two resistors to 4mV, as opposed to the 10mV +/-1mV spec.
Click to expand...
Do you have 4mV maximum, or 50mV maximum? Or do you have a bias problem in one channel only? The situation is not clear to us.
 
ecluser said:
Where is the problem?

Do you have 4mV maximum, or 50mV maximum? Or do you have a bias problem in one channel only? The situation is not clear to us.
Click to expand...
There is no problem anymore, I stated above that it worked properly (able to set bias to 10mV) after soldering the 3rd leg to the wiper, per your suggestion.

The original problem was that with the 3rd leg soldered to the wiper the first go around, I could only set bias as high as 4mV at the pots max. It's odd that both pots could only be set to a similar number, but I'm not going to question it any further since it now works correctly.
 
guessing the two ends of the pot ended up connected together rather than the wiper to one end. Not sure what the actual trace layout is on this though.
 
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