It looks like it's been since 2011 that anyone posted about this amp. I'm trying to fix mine. Relatively new at this but a background in EE that's not completely useless. The problem I'm trying to address is that one channel cuts in and out, mostly out. Starting with adjustment of bias voltages and some difficulty already. (the measures drift when the speaker switch is off- they seem to be stable if it's on, but the manual says to have it off). Anyone able to advise? Many thanks-
First off, I'm no electronics wizard but have been successful reparing a few amps. Here's a couple basic things to check, if you haven't already. Output relays are likely culprits for cutting out. Contacts get burnished or corroded. When the output is "cut out" try getting the signal by probing a point in circuit just before the relay. I use a cheap 8 ohm (or higher) speaker wired to my test leads. Be careful not to short anything or get zapped. If you get sound, you know there's an open circuit there somewhere. May also be bad solder joint.
The volume pot is also suspect, although usually accompanied by lots of other noise. Try measuring the pot resistance while turning the knob. Resistance should change smoothly.
Some amps have unstable bias. I have a HK1400 that bias and offset changes constantly due to airflow around the transistors. This is especially difficult with the cover off. If I blow air on the VAS / driver transistor area of the board, I can see the meter change rapidly. I have not pursued any fix for this as it doesn't effect the sound whatsoever.
The volume pot is also suspect, although usually accompanied by lots of other noise. Try measuring the pot resistance while turning the knob. Resistance should change smoothly.
Some amps have unstable bias. I have a HK1400 that bias and offset changes constantly due to airflow around the transistors. This is especially difficult with the cover off. If I blow air on the VAS / driver transistor area of the board, I can see the meter change rapidly. I have not pursued any fix for this as it doesn't effect the sound whatsoever.
Yes, this is probably a mechanical problem. Clean all the contacts in the signal path.
Verify that the RCA cables are good. The tape monitor switch is a frequent problem,
maybe also the selector switch, or the speaker switch, or the pre-out/main-in switch.
The bias setting won't cause this problem.
CAIG DeoxIT D5S-6 Spray 5 oz. with Perfect Straw
Verify that the RCA cables are good. The tape monitor switch is a frequent problem,
maybe also the selector switch, or the speaker switch, or the pre-out/main-in switch.
The bias setting won't cause this problem.
CAIG DeoxIT D5S-6 Spray 5 oz. with Perfect Straw
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Drifting was my bad- probe problem. This is fixed and now able to adjust biases. They are still drifting a bit but seem OK. The idling voltage adjustment seems to affect the DC Balance- in that once I got that adjusted the DC balance was out. So may need to iterate.
The biases may not affect the cutting out problem. I will opcheck after letting the unit on for a file and retweaking the biases.
Interested in the relay theory. I was unaware that there is an output relay. Where do I look for that?
Things to rule out- the volume knob is unlikely the problem since only one channel is bad. I am doing some general cleaning of very dusty boards with contact cleaner and toothbrush. Trying to get some cleaner into all of the pots and selector switches. The input selector had been problematic so that may get some traction.
The biases may not affect the cutting out problem. I will opcheck after letting the unit on for a file and retweaking the biases.
Interested in the relay theory. I was unaware that there is an output relay. Where do I look for that?
Things to rule out- the volume knob is unlikely the problem since only one channel is bad. I am doing some general cleaning of very dusty boards with contact cleaner and toothbrush. Trying to get some cleaner into all of the pots and selector switches. The input selector had been problematic so that may get some traction.
One note- I just repaired my Sansui AU-719 and the DC biases had a profound affect on the protection relay circuit. It went into protect when the biases were adjusted too far away from zero. That was why I started here with checking the biases. This HK doesn't seem to have the same kind of circuit.