Odd Problem H/K 330B: Left Channel VERY Weak SOLVED

[Johnny_Law]

Eventual solution: Bad solder job on one replacement transistor!

Problem: Weak (barely audible) left channel.

• Constant regardless of input source
• Also weak channel through headphones

Work already completed:

• Rebuild of Control Amp and Main Amp circuit boards
• De-oxit of all pots and switches

Diagnostics already completed:

• Verified replacement component orientation is correct
• Jumpers removed, L/R signal from external source fed directly into Main Amp, both channels present
• Transistor values of Control Amp measured (Q501-Q506, originally 2SC458 and replaced with KSC1815 as advised), all voltages both (a) ballpark to specs and (b) identical between L/R channels
• Power Supply output voltages all measured, all OK close to spec

Have been PMing with @dlucy about this, we are both scratching our heads here. His threads, e.g. this one, have been invaluable thus far.

The service manual is available online and is of good quality.

Next steps:

• Verify continuity through SW1a / SW1b pots (already verified this once, will do it again)
• Jump channel wiring around to try to isolate where the signal gets lost (?)
• Do whatever you guys tell me to do!

This one's a good one, it's got me right perplexed. And because a picture sometimes helps get the synapses firing... here it is pre-rebuild, already quietly teasing me:

 

[dlucy]

Fundamental approach you could take at this stage is inject a mono 1kHz 1V ptp sine wave into the AUX input, then look at both channels with an oscilloscope or signal tracer as that input flows from the in jacks to the switches then to the preamp then to the main amp and then out the outputs. At some point in that chain one channel will diverge. That should give you a better sense of where the problem lies. Cold or broken solder joints, shorting wires, REALLY oxided tongues in the switch wafers, backwards electrolytic cap, etc.

 

[cwh]

So it was all up to snuff before rebuild? You said you've separated preamp jumpers and connected directly to amp with same result?

More Bay Area 330b blues. Feeling your pain

 

[Johnny_Law]

I wish I had an oscilloscope But I had kind of a similar idea - what if, using the AUX input, I only connected the Left input, and jumped the signal to the right channel at various places - I'm thinking that it'll start playing music at some point, and then I'll be able to exclude all downstream components.

@cwh No, it was dead before the rebuild. It had a small sign of life that sounded like some kind of garbled mess but I can't remember if it came out of both or just one channel.

I don't remember if I've connected the 330B pre-amp to another amp, so I will try that this afternoon!

 

[The Fuxtor]

Yes, definitely determine which board had the issue. I had the same problem on a Sansui AU555a which after determining the preamp board had the fault, discovered after probing from good channel to bad with a scope, a capacitor was hiding a lifted pad!

 

[cwh]

As I'm sure you're aware, the typical small receiver issues (def including HKs) are switches/controls and or shorted output devices. If you've verified your OP transistors are looking good then I'd get back the controls. Single faint channel could easily be the tape monitor switch. I just spent half a day and half a can of dexoit on a 330c that wouldn't give in to the chemicals.

The pre/main jumpers allow easy separation of the stages. By running your phone into the amp, you take away the preamp and those pesky controls.

 

[hemiram]

I wouldn't be shocked if it turned out to be bad solder joints. I've seen so many things that come back to life after hitting every suspicious joint on a board. Have you tried the "tap it with a finger/stick" test? Freezing stuff sometimes shows cracks that are totally invisible otherwise. Just don't crank the volume up if you start poking around, and of course, watch the voltages so you don't fly across the room. I've had a couple of surprises when they come back to life all of a sudden at 3am.

 

[dlucy]

Any luck yet?

 

[blhagstrom]

That series of HK can have really stubborn tape monitor switch problems.

Get juice into those switches is a bitch.

FWIW, tape out is a live tap on the current audio signal. See if both channels are there. Run a cord to another unit since there is no volume control on that signal.

 

[Johnny_Law]

Some work-related travel came up and this is still on my bench where I left it. Between Detroit last week and Munich this week I've had no time to look at it ... but I will! And I will check that tape switch / tape out first thing.

I also wanted to mention that I re-flowed the solder on all the contact points on the pre-amp board, and even jumped them with extra wire, with no changes.

 

[Johnny_Law]

Ok, back at it. Tried to toggle the tape switch (it looks like a closed switch so no way to get deoxit in there) - no change, but no pop or static either. However, I ran tape out to my garage receiver and both channels went through!

So I guess that narrows it down to somewhere in the tone controls, contour, or volume?

And here's a pic of the inputs to the control amp board for good measure ...

 

[cwh]

Regardless I'd want to get those switches apart and cleaned. Once unscrewed from chassis should be four tabs holding body together.

 

[Johnny_Law]

Cleaned the vol pots and tone control pots, cleaned the tape switch, no change.

THEN - I jumped the input wire to the output wire on the tone control board and voila, music came through the channel.

(Jumping the purple or green to the closest white wire causes sound to come through the left channel)

Re-flowed the contacts of the pots and made no change. I suppose I'll pull it out to inspect it, although there's nothing on it that would categorically be suspect ... hmm.

 

[The Fuxtor]

I'm gonna take a shot and say either your switches are still dirty, or you got a lifted pad on the preamp board somewhere....?

 

[Johnny_Law]

Ok I poked around the control PCB a little more last night. I tried using a wire to jump the signal at various points to see where it disappears. It seems to disappear at the bass pot. How should I test the pots? Measure resistance across the side to the middle?

 

[dlucy]

How should I test the pots? Measure resistance across the side to the middle?

How I (just a hobbyist) would do it:

1. Disconnect all wires going to the pot or desolder and remove it from the PCB
2. Assuming it has three leads: A on one side, C on the opposite side, and B in the middle, Measure resistance from A to C (which should be THE value of the pot and remain constant,
3. then measure from A to B and adjust the pot from one extreme to the other. You should see the resistance go from zero to max (the value you got in step 2) and the increase/decrease in resistance as you adjust the pot should be smooth and not jumpy.

 

[Johnny_Law]

Well kind of back to square one.

I removed the pots and they all tested perfectly. I measured each of the resistors on the control pcb and they are all fine. I re-installed the pots, no change.

Additionally, I realized that my left channel via the tape out is weaker than the right. And, even when I jump the wires across the control board like I mentioned above, the left channel signal isn't as strong as the right.

Guess I need to go upstream.

 

[dlucy]

It really sounds like you've got a problem in a very limited area, due to your narrowing and investigation. Have you replaced the ecaps and 458's in here? If so, next step would be to lift a leg on all the remaining components in there and measure their out-of-circuit value against the schematic, right?

 

[cwh]

Did you ever test function through pre in/out?

 

[Johnny_Law]

After re-installing the tone control board, I tried again listening via Tape Out and then via Pre Amp out. After some strange observations and some messing around I found that the right side of the RCA cable I had been using was weak/bad, and my garage Onkyo TX-82 has odd tape in circuitry which was letting the right channel input bleed into the left channel, like a mono input. (Also some of the RCA outs were dirty!) Ugh.

So we took a bit of a detour because of a bad testing environment. LET THIS BE A LESSON! But at least I verified that my tone control board is OK.

After switching to a good RCA cable, I made new observations:

• Pre Amp output: VERY WEAK sound out of left channel (i.e., at full volume, you can BARELY hear the music). Right channel strong.

• Tape Out: GOOD signal out of both channels.

• Amplifier: Both R&L channels continue to work normally.

• Volume pots test good.

Looking at the circuit diagram, @dlucy, now it seems like the problem HAS to be C522, Q506, or C524. I think my next step will be to see if I can swap each for its equivalent in the right channel, one at a time, and see what happens.