Can you double-check my troubleshooting here? (old Kenwood receiver)

[cvanc]

Hello troops-

I'm hoping you can look over my shoulder on a repair I'm working on.  It is being very stubborn.  It's an old Kenwood quadraphonic(!) receiver, model KR-8840.  I could really use a sanity check.

FAILURE PATTERN:
All 4 channels work and pass signal but all 4 channels have an obvious hum.  The hum seems equal in all channels, which implies a single common failure.  I've measured about 100mvAC at the speaker terminals, and my meters say the frequency of this is 120HZ, which makes me instantly suspect a power supply problem (as opposed to a bad ground somewhere where you would reasonably expect 60Hz) .  But measuring all the supplies suggests they're OK ripple-wise.

The hum is not affected at all by the volume or balance or tone controls.  When using an external source into the AUX inputs the volume and balance controls work normally, suggesting they are not missing the ground connection at the bottom of the resistive elements.  Signal volume can be adjusted to zero routinely.  This suggests the problem is after the volume control.

The only front panel control that changes the hum is the lo filter S4; engaging this reduces the hum level.  This suggests the hum is in the signal at that point in the path.  So I must be in the right neighborhood, right?  Afraid not, read on.

STEPS TAKEN:
On the 'main amp' board I removed all 4 input capacitors and it fell dead silent, no hum (& no signal, obviously).  This suggests the power amps are OK & the rails that feed the power amp are clean.

Just upstream of the main amp board is the '20dB amp' board.  This is where the loudness, lo filter, and hi filter controls reside.  The output of this stage directly feeds the main amp stage.  The input of this stage is fed directly from the volume control.  Remember the volume control does not affect the hum at all.  So the problem must be after the volume control and before the main amp, right?  It must be on this 20dB amp board, right?  Wrong.

Looking at the 20dB amp schematic I have replaced the 47uF rail filter (even thought ripple measured OK); no change in symptoms.  Also I randomly chose the channel I had best physical access to and removed the output cap C22 - hum stopped in that channel.  I replaced it (with new) and then removed input cap C18 - hum stopped in that channel.  I replaced C18 with new; hum still the same.  This implies the problem is before C18 but after volume - and there's basically nothing there.  S5, the loudness control, does not change the hum.

Even though it's counter-intuitive I tested to see if the hum was arriving at the high side of the volume control.  This is directly fed from 2 two channel 'tone control boards'.  I went there and measured input pins 1&2 and output pins 3&4 with no signal.  All were about 1mV both AC & DC.  Seems fine, right?  So I shorted the output pins 3&4 to chassis using a clip lead.  The hum didn't change, which suggests the problem is after the tone boards.

SUMMARY:
Testing suggests the hum is getting into the path after the tone board & volume control.  It also suggests the problem is BEFORE the 20dB amp (remember yanking C18 stopped it dead).  This is one of those "both things can't be true" moments.

I have to conclude this schematic does not exactly match my unit in this area.  Unfortunately this means I have to disassemble the entire front panel to investigate further, and that is a damn mechanical nightmare that I'm trying to avoid.  It looks like hours of work.  So before I dive in I wanted to see if you had any other ideas?

Thanks, as usual, for casting expert eyes on this.

I can't post the full  schematic as it's over 8 megs but it's available online in a few different places.  I got it from hifiengine.com... which, if you somehow don't know, is a wonderful (and free) resource.  Link to schematic is below.  But here are some cut-n-pastes of the pertinent areas.  Thanks again.

https://www.hifiengine.com/manual_library/kenwood/kr-8840.shtml

[cvanc]

To add one more clue, the power going to the 20dB amp board, as measured across the leads of that capacitor Ch41 47uF is 40.3VDC and 6mVAC.

Even though the clues seem to point to the DC on this stage as the primary suspect, it's hard to imagine 6mV of ripple is a problem in a vintage piece like this. 

[David_AVD]

Does the hum change if you short the output of the volume control to the chassis ?

[cvanc]

Does the hum change if you short the output of the volume control to the chassis ?

No.

I did this directly on the volume pot with a clip lead.  I did all 4 channels with one end of the clip lead on nearby frame, and moved the other end of the lead one at a time across all 4 wipers.

Then I re-did it with the ground end of the clip lead literally on the ground terminal of the pot.  No difference either way, the hum was unaffected.

[David_AVD]

Maybe check (add a temp cap over) the main filter caps(s) ?  It could be that  the amp section is better at rejecting the hum than the preamp.  After all, the power amp does have split rails but the preamp is single.

[cvanc]

Maybe check (add a temp cap over) the main filter caps(s) ?  It could be that  the amp section is better at rejecting the hum than the preamp.  After all, the power amp does have split rails but the preamp is single.

I bodged an additional 470uF across that 47uF rail bypass C41.  Hum did not change.  Ripple across the cap leads went down from 6mv to 4.7mv.

Tomorrow I'll temporarily replace the 2 main B+/B- caps (which are 15,000uF 50V) and report.  Thanks for the suggestion.

[cvanc]

Maybe check (add a temp cap over) the main filter caps(s) ?  It could be that  the amp section is better at rejecting the hum than the preamp.  After all, the power amp does have split rails but the preamp is single.

I bodged an additional 470uF across that 47uF rail bypass C41.  Hum did not change.  Ripple across the cap leads went down from 6mv to 4.7mv.

Tomorrow I'll temporarily replace the 2 main B+/B- caps (which are 15,000uF 50V) and report.  Thanks for the suggestion.

The more I think about this, the less I suspect the problem is power on this stage.

Reason: When you pull one of the input coupling caps (1uF C17/18/19/20) the hum stops in that channel.  I think hum caused by ripple on the rail would still exist unchanged even with the input cap removed from the circuit.  (Right?)

[PKTKS]

I had issues w/some Kenwood stereos in particular w/the "protection"
block which in several models is just not published.

Question:
- Did you tried to test each of the block in separate?

Injecting a clean test signal on the power amp without the protection
Then injecting the signal only on the protection block ?

If so does the hum  persists on the protection block working alone ?

In my case that protection block had several stability issues...
In some cases without the schema the best solution was to eliminate that.

Paul

[bob91343]

This problem smacks of a ground loop.  In other words, there is some ripple current lifting the ground of some signal component.

I was given an audio mixer with a hum problem.  The factory offered a cheap fix since they admitted there was a problem.  However, their fix didn't cure it.

I offered to fix it and found that the power supply filtering was arranged in such a way that the current through the filter capacitors went to the same ground as the signal.  I lifted that and made sure there was no way any ripple current could touch the signal path.

Problem solved.

So check where the main filter capacitors get their ground, and make sure the transformer and diode currents only go to the capacitors.  You may have to lift a ground lug and reroute wiring.