Kenwood KA-770D No Left Channel

[cpenn]

Recently picked up a KA-770D for $20 on my local Craigslist. I hooked it up and found the left channel (both A and B speaker settings) is virtually silent. (If I listen closely, I can hear it very softly). The sound out of the right channel sounds normal/good.

Here's what I've ruled out so far:
- Speakers - they work fine with a different amp
- Mono/stereo switch - doesn't change anything
- Balance knob - twisting to the right doesn't change anything; twisting to the left makes everything virtually silent (again, still very soft music coming out of left channel)
- A/B speaker switch - amp behaves the same way on B speakers as it does on A
- Headphone jack - works; matches what's coming out of the speakers
- Different inputs - all inputs produce the same result
- Muting switch - works as intended
- Loudness switch - works as intended
- Subsonic switch - not sure what this does, but doesn't seem to change anything when I toggle it
- Bass/treble knobs - work as intended

I used compressed air to blow the dust off the boards and out of the nooks and crannies and then sprayed out the switches and boards with deoxit. Now letting it dry for a few hours before I try it out again.

What else should I be doing as first steps here? Anyone have any idea what could be causing this problem?

Thanks for any input you might have! I attached some pictures in case there's anything glaring...nothing I've noticed so far.

 

[avionic]

Tap on the protect relay ( K1 ) with the plastic end of a screwdriver and listen for any sound out of the left channel.

 

[avionic]

Protect Relay K1 is the black rectangular plastic box above the green wire in the photo..

 

[cpenn]

Thanks for the input. Gave it quite a few good taps from all sides but no dice. Any other tips?

 

[avionic]

Do you know how to read a schematic?

 

[avionic]

Do you see the two long narrow white dual emitter resistors with three pins sticking out of the top .

 

[cpenn]

Yes, I can read schematics. I've got a service manual that has some...they're just so dense! And yes, I see those resistors

 

[avionic]

I see those transistors

Resistors. The center pin on those two resistors is the two amplifier output test points.Measure the DC voltage (offset) with your multimeter. Ground your black test probe to the metal chassis. With the red test probe measure the DC voltage at each of those two center pins. Should only see a few millivolts.

 

[cpenn]

With respect to the metal chassis ground, the left one, CP2, measures -49 mV and the right one, CP1, measures -73 mV.

 

[cpenn]

Measuring per the idle current spec attached (from the unit's service manual), CP2 measures 14.8mV and CP1 measures 15.4mV. The spec is 13mV so I guess I don't see an issue there.

Does anyone know what the alignment points column (see attachment) refers to? VR1 is the volume knob, and VR2 is the balance knob. Am I supposed to adjust those until I obtain 13mV across both? Seems ambiguous since it's specifies to put the Volume at 0.

 

[cpenn]

Update: I can get sound out of the left channel by rotating the balance knob/shaft (VR2 in attached photo) to a specific point and wiggling it a little. My plan is to get some more deoxit tomorrow and clean the balance pot thoroughly with that, then spray it with some of the CRC lubricant (shown in attached photo) and let it dry. Does that sound like a good plan?

 

[avionic]

Does anyone know what the alignment points column (see attachment) refers to? VR1 is the volume knob, and VR2 is the balance knob. Am I supposed to adjust those until I obtain 13mV across both?

No. VR1 and VR2 would be on the driver board (X89-1020-10 or X89-1022-71) They will be small trimmer pots.

Looks similar to this.(photo below) The two on the edge of the board pictured above . They should be labeled bias left and bias right.

 

[cpenn]

No. VR1 and VR2 would be on the driver board (X89-1020-10 or X89-1022-71) They will be small trimmer pots.

Looks similar to this. The two on the edge of the board most likely . They should be labeled.Bias left and bias right.

Found them! Got them trimmed in to 13mV.

 

[avionic]

Does the balance potentiometer (if that's what that is) need more deoxit then?

Yes....

 

[avionic]

Found them!

Outstanding..

 

[cpenn]

Outstanding..

Good deal. Got those white resistors trimmed in to 13mV each using the trimmer pots. Probably was OK the way it was but might as well adjust them while I've got the cover off. I'll plan to get the balance pot cleaned and lubed tomorrow and come back with an update. Thanks for your help so far!

 

[cpenn]

Got it working! Pulled the front faceplate and plate behind that off to expose all the knob potentiometers. Sprayed them all pretty liberally several times with CRC QD (Quick Dry) Contact Cleaner and worked the knobs back and forth ~100 times. Then injected some CRC 2-26 Multipurpose Precision Lubricant into each one and worked it some more. I let it dry this afternoon and then connected speakers, etc. an hour or so ago and everything checks out so far. Pretty wild how much better music sounds in stereo. Again, thanks for the help avionic! Your suggestions prompted me to get those trimmer pots dialed in which can't hurt.

 

[avionic]

Sweeet...