Technics SE-A5 Static/Sound in Right Channel

So I'm working on an SE-A5 that powers up after replacing all 5 fuses (that were missing), however, after cleaning up circuit boards and setting voltage adjustments I'm still getting some static in right channel after it warms up (after 5-10 minutes).

I took a closer look and found that two resistors have been previously replaced, and one (R603) appears to be brown/burnt, but both resistors are for the left channel. I pulled R603 and it does not measure 2.7K Ohms, so I have ordered a replacement.

I also found a bad solder joint on one of the Power Supply cap (C408) negative leads on the right channel, so I discharged the caps and re-soldered the joint. But I cannot power things back up until I replace the bad resistor that I pulled yesterday. Would this lead to static as things warm up?

On a related note, as you can see from the pic, the board still needs some more cleaning. I used q-tips and 90% rubbing alcohol but can't get between many of the components. Any suggestions would be appreciated.
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Hmmm... cleaning a board like that.

What would I do... I'd wash the board in hot water with a little detergent using a soft paintbrush, and then rinse and dry thoroughly. If the board is to big a job to remove then I'd still wash it even if the chassis got wet. It would come up like new. The only possible problem would be water remaining trapped under components like those large caps. So you would have to be sure it was all dry. A quick dab with the iron on the pins would clear immediate moisture around the pins.

I've washed dozens and dozens of boards over the years (mainly TV chassis and VCR equipment) and they sparkle like new when done.

That resistor is well frazzled but would have to see what it did to make a judgement why it might have failed. Something around the resistor (or the resistor itself) could be the cause of the static. Something has caused that to fail though.
 
+1. AFter all, this is what they do in factories.
Or, wash in three steps:
1. 96% ethanol and rub thorughly with a soft pain brush, both sides, then rinse
2. Same as 1 but with dishwashing detergent, then rinse
3. Final rinse with distilled or demineralized water to avoid white drop stains

Drying should be on a hot summer weather, in the shade, for 48 hours. Then you will have a clear situation on your PCB.
 

Welcome

Member
2013-03-11 10:52 am
You don't need to clean the board, but as mentioned earlier, just rinse the thing in the shower. I always do this and there are no problems. Just make sure the transformer doesn't get soaked.

As for your noise problems. Make sure you have a fresh new relay. Even if that isn't the cause (it certainly can be), a new relay is mandatory in old equipment like this (especially on these more powerful amps).
 
Picture of entire board

So here is the entire board. While I can unplug most of the wires on the front side of it, the wires on the backside are all soldered on, so I'm not sure removing the board is an option.

Also, there are a few things on this board that make me reluctant to spray the board with water:

1. The two boxes labeled synchro-bias circuit are covers that are soldered on and cover up what appear to be diodes, caps and thermistors according to the service manual (not sure why the covers are necessary other than to protect them from unwanted heat).
2. Two of the relays are mounted to this board.
3. The power supply caps are mounted on this board and as Mooly indicated, drying water that might get under them could be challenging.
4. The large heatsinks connect to the board via 4 spots that are covered in white heat diffusing agent that is sticky to the touch.

So I will try cleaning it some more, in place with alcohol and small brushes.
 

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Welcome

Member
2013-03-11 10:52 am
You don't need to clean the board, if it's difficult I would avoid it. Alcohol can semi-dissolve old flux and make the board much stickier than before, so that's not recommended.

Don't touch the thermal compound on those heatsinks. It may contain beryllium oxide (toxic/carcinogenic). You never know with this old gear.
 
Hi everyone. So over the weekend I replaced the bad 2.7K Ohm resistor on the left channel (R603). I also applied the service bulletin found on last page of the service manual. (Change R347 and R348 from 68 Ohm to 100 Ohm to prevent Q311, Q312, Q315, Q316 break down due to overcurrent). I had already re-soldered the one power cap lead on the right channel that looked cracked, bad.

I then set voltages/bias/iCQ according to service manual (all voltages dialed in to spec).

Amp now plays great with no more static on right channel even after warming up.

Thanks everyone with your help and suggestions. I'm leaning towards leaving the boards as they are after my initial cleaning.