Hitatchi HA-7700 woe - one channel come and go...

TNT

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Service Manual

I have unit that has an intermittent problem on Left channels that goes like:
  • It plays fine on "Main in" (there is a split between pre and main (4 RCA)) - so problem must be in pre/line section. Its a good power amp :)
  • L ch is silent using line in like AUX at power up and after a while, 1 minute..., it start to play
  • It usually crackles a bit when it starts to play, then it can sound fine for several minutes (5-20)
  • Then it can fade out and get silent - usually when signal level gets lower
  • It can be fine, then play distorted / crackeling for a minute or so.... then come back OK
  • It feels like it needs signal to work - like some bias is not there without signal?
  • Same on AUX and Tuner so I assume som common part downstream

Happy for any thoughts or ideas...

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Speaker relays, input selector downstream. Spray them with proper type of spray.
Most likely bad caps in the main and pre amp supplies.
Unlikely is a bad solder joint around pre amp chip, must be 4558 or 5532 or similar, look for a 8 pin DIP chip (if stereo).
Since it comes back on and is intermittent, could also be thermal shut down of outputs, use a freeze spray to check.
 
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Have you used some deoxit D5 spray cleaner? It will work well on dirty contacts in preamp pots and switches.
Especially check: tape monitor switch, input selector switch, muting switch, volume control.

If no progress there, you will have to do signal tracing in the preamp line stage with a scope,
when the imbalance exists.
 
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Sounds like you've determined the problem is in preamp, but for added confirmation, try to interchange links so that left channel preamp drives right PA and vice versa. If trouble "moves" to other speaker, trouble is confirmed in left preamp.

Try to take advantage of failed periods. If you have a signal tracing tool, try to isolate where where break occurs.

Good luck!

Hint: signal tracing tool can be another amp or even the working channel. Be mindful having a blocking cap at tracer input so that you don't disturb bias of stages being probed.
 
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Maybe, but use only a very tiny bit of spray, and then exercise the pot or switch at least 10 times before testing the sound.
If there is some some progress but not enough, repeat with same tiny amount of spray. Too much spray is worse than none.
 
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ONLY clean parts in the preamp section, where the problem is:
tape monitor switch, input selector switch, muting switch, volume control, etc.

SHAKE UP THE SPRAY CAN WELL BEFORE SPRAYING.
MAKE A TEST SPRAY OUTDOORS TO CHECK FOR THE PROPER TECHNIQUE.
IF YOU LACK THE "STRAW" DO NOT ATTEMPT TO USE IT.

General maintenance, like the speaker protection relay, can be done later, after the problem is fixed.
First principle of repair: do no harm.
 
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TNT

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Joined 2003
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A long time since I own and used a switch/knob kind of amplifier. This one I got as a gift. But indeed, the isopropanol based cleaning spray for electronics did the thing aided by some intense flipping/turning of the controls at the red dots...

clean.jpg


Now plays fine without any disturbances at all. Thanks all for the tips & hints. Its quite a nice amp actually :)

Still have some faint scratching in the volume pot but that one seem quite tight so I'm not sure I'm getting any spray in there...

pot.jpg


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My 2cts worth, the loudspeaker relays. Especially older Sony amps suffer from this phenomenon as you describe.In this case the problem could be temporary solved by briefly cranking the volume up so the relay burns clean. Beware, the speaker should be able to handle the short peak.