Philips AH 7851 left channel distortion problems

[Koryj]

hello all, I’ve recently acquired a Philips 7851 and new here. I’ve been reading through threads and absorbing info about vintage audio for a while now.
That said, I had a lot of distortion and drops in almost every pot on this receiver and learning from you all I have DeOxit’d my way to near clean audio aside from the left speaker. Both a and b speakers exhibit the same distortion/low volume/static and the heafphones play fine in both right and left.
Any suggestions aside from keep cleaning the power/speaker selector pot to get things up to par?
Thank you in advance, this site has been invaluable to me!

 

[mbz]

Welcome to AK.

If I understand correctly there is some noise/distortion/static/low volume on
the left channel when played through speakers. The headphones play fine/no noise etc...

Looking at the schematic, the main deifference between headphones and speakers is
the speaker selector switch. Yep, you guessed it. This switch should be thoroughly
cleaned. It may be necessary to unsolder then disassemble the switch since the contacts
can become heavily oxidized. Suggest first do a resistance check by selecting spkr A.
Then measure resistance from Left Channel in to SPKR-A Left. Repeat for Right Channel in
to SPKR-A, Right, see pic. Amp should be powered off/unplugged.

Maybe repeat the resistance measurement at left channel in and left RED spkr post

Ideally the resistance will be 1-2 ohms (MM leed resistance)

Another difference between speakers and headphones is the load. Spkrs nominally 8ohms
and headphones hundreds of ohms, so they have a lower current draw. Will wait on the
resistance checks.

 

[sregor]

FWIW the emitter resistors (large grey or white blocks - 0.33 to 0.47 ohms) frequently go open, and will also give you no distortion with phones but distort with speakers. Could also be open output transistor. O-scope can pinpoint it quickly.