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Old 04-17-2019, 10:50 AM   #21
Xenon-Codex
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Default Re: Balance slightly off on Aiwa MX-70

There's 0,28V after two hours of not plugged in, 0,4V when I connect it to power. The volgate drops to 0,5 when muting goes off. D31 has the same voltage on both sides even with the wall socket off. Maybe it's the D31 which is broken?
The protection circuit was faulty, and the uPC1237 had to be replaced, maybe it broke something else with it.
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Old 04-17-2019, 11:15 AM   #22
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Default Re: Balance slightly off on Aiwa MX-70

If the voltage only drops to 0.5 volts going to Q21/22 base, that could be just enough to make one of them start to slightly conduct, lowering the audio signal just a little bit. Replace D31 and see if that fixes the problem
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Old 04-18-2019, 08:33 AM   #23
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Default Re: Balance slightly off on Aiwa MX-70

D31 was kaputt. I didn't have the correct one, so tried with 12V zener (other was 6.2V). Now it works, except there's some background buzzing, and volume doesn't go all the way to zero.
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Old 04-18-2019, 09:19 AM   #24
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Default Re: Balance slightly off on Aiwa MX-70

If the volume does'nt go all the way down and there is buzzing, I would suspect there is a bad ground somewhere. when the volume is turned all the way down, the signal should be taken to ground, but if at the volume control there is no ground that could explain the fault.
I think you have damaged the traces around q21 & q22, the ground circuit runs directly through those transistor collector pads, if any one of them is damaged, the ground going to the volume control will be lost.
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Last edited by R_J; 04-18-2019 at 09:54 AM..
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Old 04-18-2019, 10:19 AM   #25
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Default Re: Balance slightly off on Aiwa MX-70

Will it damage anything if I change the 6.2V zener to it? I took this 12V one from scrap box. There was no buzzing with the original, or when the muting was disabled.
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Old 04-18-2019, 10:32 AM   #26
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Default Re: Balance slightly off on Aiwa MX-70

If by IT, you mean 12 volt zener, it won't damage anything, the schematic uses a HZ9A2 8.2
The buzzing and volume issue is NOT the cause of D31, You likely damaged the traces around the mute transistors so now there is no ground path for the volume control
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Old 04-19-2019, 02:02 AM   #27
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Default Re: Balance slightly off on Aiwa MX-70

It started to work quite normally when put inside its case, volume goes to zero and that buzz is gone. I measured the gound and it's good. Some crackling was heard when using the volume pot, but that went also away after a while. Maybe the temporary transistors Q21 and Q22 are causing abnormal things.
Where did you get that super clear service manual? Mine is b&w and the numbers are hard to read...
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Old 04-19-2019, 08:59 AM   #28
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Default Re: Balance slightly off on Aiwa MX-70

You can download the manual from here: https://elektrotanya.com/aiwa_mx-70_.../download.html
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