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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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File Type: jpg Aiwa_MX-70_Ground path.JPG (309.2 KB, 6 views)

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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Old 05-11-2019, 09:16 AM   #29
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Default Re: Balance slightly off on Aiwa MX-70

I finally received the correct transistors. The amp had some kind of hum/buzz (not related to volume level), but that has corrected itself(?) when it was "off the wall" whole week... Probably that was caused by earth loop via a cable from other stereo & computer.
Thanks to all for the advice.

Last edited by Xenon-Codex; 05-11-2019 at 09:17 AM..
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Old 05-11-2019, 09:39 AM   #30
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Default Re: Balance slightly off on Aiwa MX-70

If I turn the plug other way, the hum comes back. With no hum, there's 160V AC on the cover, with the hum only 70V. I now have another identical amp, it has also quite high voltage on the cover, but no hum no matter which way the plug is on the socket.
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Old 05-11-2019, 09:54 AM   #31
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Default Re: Balance slightly off on Aiwa MX-70

I am 99% sure you broke the circuit traces when replacing the transistors, and the volume control circuit and other parts of the circuit is loosing its ground connection.
The power supply uses a transformer so the amp should be isolated, check the wires on the primary side to make sure none are touching ground.

Last edited by R_J; 05-11-2019 at 09:59 AM..
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Old 05-12-2019, 10:46 AM   #32
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Default Re: Balance slightly off on Aiwa MX-70

The phono-amp section had two white jump wires, which looked strange. That small board has a crack which I haven't noticed, it's not clearly visible except from a correct angle and powerful light. The crack disconnected both lines and the wires were to fix it. Touching the jump wires caused much more hum.
Is this a common practice to fix a fault occurred during the assembly, the wires looked like been there for 30 years? No signs of resoldering.

Last edited by Xenon-Codex; 05-12-2019 at 10:50 AM..
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Old 05-12-2019, 03:18 PM   #33
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Default Re: Balance slightly off on Aiwa MX-70

Can you hold it up to the monitor so I can see it? If the board has cracked traces I guess someone repaired them before.
I already pointed out where the hum is likely coming from. There is only one thin trace for the ground circuit on this board.

Last edited by R_J; 05-12-2019 at 03:56 PM..
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