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Old 04-11-2019, 08:09 AM   #1
Xenon-Codex
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Default Balance slightly off on Aiwa MX-70

It generated multiple faults, but now it's working except balance is not perfectly centered, at times it's hardly audible. The suspect is the transistors Q21 and Q22 (marked green). I changed them and then the balance got a lot better, and quieter channel changed from right to left.
It has new power transistors, control- and protection chips. Idle current is adjusted like the manual says.
Should I change the Q21 and Q22?
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Old 04-11-2019, 08:18 AM   #2
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Default Re: Balance slightly off on Aiwa MX-70

Have you checked or changed the caps C111,C112,C115,C116? If c111 or c112 are weak the audio will be lower in that channel
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Old 04-11-2019, 12:47 PM   #3
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Default Re: Balance slightly off on Aiwa MX-70

I inject a test tone 1kHz and use a multimeter on AC mV to compare the L, R signals. You can get precise down to 1/10's dB.
Usually it is crappy potentiometer is part of an imbalance.
Some DMM's do not like measuring ACV when there is lots of DC present, so know your multimeter.

You can disconnect the 2SC2001 muting transistors (emitter) to see if they are partially on. I think they need -ve base voltage to turn off. A shorted coupling capacitor would also make the muting circuit malfunction - but they usually add distortion.

Last edited by redwire; 04-11-2019 at 12:51 PM..
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Old 04-11-2019, 12:54 PM   #4
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Default Re: Balance slightly off on Aiwa MX-70

I think the capacitors are ok, the fault would still be in the right channel if it was weak. Is it possible that there's diffrerence between the Q21 and Q22? I only changed these two components vice-versa and the fault moved to left channel. Resistors 137 and 138 are slightly different, R137 is 2,9kohm, and R138 is 3,3kohm.

Last edited by Xenon-Codex; 04-11-2019 at 12:56 PM..
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Old 04-11-2019, 01:02 PM   #5
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Default Re: Balance slightly off on Aiwa MX-70

resistors don't usually go down in value, and it is within 10%, If you switched the transistors and the problem moved with the transistor, I suspect the transistor is leaky, It shoul'nt be hard finding a NPN 30v 700ma replacement to try

Last edited by R_J; 04-11-2019 at 01:04 PM..
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Old 04-13-2019, 08:40 AM   #6
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Default Re: Balance slightly off on Aiwa MX-70

C111 and C112 changed, the Q21 and Q22 were replaced by BC141. Balance stays centered about 5 minutes and now the right channel is weaker, before changing the caps it was the left channel(?). Heavy distortion also appears.
There was some esr-difference between the caps, and new ones are only 2,9/3,3 ohms.
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Old 04-13-2019, 10:56 AM   #7
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Default Re: Balance slightly off on Aiwa MX-70

Is the distortion on both channels? by changing 4 parts, its hard to tell where the problem is. reinstall the old caps or transistors and see what happens.
Did you install the transistors the right way? 2sc2001 = E-C-B; BC141 = E-B-C
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Old 04-14-2019, 05:33 AM   #8
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Default Re: Balance slightly off on Aiwa MX-70

I changed the transistors first, and after testin the caps, as the fault was still there. It works perfectly for a minute, and then the balance starts to shift and distortion appears on higher volume.
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Old 04-14-2019, 08:08 AM   #9
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Default Re: Balance slightly off on Aiwa MX-70

I changed the 141's vice-versa, no change. But cold spray to the quiet one (Q22) makes the volume to rise, so this is temperature dependent. All capacitors measured fine, so what makes the Q22 run hotter? I didn't find any offset voltages from IC5 or IC6 inputs.

Somehow that distortion is gone, so far.

Last edited by Xenon-Codex; 04-14-2019 at 08:17 AM..
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Old 04-14-2019, 12:01 PM   #10
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Default Re: Balance slightly off on Aiwa MX-70

There is no way for the muting transistors to warm up, it is not possible. I think you are seeing a side-effect to something else.
BC141 is not a special audio-muting transistor. These have high reverse hFE, like 2SC2878. So expect distortion and odd effects.

I would ensure the uPC1225H input pin 4 has no DC offset. It should be 0.1-0.2V and it should be a very close match to feedback pin 5 for DC. Otherwise, if pin 4 has a lot of DC it could reverse bias C111 or C112 causing leakage currents and then upset the muting circuit. If there is DC working back from pin 4, the treble potentiometer would also get very scratchy.

The rule is "change one thing at a time" to troubleshoot a problem.
It doesn't make sense when you switch parts from one channel to another and the problem either moves or stays. This can mean nothing.
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Old 04-14-2019, 12:13 PM   #11
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Default Re: Balance slightly off on Aiwa MX-70

Maybe there is just enough voltage on the muting line to cause Q22 to turn on slightly before Q21. D25 or D31 may be causing the issue?
When you mute the amp and then un-mute it does that effect the audio level (ie comes up ok then drops lower)
also try pluging/unpluging a headphone plug into the headhphone jack and see if that has any effect
try lifting one end of D25, then try lifting one end of R171

Last edited by R_J; 04-14-2019 at 12:35 PM..
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Old 04-15-2019, 01:36 AM   #12
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Default Re: Balance slightly off on Aiwa MX-70

No change if I lift off the D25 or R171, except R171 caused huge buzzing, the mute itself still works. Pin 4 (and also 5) has 151mV on R-channel and 200mV on L-channel. Something lowers this voltage
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Old 04-15-2019, 07:20 AM   #13
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Default Re: Balance slightly off on Aiwa MX-70

The voltage difference is there also when the balance is centered. It takes about a minute (from cold start) when the balance wanders to left. How the cold spray corrects it for a ~30-60 seconds? I don't have the correct transistors yet for the Q21 and 22.
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Old 04-15-2019, 01:03 PM   #14
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Default Re: Balance slightly off on Aiwa MX-70

With D25 AND R171 lifted, the muting circuit is disabled - although the two channels still have a connection together (Q21, Q22 bases).
Do you still get a difference in volume?

You could look at the muting control voltage. It has three sources.
I think the base voltage for Q21, Q22 is +V when muting is on (25V - 9V zener D31 to give +16V to R171), muting activates when AC mains is on (D32/C129) until the speaker protection relay is on from IC-7. Normally IC-7 has the loudspeaker protect relay on, which pulls down the cathode of D31. So Q21, Q22 should have no +ve base voltage when muting is off.

Muting also comes from IC-2 input selector pin 5, through zener D1 to D25.
Usually muting circuits have a little -ve bias voltage to ensure the transistors are off, like -1V. But in the Aiwa I see no resistor for that.

C111, C112 do get a hit of reverse polarity because there is +0.2V on the uPC1225H side and 0V (muting off) or +ve several volts when muting is activated.

If it isn't the muting circuit, then the problem is somewhere else.
The power amp driver IC drifting as it warms up, I would keep checking idle current and DC offset. Pin 4,5 should stay around +0.2V
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Old 04-15-2019, 01:16 PM   #15
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Default Re: Balance slightly off on Aiwa MX-70

I didn't have them off same time, so thats's why the muting system still worked? Need to take both off and test. The uPC1225's get quite hot, is that normal? Pins 4 and 5 stay below +0,2V.

So these transistors related to speaker relay, and not the volume muting system? I have thought that these are somehow related to the volume lowering button (SW6). It's a bit confusing.

Last edited by Xenon-Codex; 04-15-2019 at 01:20 PM..
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Old 04-15-2019, 02:36 PM   #16
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Default Re: Balance slightly off on Aiwa MX-70

Yes, it is confusing because there are many things that can affect volume.
I would disable the muting on Q21, Q22 and see if the volume change is still happening. Lifting a leg on both D25 and R171.

The Q21, Q22 muting circuit is only at the power amplifier input, and it gets activated for any one of three reasons. It is OR'd to three circuits:
-Mute on power up, until the protection relay pulls in after a few seconds.
-To stop thumps from IC-2 touch-switch, whenever you change inputs (tuner, tape, CD etc.)

The volume control and SW6 mute pushbutton switch are upstream of the power amp and muting circuit.

There is yet another muting circuit internal to the power amplifier driver uPC1225H at pin 3. It seems to just be a power-up mute delay with C119, C120.
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Old 04-16-2019, 07:41 AM   #17
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Default Re: Balance slightly off on Aiwa MX-70

I lifted the R171 and D25, now the balance stays centered. Headphone jack worked like the speakers.
This does not affect to the sound very much, no thumps when changing source. But it would be nice to have it fully working.

Last edited by Xenon-Codex; 04-16-2019 at 08:20 AM..
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Old 04-16-2019, 10:12 AM   #18
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Default Re: Balance slightly off on Aiwa MX-70

Seems like the mute line is slightly turning on the mute transistor, and because the two transistors (21,22) are slightly different, it effects one channel and not the other, likely only a couple milivolt difference.
You could have a leaky D1, D25, D31 or maybe... C129 47f/50v could be causing the problem?

You should have 0 volts on q21/22 base un-muted and over .6 volts during mute, if you get about .4,.5 it will start to turn on the mute transistor, and start to lower the audio on that channel
When the amp turns on the speaker relays, ic-7 pin 6 should be low, less than 1 volt I would guess so D31 (8.2v) should be blocking any voltage, so maybe check what voltage you get on D31 cathode & anode

Last edited by R_J; 04-16-2019 at 11:20 AM..
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Old Yesterday, 08:57 AM   #19
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Default Re: Balance slightly off on Aiwa MX-70

I get 0.50 V on both sides of D31 and on both transistors (base). C129 changed but this no effect. The transistors have just over 0.6V when muting. D1 has 0v on cathode side all the time, about 0,44 VDC on anode (1V AC is also present there, if the meter shows that correctly).
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Old Yesterday, 09:40 AM   #20
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Default Re: Balance slightly off on Aiwa MX-70

What do the transistors have when not muting? 0.6v turns them on, but do they fully turn off?
0.50v on both sides of D31 does'nt seem right, I would think it should be 0 volts on the anode
The line coming from ic-7, pin6 is used to mute the audio before the relay closes for the speakers, when the speaker relay is not energized, there should be about 24 volts on D31 cathode, the 8.2volt zener will conduct and have about 15v on its anode, through R171 turning on the mute transistors, when the relay is energized, that voltage on the cathode drops to (measured 0.5v) so D31 should not be conducting and should have 0 volts on R171

Last edited by R_J; Yesterday at 09:50 AM..
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