Powered mixer output distorting

[Yamin]

I've got a phonic powered mixer in for repairs. The original issue was that the output was distorting. Previously two techs has been inside. I noticed that the two big filter caps C3& C4 has been changed to 6700uF and op amp IC502 has been replaced.
When I got the unit the output also was very low. After retouching some solder joints around the op amp which was previously replaced. The dropped output issue was resolved. I used my 8 ohm woofer and it seemed to work and I didn't hear any distortion. It is when I connected a 4ohm two way speaker I heard the distortion, the distortion is also specially in the HF region.
I looked for obvious culprits like bulged capacitors and dried joints and I couldn't find any. DC offset measured across speaker outputs were also very low and it settles around 1 to 2mV.
The service manual doesn't show the voltages which should be present and it doesn't give any instruction on any adjustments. I was hoping that you could share some ideas on what to look for. I am going to scope out the audio path tomorrow.
Also I would like to ask about the potentiometer API 500 what it be adjusting?
Thanks in advance.

[Audiorepair]

Typical suspect is Inserts.

In this case the Insert socket is marked "Mast out Amp in"

Spray with switch cleaner and push a jack plug in and out a few times.



Next on the list is the speaker relay.

[Yamin]

Thanks for the response. So far I have cleaned the contacts of the speaker outputs and the insert plug. I have cleaned off some of the glue on the PCB, the sort of glue which sometimes becomes conductive (it was not conductive yet). I should remove the relay and measure the resistance between the contacts with the relay engaged right to rule it out?
I have measured the DC voltage around the output section and it seems that its all ok, and with in tolerance (measured referenced to ground). However I want to point out I'm measuring 1.8V at pin1 of op-amp IC3A - is this ok? I'm assuming that its providing the bias for the next sections. All measurements done with no input signal.
When signal is applied and the master volume raised the DC voltage at pin1 of IC3A also raises and when the voltage goes above 2.5V I start hearing distortion from the speaker.
Is the VR AP1 used to adjust the DC offset?
I couldn't get access to a scope today. Hopefully over the weekend.
What else could I look for in the meantime.
Thanks in advance.
P.S I'm using the "tape in" to input my signal.

[Audiorepair]

You should really be using the Amp In jack as your input.

And monitoring the speaker output from before the speaker relay briefly to prove it is or isn't the relay.


You could change the electrolytic C13, it would be cheap and easy to do so.

AP1 sets the bias current, don't touch this for now.

[Yamin]

You should really be using the Amp In jack as your input.

And monitoring the speaker output from before the speaker relay briefly to prove it is or isn't the relay.


You could change the electrolytic C13, it would be cheap and easy to do so.

AP1 sets the bias current, don't touch this for now.

Thanks @Audiorepair. I'll do that.
Can I ask why C13?

[Audiorepair]

[/quote]
Thanks @Audiorepair. I'll do that.
Can I ask why C13?
[/quote]


Because you seem to think there may be a dc problem, and this point sets the DC gain.
Likewise you could change C17 and C9, as electrolytics are well known to go bad, and it is cheap and easy to replace 3 of them to rule them out, as you currently have no real data to go on as to what is going on.

While you are fault finding, I would remove the mute JFet Q1, it is very common for these Fets or the mute circuit itself to go partly faulty.
(The electrolytic C8A would be a prime candidate for this)

[Yamin]

Thanks mate, the mute circuitry was not populated on this board. Thanks for the tips.
Turns out in this case it was one of the power resistors leg was not properly connected, visually it looked as if it was. The amp works fine now. Didn't get a chance to scope it though as the customer wanted it right away.
Still wondering why the voltage measurement was not way off when I measured it before.