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.
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.
Please also post the preamp section, your distortion might actually be instability and coming from there.
Not the full manualbut at least one sample channel.
Amplifier as shown has crisscrossing signal paths, your schematics also include the "Reverb"/Delay system, part of the Master, the signal Limiter, "Tape" send/return, etc. let´s see all together.
Please measure and post rail voltages (+V and -V) and I´ll estimate expected values based on that.
You can self-report your first post (red triangle bottom left) to get a Moderator attention and ask them to move this thread to the Musical Instrument section.
Do you have a Scope and generator?
A load resistor to avid using speakers?
Although at some monent you´ll need to use them too, they are a more complex load than a plain resistor.
You *might* also be triggering short protection (Q4 - Q9) with the 4 ohm load, the resultant clipping sounds NASTY.
Not the full manualbut at least one sample channel.
Amplifier as shown has crisscrossing signal paths, your schematics also include the "Reverb"/Delay system, part of the Master, the signal Limiter, "Tape" send/return, etc. let´s see all together.
Please measure and post rail voltages (+V and -V) and I´ll estimate expected values based on that.
You can self-report your first post (red triangle bottom left) to get a Moderator attention and ask them to move this thread to the Musical Instrument section.
Do you have a Scope and generator?
A load resistor to avid using speakers?
Although at some monent you´ll need to use them too, they are a more complex load than a plain resistor.
You *might* also be triggering short protection (Q4 - Q9) with the 4 ohm load, the resultant clipping sounds NASTY.
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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 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.
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.
Not too tidy but fine, drive is notb symmetrical,OpAmpout is tied straight to one half the output section, not to the other (which is over 3V away) snd takes whatever value is needed to get 0V at speaker out.
You are not measuring a real DC voltage but signal DC component,no big deal.
NO! Please don´t touch it, it adjusts idle current.
Too low you get ugly crossover distortion, too high amp overheats.
So far, nothing else.
I guess scope will show output clipping above a certain volume.
If you get clean 18-20 Vrms it´s fine
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