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I recently purchased a used Fender M80 Chorus that has a problem on the left channel. When the amp is first turned on after sitting for 15 minutes or so. The amp makes a super loud high frequency squelch through the left channel speaker. This will go on for about 2 minutes or so and then the amp works fine. If I plug in headphones while the amp is making the squelch sound it is NOT heard even on the left channel of the headphones. It appears to be something in line after the headphone jack. Any suggestions would be much appreciated.
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Maybe the headphone jack itself is the problem.
The contacts may be dirty or the tangs got misaligned.
The jack 'passes' the output signal to the speakers.
Inserting headphones breaks the path to the speakers & sends the signal only to the headphones.
So that explains why inserting a headphone plug stops the speaker noise.
If it was an amplifier problem you would expect to hear it also in the headphones.
The service manual can be found here: http://music-electronics-forum.com/t33934/ -
Thank you for your insight. In looking at the schematic it looks like the headphone is driven by the drivers and not the Darlington outputs so I'm thinking that maybe it is a capacitor as it is like clockwork. Give the amp two minutes and the sound slowly fades. Maybe it is a leaky cap on the bias of the darlingtons. The voltage levels on the base of the darlington outputs is much different on the the faulty channel than the channel that is working correctly. I will look into that and then repost.Comment
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I believe that the TIP outputs are 'driven' by IC 12 & 13.
Q10 & 11 appear to be part of the protection circuit.(which could be the problem)
The headphone comes directly off of the speaker.
The Left Channel CP4 is ground (well .47 ohms from)
CP5 is the plus signal, which gets interrupted by the headphone plug & routed through the resistor network R173 /174.
You may be onto something with the TIP base voltages being different.
Still, it doesn't explain why the headphones don't make noise.
If you want, post some basic voltages.Comment
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I did a couple things just now. I timed how long it took from initially turning on the amp for the loud hum (no squeeling sound this time just a much louder than normal hum) to subside. It took about 3.5 minutes. I also took some voltage readings.
Q12 B=1.623V C=40.4V E=455mV
Q13 B=-698mV C=-40.6V E=414mV
These voltage levels are much different from the good (right) channel.Comment
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Simply because you have the amp, can you post the same voltage points from the working side?
Is there any voltage reading difference when the amp is initially turned on until the 3.5 minute mark is reached?
For now & until you have the output section more stable, I would remove the speaker.
Your idle condition is off so there is no need for a signal either.
For sure the base of Q13 is low.
Now you have to find out why.
I would start by lifting one end of CR40 & CR41, trying to eliminate the protect circuit.
If there is no change then put them back.
Also, what are the readings for TP 32-34 (VDC)Comment
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normal readings on the good channel for Tip 147 are: B=-1.46V C=-40.6V E=-28mV
I am going to go do the other suggestions you made right now.Comment
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Also Tip142 on the good channel readings are:
B=1.17V C=40.6V E= 32mVComment
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It's going to take a little bit to remove the circuit board so here are my preliminary findings. When I first turn on the unit the base of Q13 was -10.5 volts then as it climbed higher and higher the volume of the hum got lower and lower. I have a very cheap speaker that I am using for testing so I left it connected so I could hear what was going on. As the hum reduced to a very slight sound the voltage returned to -698mV.
TP 32 reads 26.36 volts
TP 33 reads -26-60 volts
TP 34 reads 42mVAC
Now I will lift the legs of CR40 and CR41 and see what happens.Comment
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raising the leg of CR40 and CR41 did not change anything. Same voltage levels and sound.Comment
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TP 34 reading should also be in Vdc.
That one on the schematic is a signal test voltage.
So where is -10 volts coming from? (Q13 base)
Monitor TP 34 when you turn on the amp.
Volts Dc & then Volts ac.
The + & the - base voltages should meet at TP34 & basically equal zero volts.
You may have to pull IC 13.Comment
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I pulled Q13 and took measurements.
B=-990mV C=-40.6 E=220mV
When I first turn on the amp TP34 starts at roughly 450mVdc and pretty quickly goes to 1.017Vdc and waivers right around that value. The AC voltage on TP34 is right around 47mVac
Q13 is still pulled.Comment
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Is it possible that it could just be a bad TIP147. Even though it tested good on a meter could it be some thermal problem that changes as the device heats up or something?Comment
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Short of replacing Q13 I do not see where much can be learned by puliing just it.
The whole output section wants to balance around zero volts.
My suggestion was that if nothing else can be learned, pulling IC13 may be best.
I don't see anything else that could drive Q13's base to -10 volts at start up.Comment
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