Power amp right channel running hot.

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bugeyed

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Joined 2012
#1
I am just putting a Classe Seventy power amp back into service after several years in storage & the right channel is running much hotter than the left. Not running it very hard, so it may be operating in class A, but both channels are equally loud. Around .4v max into 4 ohms.
Since I have an assumed "good" channel, should I compare bias readings between them? I think I have a schematic too. What sort of problem should I be looking for?

Thanks,
kev
 
#2
well i'm not intimately familiar with the amp in question but i do have some questions about the storage conditions was it allowed to freeze or did the ambient temp get above 110 fahrenheit which can cause filter caps to fail especially if you just hooked it up and started to run it without allowing the caps to reform in a current limited fashion it's probably oscillating at an ultrasonic frequency creating the heating
a good friend once watched his mint condition vintage marshall plexi go up in flames for the same thing
 
#4
turk 182 said:
well i'm not intimately familiar with the amp in question but i do have some questions about the storage conditions was it allowed to freeze or did the ambient temp get above 110 fahrenheit which can cause filter caps to fail especially if you just hooked it up and started to run it without allowing the caps to reform in a current limited fashion it's probably oscillating at an ultrasonic frequency creating the heating
a good friend once watched his mint condition vintage marshall plexi go up in flames for the same thing
Click to expand...

Not sure what temp extremes it endured. Probably not freezing. I did turn it on for a day or so before hooking it up & using it. The unit sounds fine. No hum or anything audible. I measured around -.1 VDC at both outputs before hooking it up.
I will pop the top & see what I can find comparing channels.

Thanks,
kev
 
#5
i'm not familiar with that amp either, but there should be some ceramic or flameproof resistors near the output transistors. typical values are less than 0.5 ohm 5 watt. measure the voltage (in millivolts) across these resistors. compare the good channel and the bad channel. depending on the amp design, output transistor types, and resistor values, typical readings across these are 5 to 30mV. so if the cool channel shows 10mV for instance, and the hot channel shows 60mV, turn the bias down on the hot channel to match the cool channel. the voltage across these resistors should not be 0mV, so if you see this in the cool channel, you have no bias, and the warm channel is probably the one that's ok. the way to tell which of the two scenarios you are facing is to play some music at about a watt and see if the cool channel sounds distorted, and the distortion reduces as the level goes up. this indicates a no-bias condition. if the cool channel sounds ok at a watt or so, then the warm channel is overbiased.
 
#6
unclejed613 said:
i'm not familiar with that amp either, but there should be some ceramic or flameproof resistors near the output transistors. typical values are less than 0.5 ohm 5 watt. measure the voltage (in millivolts) across these resistors. compare the good channel and the bad channel. depending on the amp design, output transistor types, and resistor values, typical readings across these are 5 to 30mV. so if the cool channel shows 10mV for instance, and the hot channel shows 60mV, turn the bias down on the hot channel to match the cool channel. the voltage across these resistors should not be 0mV, so if you see this in the cool channel, you have no bias, and the warm channel is probably the one that's ok. the way to tell which of the two scenarios you are facing is to play some music at about a watt and see if the cool channel sounds distorted, and the distortion reduces as the level goes up. this indicates a no-bias condition. if the cool channel sounds ok at a watt or so, then the warm channel is overbiased.
Click to expand...

Thanks, both channels sound great at low to moderate levels. Just noticed that the "Hot" channel is hot at idle too. Anyway, I just found the service manual for the 70, although it appears to be an older model. Hopefully they didn't change the ckt. too much. The manual does have the calibration & adjustment procedures. Bias & DC offset. Let the fun begin!!

Thanks,
kev
 
#7
OK, looks like I will be in business, I mean music soon. Went through the bias adjustments & DC offset adj & it seems to be holding the levels steady. Temps of the heatsinks are within 1deg. of each other. Not really sure what the settings were, 'cause I just followed the service manual & turned the pots to 0 & then set bias to 14mv for 24 hr, then bumped it up to 21mv. Will need to run it for a while with a load & signal & then check the levels again.
Right now this is my only power amp & I was bummed 'cause I just picked up an Audioquest Dragonfly USB DAC & wanted to try it tonight. Hope the amp works now, since I've been fiddlin' with it!!!!😱


Thanks for all your input,
kev
 
#8
Well I lied, not working at all now. I had trouble adjusting the DC offset. The reading bounced from -3mv to +5mv & would not settle down. Now I realize that this adjustment is touchy & needs to be done in small increments, let it settle & then adj a bit more, but it still swings a lot. I read in the manual that the output protect circuit senses excessive DC offset to trigger protect mode. That's probably why I have no output. Checked the rail & pre drive supply voltages & they are ok. Any idea what I am looking for with this DC offset issue?

Thanks,
kev
 
#11
bugeyed said:
Well I lied, not working at all now. I had trouble adjusting the DC offset. The reading bounced from -3mv to +5mv & would not settle down. Now I realize that this adjustment is touchy & needs to be done in small increments, let it settle & then adj a bit more, but it still swings a lot. I read in the manual that the output protect circuit senses excessive DC offset to trigger protect mode. That's probably why I have no output. Checked the rail & pre drive supply voltages & they are ok. Any idea what I am looking for with this DC offset issue?

Thanks,
kev
Click to expand...

A couple of mV offset is no reason for the output protection circuit to cut power to the speaker relay. I also don't think that a couple of mV of offset-drift is a problem. If we were talking a couple of hundreds of mV, that would be different.
But didn't you write that both channels sounded equally loud in the OP? How can you know that if you have no output?
 
#12
jitter said:
A couple of mV offset is no reason for the output protection circuit to cut power to the speaker relay. I also don't think that a couple of mV of offset-drift is a problem. If we were talking a couple of hundreds of mV, that would be different.
But didn't you write that both channels sounded equally loud in the OP? How can you know that if you have no output?
Click to expand...

I think that, after making the adjustments, there was a cockpit error when I tested it in the system. I probably had something upstream configured incorrectly. My pre-amp has a Digital/Analog setting for each input, so I may have had that set wrong. Didn't want to leave the amp connected to y good speakers if there was a chance that it would kill them!!!! I have some less expensive speakers hooked up now & am running it in. I will check the levels again before I button it up. Probably listen to it until my main amp gets back home & then maybe go through the Classe & check for degraded components. At least it will be a nice project for me to get back into working on this stuff.

Thanks,
kev
 
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