OK I installed a DIP socket to substitute for both pot gang resistances, I have several resistor arrays I can use.
I tested 47K (minimal gain) and 12K (higher gain, less than half). In both cases, the channels are now within 5% or less of each other VAC while playing a test tone.
So this would seem to indicate the pot itself, or as it was installed, had some problem, and nothing otherwise is wrong?
I tested 47K (minimal gain) and 12K (higher gain, less than half). In both cases, the channels are now within 5% or less of each other VAC while playing a test tone.
So this would seem to indicate the pot itself, or as it was installed, had some problem, and nothing otherwise is wrong?
If they are equal with the fixed resistors, I don't see any problem with the rest of the circuit.
You may want to check the DC voltage on the 3rd leg of the muting transistors. It should go down to about -15v consistently after a few seconds each time the amp is powered up.
Black probe on RCA shields when measuring the voltage.
You may want to check the DC voltage on the 3rd leg of the muting transistors. It should go down to about -15v consistently after a few seconds each time the amp is powered up.
Black probe on RCA shields when measuring the voltage.
Indeed they both do that!
Thanks very much Perry I will update the thread after I reinstall the new pot. Can I donate to BCAE somehow?
I received and installed a new gain pot today.
The problem appears to be resolved. Both channels respond to the gain, the sound is fine.
One channel is consistently about 8% higher than the other VAC on a test tone. I don't think I'll worry about it - but I'm curious how typical this is, or if I should be concerned about it for any reason.
I have Dow 340 on order, I don't have a great source on Kapton MT. Is it a sin to reuse the old tape? I was thinking of cleaning off the old grease, reapplying, and just remounting.
THANK YOU
The problem appears to be resolved. Both channels respond to the gain, the sound is fine.
One channel is consistently about 8% higher than the other VAC on a test tone. I don't think I'll worry about it - but I'm curious how typical this is, or if I should be concerned about it for any reason.
I have Dow 340 on order, I don't have a great source on Kapton MT. Is it a sin to reuse the old tape? I was thinking of cleaning off the old grease, reapplying, and just remounting.
THANK YOU
It looks like it would be best to replace the tape. Does anybody have a good source for Kapton MT 3/4" in a reasonable quantity?
Amazon has a variety of stuff that might be fine, but in many cases it is hard to source the manufacturer, datasheets, etc - hard to know what it is or what you'd be getting.
Amazon has a variety of stuff that might be fine, but in many cases it is hard to source the manufacturer, datasheets, etc - hard to know what it is or what you'd be getting.
OK, currently there is tape stuck to the heatsink, and grease only between the device bases and the tape.
The tape looks to me like it might as well be replaced.
So I should just remove the old tape, grease the device bases, use grease to seat the new film in place on the sink, then clamp in place - correct?
Thank you
The tape looks to me like it might as well be replaced.
So I should just remove the old tape, grease the device bases, use grease to seat the new film in place on the sink, then clamp in place - correct?
Thank you
So this amp is still acting up. When playing, one channel will cut to minimum gain off and on depending on how I touch it, twist it, etc. Very sensitive. It will play fine for a few days, then decide to cut a channel to minimum just because.
I cleaned the gain contact points on the board, cleaned the board on both sides wherever I thought there might be a problem. This seemed to help for a few days, don't know what gives.
I have two other 250s I've dropped in place, and they are just perfect. I cannot get static in either channel when playing no matter how I wiggle the input or the gain dial.
I cleaned the gain contact points on the board, cleaned the board on both sides wherever I thought there might be a problem. This seemed to help for a few days, don't know what gives.
I have two other 250s I've dropped in place, and they are just perfect. I cannot get static in either channel when playing no matter how I wiggle the input or the gain dial.
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