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Yesterday my 240 has begun distorting ,especially noticeable at low levels. It seems to clear up with the pre turned up.
Began after adding a Bugle 2 to the system. I would note i had misread one pair of resistors and had lower than expected gain. Once corrected ,the gain was as I expected,but I then noticed the amp sounded poorly.
Im sure its coincidence.
I have since isolated the amp with a single line level in(ipod),and different speakers.Tubes have been checked,a full new set installed to no avail.
I had rebuilt the amp with Jim McShane's level 2 power supply,with additional tweaks with the help of Mike Samra.
All resistors were replaced with hand matched new.Essentially every part.
I have had s slight hum when first warming up.It rises on power up then falls. A total of about five seconds.
My scope trace is very clean and stable. Close visual inspection shows a slight rise in the center of c24 as compared to c23. That's all i can notice.
Any input appreciated.
sf
Edits: 12/05/14 12/05/14 12/05/14Follow Ups:
Those symptoms match a few things, but first I would try to exercise the knobs and the mono/stereo selector switch. You may have a little grunge in there that is turning it into a resistor.
Measure all voltages, especially at the input and driver stages. Something might be low.
Also, there is always the possibility that there is a duff solder joint from the last round of repairs that has gone unnoticed until now.
Thanks. Ill do those things as well later today.
sf
Those Panasonic caps are superb quality, I have had one fail in use in 10+ years and that was likely due to a power conditioner that was beating the hell out of the amp. The symptoms you describe are not likely to be caused by a doubler cap issue.
Sometimes the insulating shrink wrapper on the caps will shrink a little bit if it is warm enough where the cap is located. If it’s just the sleeve/insulator that isn’t perfectly flat then there’s no concern at all. A bulging can could indicate an issue, but the caps are 105 C and VERY high ripple current rated - so that's not likely at all.
Is the distortion in both channels or just one?
Thanks Jim. I appreciate the info.
sf
that is glued on and unless it's buldged,I'm the cap is fine..I will call you later and walk you thru some tests.
"
was all. Hard to understand for me how one day all is good then not.
Thanks for the time,Mike.
I've wired in a jumper on the c23 to sr3 grounds and it is sounding fine again.
C23 had been reading 300uf during all this troubleshooting. So I have ordered a new pair to put in as a precaution.then ill add a negative buss just to make sure I have not overlooked anything.
Anything I'm missing?
Ill call you once its done and we'll talk about any other tweaks .
For now though, a Sierra porter and Ernie Heckscher. By the way this Hagerman Bugle2 is a pretty impressive little phono pre, given the cost.
sf
Steve
It's fine but you need to do the buss bar ground that is on McShane's site..That was a temporary fix I got you going with by having you solder a jumper on the lower half doubler diode and the 820uf cap,the one that goes to ground directly..This is most likely the reason the one 820uf cap failed because I've never had one of those fail..I'm not saying they can't but I haven't seen it.I know it was working before and you did a nice job but the grounds are so critical on these amps,especially Macs and Citations.Their bandwidth is so wide that any noise or hum will show up like a rabid dog.
"
You're the best.
sf
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