No PbF statements anywhere but I assume it is. Reported as consistent intrusive mains hum with any settings of controls, but of course with me it is perfect. Expecting a loose cap or input socket problem, but nothing found. Before dismantling the amp, anything to check , from access side only at the moment. So far,cursory comparison, circuit looks like this one
Sounds like a polarity problem with the AC input. Is there are polarity switch on this unit? Floating ground?
No polarity or ground lift options. There's always a question I forget to ask the owner. This time how prominent was the hum, like unplugging a guitar and touching the pin of the plug, or just noticeable with no signal through. I'll have to email, while I dismantle the amp.
Thus, a likely hypothesis is that it is picking up some EMI in its regular location, with the regular instrument and other signal chain components that it is hooked into: stomp boxes, and their power supplies, cabling.
Step one is to reproduce the problem. Get the user to demonstrate it, if possible. Re-create some of the original conditions, if necessary.
You can hardly be effective in investigating a problem that isn't reproducing.
Not to mention fixing it; how can you be sure something is gone, that you never observed!
Even if you *do* reproduce a problem, doing something that apparently makes it go away isn't necessarily a proper fix. So far, one thing that makes the problem go away is to relocate the amp to your polace. See? Not a proper fix.
If it is a polarity problem, it could be in the outlet that the amp is plugged into at home.
** Long as the amp exhibits no AC frequency hum when used alone, it has no fault. ** Of course, but few customers are capable of comprehending even that simple idea. ** Of course, but having outlined a perceived fault customers expect it to be fixed and never return.
Only thing is to write on the invoice:
" Fault not seen, not fixed and not warrantied "
.... Phil
** This recent British made amp uses an AC supply ground conductor so plug polarity is irrelevant.
Been a long, long time since guitar amps were sold with no safety ground. Some mad Yankee idea, I believe.
... Phil
Definitely PbF used, but nothing obviously wrong in the way of ring cracks etc. "digital" checking of caps (with fingers) a Sanwha SD, 47uF 450 V , C16 designation ,would move like that situation of pad and pin fully soldered over so you cannot see the metal of the pin at all, but there is movement of the cap as the pin is moving fully enclosed in a PbF solder "socket". Ground back the solder on both pins to show discontinuity of metals and still no movement seen. A physical break inside the cap on one pin of 'D' anode supply to the preamp, of the R/C, B to E HT chain. I've yet to hear back from owner , how much hum there was.
Just yesterday had a hum problem caused by the infamous "yellow glue turned brown"
The glue was apparently conductive enough to couple to a signal lead right at the edge of the goop.
Strangely, the hum was 60 hz instead of 120. Still haven't figured out that part.
Mark Z.
If the glue became conductive - then it would be due to a chemical salt within its composition. Many salts, by nature are diodes - so it would rectify only half the wave-form - 60 hz.
Just a random thought.
Peter Wieck Melrose Park, PA
I did wonder about that but it seemed a bit much. There was also an important but not necassarily the same circuit ground in the affected area. Possibly that coupled over somehow.
mz
Have something to add? Share your thoughts — no account required.
Ask the community — no account required