So here’s a theory … Suppose a miniscule amount of supply hum gets into the Q4 of an EQ board and the board feedback is unable to completely keep it off the collector. This hum will pass through any subsequent EQ boards unaltered. It then goes to the output pot and finally to 7Q1. Since the supply has some resistance, if the caps on the board are unable to keep it perfectly clean, the hum in 7Q1 (and 7Q3) will return through the supply to Q4 as positive feedback. While there are multiple measures to prevent it, apparently if Q4 has enough gain some kind of loop like this can come into play.
...The power supply to 7Q* is electrically isolated from the power supply to the other boards in the 81...
Zmix was saying that the bias current is set too high for the transistors, which (I think) makes sense if the original design was for parts with a much lower Hfe. That said, I'm not really clear on how Hfe, bias current, etc. all interact, so I could be completely off there.
It should be apparent from these facts that while zmix’s fix works, it was just a happy accident based on flawed deduction. The buzz is clearly not related in any way to overheating, excessive current, or transistors with insufficient current or temperature specs.
There still remains the question of the 70MHz RF signal/oscillation present with the original transistors, which disappeared along with the broadband EQ noise with the transistor swap. This could be coincidental, but they seem suspiciously related to me. I don't know whether anyone but myself and Steve Hogan has documented the presence of this in their unmodified 81's.
...Still, I'm curious why it does seem to significantly diminish the buzz in most cases---maybe the shielding effect of the metal cans on the transistors?
I live in the boonies with no TV reception and one radio station that doesn't come in well. And the signal disappeared with the transistor swap. Maybe coincidental, but I suspect they are related. Maybe I'll have to get out my virgin 81 and check it out.crazydoc, We live in a sea of RF and any piece of metal is an antenna. It is present in every piece of audio gear. You cannot tell if something is oscillating simply by putting a probe on it.
I live in the boonies with no TV reception and one radio station that doesn't come in well. And the signal disappeared with the transistor swap. Maybe coincidental, but I suspect they are related. Maybe I'll have to get out my virgin 81 and check it out.
No need to get snippy.Maybe you should. If you were able to prove you had measured an oscillation or more importantly demonstrate that there is a mechanism by which low level rf can increase supply hum, it would be remarkable.
My experience tells me the probablility of success is zero. I would list more reasons, but I'm sure you could come up with a clever counter for each without leaving your keyboard and it would get us nowhere.
That certainly could be - they do seem to be related - but the causal factor is still unclear.Doesn't mean the high frequency oscillation caused the hum, though. It is much more likely, IMHO, that they both have a single common cause---