Dyanco problem
Ian:
Note the interpolations:
Ian Iveson wrote:
I wonder if this is a wise strategy. Seems good for someone in the
business of overhaul, but not necessarily best for an end user.
Anyone replacing a driver board (functional equivalent of replacing a
car engine or transmission) is in the business of overhaul. And as much
as one would not want to put a new engine in a rotten chassis or
coupled to a bad transmission (or vice-versa), one should _never_
undertake an overhaul until one has done 100% of the "other"
maintenance first. It is part of the learning process if nothing else.
Considering the amp did work before something went wrong, I would want
to find the cause of the failure first. Pulling at all the wires and
replacing the bias rectifier may obscure the original problem.
That failure could be anything from the board itself, to a broken wire
to a coincidental 'elsewhere' failure. So, the point is to eliminate
the potential for those 'elsewheres'. See my first paragraph above.
I note that the OP said that it worked briefly after the mod, but then
"faded". I wonder how soon, and how quickly, this fading happened.
Could be a clue. Perhaps: a failed power transformer; open or poorly
connected small-valve anode or HT dropping resistor; ditto cathode
resistor; failed small valve or heater supply to same; failed bias
rectifier diode.
Again, new board, let's isolate the difficulty to the board. Or not.
Other than the bias supply and the filter caps (and associated
resistors), there are no active components except on the board.
Also, the bias measurements were good before the failure. I wonder if
we can assume that they remained good afterwards? And do they apply
only to the power valves?
Were they good? Or were they assumed to be good? And, no, you may not
assume that they would be good after the driver board is replaced. Bias
voltage will change with a change in transformer load (any winding),
line-voltage, almost with the phases of the moon, it is that sensitive
on that beast.
So, OP, I advise if you haven't already done so to recheck voltage
measurements, including for the small valves. Start with HT and power
valve grid voltages, then output stage bias current, just so you know
you aren't risking burning out a transformer. There is no indication
that the problem is intermittent, so this process is likely to reveal
the cause.
Once you have it working, then swap the rectifier if it's selenium,
and do whatever other overhauling seems necessary.
Are the 'new' tubes in the 'new' board good? How did you verify
this?
Last: What happens if you put the old board back in? Does the amp
come
back?
I would measure the voltages on the new board first. If he takes it
out, and finds that the amp works, what does he do next?
He has three choices of what to do next. They are in order of what I
would suggest first:
a) have another person check the new board top-to-toe for any errors.
Check the traces as well. Often the original assembler will overlook a
mistake over and over and.... Correct anything found and try again.
b) check the new board himself. Painstakingly. Check _EVERYTHING_. It
could be something as simple as a mechanical connection or cracked
trace that fails upon heating. Thus accounting for the fading as
described.
c) stick with the OEM board.
But in any case, he has isolated the problem to the new board. As it is
now, it could be anything up to and including multiple coincidental
failure... which can be terribly frustrating. Another bad analogy: This
would be like moving a door in your house by say half-a-meter (~20" for
in the old currency). Seems simple enough. Would you do it without a
thorough investigation and understanding of both the wall and what may
be in it? I wouldn't.
Peter Wieck
Wyncote, PA
|