There are several possible failures that will blow the fuses. Blowing
more fuses won't fix the failure and may damage more parts.
The failure could easily be a shorted rectifier and filter capacitor.
One caused the other, but its never clear which went first. If a diode
shorts from a transient voltage, that puts AC on the electrolytic filter
capacitor which can destroy the capacitor. If the filter capacitor
failed shorted, that excess current can melt the rectifier destroying
the junction making the rectifier into a short.
Loss of protective bias can let the PA tubes draw excess current. If
that only blows the fuses you are in luck, but before you apply plate
voltage you need to know that didn't damage the tubes, like burning
loose a grid wire to flop over inside the place to make a short. Or a
filament strand that can break and short the grid, when then makes the
tube draw excess current and damages cascade. Its possible for a tube to
fail without outside influence other than a poor weld at manufacturing
time, or too much shock somewhere in shipping or operation.
Or like a certain series of Ameritron amps, the PA could go into a LF
parasitic oscillation somewhere below 100 kHz, that wasn't bypassed
adequately and got into the plate transformer high voltage winding and
broke down the insulation because of a matching resonance in that winding.
We would wish that the fuse would blow before other parts are damaged.
Unfortunately that's inconvenient because there are legitimate turn on
surge currents that the fuse has to carry. Then we wish for the fuse to
blow quickly, but the slow blow fuse that allows charging the filter
capacitors can't be a fast acting sensitive fuse. Investigating the
choices of fuses for a 1 KW AC solid servo amp about 1965, I came to the
conclusion that even the fastest available solid state rated fuses (very
expensive because the fuse wire is solid silver) were still only
reliable indicators of a failed semiconductor device. That's because the
fuse took longer to blow than the power transistor.
To trouble shoot, I would remove the tubes first. Check them for shorts
between filament and grid and plate. Its least likely there will be a
filament to plate short, but it not impossible even without showing a
connect to the grid also.
Then a classic troubleshooting technique is to apply power through a
current limiting resistor. For a power amp, a 1 KW incandescent lamp
would be suitable. It limits the current and glows brightly until the
short is found and cured. But that exposes you the high voltage which
can do more than hurt you. It can do permanent damage including kill
you. Its a bit safer to apply 5 of 10% of rated line voltage. The only
circuits that won't work at low voltage are voltage regulators and the
tube filaments. All the other circuits will rectify and make DC voltages
but at about the same 5 or 10% of normal operating voltage. When all the
bias and supply circuits show the proper fraction, you can increase the
voltage with a variac but then the lamp is a very good protector (AND
VIVID indicator). Shorted filter capacitors and rectifiers and maybe
transformers will show their failure at the low applied voltage, though
there is some chance they won't break down until nearly all the normal
voltage is applied. That makes the current limiting lamp vital.
Electrolytic capacitors don't store charge long, but you need to be sure
they are discharged before you get body parts across them by shorting
them with a sturdy and well insulated screwdriver or purpose made ground
stick. If there is charge you may well make divots in the screwdriver
and whatever components you shorted. After the flash, shove the
screwdriver back in contact again to be SURE the discharge is complete.
These are the techniques I'd use to trouble shoot, I keep a bench supply
with fixed 6.3 and variable 6.3 volt outputs that are handy for such
tests. The lamp I don't use as much as I should.
73, Jerry, K0CQ
On 3/22/2010 3:50 PM, Art Trampler wrote:
> The responses have confirmed what I was already thinking/my gut was telling
> me.
>
> I guess I was hoping some fount of wisdom would show me why replacing the
> fuses at this point made some sense!
>
> Stuart--thank you for the thought on making a diagram of anything I
> disconnect to isolate a component. So maybe this weekend I'll set up a
> suitable workspace and begin the process...safely. I don't know how long
> those capacitors could hold a charge, but I won't be taking any chances with
> them.
>
> Doug...sorry about your experience. Ouch doesn't begin to describe it.
>
> I just hate it that I'd left the shack at that point, though in all
> probability it would not have told me much to have been there.
>
> 73,
> Art
>
>
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