Hello, All,
I am asking for some advice.
A friend gave me his SB1000 to look at as it started [after working well for a
number of years] blowing the line fuses everytime it was powered on, and has
made the rounds of 4 or 5 folks who couldn't figure out what was wrong over the
past 1-2 years [!]. It is wired for 220 V operation. It uses a
multi-secondary transformer with 700 VAC winding for the 3-500Z, 5.2 VAC
winding for the filament, and 10.5 VAC winding to power the metering circuits.
Before it got to me, it had a new set of filter capacitors installed, the
diodes checked, a new meter circuit board added, etc. and 'everything checked
out OK' when individual parts were tested, but yet when the HV supply, metering
circuit, etc were connected together for operation and it was powered up, it
would immediately blow the line fuses every time.
I got it and confirmed that the recitifier board and filter capacitor boards
were wired correctly and left the power to the metering circuit disconnected
[by disconnecting the 10.5 VAC secondary leads from everything], and found that
in that state the HV supply works fine, and the appropriate voltage appears at
the plate of the 3-500Z.
I also found that:
If the 10.5 VAC secondary leads are connected to their appropriate connections
at the metering circuit and the unit is powered on, the fuse immediately blows.
If the AC voltage at the 10.5 VAC secondary leads is measured between the two
10.5 VAC leads as they come from the transformer, with the leads not connected
to anything, there is the correct voltage present.
If one measures the potential between either of these 10.5 VAC secondary leads
and ground, one finds that there is well over 1000 volts DC on both of these
leads, when they are connected to nothing [except of course the innards of the
transformer].
The amp uses a full wave voltage doubler to generate the HV, and so I think it
is normal for about half the total DC voltage to appear on one of the HV
secondary leads in this circumstance.
So I assume that the transformer has a short when there is high voltage present
between the HV secondary and the 10.5 VAC meter circuit secondary, and that
when everything is connected one is essentially grounding the lower half of the
voltage-doubler HV supply; hence the fireworks.
I don't measure a short between the two windings with an ohm-meter, but I
assume that I would if I had a Hi-Pot device that could measure the resistance
at high voltage.
I am not an amp expert. I play around with microwave stuff mostly, and its all
solid state with no high voltages [no TWTs]. So I am checking in here to see
if my thoughts on this are correct.
I think I should tell my friend to buy a new transformer [if he wants to fix
his amp]. Is that likely to be the problem, as I suspect?
I apologize if this question is too lowbrow for the list. I tried to find an
answer in the archives and elsewhere but didn't succeed, likely due to my
inexperience at looking for such answers.
Thanks in advance, and
73,
Roger Rehr
W3SZ
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