Have happily been running my Alpha 91b, day before WPX SSB it made a bang
and blew its 20A Fuses.
All voltages OK as long as I do not connect the HV winding of the
transformer to the Power Supply (PS) Board (J1). J2 with the other wires is
connected all time. Tubes taken out and HV to tube compartment disconnected
(J1 Filter Board). Crow Bar disabled and Cover Switch activated.
When I connect the J1 to the PS Board I get a proper shortcircuit, usually
the house fuse blows!
Have also disconnected the filter capacitor string (c2-C10).
If I disconnect HV+ (J3) to the HV Filter board I still get the short.
If I disconnect HV- (J6) to HV Filter board the short disappears. The short
is obviously through the 10ohm/16W (R1), have disconnected all other
components: Filter Caps (removed C10, 220u/450V) as well as R11 and R12.
Again note that HV+ (J3) is also removed.
The only components that I now can suspect are the Trafo and Diodes.
Trafo, measured resistances between windings including primary, no short can
be identified, measured vith normal V/ohm meter.
Diodes measured, (D12-D31) as well as the resistors (R4-R23) with V/ohm
meter all seems OK.
Can it be that there is a short in the trafo that only activates at HV?
Think not as if short, and after all repeated test-shorts windings should be
properly welded together.
Diode strings, can it be short in one of the legs, that needs HT to be
activated to show and can not be seen with ohm meter? How do I identify
that?
Also did another test. All disconnected as listed above, also removed the
HV- to Filter Board. At this point fuses do not blow.
Then I connect a 1K/10W from HV- to ground. The fuses did not blow but the
resistor glowed properly.
Have checked boards and wiring, NO show of damages or burn-marks. Only
sparks I can see is at contacta of K1 and K2 when I try to switch on.
Hints are welcome
73,
--
Thomas, PY2ZXU/SM0CXU
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