The purpose of this post is to alert Alpha 9500 owners of what's likely an
inevitable repair if the 8877 develops a cathode-to-grid short. Hopefully,
this information will assist other 9500 owners who conduct a web search on
the subject.
The Alpha 9500 uses an 8877 in a grounded-grid configuration. The RF deck
design is not too different from other Alpha GG amps although some unique
MOSFET circuitry is used for bias control. Last week, the 9500 used at the
remote site of N4CC and myself had developed a symptom where Ig read
excessively high upon key line activation and without RF drive. Ig remained
off-scale even without the 8877 installed in its socket.
In localizing the problem, the 8877 likely developed a cathode-to-grid
short, causing two components to fail. R93 is a 10-ohm, 1W SMD resistor. It
failed open with a burn hole punched through the center of the resistor.
Q16 is a P-channel MOSFET. It failed with resistance readings that showed
high leakage between the gate and both source and drain leads. It is also a
SMD component. When these components fail, Ig reads excessively high when
the amp key line is activated.
In looking at the schematic, these two components will always be subject to
failure with such an internal tube short, even if intermittent. When
replacing the components, it's important to change the 8877 at the same
time. Otherwise, the same components will keep failing if the faulty 8877
is reinserted back into the 9500.
Surprisingly, the on-line schematics from RKR Designs do not show the 8877;
the page connectors must be studied to understand the 8877 connection
points.
Protecting these components from future failure is possible without
affecting amp performance, but that takes amp modification. Despite the
hassle of changing out these parts, the good news is that much of the
control circuitry is easily accessed inside the rear panel antenna switch
cage. In fact, quite a bit of diagnostic amp testing is possible with the
amp's cabinet completely attached and there's no exposure to PS HV while
testing.
Replacement parts:
Q16: P-MOSFET, VISHAY SEMI IRF9620STRLPBF SMD - MOUSER P/N:
844-IRF9620STRLPBF
R93: 10-OHM, 1-WATT, 1% THICK FILM, 2512 SMD, VISHAY/DALE
CRCW251210R0FKEG - MOUSER P/N 71-CRCW2512-10-E3
Paul, W9AC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|