Hello friends, I am in need of some info and/or suggestions regarding a fault
17 problem.
I am helping a friend restore his 87A to operation after he changed the blower.
Immediately prior to the blower change, the amp was inoperable with a series
of fault indications. Unfortunately, I do not know the faults he was getting
but he does not think fault 17 was one of them.
I have only been inside one other Alpha 87A which had a loose wire on the Rbias
terminal on the HV board. Prior to this case I had no experience with
operating or repairing an 87A. This is a different animal from my other amps,
all of which I maintain myself.
To repair this one, I studied the schematic and did research online about pin
diodes, to figure out how the amplifier is supposed to work. Was able to
figure out the bias sources and how the thing goes from receive to transmit.
Next I reviewed many posts in the Amps archive, plus the conversations on the
Yahoo 87A board.
Several issues were worked through and corrected, including blowing step start
fuses, a blown bypass capacitor on the plate choke, and replacing a control
wire that had been cut (apparently during the blower replacement) which was
preventing the amp from going into operate mode from standby.
HV is normal. All the bias sources (-109 volts/ Tbias + or - 30 volts/ Rbias
960 volts ) are there. The bias sources switch off/on as they should between
transmit and receive, and the Tbias switches polarity.
Without drive and keyed with a footswitch, using a terminal program to
communicate with the 87A, the tubes have 60 ma resting current, the same for
both sets of tubes I have.
With the radio (FT5K) hooked to the 87A, receive is normal with the amp in the
operate position. My 20M yagi has a flat SWR. On transmit, the first dit
sent causes the 87A to go to 20M (if it is on another band). The tune and load
capacitors change position. On the 2nd dit sent, the amp immediately gives
fault 17 with red plate led and goes offline. Fault 17 is a soft fault, and
the message on the computer monitor is severe mistune or very low gain.
Online, I found a reference to input and output wattmeter readings being
different causing fault 17 (among other potential causes). With the amp in
standby and the radio keyed, I used the GPIO command in the Alpha software to
get a read on the input/output wattmeter. With the input wattmeter reading
103 watts, the output wattmeter reads 79.
With lower drive, the input wattmeter reads 19 and the output reads 13.
My question to the gurus out there with experience on this amp: What
differential reading on these wattmeters is enough to cause a fault 17? Does
anyone know what the actual tolerance is?
I checked the diodes in the output wattmeter in circuit with an ohmmeter and
get a standard diode reading on all 4.
This does not mean they are good, but none are shorted.
I see a variable capacitor in the wattmeter, the only adjustment. Is this a
calibration adjustment?
Knowing if the wattmeter differential is outside of the Alpha operating range
would be helpful. If it is, I can focus on the wattmeter. If my variation is
acceptable, I need to keep looking for the problem elsewhere.
This fault 17 happens so fast, it is hard to tell with the led meters what is
going on. I see the grid current flicker up, so I think the tube is being
driven. What are the symptoms of a failed input pin diode? If there is a
plate current spike, it is too fast for the led's to respond.
There is no noise or arcing detected, with the room lights turned off.
It does the same thing on every band, so it is not band related.
I was wondering if maybe the input bandswitch out of synch during the blower
change, but this seems unlikely. I have not tested that yet.
I have not unsoldered one end of the pin diodes and tested them yet. The
receive functions OK so I am pretty sure the rx pins are OK. I suppose if the
TX pins were not turned on or open during transmit, the wattmeter would see no
output.
IF the tx pins were open, it would be like transmitting into an open circuit.
Maybe the fault system is so fast
it prevents a plate current spike or high voltage arc.
I have had no other fault except 17, after I repaired the cut wire. Before I
fixed that, it gave hard fault 1 when the operate switch was pushed. The Rbias
monitor voltage was missing.
This has been educational. However, I would really like to give the amp back
to my friend in operating condition. Any help would be appreciated.
73 Charlie N8RR
Charleston WV
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