Another takeaway from your experience is that a new problem is often a
result of the last change that was made!
I blush to admit that while repairing one thing I've inadvertently broken
something else.
73, Dick WC1M
-----Original Message-----
From: Charlie Young [mailto:weeksmgr@hotmail.com]
Sent: Friday, January 6, 2017 3:37 PM
To: amps@contesting.com
Subject: Re: [Amps] Alpha 87A Fault 17
I have good and bad news friends. The good news is the amplifier works
with a temporary clip lead jumper in place of the open control wire between
the input and output relays. Seems to have no issues, at least on 20M. I
am shutting it down now to address the wiring harness. Thanks to everyone
for the suggestions, advice and encouragement. I am glad to see this baby
play, and I know my friend will be happy as well.
The bad news is this confirms to loss of my short term memory. Oh well, I
was aware of the problem and can live with it.
The moral of this story is: no matter how complex the equipment, check the
simple stuff first. I made an assumption apparently that operate mode on
the computer and amplifer front panel means everything was in operate.
Wrong!! Not necessarily.
I knew the above troubleshooting rule, but think I let the complexity of
this amp and my unfamiliarity with it overwhelm my common logic. In any
case, this is a nice amplifier and I hope parts continue to be available for
it. I have 3 other friends locally who have one. One thing is for sure, I
am familiar with it now!!
Again, thanks for the help and the bandwidth.
73 Charlie N8RR
________________________________
From: Charlie Young <weeksmgr@hotmail.com>
Sent: Friday, December 30, 2016 9:40 PM
To: amps@contesting.com
Subject: Alpha 87A Fault 17
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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