The diodes in the 87A output power/swr measuring circuit are a common failure.
- Paul
At 10:05 AM 12/31/2016, Charlie Young wrote:
Thanks for the response Jerry.
I have the computer hooked to the CPU. Because of the difficulty in
finding the cause of Fault 1, I have had to reset the Fault 99
complete lockout twice clearing all the faults, using Alpha's reset
instructions. Presently there are no hard faults in memory, only
the Fault 17 soft faults.
I have performed the GPIO test you suggested, and do see an
approximate 23% lower reading on the output wattmeter compared to
the input wattmeter. The input wattmeter compares favorably with
the drive power from the rig. This seems significant and should be
addressed, even if it did not cause the amp to give a low gain Fault 17.
The examples I have seen online have had shorted diodes in one or
the other wattmeter, and the resulting wattmeter reading has been
very low, like in the single digits or zero. the present reading is
low but not nearly as low as these other examples I have seen.
The output wattmeter does not have a shorted diode but obviously
there is some other issue causing it to read low.
I will check the microprocessor board and see if some other
component there may be loading the reading and I guess the next move
is to pull the board, change the diodes and check the values of the
other components.
Perhaps my next logical move would be to devote time to resolving
the wattmeter issue, which should be addressed in any case. This
may be the only way to make sure the wattmeter disparity between
input and output is not the problem.
Wattmeters have tolerances. Somewhere I have seen that 15%
difference between input and output wattmeters is not an issue, but
it would be good to know the tolerance limits. Alpha had to have
set an operating parameter which faults the amp if the parameter is exceeded.
Thanks again for your helpful response, and Happy New Year to you
and other members of the Amps Reflector.
73 Charlie N8RR
________________________________
From: Jerry O. Stern <jsternmd@att.net>
Sent: Saturday, December 31, 2016 9:28 AM
To: 'Jim W7RY'; 'Charlie Young'; amps@contesting.com
Subject: RE: [Amps] Alpha 87A Fault 17
Fault 17 could be due to several issues, the fault reporting only
shows the last fault and sometimes the actual triggering cause may
precede the 17. You will need to hook up the 87A to a PC with
hyperterminal or similar and look at fault history.
Three top causes : 1) bandswitch misalignment; 2) GPIO failure and
3) arc to L1 with carbon trace
1) If it happens on every band when manually selected (mid band)
then unlikely to be bandswitch unless someone really knocked it
around during repair
2) For the GPIO then due to a failed diode (1N5711) in one of the
wattmeters. There are Input and Output wattmeters.
Here are some tests.
a) Connect PC to serial port with a modem program communicating with
the amplifier.
b) With the 87A in STANDBY, transmit through the amplifier with 100
w from the radio to a dummy load (or antenna with low SWR).
c) On the PC, type EXT ON.
d) On the PC, type GPIO (radio still transmitting 100w). This
command reports Grid current, Plate current, Input power, and Output
power each time it is typed. It should be showing about 100w input
and about 100w output. If one of those is not reading correctly,
that indicates the wattmeter diodes are damaged.
3) requires removal of L1 and close visual inspection
73 es HNY
Jerry NY2KW
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim W7RY
Sent: Saturday, December 31, 2016 9:41 AM
To: Charlie Young; amps@contesting.com
Subject: Re: [Amps] Alpha 87A Fault 17
I see a variable capacitor in the wattmeter, the only
adjustment. Is this a calibration adjustment?
This is called the Null adjustment. Which you set for lowest
reflected power with the amplifier into a KNOWN good 50 ohm (very
low return loss) load.
Most good wattmeters have them. If you look in any of the later
handbooks, or on various commercial wattmeters, you will find the null cap.
73 and good luck with the 87A. (I've never owned one).
Jim W7RY/0
-----Original Message-----
From: Charlie Young
Sent: Friday, December 30, 2016 9:40 PM
To: amps@contesting.com
Subject: [Amps] 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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