Charlie,
I don't think it's the wattmeter as the amp can easily handle VSWR to 1.5
and with some tuning up to 2:1. Did you only test into your Yagi. Have you
tested this on all bands into a dummy load? It may only be the 20m
bandswitch that is off.
73 es HNY
Jerry NY2KW
From: Charlie Young [mailto:weeksmgr@hotmail.com]
Sent: Saturday, December 31, 2016 1:05 PM
To: Jerry O. Stern; amps@contesting.com
Subject: Re: [Amps] Alpha 87A Fault 17
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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