[Amps] Alpha 87A Fault 17
Paul Baldock
paul at paulbaldock.com
Sat Dec 31 13:53:20 EST 2016
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 at att.net>
>Sent: Saturday, December 31, 2016 9:28 AM
>To: 'Jim W7RY'; 'Charlie Young'; amps at 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 at contesting.com] On Behalf Of Jim W7RY
>Sent: Saturday, December 31, 2016 9:41 AM
>To: Charlie Young; amps at 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 at 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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