Hello Charlie,
It sounds like your troubleshooting skills are up to the task, but if you need
the input of an engineer very familiar with the Alpha 78A, I wholeheartedly
recommend Glenn Pladsen (AE0Q) gm5bkc@gmail.com. Glen's patient and
knowledgeable assistance helped me through an issue with my 87A a couple of
yeas ago. He is often eager to years of 87A experience with other 87A
operators in the Yahoo Alpha 87A group Alpha87a@yahoogroups.com. Glen's depth
of experience with the 87A is a valuable asset to the Alpha community.
Best of luck with your amp, and throughout the new year.
Larry
WB6BBB
On Sun, 1 Jan 2017 16:39:22 +0000, Charlie Young <weeksmgr@hotmail.com> wrote:
>Bruce, thanks for the thoughtful response.
>
>
>Yes, I was incorrect about the wattmeters being the same. Last night at
>dinner I thought about the CT ratios and checked that after coming home, but
>missed the difference in the voltage divider resistors after the diodes.
>Thanks for pointing that out.
>
>
>I have two pairs of tubes, 1990 date code and 2012 date code. The current
>condition of these tubes is unknown to me but the 2012 pair was being used by
>my friend. He acquired the amp from a SK estate a few years ago. He has not
>tried the 1990 pair in the amplifier.
>
>
>The first thing I did after fixing the fault 1 problem that would not let the
>amp go into operate mode was to run the amp with HV on the tubes and no RF
>drive. Yes, I have checked the B+ path from the tube plate back to the HV
>board. All 4 tubes were static tested with HV and no problems. HV was set
>for the high position and appears normal.
>
>
>Next, I keyed the amp with a footswitch , still with no drive, and tested the
>various bias voltages to make sure they were proper and that the amp was going
>from receive mode to transmit mode. All appeared normal. No faults.
>
>
>Next, I used the computer terminal to monitor the Ip and HV in the transmit
>and receive state. 0 Ip on receive and 60 ma
>
>Ip idle current in transmit, with no drive. HV drops a bit in transmit from
>the standby state due to the load from the tubes pulling idle current. This
>all looked normal to me. Both pairs of tubes were tested in this mode with
>the same idle current result.
>
>
>At this point I was ready for RF, or so I thought. Hooked the FT5K to the
>amp, and the antenna switch matrix to the amp output.
>
>
>First up, with the amp in the operate mode, I confirmed the receive was normal
>with no attenuation. The rx pin diodes were working.
>
>
>The amp was on 17M. I put my very low SWR 20M yagi on it with the RF output
>on the rig set all the way down, about 10W. The first dit caused the amp to
>go to 20M, and both the tune and load capacitors moved to a preset position.
>
>The next dit caused a fault 17 with red plate led indication, and reversion to
>standby from operate.
>
>
>If the amp is on a different band from the rf drive source, it stays in
>standby until retuning/band change is done. It does not go to operate mode
>until the band and tuning have been reset.
>
>
>After the fault 17 on 20M, I tested the amp on some other bands, including
>80/40/30 and 17 meters. All of these antennas have very low SWR. With about
>10 watts drive, the result was fault 17 on each band on the 2nd dit, after the
>retuning/band change was complete.
>
>
>Next, I used the computer terminal program to communicate with the CPU and
>read the input and output wattmeters with the GPIO command, with the amp in
>standby. The input wattmeter appeared to be accurate and reads approximately
>the same power as the exciter. The output wattmeter reads consistently low,
>checked at drive levels of minimum to 100W from the FT5K. For example, with
>the rig set for 100W, the input wattmeter read 103 and the output wattmeter
>read 79, a 23.3% disparity. With reduced drive, the input wattmeter read 19
>with output at 13, a 31.5% difference.
>
>
>Last night, on the microprocessor board, I measured the DC voltage output
>from both the input and output wattmeter, directly at the plug. With minimum
>drive from the transmitter, somewhere in the approximate 10W range, the
>output wattmeter read .13V and the input wattmeter read .49V. Because of
>the different CT ratio and, as you pointed out, the different output voltage
>divider resistance, these readings were expected to be significantly different
>from each wattmeter.
>
>
>The next test was to disconnect the microprocessor board loads from the
>wattmeter outputs to see what, if any, loading effects the microprocessor
>board electronics might be having on the voltage readings from the wattmeters.
> With drive level untouched, the output wattmeter voltage remained completely
>unchanged at .13V but the input wattmeter voltage increased to .55 from .49
>volts with the microprocessor board disconnected.
>
>
>We know the input wattmeter is approximately accurate compared to the drive
>level, so the increased voltage with no load is likely the norm. Perhaps the
>fact the output wattmeter was unchanged from load to no load conditions is the
>sign of an anomaly with the op amp circuit on the microprocessor board. Not
>sure.
>
>
>I am going to do some more testing. The test setup is a 1KW 50 ohm dummy load
>on the amp output, to verify the wattage levels. I will test the wattmeter
>output voltage levels at the microprocessor board for various drive levels,
>from about 10 to 200 watts and see how the power ratio holds with the internal
>computer measurement. I will repeat these tests with the microprocessor
>board disconnected to see what the internal load does to the wattmeter output
>voltage.
>
>I will attempt to measure A/D:0 and A/D1 voltage levels and see what that
>shows.
>
>
>No, I have not verified the input circuit is OK, other than to check and makes
>sure there is continuity in the RF path through the input band switch. I
>would like to get around this wattmeter ratio question first if possible,
>because we know if the difference between input and output power is too great,
>it triggers the fault 17. We just don't know how great this ratio difference
>must be per the design of the amp.
>
>
>73 Charlie N8RR
>
>
>
>
>
>
>
>
>
>________________________________
>From: bruce@bubble.org <bruce@bubble.org> on behalf of Bruce W2SE
><W2SE@QSL.net>
>Sent: Sunday, January 1, 2017 2:24 AM
>To: Charlie Young
>Subject: Re: [Amps] Alpha 87A Fault 17
>
>
>
>On 31-Dec-16 23:25, Charlie Young wrote:
>
>Happy New Year Bruce.
>
>Ditto
>
>
>The 87A has two built in wattmeters, one on the input and one on the output.
>Only the output wattmeter reads on the front panel LED display. The input
>wattmeter is used for the CPU to monitor the amplifier gain. If the CPU does
>not see an output level it believes is commensurate with the input level, it
>immediately puts the amp into a fault condition (17).
>
>
>As soon as the first dit is transmitted, the amp faults giving a low gain
>fault 17 indication. So, I have yet to make RF with it that can be measured.
>
>
>The two built in wattmeters can be read with an external computer terminal
>plugged into the CPU, with the amp in standby. This monitor says the output
>wattmeter is reading 23% less than the input wattmeter, with 100 watts applied.
>
>The wattmeter transformer ratios are different as are the voltage dividers
>after the D1 & D2 1N5711 rectifier diodes for each wattmeter. A/D is done by
>the CPU (2 multiplexed analog inputs). Software makes all the decisions.
>
>Try again with 50W and see what the percentage difference is as reported on
>the external PC interface async serial link.
>
>Can you measure the DCV outputs on the microprocessor Analog I/O board in
>standby with 100W and 50W drive? microprocessor schematic sheet 6 right side
>of page for the output signals: A/D0 and A/D1 ?
>I think that you may have some of this data already.
>
>I can think of one way to dummy-up the input and/or out-watts voltage to help
>you figure out the ratios for fault 17 but it will mean a temporary circuit
>mod with a small pot.
>
>
>
>By design, the amp will kick out on fault 17 if there is too great a disparity
>in what the CPU thinks the readings should be. So far, I have been unable to
>find the factory parameter that is programmed in to trigger a wattmeter
>related fault 17.
>
>Certainly OutWatts > InWatts for normal and correct operation. It has
>to be a ratio since input drive can vary.
>I would expect #17 generation is disabled (in software) during tuning.
>I would guess that #17 should expect to see an A/D0 : A/D1 voltage ratio
>that equates to at least 6 to 10db amp gain, that's just a guess. Anything
>less is either: soft tubes, bias issue, low HV, input issue (which you have
>probably ruled out), output tuning/matching issue. and of course wattmeter
>issue. I have no idea what this means for the A/D0 and A/D1 voltage ratios
>and values which would require some testing.
>
>With the amp turned off and unplugged, and the HV bled down and fully
>discharged, have you tested continuity between the HV supply and the plate
>caps? (this tests DC continuity of the chokes feeding HV into the plate
>circuit, which the amp does not measure). Be sure to check the HV R34 10 ohm
>25W glitch resistor.
>
>
>
>
>So far, I have run 100W through the amp in standby with a dummy load on the
>output, and measured the voltage output directly at each of the two
>wattmeters. The wattmeters are
>
>NOT
>
>identical but fed with a different ratio current transformer on each input. I
>have run the calculations and they confirm the readings on the output
>wattmeter are consistently low compared to what they should be. This confirms
>what the computer monitoring shows.
>
>
>The amp does this on every band.
>
>
>I am about ready to pull the output wattmeter board and check all the
>components.
>
>
>Fixing the wattmeter may not solve the fault 17 issue, but in order to rule
>out the wattmeter as a problem, I have to get it reading up to normal level.
>So far, no one has been able to tell me how much difference in wattage reading
>will trigger the CPU fault.
>
>It has to be in an algorithm in the software for the CPU.
>
>If you want to brain-storm via Echolink or telephone let me know. I like
>puzzles.
>
>
>73
>Bruce W2SE
>
>
>
>
>I am optimistic we will find the problem, but the fault 17 may or not be the
>wattmeter. It certainly could be.
>
>
>I will post on the Amps Reflector what is found.
>
>
>Thanks for the response.
>
>
>73 Charlie N8RR
>
>
>________________________________
>From: bruce@bubble.org<mailto:bruce@bubble.org>
><bruce@bubble.org><mailto:bruce@bubble.org> on behalf of Bruce W2SE
><W2SE@QSL.net><mailto:W2SE@QSL.net>
>Sent: Saturday, December 31, 2016 7:40 PM
>To: Charlie Young
>Subject: Re: [Amps] Alpha 87A Fault 17
>
>Hi Charlie,
>
>I haven't worked on an 87a yet but ...
>Just a thought: Did you check/compare the amps' power output to a known good
>wattmeter connected to the 87A's output?
>Both standby and operate.
>
>Did you get any key-down current and voltage measurements during key-down that
>confirm that you are getting normal output power?
>
>If you can verify normal power output, the first place that I would look would
>be the output wattmeter circuit.
>
>Good luck.
>
>73
>Bruce W2SE
>
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