[AMPS] 87A Fault 17

[Dick Green]

For a couple of years, I was getting intermittent fault #17 trips on my 87A
while operating in contests, mostly on 10M and 15M. The manual describes
this fault as "Abnormally low or high amplifier gain. Usually due to extreme
mis-tuning or possibly an RF arc" (don't get too excited, Rich...)

The fault almost always occurred on the first transmitted element or
syllable. I could prevent it from happening by pulling the power back to
about 1200W from the full 1500W. I first experienced the problem with a GAP
vertical, which has been known to arc in wet weather. But later the faults
continued to occur with a TH-7. I was advised that TH-7 10M traps are
notorious for arcing in wet weather, so for a while I thought that was the
cause. Later, I found that I could generate the fault by transmitting into
the dummy load mounted near the tower. I even considered that my lightning
suppressors might not have a high enough rating and were arcing internally,
but the problem didn't go away when I bypassed the lightning suppressors.
Could it have been the dreaded parasitic oscillation?? Well, I thought that
was a long shot and there was no physical evidence of arcing inside the amp
(clean tune cap, clean bandswitch, etc.)

At about the same time this started happening, I began to see momentary
infinite SWR spikes on the LED meter in the TS950SDX I was using to drive
the 87A. I could reduce the frequency and amplitude of these spikes by
increasing the rise time of the CW waveform from 4 ms to 8ms (a
menu-adjustable parameter on the 950.) It also seemed to me that this
reduced the frequency of the fault 17 trips, but it was difficult to
correlate.

I believe I posted a note about this on another reflector, and got a
response that the problem was probably being caused by RF spikes from the
950. Actually, I think the person who told me that was Tom, W8JI. I saw a
more recent post from Tom that he measured 240W spikes on a 950 set for 150W
output. He said it was a common problem on modern solid state rigs: the ALC
is off in receive mode and ramps up too slowly when transmission begins. The
timing diagrams in the 950 service manual seem to bear this out -- the CW
waveform rises quite rapidly, while the ALC slope has rounded corners and
rises more gently. You'd think the designers would have been concerned about
this.

It seemed reasonable that spikes were causing the faults, especially since
increasing the CW waveform rise time reduced the SWR spikes and maybe the
fault 17 trips, too. But I just couldn't buy the explanation completely
because the amp has a separate fault status for the overdrive condition. I
couldn't understand how a 240W spike would generate an abnormal gain fault
instead of an overdrive fault!

Well, I had a conversation with Karl at AlphaPower that finally cleared this
up for me. He said they often see fault 17 instead of the overdrive fault.
Fault 17 is detected by measuring the ratio of the input power and the
output power. If the gain is too high or too low, the amp trips. Karl wasn't
sure about the amp's coding on this, but my background in software suggests
a possible timing problem: the code that checks for fault 17 probably gets
executed before the code that checks for overdrive. Since the amp does
report overdrive faults if you manually increase the drive power too much, I
suspect that the trip level for overdrive is set lower than the trip level
for abnormal gain. Manually increasing the drive takes place on a time scale
that is very slow relative to the CPU's clock cycle time, so the software
sees the trip point for overdrive before the trip point for abnormal gain.
But on an input spike with a very rapid rise time, the amp could get to the
trip point for abnormal gain almost instantly relative to the clock cycle
time, and if that condition is checked first, a fault 17 will be reported.
(If the fault detection system is interrupt-driven, the same sequence would
occur if the abnormal gain interrupt has priority over the overdrive
interrupt.) The easy fix would be to check for overdrive first. A better fix
 might be to leave the order as is, but check for overdrive immediately
after abnormally low gain is detected and report the overdrive fault
instead, appending "possible transmitter RF spike" to the error message (or
perhaps assigning a new fault number to this condition.)

Recently, I happened to catch a fault 17 while monitoring the RS232
interface. The message said, "Gain abnormally low." Karl wasn't sure if the
code really distinguishes between high and low gain (i.e., it might display
the same message in either case), but I think it's more than a coincidence.

The final proof is that, so far, I have been unable to drive the fault with
either a Yaseu FT-990 or an FT-1000mp that were recently added to the shack.
I think the answer will be definitive if I can get through an entire contest
with heavy 10M use without the fault being generated. I'm having the
TS950SDX realigned by an experienced service technician, and I asked him to
carefully check the ALC, especially the frequency response adjustment. It
could be that it's out of whack on the higher bands, but I'm not optimistic
about that.

I think this illustrates how troubleshooting amps is getting harder as both
the rigs and amps advance.

Looks to me like Tom was right. (Sorry, no parasitic this time...)

73, Dick, WC1M



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