[AMPS] 87A Fault 17; cause & cure
Richard W. Ehrhorn
w4eto@rainbow.rmii.com
Sun, 11 Apr 1999 23:18:41 -0600
Hi Dick & all...
Dick, I believe your timing explanation as to why you see Fault 17 (low
gain)rather than Fault 13 (overdrive) is precisely correct. I remember
discussing the exact same issue with the guy(s) who knew the 87A code best
- probably 7 or 8 years ago.
Your discussion of transceiver spikes is right on, as well. I've
encountered large initial rf output spikes on the IC-781, TS-940, -950SD &
-950SDX, and FT-1000D. ETO/ALPHA lab techs used to reset a TS-940's
internal ALC to get more output for various tests, and discovered that they
could put out brief initial spikes up to 400W!
With the TS-950SD & FT-1000D, the cause is usually, in my experience, a
cockpit problem resulting from (a) not reading and following the
transceiver manual OR (b) the manual's giving inadequate or no instructions
as to how to set the xcvr DRIVE control (the Yaesu label; I think the
equivalent front panel knob on the TS-950s is called something different,
but don't have one here to look at). In any case, it determines the amount
of xcvr INTERNAL ALC feedback required to deliver desired Pout from the
transceiver.
The xcvrs have ALC meter functions and it's easy to set DRIVE during actual
operation with the xcvr delivering the desired Pout. Just switch the meter
to ALC and, either key-down or while talking normally on SSB, adjust DRIVE
so ALC peaks are around the middle of the normal range marked on the meter
scale. This is probably in the area of 6 dB. If the transceiver drive
control is set too high relative to actual operating Pout, a big initial
spike does in fact get through to the output. This is because of the finite
time (on the order of a millisecond or two, typically) that it takes the
xcvr ALC to respond and bring Po down to the preset level.
We've found that many guys tend to crank up the exciter drive control by
the seat of their pants and then set its Po with the RF OUTPUT control -
never, or at least never again, checking the ALC. In a common situation,
e.g. running ~50W to drive an 87A to maximum legal output, using an xcvr
capable of 150-200W, most modern SS xcvrs will put out substantial spikes
if their peak ALC exceeds 6 dB or so.
The GOOD NEWS is that when transceiver ALC is set reasonably close to
normal by proper adjustment, the problem goes away. I used a TS-950SD for
~2 years with my 87A, then an FT-1000D for the past 6 years or so. Unless
the operating conditions (mainly exciter output power) are changed fairly
drastically, it's generally sufficient to check exciter ALC only when there
seems to be a problem. I do it maybe once a month just to be safe.
NOTE that every reference to ALC above refers to ALC within the transceiver
itself. Personally I never use external ALC feedback from the ALPHA to the
xcvr, and we do not recommend it in general. It's completely unnecessary.
Hope this is of interest, and maybe even helpful, to somebody. Sorry to be
so wordy - don't know how to explain this particular issue briefly!
73, Dick W0ID (formerly W4ETO)
Following is the original message to which the above responds:
From: Dick Green [SMTP:dick.green@valley.net]
Sent: Saturday, April 10, 1999 10:00 AM
To: Amps
Subject: [AMPS] 87A Fault 17
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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