>
>Hi folks. Here's a problem I've encountered with amateur radio's most
>complex amp. Hopefully, I can resolve this tomorrow with some nominal test
>voltages from Alpha Power, but I thought perhaps the experts would like to
>speculate a bit (not that they ever do that...<g>)
>
>I've had my 87A for about 4 years and it's always done a superb job. I know
>there's controversy on this, but I think it's the best amp made today. I've
>had a few minor problems with it, but have slowly managed to track down
>explanations and/or corrections for them (found the plate choke relay
>firmware bug, discovered that input coax length really makes a difference to
>input SWR ....
? A given with solid-state transceivers driving a g-g amp. .
>.....and discovered that my TS950SDX doesn't have the best ALC in the
>world.) Here's a problem that's been bugging me for a while:
>
>A couple of years ago, the lowest LED segment on the plate current meter
>began to flicker intermittently. At first it was so infrequent that I
>wasn't really sure it was happening -- I would see a flash in my peripheral
>vision but never caught the flicker itself. Over time, the frequency of the
>flicker slowly increased, until I could finally confirm that it was actually
>happening. It began as soon as the amp was turned on, even before the warmup
>period had completed. After warmup, the LED flickered whether the amp was in
>standby or operate mode. As still more time passed, the flicker became
>increasingly more frequent and increasingly more annoying. The amp was
>working fine, so I speculated that it might be the reference voltage
>adjustment on the LM3419 LED bar/dot drivers used to run the display
>(there's a mini pot for this, but it's got a drop of nail polish on it.)
>
>I happened to mention the problem to Carl at Alpha Power when calling about
>something else, and he had me run some voltage checks on the D/A converter
>chip used to drive the display. We found nothing wrong with the chip, so he
>speculated that perhaps I had a leaky tube and that the plate current
>indication was real.
? A very real possibility with 3CX800s. I would use a high-pot. (the
tube should be upright during testing) with pos. on the anode and neg.
on the grid. Normal leakage is under 5uA at 5kV. If the leakage is
higher than 5uA, I would reverse polarity and measure the neg. leakage.
If the leakage decreases substantially with neg. pol., it indicates the
presence of loose gold-meltballs. Gold-leakage tends to fluctuate, esp.
when the tube is tapped.
> But he said the leakage would probably be accompanied
>by the tube getting soft,
? Agreed. "Soft" is quite possible since some of the gold sputtering
from the gold-plating on the grid can land on the cathode, and thereby
poison emission. [see "Eimac letter" on my Web site].
>and I had noticed no such problem (still 35-50W
>for 1500W output, with plenty left over.)
? Lesser amounts of gold-sputtering barely affect emission. Moderate
amounts of gold-sputtering noticably reduce emission. However, if such a
tube is tapped on top, anode up, to resettle migrated gold into the base
before plugging the tube in, normal emission can often be restored. .
(see "Parasitics Revisited" Sept.-Oct. 1990 *QST*)
>
>Now the LED is on steady. I figure I'd better start treating this more
>seriously in case it really is a leaky tube or some gradually deteriorating
>component that might do some damage when it fails.
>
>I've been conducting a series of measurements and tests. Here are the
>results:
>
>1. The CPU and the bar graph agree on the thee amount of plate current while
>in receive (standby or operate). The RS232 interface says it's .070A, and I
>compute each LED segment at about 75 mA. Certainly, this is too much plate
>current for receive mode.
>
>2. I tried running with one tube, then the other. As I expected, the amp
>tripped out with the "filament open" status, but the first LED was still lit
>and the CPU still reported 70 mA plate current. The reading was the same
>regardless of which tube was in the amp.
? In the amplifier I worked on, both 3CX800s were gold-sputtered.
However, after the (admittedly ridiculous-sounding) tapping procedure,
leakage decreased to normal and the amplifier functioned normally. To
reduce the chance of additional gold-sputtering, the vhf gain of the
tubes (at the anode resonant frequency) was reduced by lowering the vhf Q
of the vhf parasitic suppressors. . (note: with moderate amounts of
loose gold, anode-grid flashovers are pretty common -- and they ain't at
all pretty.)
>
>3. I found a better way to test for tube leakage -- I disconnected the HV
>lead from the HV power supply board, completely disconnecting the plates and
>all associated circuitry from the HV supply. This time the amp didn't trip
>out, but the first LED segment was still lit and the CPU still reported 70
>mA plate current.
>
>4. With the HV still disconnected, I measured the voltage drop across the 10
>ohm shunt resistor, the same place that the 87A gets its plate current
>reading (some things never change...) Here, I found a drop of about 15 mV,
>which I believe translates to about 1.5 mA. I can't quite figure out why any
>current is flowing in the resistor at all with the plates disconnected, but
>perhaps it has something to do with how the cathode bias supply is arranged.
>Maybe somebody can explain this to me.
>
? ¿?
>5. Just to make absolutely sure it wasn't related to the tubes, I
>disconnected the cathode bias voltage at the HV power supply board and got
>exactly the same 70 mA Ip reading from the LED meter and CPU. I don't think
>I have a leaky tube.
>
>6. With the HV reconnected to the plates, I measured the voltage drop across
>the shunt resistor at about 30 mV, indicating a plate current of about 3 mA.
>Deducting the 1.5 mA that was already there, perhaps the tubes are each
>leaking about 750 uA. Rich says it should be 500 uA max,
? Roughly 500uA is for 20v of cutoff bias.
>so it's high, but
>this is a lot closer to nominal than the 35 mA per tube the LED Ip meter and
>CPU would have me believe!
>
>7. FYI, cathode bias is about 33 volts in standby
? With 33v of cutoff bias, anode-current in healthy tubes should
probably be under 50uA. My guess is that there is a leakage problem
with both tubes. To find out for sure, a high-pot test is the best.
If you need to have the tubes high-potted, I can do.
>.........
- good luck, Dick.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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