>
>
>Looking for some suggestions on what to check before I send my Alpha 86 to
>Colorado for repairs. It's been reliable workhorse since around 1990.
>
I would high-pot. the tubes first. Why ship the whole thing to Colorado
when the problem is just a tube?. The test potential should be 2x the
plate/anode supply volts, or about 5kV. Normal cold leakage is approx.
5uA at 5kV. // If you don't have access to a high-pot. tester, I will
be delighted to test 'em for free and ship them back to you the same day
via Priority Mail (maybe) 2nd Day delivery. // I have been trying to
test a pair of 3cx800A7s from a kaput Alpha 86/87 for years, and so far
no luck.
>In CQWW SSB it was being used as the run station on 10m SSB (28.7 or so,
>but SWR is fine) when smoke started to pour out of the top (through the
>blower exhaust holes above the finals). It was quickly turned off - the
>protection circuits didn't kick in. Inspection showed only one obvious
>problem inside. The plate choke had overheated - and near the middle of
>the choke it looks like the winding got very hot and the core material
>(plastic of some kind) smoked. A particularly pungent smell...
>
Heating of a narrow band of wires in a hv rf choke is indicative of the
presence of a frequency that is above the normal operating range. Henry
3K-A HV chokes are notorious for this one. There is a natural resonance
in the Henry choke around 95MHz and when the phase of the moon is right
and its a contest night, downtown Smokeville. . .
>So, I ordered a replacement plate choke the next day which arrived a few
>days ago. Tonight I replaced the bad choke with the new one and turned the
>amplifier on. After warmup, I fed it a 25w from my FT1000mp and things
>seemed just fine. Increasing the power slowly it went past 600w output and
>then blew the two 20a 250v fuses. The protection circuits didn't kick in.
>
>So... I'm looking for some suggestions on what I might be able to check
>out.
Using a dipmeter, record the resonances in the choke between 40MHz and
160MHz. measure the lowest resonance in the anode's vhf-resonant
circuit. If you find a match, there may have been an intermittent osc.
that caused the choke to smoke.
> Obviously I don't want to ship it to Colorado
Amen to that, Tom
- Rich..., 805.386.3734, www.vcnet.com/measures.
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