> 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.
>
> 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...
>
> 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. Obviously I don't want to ship it to Colorado (what kind of
> turnaround time can I expect if I have to?) and would like to have it for
> CQWW CW in two weeks... The amplifier has the original pair of 3cx800
> tubes. The power supply capacitors check out OK. The power supply
> voltage is fine at 2500v+. No excessive grid current.
>
> 73 Tom
Hi Tom,
I would suspect the plate blocking caps are failing when they get hot
from all the 10m current. A significant amount of tank circulating current
passes through the caps on 12-10 meters, as the anode C is a good
percentage of C1 in your Pi-L.
Worth a check, anyway. It is very hard to diagnose from 1500 miles away!
GL!
Phil, K5PC
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