KPA100 T/R Switching Failure

[Don - W6CZ]

I'll try this again - sorry if it appears twice

During SS, one of my K2/100s (4029) failed. No RX and as I recall no TX.
It was intermittent at first (and had intermittently shown this symptom
before) and to be honest, between fighting a cold and trying to work at
least a sweep between family commitments for my father's (W6ARM) 85th
birthday (I missed - if you can believe it - LAX & SDG - doh!) things is
a bit fuzzy.

I did some poking around and another thing I noticed was failure of it
to communicate with the KAT100 (if I recall correctly it was talking to
the computer correctly).

This morning, I took the PA off and hooked just the k2 up to the antenna
and all seems fine. So I started checking voltages in the PA, and the
problem seems to be in the High-Voltage Bias Supply. All the voltages
are low - with the higher voltages by about 30% and the lower ones by a
lot more. i.e.:

        Anode Cathode
D1 7.6 93
D2 65 7.5
D3 5.8 65
D4 31 5.8
D5 4 31
D6 13.7 4
D7 -18 *
D8 * 0

* Undetermined - the display my cheap DMM is flashing between odd values
an -1 as if it were an AC voltage or out of range - better test gear at
the shop would help - but it is there and I am here and I don't feel
like going if I don't have to. It can wait.

Power set above 100 W:


        Cathode RX Cathode TX
D11 80 .8
D12 .8 52
D13 80 .8
D14 .8 52

So while they seem like they are doing what they should - the voltages
are way off. The notes in the construction manual on page 43 indicate
the voltage test was within range when built - and it has been working
fine for about 6 months of light use - though it was acting up
intermittently.


So I looked at Q8 - the collector voltage is at around 2.3 - 2.5 volts
instead of 11.6 (It is 13.6 at the input to T3). The base voltage is -28
or about 10x what it should be - although the chart says that is
approximate and may fluctuate (and it is fluctuating) - but it isn't
anything close to -2.5 - so I am thinking Q8 went south (open emitter?)

Since it is rather a pain to replace - I would like to get opinions from
others - there may be something I am overlooking (and I AM under the
influence of cold medications - so things are a bit fuzzy).

Thanks

Don
W6CZ



               


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