The eight transistors are matched. Swapping just one or two out for unmatched
replacements could spell trouble.
A note I added to figure 4-6 of my Herc manual says set the bias for an idling
current of 500 mA.
In my particular case, another note, this time scribbled on the front cover,
(don't ask) says the base voltages are all 0.67 when the bias is set for 500
mA.
When in doubt, ask TT.
Unfortunately they have not seen the intermittent problem I have on 10/12m.
Sometimes there is normal operation, a good 500w out on either band. Then
sometimes the power is way down on 12m and only up to 80/100w on 10m. This is
unpredictable, in the sense that I cannot make it happen. It just happens.
Guesswork suggests a relay contact (K12/K13) or a so far unidentified bad
joint, but if anyone else has seen this one and has a fix, please let me
know.
-------------------------------------------------------
The vacuum relay in the 253 tuner is an expensive item. Easy to get one and
install it; OUCH to pay for it. But as it is not hot switched, I will try
replacing mine (when I have to) with something a lot less expensive. There are
12 v relays around with 7ms operating speeds, coil to contact insulation of
several kV, and a current handling capacity of around 8-12 amps AC. I found
some costing as little as $5 each, slightly better than $100 plus for a
vacuum relay.
A probably very rare relay problem I had in my 253 was that a back contact on
one of the relays (I think it was K3) on the bridge pad board did not
actually close completely. It was intermittent or even open circuit. I found
the problem when I managed to get the relay cover off so I could operate the
thing by hand and watch the contacts close. I fixed it by tweaking the blade
very gently so the back contacts closed firmly. I then cleaned the contacts to
remove the product of any arcing. End of erratic operation and irregular SWR
detection.
Yes, that was a rather crude fix, but it has worked for several years without
needing a new relay. I am too far from TT to take the thing back, although I
did just that once with the Herc II, during a business trip to TN. In those
days, no-one asked what was in my hand baggage. I was asked only once to open
my bag .... at Nashville on the way home. The lady took one look at the top
of the Herc and decided I should shut the bag and get on the plane.
73
John G3JAG
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