if each pair is to be biased to the correct spot on the curve for
linearity, ( which I think is the goal) and that results in the
output being different from the other pairs, which do you choose. Or
is that why the "matched pairs " is a good idea.
If the pairs are similar "gain" , that should equal the same output
while set at the appropriate bias setting for linearity ????
sounds good , but am I right ???
bill ( who did swap all eight two years ago,,,,)
At 01:20 PM 11/14/2005, you wrote:
>Interesting....
>
>I have never believed it necessary to change all eight when only one is bad.
>
>It is only necessary that individuals in pairs be matched, then balance all
>four of the push-pull amplifiers by way of adjusting the bias. Also be sure
>also that the voltage on each amplifier is the same.
>
>If you have one bad transistor , I would change it and its mate on that
>amplifier. Doesn't make sense to change the other six.
>
>73
>
>Phil K4DPK
>www.k4dpk.com
>----- Original Message -----
>From: "JACrux" <G3JAG@patents.freeserve.co.uk>
>To: <tentec@contesting.com>
>Sent: Monday, November 14, 2005 11:16 AM
>Subject: [TenTec] Hercules II repair: 253 tuner relay
>
>
> > 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
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
>
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