> Hi Guys,
>
> I'm in the process of repairing a 77SX, for those of you that may be
>familiar
>with the unit, CB-2 contains among other things, the T-R lockout timer
>circuitry
>and the manual I have dated 1980 shows a mosfet as a Hi-Z input switch
>with a R-C time constant at the gate. However, on the actual board in
>this unit
>instead of the mosfet they use a unijunction transistor driving a small
>scr to
>trigger the relays. My question is, is this board the latest or is the
>system outlined in the manual the latest? The delay doesn't cycle
>properly, in watching
>the gate voltage on the timer transistor it seems that there is leakage
>that
>doesn't allow the gate to come up to the turn-on point reliably. I've
>replaced
>the transistor, and the tant capacitor used in the time constant portion
>of the circuit and it still acts the same way.
> I can make the circuit play right by
>taking the
>10 meg time constant resistor down to about 5 meg, but then the delay is
>too
>short, I know I could at this point increase the value of the capacitor
>to get the
>charge time back up to about 3 minutes, but I don't see this as really
>FIXING
>the problem. Has this been a problem with this amps?
>
Yes.
- I would go with a monostable 555 timer circuit. A number of owners
have reported that their diagram does not match what is actually in their
77.
- There has also been trouble with the RF acutated T/R bias switching.
The 77 automatically switches to non-linear bias during softly spoken
syllables. The result is chopped sounding audio and bursts of splatter
on adjacent frequencies. A simpler T/R system would solve this problem.
(page 33 in 1/94 QST) also, Figure 7 at:
http://www.vcnet.com/measures
- Yet another problem with the 77SX is that the stock tank will not work
right on 160m unless the anode current is reduced to about 1.2A. Also,
there is a 160m doorknob cap. padder that is being operated above it's
current rating. As a result, it overheats/drifts.
Rich---
R. L. Measures, 805-386-3734, AG6K
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