[AMPS] Alpha 77SX

[Mark S Graalman]

    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?




Mark Graalman
Toledo, Ohio
Callsign: WB8JKR
Member: Society of Telecomunications Engineers
TCBA member # 1399

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