[Amps] Alpha 78 failure

[Paul Gerhardt]

After a couple of years working well and many thousands of Q's the
Alpha 78 quit.  It still switches to xmit, I can hear the relays,
there is a small amount of grid current but no plate current so I am
thinking that the bias is not switching.  The HV reads normal.  Last
time I worked on it I replaced and reworked some parts on the QSK
board two or three years ago.


Alpha 78 failure


Thanks to a lister for selling me an extra QSK board  I still do not
have the '78 running but I made a few steps forward.  The amp was
moved to the 'service area' and  new QSK board was swapped out to the
old one and I changed a couple of caps on the old on  I have got the
same condition now with both QSK boards.

This condition is intermittent and getting worse,at 1st I noticed it
would not come on a couple of times but now it is  usually NOT working
but it has worked now with BOTH QSK boards after I moved it to the
service area  and NOT Worked with BOTH QSK boards with the same
symptoms.


When NOT working
NO Plate idle current but slight 'normal' movement of grid meter
Timer works and K6 IS grounding the T/R Enable line.  Lights change color.
HV is good and switches from CW to SSB fine




Repair history
Found some burned traces on bottom of Power supply board from a
previous failure this was related to failed D111
I replaced D111 a diode when I received the unit but seems ok very
high resistance one way and about 30k ohms in other direction
D110 the 5.1V Zener was replaced before I got the unit but seems to
read 'OK' on ohm meter.


 Next I will try to measure the 5V during switching from 28V.  I am
thinking of running a temporary lead to the meter so I can ck it with
the cover on and the amp powered up. I am assuming that the -28 bias
is ok as the tube appears to be 'cut off'

-- 
Paul Gerhardt
K3PG
http://pgerhardt.blogspot.com
QRP ARCI 6674
FP 274
"You must do this work with love or you fail." -- John Muir, from How
to Keep Your Volkswagen Alive