Message: 7
Date: Wed, 15 Feb 2012 11:34:35 -0500
From: Paul Gerhardt <phgerhardt@gmail.com>
Subject: Re: [Amps] Alpha 78 failure
To: amps@contesting.com
Message-ID:
<CACkHHUKBnJqKzt0_EpK5CfXW4qpLsEH6soaLDnVLkTm-81HYJQ@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
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'
Today finally measured the bias at the tube socket and is 'OK' I was
afraid to do this 1st but I sure missed the obvious. The filament
voltage was less than a volt and I could see it changing. OK then the
transformer is failing so I went back to the molex connector and it
was the same there on both sides of the connector. I looked up Gary
in ME at
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/.
He answered email and is still in this biz. Next I put a pin in the
wire about an inch from the connector and there was the full voltage.
The one pin has been 'hot' but the main resistance must be at the
crimp on the transformer side because the voltage is NOT getting to
the pin. A proper repair would be to replace the connector housing
and install a new pin but I will probably put some sort of splice in
it for now.
Back to looking at the Molex list to see if the connector is still available.
--
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
------------------------------
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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|