At 11:56 PM 12/10/97 -0700, you wrote:
>
>Alpha 77D problems..
>
>Background...77D (says PA-77 on rear, s/n 2986) Has been in use for
>several years, treated well (I hope) and behaved well. Sometime ago,
>occasionally it would fail to work (no RF out, 40m grid, high SWR to
>exciter) when first turned on (after 3 min warmup of course). Usually
>turning it off and back on would bring things to life and it would perform
>fine all weekend in a contest environment. Later it became increasingly
>harder to get it to work, sometimes having to give it a right jab, and
>later by removing and reinserting the CB-2 and QSK control boards. But
>once working, it would perform fine all weekend (contest, never turned
>off)
The obvious problem is that the tube is not turning on.
If you will notice, CB-2 has only one ground connection to chassis. (pin 22)
Use a blank pin (10, 12, 15, or 16) and run jumpers from the ground lands
on CB-2 to one of these pins, then run a wire from the corresponding pin
on the CB-2 socket to chassis. While you are at it, replace D3 and D4;
check R2 for proper value, check C16 and C17 for short/proper value.
Several changes to CB-2 were made over the years. A 1N4004 diode, D-216,
was added from the collector of Q202 to ground; a .01 @ 50v disk ceramic
cap. was added on the back side of the board from the base of Q202 to
ground.
>Manual toubleshooting tip number 6 seems to desribe my problem, and points
>to damaged RF sensors (D3 D4 area) or bias switch Q202, Q203. Some
>measurements: cathode bias line (CB-2 pin 2) seems to work as it should
>60v to 20v when keyed, then drops to about 8.5v when a small amount of RF
>applied. I think this indicates the rf sensors and biasing works - it
>knows the difference between no RF and a little RF.
Look at the bias line at the feed thru cap (C1) where it enters the RF
deck to see that you get the same readings as above. The cathode choke
(L2) is only 15 uh and microscopic in size. I always replace this in
77's with an Ohmite Z-14 (orange) choke. (44uh @ 1 amp)
>Continuity: OK between the plate and the B+; on the return side (cathode
>choke L2 back to B-) is 55 ohms plus D203 which adds up. Therefore nothing
>obviously wrong in the HV and return
You need to measure bias at the tube end as you step it through the keying
and drive process; not a just a continuity check with an ohmmeter. Something
is probably breaking down with voltage applied.
Good luck, Don. It is hard to try to trouble shoot by long distance.
The best guy I know to do this with an Alpha is Ray Heaton. I don't think
he made the move when ETO changed hands. Perhaps someone at ETO is as
knowledgable as Ray in about the 77 series...I don't know.
(((73)))
Phil, K5PC
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