[Central-Electronics] Central-Electronics 20A project

[Alan Gutfrucht]

Hello All,

I finally got back to my 20A rebuild project recently and have run into a
problem. I thought I was almost all done and was working on final tune up
and the failure of the 458 VFO to triple at 16MHz (another problem) when I
noticed that the 20A would now drop significantly in power after 5 minutes
or so of warm-up. At startup, I was seeing 12 to 13 watts on 20M and this
would drop back to about 2 to 3 watts indication on a Heathkit HM-102 SWR/RF
meter.

I suspected a heat related issue so I started poking around looking for a
burnt resistor or something that was obviously wrong. I found nothing at
first check. Subsequently, the low power problem became permanent and
exhibits itself right from startup. I'm seeing low power on all bands.

I am currently back at it checking all resistor values over again and have
found nothing. Previously, all suspect resistors were swapped out along with
all electrolytics. Some of the upgrade mods were not completed on this unit
(serial 4345) but they have all been completed now. Initially, I had been
through the wiring from start to finish on this checking all and found
nothing miss-wired (but you never know what might have been overlooked). 

One thing I have noticed is that the when switching from STBY to MAN the
relay makes immediately but the RF now lags by a half a second or so. That
did not previously happen.

Also, in checking the voltages to the chart I see that quite a few of the
specified negative voltages are off. For example, pin 2 grid of the 12BH7
cathode follower it is specified at -28 volts. I measure -7 volts.

Anyways, I will be back at this over the next few days and am hoping that
someone out there has some ideas of what I might look for. 

73, Alan, WB2FZC