Screen voltage drops from the 2000 VDC supply through two 100 watt
resistors. I think they are 47K each. Then they feed three zener dodes
in series to get the approx 390 VDC for the regulated screen voltage.
One of tjose big resistors will open up and you lose scren voltage. I
have had many failures like that. You can get away with using just one
47K, but I would advise against it as it serves as a current limiting
device. I think I have some carcass parts here if you need anything. I
have a few modules like the fan and bias etc.
You could also have shorted zeners. I fixed an amp for WA1T and he blew
out all the zeners so we needed to replace them. The power resistors
are the typical failure mode.
Dave K1WHS
On 7/19/2023 10:48 AM, Donald Fox via VHFcontesting wrote:
Good afternoon gents:
We all have our specialties. I'm looking for someone well versed with the
AM-6154/55 series of amplifiers. In my quest for an RF output increase for my
144/222 stations, I came across a few of them at an excellent price. Of course,
they are in need of some work. So far, it appears that I am missing screen
voltage on 2 of them, hence no output. I have done some tracing of the screen
voltage circuit, but it's a bit of a challenge since the screen power is
stepped down from the 2kv plate supply.
Feel free to reply here, or offlist @ taurusshoguy@yahoo.com
TIA Don N8ECH
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