"Graham" wrote in message
...
On Oct 6, 12:03 am, "Richard Knoppow"
wrote:
I just aquired a Drake R4B which is in quite good
condition. I've found one problem which I am hoping may be
familiar to someone, namely hum when using AVC-Off. This
is
appearent on CW signals or the calibrator. There is no hum
when using the AVC. This _should_ be easy to track down
but
the schematics in the BAMA handbooks have such poor
resolution that its difficult to see where the connections
go. I may have to buy a handbook but I would like some
assistance, if possible, before I have to do that. Its
probably a tired capacitor somewhere, but _where_?
Also, does any one know of a freebee handbook that is
better than the one at BAMA?

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Richard Knoppow
Los Angeles
WB6KBL


That sounds liker a 'hum dinger' of a set you have there !
...... It
all depends how Drake impliment the 'avc-off' function ...
is the avc
fed from a -ve bias supply , and altered by the rf/if gain
control ,
to bias the vlaves off .. or dose it set the gain to max
and cathode
bias one of the valves - rf ?-

could be hum on the bias pack ... or heater cathode problem
when the
tubes are biased fully 'on' ? ... carrier borne hum
reaching the
detector ? shold show on a 'scope' at that level ??

G ..


I haven't fully analysed the bias circuit, I just got a
decent schematic thanks to one of the members of this group.
I replaced the filter capacitor in the bias supply but I
don't think it made much difference. I am more suspicious of
a tube causing the problem. It is not apparent when using
AVC although the bias is still there. The hum is not serious
but noticable. I was hoping someone in the group had
encountered the specific problem and knew what caused it.
If/when I find it I will post back. In general this is a
very nice little receiver


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Richard Knoppow
Los Angeles
WB6KBL