Problem with Heathkit VHF-1 Seneca

[Glen Zook]

I just got around to getting my old Heathkit VHF-1
Seneca transmitter ready to go back on the air and
"discovered" a problem that has me rather "stumped". 
When transmitting on 2 meters maximum plate current
occurs at minimum capacity on the "load" capacitor
(maximum loading).  This is with the link coupling
also at maximum.  Unfortunately, the plate current at
this point is only around 60 mA instead of the 230 mA
that is the "maximum" current specified for the
transmitter.  The loading capacitor has a maximum
capacitance of 75 pf and a minimum aroud 5 pf.

There is plenty of grid drive but the transmitter just
won't draw anymore plate current than 60 mA which
gives a plate input of around 36 watts (the plate
voltage is right at 600 volts).  Now the Seneca reads
true plate current rather than cathode current like
most boat anchor transmitters read.  At that point the
output is almost 15 watts (read on a Bird 6154
"Termaline" wattmeter) which gives an efficiency of
40% (not bad for a pair of 6146 tubes on 2 meters). 
The rated input power to the Seneca is 120 watts on 2
meters which should give an output power of around 50
watts which represents not quite 42%.

Unfortunately, there is no more possible coupling in
the link.  On 6 meters the output is right at 85 watts
(again checked on my Bird 6154) at the rated input
power of 140 watts (almost 61% which is great).  But,
on 2 meters there is nothing that seems to get the
power to increase (both input power and output power).

I have replaced the 6146 final amplifier tubes, the
2E26 driver has been replaced, the 6DE7 modulator tube
has been replaced, and the 6AQ5 clamp tube has been
replaced.  All with new tubes.  

Both the driver tank coil and the parallel lines in
the plate circuit of the 6146s grid dip fine with the
variable capacitors well within range.  When the
transmitter is operating the frequency is "right on"
when observed using a service monitor and the grid dip
meter does not show any energy on either harmonics or
sub-multiples.

Has anyone run into the sort of problem before with
either the Seneca or another transmitter that uses the
same tuning configuration (the 6 meter coil is across
the end of the 2 meter tuning lines and is shorted out
when the transmitter is operated on 2 meters.  I am
sure that I can eventually locate the source of the
problem.  However, if someone has experienced a
similar problem and know the "cure" I would definitely
appreciate hearing about it.  I have enough gray hairs
and don't need any additional ones if I can help it.

Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco


		
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