Problem with Heathkit VHF-1 Seneca

Brian Carling bcarling at CFL.RR.COM
Sun Nov 20 07:56:32 EST 2005

Glen, what is the grid current?

Are you sure you are getting enough drive?

Also, have you measured the plate voltage under load?

On 19 Nov 2005 at 19:57, Glen Zook wrote:

> 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
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