Problem with Heathkit VHF-1 Seneca

[Brian Carling]

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
> 
> http://home.comcast.net/~k9sth
> http://home.comcast.net/~zcomco
> 
> 
> 	
> 		
> __________________________________ 
> Yahoo! Mail - PC Magazine Editors' Choice 2005 
> http://mail.yahoo.com
> 
> -----------------------------------------------------------
> This list is a public service of the City of Tempe, Arizona
> -----------------------------------------------------------
> 
> Subscription control - http://www.tempe.gov/lists/control.asp?list=BOATANCHORS
> To post - BOATANCHORS at LISTSERV.TEMPE.GOV
> Archives - http://listserv.tempe.gov/archives/BOATANCHORS.html

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.asp?list=BOATANCHORS
To post - BOATANCHORS at LISTSERV.TEMPE.GOV
Archives - http://listserv.tempe.gov/archives/BOATANCHORS.html