[AMRadio] Need some help here

Mon Mar 4 17:47:21 EST 2002

Hi, Dan, will comment throughout the text of both messages.

Bill:
I have been resisting putting in filament chokes because the cathodes are
directly heated. You have any feel for that? The main area I'm going to look
at is the exciter tank I guess. I have set it up with a multi-tap coil and
switch so that it resonates on 160,75, and 40; at least with the dip meter.

Well, Dan, I am of the firm belief that these transmitters are
single-frequency beasts only.  I wouldn't try to multi-band them, you'll run
into all sorts of difficulties.  If you want a ham transmitter, it is better
to home-brew one with all the bells and whistles than to try to convert a
well-engineered piece of equipment into something for which it was not
designed.  There are all sorts of subtle considerations built into broadcast
transmitters which make them excellent for what they were designed to do -
transmit reliably and continuously on one frequency.

Clearly its resonating all kinds of places. I'll rip all that out and use a
plug in coil to eliminate some of that, one hopes. I suppose I could look at
the plate choke too.

Unfortunately, I don't have a schematic of the Gates, and as mentioned,
haven't worked on them.  I have had experience on converting a number of
others, notably RCA, GE, CTC, Western Electric, Bauer and Collins.  I have
tried to stay with the existing circuitry as much as possible, even to the
extent where in most cases to the transmitter can be retuned back to the
broadcast band.

Sufficient drive is typically the problem when pulling them up to Amateur
frequencies.  Other hams try to grid-block key them, too, which I am much
opposed.  You should key the transformer primaries (usually already designed
into the equipment).

Dan
----- Original Message -----
From: "Dan Hall" <kj7fx at tdn.com>
To: <amradio at mailman.qth.net>
Sent: Monday, March 04, 2002 11:32 AM
Subject: [AMRadio] Need some help here

> Greetings Gang:
> I have just about gotten my BC1-J up on 3870.  I'm using crystal control
and
> have wound a plug in coil for the oscillator. It's on frequency and dips
> well. The Gates manual says it should make about 2-4 MA of grid drive for
> the 6146 exciter pair. It does. I have re-tapped the exciter coil and get
a
> dip when it is tuned, though no where near as sharp as I would like to
see.

I have found that some coils can't be tapped down without excessive loss.
The unused turns should be shorted out with a strap.  Can you look at this
signal with a 'scope?  If it isn't a clean sine-wave, then you may have to
use other sets of taps on the coil or replace the coil.  If you see a
sine-wave with a big dip where the peak belongs, you indeed have problems
with the unused coil turns.

> The manual says it should provide about 140 MA of grid drive to the 833A
PA
> tubes. It does not. I'm getting about 25 MA.

This is indication of your problem.  You need something close to the
specified drive.  Sorry I don't have any specific recommendations here, but
the transmitter should end up with operating parameters identical to the
"typical operating conditions" published in the book.  Although I discuss
the final circuitry below, I would concentrate on proper drive first before
worrying about the final.  For a good sounding signal, you need lots of
drive in the AM modulated stage, e.g. your final.

I have shorted the low grid
> relay so that I can put plate voltage to the finals and it will tune, with
> 2000 VDC on the plates to about 5-600 Watts. The problem is that the
> frequency is all over the place depending on the variable coil setting. By
> that I mean , when moving toward the dip the frequency will rise from
> 3869.667 (funky crystal, thanks Jan) to around 4050 KC. The frequency
shift
> is not in jumps either but seems relatively smooth.

Sounds like the PA is oscillating.  It shouldn't, ever.  If it does, it will
generate spurious signals under modulation.  Think of the final as a bell
that is hit by the PA tube.  It rings and generates RF output.  If something
else is ringing also, it will also produce RF output, but of course that is
undesirable.

2000 Volts?  That sounds low, but perhaps that is correct for 833's (I have
more experience with 4-400's).  At any rate, they should put out well over
1000 watts at full output.   Anything less means something is adsorbing
power, or power is developed in something other than RF tank energy.   Check
the output tank circuit.   I favor a PI-L arrangement if you have a choice,
and can help you configure one if you need.  One GE I have uses a
permability tuned coil, and of course I didn't change it.

Again, check the tank coil and make sure unused turns are shorted.  You may
have to also short intermediate turns so the coil can't adsorb energy in the
unused portion.  Use a scope to verify the output waveshape is a good
sine-wave.  As a last resort, you may need to cut the tank coil and reduce
the number of turns.  I hope you don't, and typically you shouldn't have to.

Another problem may be the RF choke in the B+ line.  Make sure it isn't
resonant.  I have had to replace the RF choke in several transmitters.

> There is no decent dip
> at all until the PA begins to act like a doubler, then she dips good.

Not sure why the PA would ever act as a doubler, but sounds like tank
circuit problems.  But double-check the RF choke.  It should NOT ever get
warm.  If it does, it is adsorbing power.

> When
> tuned and loaded through much trial and error to about 350 Watts, and on
> frequency, the frequency is not stable and can jump to 3900+/- or higher
> when switching between transmit and standby (exciter and PA plates turned
> off). On top of all that, the modulator isn't modulating. I get the 80 MA
> idle current and the current jumps around when speaking into the mic, but
> nothing out on the peak meter.

Don't know what to say about the moduation circuit (yet).  Regarding the
tank circuit, how are you tuning it?  I calculate the tube impedance from
specified voltage and current, multiplied by a factor, think it is 0.8.  I
then take a film resistor of somewhere around that value (surprisingly it
has turned out to be 3300 ohms in many cases) and, with power disconnected
to the transmitter, attach it between one of the PA plates and ground.  I
then connect an Autek RF-Analyst to the antenna terminals, and adjust the
plate tank circuitry for an SWR of 1:1.  It must be 1:1, anything higher is
not acceptable.   MFJ units don't seem to work in this application.  I then
remove the resistor (important!), turn on the power, and the trasmitter is
tuned and ready to go!

Of course, this presumes the drive circuitry is working, and from what you
have indicated, that is still a problem in your set.

Hope this helps.  Let me know your progress, I am very interested.  If you
supply the operating voltage and current I can calcuate the tuning resistor
value, or you can look it up in the 1990 ARRL Handbook where they discuss
amplifier tank designs.

73 de Bill, AB6MT
billsmith at ispwest.com

>
> ????
> Dan
> KJ7FX
>
>
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