Paul -
Yes, that was one thing that should have been kept in the AC-4. The
voltage at the test point shouldn't go over a half volt, (0.35V to set
the Bias for the T-4(any) transmitters.) Unless the resistor opens up!
I think if I was going to bring the TP outside the cabinet I'd use a BNC
connector, and make a BNC shorting plug to keep over it. Probably
wouldn't hurt to put a power diode (1N5408,) across the jack as well, so
the most voltage that could appear across the jack would be ~0.6VDC.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Ron wrote:
Garey,
I took a different approach to setting the bias. I installed TP of the
original AC3 power supply into my AC4 (actually my AC4R). It was an easy
modification and now I don't care how accurate the meter is, or how difficult
the hash marks are to see on the meter.
The original AC3 had a resistor in the ground side of the HV diode string. It
also had a shorting jumper to remove the resistors after you set the bias. The
bias adjustment on my AC4 is now a simple meter reading made with any good VOM.
The only negative I see with this modification is that you are exposed to the 650 volt B+
high voltage while doing the bias adjustment, so care MUST be exercised. You could
actually bring the TP outside the AC4 "cabinet" if you wanted and avoid the HV
exposure risks.
I sent Ron WB4HFN information to post, if he wants, when he gets the chance.
The modification was more difficult to describe accurately than I thought it
would be. Hopefully the pictures and AC3 schematic will help.
73,
Ron WD8SBB
--- On Wed, 1/13/10, Garey Barrell<[email protected]> wrote:
From: Garey Barrell<[email protected]>
Subject: Re: [Drakelist] T4Xb meter shunt
To: "Drakelist"<[email protected]>
Date: Wednesday, January 13, 2010, 2:20 PM
Paul -
The resistor is ok, but you need to calibrate (with R38) to
get the BIAS
setting of 0.07A correct on the meter. Obviously the
idle current needs
to be set correctly for linearity. The 1/4W is a
little small, but as
long as you keep the final dipped it may be
ok. Typical plate current
of 0.340A is just a little over a quarter watt in the
resistor. The
concern is that as the resistor is overheated it will go up
in value.
You never know where you are!
I like to use 1W or, even 2W if I can find them,
"flameproof" metal
oxide resistors. They don't change value so once
calibrated you don't
have to worry about it any more. The finals can
easily draw an amp or
more of current off-resonance, which would leave 2.2W in
your resistor!
Even a half amp is over a half watt in the resistor.
But at least now you know HOW to fix it
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement
CDs
<www.k4oah.com>
Paul Gerhardt wrote:
Garey
Thanks for the help on this one. The meter shunt
on the vertical
board behind the three power resistors was open!
Yea...
I replaced it with another 2.2 it is probably too
small at 1/4 but it
seems fine for now and I can order a higher wattage
one for later.
May still need to get the resistor on the slide switch
changed as I
think the meter is now reading a bit low but the
output is way up from
yesterday. I was able to load it up to 100 watts
out and it may go a
bit so it still needs work but yesteray it was 7-8
watts so it is up a
LOT!
Will try to get a pix of the 'bad' part up on my
blog.
73's
Paul K3PG
If I screw the bias back I see up to 80 watts with the
plate meter
pegged hard over.
With the bias set 'properly' I only see power output
if I turn the RF
Gain up to where the meter is again pegged to the
right. When it hits
the pin I am getting 5 - 7 watts.
When the modulation 'looks' right on the meter I am
getting 5-10 watts
pep out but the plate meter is kicking up all the
way.
I am hearing anti vox coming out the speaker while in
SSB is this
normal? I would think not?
The R4B seems to be working well.
The two VFO are transceiving using either or both with
the inj cable connected.
I am going to start 'dusting, cleaning and visual
inspection'
Paul Gerhardt
K3PG
http://pgerhardt.blogspot.com
QRP ARCI 6674
FP 274
_______________________________________________
Drakelist mailing list
[email protected]
http://mailman.zerobeat.net/mailman/listinfo/drakelist
