[ARC5] [Milsurplus] GO-9 Progress Report. 4 Nov 19, First Contact
Mike Bracey
mikebracey at att.net
Mon Nov 4 21:43:57 EST 2019
Thanks for the update David. I love to follow along with your military radio restorations. I'm looking forward to more updates.
73, Mike, KE5YTV
On Monday, November 4, 2019, 5:13:25 PM CST, David Stinson <arc5 at ix.netcom.com> wrote:
GO-9 Progress Report. 4 Nov 19
On the bench:
https://photos.app.goo.gl/CVEUCCRUNXiVzgRs8
Thank you to everyone who is contributing discussion and knowledge to
this project.
You're invaluable!
Home-brewing stuff from scratch has never been a "strong point" for me.
I begin with good intentions and a vision of this "neat and sleek"
design with laced-cables and right-angle wires and everything easy to
reach. Then I install the Transformer sub-chassis and discover my "neat
and clean" vision has collapsed into smouldering ruin; I built the
Transformer Deck backwards, the hardware holes don't line-up and the
"tidy" wiring harness is now a fantasy. Not about to pull everything
out and rebuild it; I ain't gonna live forever. So the orderly wiring
is now a rat's nest and spider web, which is why I'll carefully crop all
these photos to hid the most embarrassing jumbles. Hey- at least it's
admitted. And it works, so "all's well that ends well," right?
The plan for the power supply is to build it in vertical layers, using
"floors" or sub-chassis. Some aluminium angle-stock easily adds a
"floor" mount. I'm no MIke Hanz when it comes to metal work, but I did
manage to mount it right-side-up:
https://photos.app.goo.gl/CKjydzEpxT1JKtyWA
The build-space is only 7 inches wide. The Transformer deck is cramped
and crowded- hard to work in there since my "master plan" turned into a
"Mess-ter plan." Worse- there's HIGH voltage running-around in there.
With the gas pedal (variac) mashed to the floor, we're talking 1700
Volts or more. I'm paranoid about such voltages- a lot. Have rebuilt a
couple of Heathkit DX-100s, recovering them from the fried and ashy
results of careless design around High Voltage, and that was only
8-or-900 Volts, so I kinda over-compensate. The bridge rectifier wires
from the HV transformer secondary are triple-covered in heat-shrink
tubing and are spaced at least an inch apart, with no sharp bends or
proximity to grounds. The solid terminal strip that connects the HV
leads of the transformer to those leads will get a double coating of HV
Varnish. The bridge rectifier is mounted on semi-flexable Lexan sheet
with holes drilled for the HV leads.
https://photos.app.goo.gl/Uo8rCNamzWooaimN9
The Low B+ ("Low" - That's funny. It can go as high as 700V+ ) filter
caps and future regulator mount on another Lexan sheet above the bridge,
the center-tap wire passing through drilled holes in both sheets.
Lexan is hard to cut; it's tough and semi-flexible. You can't just
score it- even half its depth and break it. I ended-up using a power
hand grinder to cut it. Drilling is no problem if one uses light, steady
pressure and patiently waits for bit heating to do most of the work.
You can break the stuff if you force it. The two B+ output leads also
pass through holes drilled in the Lexan before connection to the power
supply/Transmitter spring contacts. Update: The Low B+ regulator may
not be needed. Will depend on any problems with FMing on modulation.
We'll see.
Speaking of the connections between the Power Supply chassis and the
Transmitter chassis: The transmitter has knob-looking posts which are
connected by spring-contacts on the power supply:
https://photos.app.goo.gl/k4w2VmpW6UgZvyU68
I had to remove these spring contacts to mount the backwards-Transformer
deck and wire it up. If you take these things off, have a special
holder for the hardware and remember - Long stand-offs on top, short on
bottom. These strips have four mounting screws with star-washers and
nuts on the back side. View with the right-hand strip removed:
https://photos.app.goo.gl/Xs3wvXbhYsim9NH8A
When you go to put these back on, it's nearly impossible to reach the
nuts on the back of the one on the right. I wasted lots of time and
frustration trying to start those blasted nuts. Hit upon an idea: I
layed the strip flat and put a drop of glue to the side (not in!) of
each of the holes. Then carefully aligned the washers and nuts over the
holes:
https://photos.app.goo.gl/jsEEVjxniDU7nqkr7
The glue set in a couple of hours and held the washers and nuts just
enough that, if you're gentle, you can get the nuts started. Only one of
the four failed to start (the hardest to reach, of course) but did
managed to get the strip properly remounted.
Oscillator-Intermediate Amp Filaments:
The ferrites across the High-IR-drop filament chokes did not work; it
introduced 60Hz FM to the Oscillator. I'm pretty sure some of you
expected that, LOL. Was worth trying. With only 10 Volts on the Osc.
filament, the drive from it is reduced quite a bit. Not about to try to
disconnect the wire snaggle and pull that heavy transformer deck back
out to shoe-horn in a 15 VCT transformer, if I could find one. Hit the
junk box and found a little "Radio Shack" 6V transformer and hooked that
up as a boost winding (note winding phasing). This haywire delivers
11.9 V to the Osc and IA filaments and has improved drive to expected
levels It is small enough to tack on top of the original filament
transformer. "More than one way to skin a cat," they say. Here's the
idea being tested:
https://photos.app.goo.gl/xBjmLr2WvuUzaSuQ6
Here's a rough diagram of the power supply as it stands now:
https://photos.app.goo.gl/4VxtZWkMEkwQhaUh7
Haven't yet built the Grid-Block bias supply or any of the control
circuits. The High Voltage design uses the "Economy" principle, where
a center-tapped transformer uses a bridge rectifier and the normally
unused center tap provides a second DC High Voltage of one-half the
value of the full secondary. It needs only capacitive filtering. High
B+ uses six 330 uFd 400V caps with 100K resistors across them in series
for filtering. Low B+ uses two 120 uFd caps with 100K resistors. This
seems to be adequate- no hum or FM in the transmitter signal or chirp on
keying. I changed the panel-mounted Plate Current meter connection.
It was originally in the Hi-B+ negative lead, which prevented having a
meter on the front panel at full B+ (as long as the meter doesn't
open-circuit). I couldn't just leave it in the negative lead from the
bridge because this is a solid-state rectifier. The inrush current from
charging the caps would insta-smoke the meter. Putting it in the PA
Cathode circuit (with long, bypassed leads) solves that issue, but it
does mean the meter is reading the sum of Plate, Grid, Suppressor and
Screen currents. I very carefully insulated a temporary current meter
and put it in the Hi-B+ lead to read actual Plate Current and compare it
to the panel-mounted meter. At maximum rated Plate Current of 175 mA,
the panel-mounted meter reads 260 mA. I can live with this.
You probably remember that the "keying" lead is power supply pin 17,
which grounds the grid circuits of the Osc., IA and, in the original
owner's "grid block keying" change, the PA grid circuit to ground.
Review of the simplified diagram and the keying points:
https://photos.app.goo.gl/BkRAsT5X54ozFwdGA
I mention it at this time because, with the "keying"point left
unconnected and no negative cut-off bias at this point, the Oscillator
stage will start working at a low level, as do the other stages, and a
few Watts of "Backwave" get to the load. One of our members kept the PA
grid circuit grounded and keys only the Osc/IA point, leaving the PA
running unbiased during key-up. That configuration does not exhibit
back-wave. Since we'll eventually be using grid-block keying, this
won't be an issue. Just mentioned it in case someone else runs-into
this situation.
Grounded pin 17 (keying), fired-up the power supply and the transmitter
seems to be cooking. Cranked on the B+ variac and "put the pedal to the
metal," bringing the Low- B+ to the rated 550V and the Hi-B+ to about
1400 (estimated- my meter won't go that high). This maxed the PA Plate
current at 175 mA. Tuned-up into a 50-Ohm load, measured with both my
wattmeter and with my calibrated scope, the transmitter is delivering
200W+ out on 3890 KC. This seems a lot but both instruments agree.
First QSO:
Powered the B+ off and hay-wired the large Line-to-Voice-Coil
transformer into the Cathode lead, fed it's 4-ohm tap with a small audio
amplifier and a lo-Z dynamic mike. Powered up and tuned-up the
transmitter to 150W out, then cranked-up the audio until I got a nice,
clean 100% waveform. Cathode modulation doesn't add to the PEP of the
carrier but that's OK. The simplicity is worth the trade-off. First
contact was with local AM guru Mason, K5YHX, who reported excellent
audio. Kinda funny to watch as my "keying" was turning the variac on
and off, switching the B+ just like in the original circuit. Yes, it
did "swoop away" for about half a second. Didn't mind. Was too happy
about the contact.
An important point:
It is very easy to mis-tune this transmitter and be wind-up on the wrong
frequency. My first tune-up turned-out to be on 5 MC. The tuning charts
give a "ball park" for settings and should be consulted. Settings for
operation of this specific rig on 3890 KC are:
A=2 B=600 C=2 D=50
E=1 F=80 G= Just under 4MC marking
H=Current I=88 J=655
Things are coming together. More later.
And I have no idea where I'm going to put this big beast!
GL OM ES 73 DE Dave AB5S
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