[Milsurplus] Tips for Reviving a RadioMarine Corp AR-8506B Receiver

[David Stinson]

The RadioMarine Corporation AR-8506B receiver was
used in the RMCA "4U" radio console in Liberty Ships.
https://photos.app.goo.gl/cvW7xMy1W4Mdzm5K6
This one was built in September of 1944.
https://photos.app.goo.gl/V6MfDxCxtVbXHYo88

Excellent background on these receivers and the
  4U station in which they were used can be found
at Henry Rogers - Radio Boulevard,
Western Historic Radio Museum web page:
https://www.radioblvd.com/WWII_Communications_Equipment_Part2.htm

Geoff WB6NVH has compiled invaluable repair notes at:
http://www.wb6nvh.com/AR8506.htm

Bama has the manual, which I urge you to get:
http://bama.edebris.com/manuals/radiomar/ar8506b
Not a great manual but way better than nothing.

My thanks to Geoff, Henry, Bama, Mike Hanz and our
list members for making this project possible.

Here are some tips for resurrecting an AR-8506B:

First- Slow down.  Look close.  Check twice.
Keep notes.  Take photos.
Read the available documentation and manual.
"Your patience will be rewarded."

Rolling the rig on its back:
Look at this photo:
https://photos.app.goo.gl/iri8n3bpq7SamXGJA
The tall can on the left with the knob is
the BFO tank. The knob adjusts the capacitor.
If one carelessly rolls the rig upside down,
this capacitor shaft will be bent.  I did get
it "straight enough" to work OK.  Was afraid
of the shaft shearing or the ceramic cracking
if I got too "froggy" with it.
The shorter can to the right is the 1700KC IF
wavetrap.  We'll get to that later.

This is an AC/DC radio.  "Signal (Chassis) Ground"
is not the same as Power Negative.
The B- buss sits at -6V to provide bias.

That odd looking bulb behind the front panel
"Power On" jewel:
https://photos.app.goo.gl/dF9QQvXvXfURjkCb7
This is a GE NE-32, which regulates the voltage
supplied to the LO.
https://photos.app.goo.gl/uMUsvL8m69fKWnTq6
It is fragile, scarce and expensive.
When they are on Ebay, they're usually $30 each.
Of course, I had to learn the hard way (as always),
dropped a screwdriver and *POP!*
Carefully remove it and protect it
like you would a bottle of nitroglycerin.
A BC-348 991 neon will not work.

If you must replace it with a new one, you'll note
in Geoff's article that the bulb usually won't fit
in the socket- that the two solder "bumps" on
the bottom are too tall to slide over the contacts
in the socket to engage the bulb.  Don't force it.
Geoff speaks of pulling the socket and two screwdriver
adjustments which will lower the contacts and allow
the bulb to engage.  I didn't do this.  I used my
soldering gun to "flatten out" the contact bumps on
the bottom of the bulb, which then engaged with no trouble.

Disassembly - Hardware.
The rig has unusual hardware just about everywhere.
If you lose screws, you'll pay the devil to find more.
Geoff's service notes about the oddball screws, from
the cut-down ones that secure the panel to the two
tiny counter-sunk screws with tiny nuts that secure
the frequency scale and the weird-ohs that secure the
dial cover.
Put a towel or rug or something on the bench
to "catch" screws. Take lots of pictures/notes.
Use one of those plastic sorting trays to
keep them separated.

If you remove the front panel- there are three washers
used as spacers.  Very important these go back where
they came from.  Photo with the washers marked in red:
https://photos.app.goo.gl/LXrVtfzFM7ad7wX17

Changing Caps.
Plug-in Power Supply electrolytics:
These plug-in caps, located behind the speaker,
have an octal plug on the bottom.
Each of them contains two 40 uFd at 150V caps.

These are isolated in the cans- the negative leads
are not tied together.  I opened the cans to "restuff"
them and found that the connections to the octal pins
are soft aluminum that's "crimped" to the cap leads.
Cap one is Negative pin 1, Positive pin 3.
Cap two is Negative pin 5, Positive pin 7.
Cannot get solder on that aluminum, so I glued the cans
  back empty and mounted the four caps under the chassis.
Check the wiring diagram closely- only one of the cap
negatives goes to chassis ground.

The "real bear" are the paper and oil-filled caps.
All the paper caps and oil-filled caps
in this one were bad.
Under the chassis, there are two shields over the
ANT/RF/LO trimmers.  Remove these carefully.
https://photos.app.goo.gl/SSttwoBQeLx6FNzw9
Removing these covers will also make changing the
paper caps outside of them much easier.  It's
crowded under here.

Note that there are many .01 and .001 paper caps
under all those wires and a couple under the band switch.
Don't panic.  It looks hard, and a couple of them are,
but you can do this.  I did, and I ain't no magic man.
You need some good haemostats and narrow, small wire nippers.
Most of these "buried" caps bypass points to chassis ground.
You'll see a cluster of them with one side connected to
a ground point.  The other ends usually connect to a tube
socket pin.  You can nip the ground end of the caps and run
a buss wire to connect one side of the replacement caps.
Nice thing is that 150V caps are small.  The .01 discs
are about the size of the end of your finger.
All the low-voltage RF caps on the band switch and the
two big yellow rectangular caps under the covers were good.
Didn't need to change any of them.

The oil-filled .1uFd caps were all bad, as was the
one on the grid of the audio output. Leaky .1 bypasses
put voltage on the chassis.  I used 4-terminal
strips (one terminal grounded) to make replacements
for these, since trying to remove the bottom cover
for "restuffing" resulted in hot oil shooting out
from the cover and just missing my face (eye protection).
Since we're only talking 150 Volts B+, you can use
some pretty small .1 caps.  Some of mine were the size
of a pencil eraser.  Not enough room for those
hulking orange drops here.

When you get done with all the caps (and resistors of course),
  be careful replacing those two shields.
It's easy to pinch a wire under the cover.
Check the edges before screwing them down.

Look at this photo:
https://photos.app.goo.gl/B7nTiSygeWSjMF7E7
Mounted on the chassis wall at the back, there is
a large, silver bar with two connections- upper right
in the photo.  The tubes are series filaments and
this is a 10 Watt, 50 Ohm dropping resistor. It's supposed
to drop 15 Volts when the tubes are warmed-up.  Mine was
fine but this is often open or intermittent. If your filament
voltages are too high, you can adjust the value of this
resistor.  The tube fils add-up to 100V (at 300 mils)
and the resistor drops 15.  If your line voltage is 120,
add 15 ohms at 5 watts.  If 125, add 33 Ohms at 5 W.

Tuning Gear Train Fun and Frolic:
The main tuning gear box has two small, pot-metal
gears that are press-fit on shafts.  If you tried to
rotate the main tuning and it only goes a fraction of
normal travel, you're about to have some fun.

Inside cover removed so you can get to the gear box:
https://photos.app.goo.gl/AvZtpoPJyBciTbi68
The gear located here was fractured.  There is a
similar gear on the lower shaft which was intact.
https://photos.app.goo.gl/TqcDsGL56GoYNweX6
https://photos.app.goo.gl/SnUWSpGi4LCE4ogs7
The gear after removal:
https://photos.app.goo.gl/s7fTUg2gQRtBiGTGA

Looked around for a replacement gear, but no joy.
After some "brain-sprinkling" (since Covid and
Cancer, I ain't got enough brain left for "storming),
I loosened the set screw to the variable so the
gear could free-wheel.  I used some thin, 1/4
inch wide electrical tape and carefully wound a
few layers around the shaft where the gear went.
Wound enough that the upper split gear teeth made
dimples in the tape deep enough to stay engaged,
but not so much they bound.  Took some fiddling
but finally got it to work well.  Used a sharp
Xacto knife to trim the excess from each side
of the gear space to prevent rubbing.  Yes,
this does defeat the upper backlash gear
but this hasn't been any problem. It's
working fine.

IF can trimmers with "busted nuts:"
As with many of our treasures which use
ceramic air trimmers, the pressed-on nuts
which hold the rotor shaft in place can
fracture and the plates fall down and short.
In one of the IF cans, both trimmers had this
problem.  Bottom one in this photo:
https://photos.app.goo.gl/As6XPqbq9WLSK9yz7
I know many people glue the nuts back in place,
but I don't do that.  I cut stiff clear plastic
shims (the stuff all the WallyWorld junk is
sold in), set the caps at 50% mesh, install the shims
and reinstall. I tune carefully during alignment
since doing a "gorilla" twist might offset
the shims, but I haven't had a problem.
If you must do this, be careful removing the
cover.  It would be easy to break the fine
wires in there.

NOTE: The IF trimmer shafts are at B+ and
will bite you good if you get between them
and the grounded IF can.

Alignment:
Follow the manual procedure.

That 1700 KC wave trap:
I have a strong local on 1700 KCs, and
the receivers IF is 1700 KCs.
Many have had trouble and have even moved
the IF freq to help.  My trap was completely
misaligned.
You cannot get this trap right "by ear."
Follow the manual procedure. The null is
very narrow (intentionally so) and you
need to approach it slowly.  Once properly
adjusted, with the receiver connected to a
75 meter full wave Delta loop, it takes
some effort to find any leakage from the
local 1700 KC station.  If one takes the
time and follows the procedure, the trap
works surprisingly well.

After warm-up, the rig is quite stable.
If you don't have an isolation transformer,
you may run into the "AC-DC" problem which
anyone who has used a Hallicrafters S-120
knows: plug the thing in one way and lots
of hum, the other way and it's OK.
Either way, there should be no significant
voltage between the chassis and ground unless
you missed a leaky bypass cap.
You should be able to connect the chassis to
earth ground to reduce noise without issues.

The rig does sound better with an external speaker.

I hope this is helpful to someone.
Some videos:
https://photos.app.goo.gl/uHEbfu2bgYf19inM7
https://photos.app.goo.gl/dPVktQwzAHwVHQwJ6

P.S.
Got some volunteer help getting the last little "bugs."
https://photos.app.goo.gl/31EFh3iGSLsxrFtJ9

GL OM ES 73 DE Dave AB5S







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