The GRC-106 Info Page

This page is hints and kinks for the GRC-106 Radios...

After going round and round with the RT unit (RT-662/824) of my GRC-106, due to it suddenly going 'deaf and dumb' after a megacycle change, I determined that the MHz drum was not in the correct position, that is, the proper MHz numbers did not show up in the turret window. The MHz synthesizer was, however, in the correct position.

The cause was the large (2-3"?) gear in the bottom of the radio. It was loose on the shaft because the setscrews were loose. Since the MHz seek switch is on the same shaft as the MHz synth, the synth shaft will always be in the correct position during this failure, but the motor, gears, and drum will have turned past the correct position. Most gears and shafts in this train are pinned, but the large gear is set by setscrews.

Fixed by setting the unit to 15 MHz, powering off, and with the setscrews loose on the large gear, manually rotating the worm shaft of the motor until the drum shaft points to 15MHz, and then aligning the MHz synth shaft to match. Tightened the setscrews, and all is well. This was a field repair by the way :)

If the amp keys up and immediately the high voltage cuts out, you may have a shorted 4CX350 final tube. The short may not show up with an ohmmeter, but the tube is arcing inside. Pull the tubes and re-try. Also measure the screen voltage and grid bias to see that they are present. If the HV stays up, put one or the other tubes back in, and determine which one (or both) may be shorted. Be careful, the deadly high voltage is about 2000 volts.

The amp will not complete the operation of the TUNE function.

Check to see of one of the two DC gearmotors which rotates the capacitors and switches in the PA tank assembly is not operating. If the elements are not in the right position for the MHz selected, the amp will never complete the tune operation.

Additional Sources for GRC-106 parts:

Jim Williams Surplus:

Mike Murphy :

Starks Electronics :

Barans Surplus, John Baranosky :

American Trans-Coil :

Mark Masin :

Stuart Goldberg :

Ken Winheim : (better results making a phone call!)

Steve Haney Haney Electonic Company 8012 CR 1009 Godley, TX 76044-3688 817-309-2220 Best hints I can give you on the GRC-106 is:

1). carefully take the radio drawers out of the cases, carefully oil every bearing, gear, busing, and thru-panel busing. The radio will change channels much faster, more reliably, etc.

2). Go to STBY (stand by) BEFORE changing freq selectors to a LOWER frequency!

3). Make sure that all fans are functioning and that the heat exchanger is CLEAN

4). Use headset when communicating

5) if you have the TTY modem, use it to get a "closing" of the audio passband, it sounds better!

Larry, 1). sounds like your antenna output is not in a "tuned" state, meaning that there is too much reflected power. Try tuning into a 50 Ohm dummy load then transmit and see what happens

2). oil all shaft bearings, bushings, gears in both exciter and PA

3). determine that the band switching in the PA 'tracks' the exciter freq setting. Change freq UPWARD ONLY while the PA is in operate, listen as the drive changes the lumped elements and STOPS!

4). Check that the PA lumped elements drum stops with ALL contacts aligned and making contact. This requires setting a freq, shutting down, open the PA and look at the drum & contacts.

The PA can be operated out of its case but you have to make a connection between the back of the chassis and a voltage controller board mounted on the case. Its std DB connector with all contacts made up as straight through. I purchased a cable from the computer store.

--- In, "Larry Tighe" wrote:

Does the AN/GRC-106 NEED 27 VDC? I'm operating mine on two car batteries delivering about 25 VDC.

Should be fine. 27 volts is listed but 24 to 28 volts is ok. Use the AM-3349's built in test meter to check primary power. It should read in the proper green scale at all times (27 volts is about midscale). At first turn on of the power amp there is a 60 second initial time delay and no operation is possible (AM-3349 warm up).

When I go into tune or operate and key with the exciter, the RF comes up and drops out in a few milliseconds on the AM 3349. The meter readings head toward normal. There is good momentary RF output.

During a tune procedure RF output is lower than the normal 200 watts (400 PEP) ; about 50 watts. The tune and load meters are also made more sensitive (electrically) during tuning.

Is the RF output of the RT remaining constant? About 100 mW is normal. Check if the fault condition is present on all settings of the MC (MHZ) control; 2 to 29. I have seen a bad 100-kc module 1A2 in the RT make all kinds of problems but it (usually) shows up only on certain combinations of frequency settings. Also RT modules 1A5, 1A7, 1A8, 1A12 can cause TX troubles.

Lower rear corner of RT (remove from its main case) there is a toggle switch, S11, it should be down when the RT is used with the AM-3349. Up only for testing the RT transmit when no ALC signal from the AM-3349 is present.

I'm wondering if it is sensing low voltage and shutting down. I expected the unit to operate on a nominal 24 VDC but the book references 27 everywhere.

Unless the AM-3349 is overly sensitive to an undervolt condition (electrical fault) I don't think it is your problem... but do the batteries hold up ok? Put a voltmeter on them when you try to run the amp to make sure.

When you have the RT service selector in any position other than oven or off and the AM-3349 power switch on the two fans should be running immediately. If not there is a DC-AC inverter and/or power supply fault or problem in the RT/PA control circuits.

There are two protections built in to the internal DC-DC converter of AM-3349. Under volt and over current. The DC-DC converter only operates when the amp is "keyed" or in the tune mode. Over current is when the PA plates draw more than 450 ma for about 200 ms. Relay A3K1 is the current sensing relay in the tubes plate circuit. Reset is done by moving the tune-operate switch from one position to the other and back. If the voltage at capacitor A7C3 falls under 10 volts, diode A7VR2 will stop conducting and the under volt shutdown is initiated. Reset is done by moving the tune-operate switch from one position to the other and back.

***Do you have to do a reset in order to try transmitting again? If so then the AM-3349 itself is shutting down by fault or to protect from some type of internal/external problem. If not then there is lack of RF drive from the RT, tuning code problem, etc.

The PA's main DC power input is circuit breaker protected by A2CB1. The primary power on/off control will move towards the off position if the breaker trips.

The best way to check the GRC-106 set up is to have the long PA control cable CX-10099, an appropriate length of coax for the RF drive, and build a test cable to connect the AM-3349 to the rest of the electronics when it is pulled from its case (connectors 2A6XA1 and JA1A1 rear right looking into case). Have the RF output connected to a 50 ohm dummy load. Set the RT to several random MC frequencies (KC knobs don't matter for this check) and each time set the AM-3349 to tune. Check if BOTH the RT and AM-3349 turrets are keeping up with the selected MC (MHZ) selection. Also check the AM-3349 rough tune (2 motors) on the antenna coupler 2A3 and associated gear train, switches, inductors, and variable capacitor C27. Insure the (large) vacuum variable capacitor C26 is not shorted and it as well as inductor L1 is properly timed with the front panel counters (load and tune controls).

Is the GRC-106 output power poorly adjusted causing an overdrive condition? In the RT, on the inside of the front panel are three multiturn pots. APC, PPC, and TUNE. APC is average power control and greatly affects the output power. PPC is peak power control and affects the peak power. Tune is the tuning procedure power control. All three are very important and the TM should be followed to properly set them.

I advise, if you have not already, obtain TM 11-5820-520-35. It will help you. The radio is all but impossible to work on without it.

Last night I was curious about a metal plate marked "A2" screwed to the bottom of the chassis and located just below the two PA tubes. I removed the six screws and removed the plate. On that plate was a sub-module with several wires going to terminals marked E-1 thru E-6. One wire, white/black/brown was not connected....apparently had broken away from the module. I cannot locate this 5 transistor module on the prints. Started tracing that wire and will probably find where it goes tonight. Any idea what the module is and why I can't find it on the schematics?

Update to my previous post: switch, labeled as S1 (actually 2A1A2S1) is an air pressure activated switch to detect failure of the internal blower fan. It closes its contacts when a slight pressure is produced in the plenum. You should be able to lightly blow into the switch (plenum side) and activate it... less than 1 psi will easily do... if it takes more pressure than that the switch is no good. The small circuit board is (from what I can tell) an interface for the switch to the rest of the AM-3349 protection circuits. You are correct in that the module is hard to find in the schematics.... because it is not printed in them. This air pressure switch appears to have been a revision to the original design. There is a picture in the manual of the plenum cover and if you look closely there is no rubber grommet for the switch and the two mounting screws are not shown. The parts location data is also not printed. Apparently a later manual would have a revision change but I've not seen it. A loose wire on the board, or switch, as you indicated you have in your unit could easily shut the DC-DC converter down. Here are the wire colors and where they go on module A2 on the plenums cover:

blk/brn E3

blk/org E6

blk/yellow E5

red/wht E2

brn/wht from air switch S1 E1

blue/wht from air switch S1 E4

red/blk E4

As I'm sure you figured out the other parts listed as A1 on the plenum cover are the components inside the plenum chamber. Board A1A1 is the power level control board.

SSB Filter kit Info

Fumio Miyato of B.M Electronics Co., Ltd. Makes a very fine set of replacement filters for the GRC-106's I bought and installed a set of these filters in my RT-662 and I am very happy with them! I used a small relay from a RT-524 to switch between USB and LSB. I used a switch and matching knob from a DOA Ls-166 speaker. I drilled the hole for for the switch where the NB switch in the front pannel would have been. Looks like it belongs there. I run RTTY every weekend using a MD-522A and TT-4A, the filters work very well and there is no change other then to check carrier balance and RF drive levels to the Amp. Here is the link to his web site, scroll down to the bottom of his main page for the filter info...


Info on replacing the meters in the GRC-106 Radio
If you one of the lucky folks that gets your GRC-106 minus the meters, Here is the info for soldering in your replacment meters. This is a list of the wire colors and my code for them.
  1. Black = Blk
  2. White = WHT
  3. Yellow = Yel
  4. Green = Grn
  5. Purple = Pur
For the AM-3349 Amp
  1. Ant Load - - Meter + WHT - - - Meter - Wht / Blk - Grn Trace
  2. Test Meter - - Meter + Wht / Pur - Grn Trace - - - Meter - Wht / Pur - Blk Trace
  3. Ant Tune - - Meter + Wht / Pur / Grn Trace - - - Meter - WHT

For the MD-522

  1. Meter + Wht / Yel - - - - Meter - Wht / Grn
If you end up with a solid black wire that is not connected to eather of the other wires in that meter spot in the amp it conectsto the solid white wire. Or at least my amp it was done that way with the blk wire soldered to the wht wire and covered with heat shrink tubing

Updated 06-01-08