[MRCA] [MMRCG] GRC-106 AME problem

Mon Jan 4 14:37:00 EST 2021

Remember that the GRC-106 has no AM, they make a fake AM by running USB and also injecting a carrier thru R14 on 1A5
Power control is by the PPC and APC voltages. A1R15 controls overall PPC voltage and is derived from the amplifier. It works with A1R14 the ALC level by the fixed resistor network on the mode control. The other adjustment under the cover is A1R13 that sets the power level in the TUNE mode.
If the exciter is overdriving the amplifier and is causing plate trips it would be interesting to see what the PPC and more important the APC voltages are doing. You can see the test points for each on top of 1A5
PPC is more audio derived and used to control distortion, APC is derived from the 1A1A2A5 located under the chassis from what I recall and derived from the exciter output.
Try to avoid the temptation to just crank on R14 on 1A5 (AM Carrier Level) because if the PPC and APC are not working that can result in the radio always running wide open.
Think the manual refers to both voltages being around 3 volts under normal operation but would not swear to that.

Ray F/KA3EKH

From: Meir Ben-Dror WF2U <wf2u at ws19ops.com>
Sent: Monday, January 4, 2021 10:17 AM
To: Ray Fantini <RAFANTINI at salisbury.edu>
Cc: mmrcg at groups.io; MRCA <mrca at mailman.qth.net>; Milsurplus <milsurplus at mailman.qth.net>
Subject: Re: [MRCA] [MMRCG] GRC-106 AME problem

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Ray, and all who replied so far,
Thanks for the replies and input.
I don't have a second transceiver, but thinking of getting one.
It works fine in all modes except AM.
What threw me off is that the "tested" 1A5 module (transmitter IF/audio) I swapped the original, which works fine in all modes, except AM, also does the same thing, but works in all the other modes. When I ordered it from Fair Radio, I specifically asked to test it on AM as well.
The first thing, before I go through the APP/APC settings is to connect an RF step attenuator between the RT-834 output and the amplifier input, to set and measure the drive level that won't trip the overload, hoping that the problem is as simple as that, and it's not some kind of spike introduced into the exciter output from another fault when switching to transmit in the AM mode.
73, Meir WF2U
Landrum, SC

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On Jan 4, 2021, at 9:53 AM, Ray Fantini <rafantini at salisbury.edu<mailto:rafantini at salisbury.edu>> wrote:
Is it working correctly in SSB? You should see the meter on the receiver exciter modulate with voice in SSB transmit. The APC/PPC pots under the little plate may be dirty but that will also affect your SSB/CW and FSK operation, if all that’s working you will see more power output in CW/FSK then in SSB or AM. The receiver exciter only need to put out 100 Mw for full power on the amplifier and if the meter on the receiver exciter pegs you are way overdriving the amplifier and that’s not a good thing.
Don’t you have a second receiver exciter with the 122 shelter? Did you try that with the amplifier? The amplifier pumps PPC back into the receiver exciter but if its working in SSB/CW that’s all good.

Ray F/KA3EKH

From: MMRCG at groups.io<mailto:MMRCG at groups.io> <MMRCG at groups.io<mailto:MMRCG at groups.io>> On Behalf Of Meir Ben-Dror WF2U via groups.io
Sent: Sunday, January 3, 2021 7:50 PM
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Subject: [MMRCG] GRC-106 AME problem

CAUTION: This email originated from outside of Salisbury University. Please exercise caution when clicking links or opening attachments from external sources.

HNY all,

I wonder whether anyone experienced a problem with trying to transmit on AM (actually it’s AME) with the radio. My receiver/exciter is the RT-834. The system tunes, loads up and transmits (and receives) normally in all modes, with rated power, except when I’m attempting to transmit on AM, the amplifier kicks off at overload. The exciter meter pegs, and the amp center meter set on PA cathode current kicks up above the normal current momentarily as the amp kicks off. It is immediately resettable to Tune mode, where it works normally in all modes and the normal tuning carrier output. I suspected the 1A5 module (transmitter IF and audio), and I got a tested one from Fair Radio. I put it in and it does the same thing. I thought maybe the mode switch has a problem, but visually it looks OK and after applying contact cleaner just in case, it does the same thing.
It looks like the exciter sends a spike into the amp when pressing the PTT only on AM (does the same on VOX as well) and that puts the amp in overload condition.
I was even thinking that maybe the radio has a modification to prevent it from transmitting AM, but no ham messed with it before I bought the GRC-122 shelter complete with all the installed equipment directly from a surplus dealer’s lot about 20 years ago. Everything in the shelter is still installed as I got it, and I operate the radio in there periodically. I never tried to use it on AM before the last few days, and I never had issues with it in sporadic use on SSB and CW.

Any input is appreciated.

Thanks,

73, Meir WF2U
Landrum, SC
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