PRC 6

[W1ZFB]

I have a working PRC 6 and it has the crystal for 51.00 Mhz installed.  I was checking the frequency after got the power supply for it and found that it to be 51.020 both RX & TX. I've looked at the schematic which shows the oscillator section with the crystal. There is no adjustment that I can tell to bring it back to 51.00mhz. I was wondering if anyone here has any experience with this radio. I realize its FM but I thought someone might be able to help.
Thanks
Mike 

[Pete, WA2CWA]

Google says:
How do you lower crystal frequency?
Adding capacitance across a crystal causes the (parallel) resonant frequency to decrease. Adding inductance across a crystal causes the (parallel) resonant frequency to increase. These effects can be used to adjust the frequency at which a crystal oscillates.

Most FM activity on 6 meters generally is above 52 MHz.

[W1ZFB]

Thanks Pete! yea I know the simplex 6 meter frequency is 52.525 but the military guys use this 51 mhz frequency when they have their field meets.
Mike

[KA3EKH]

The PRC-6 is a second generation tactical FM radio, things like SCR-300 and other WW2 sets were first generation sets, the PRC-6,10 and all the RT68 type stuff built in the late forties and early fifties were second generation sets and the VRC-12/PRC-25/77 sets are third generation radios built in the sixties and seventies.
First generation sets were super wide deviation, 60 or 75 KHz and had no real channel structure, the receivers were super wide too. Second generation sets like the PRC-6 and PRC-10 were anything from 20 to 30 KHz and did not operate on fixed channels, its not until you get to third generation sets like the VRC-12 or PRC-25 that they operated in 25 KHz channels and were down to about 15 KHz deviation.
As long as you are netting with other military sets don’t think that 20 KHz offset will make any difference until you try to net with a newer SINCGARS radios like a PRC-1439/VRC-119, and being the PRC-6 has no 150 Hz tone that limits it to working with mostly older first and second generation sets.
Modern “Ham” handhelds or radios all are built around 5 KHz plans so you can hear a modern Yaesu or Kenwood but it will be week and your deviation will be so wide unless you whisper they won’t hear you or at least understand what you are saying
That being said there is lots of fun that you can have with the radio talking with other military sets at military vehicle shows or at Hamfest like the Dayton Hamvention where we have a net on 51.0 on Saturday. Several people have operated PRC-6 radios at that event. A PRC-10 backpack radio would be a fun radio to pair with the PRC-6 and it won’t need any crystals! The RT-68/VRC-10 is a vehicle mounted set that I have seen sell cheap at shows and it will net really well with that radio.

[W1ZFB]

Okay on the Deviation I never really thought much about trying to listen to a 5kc signal over a 15 or more kc wide receiver or vise versa.  It makes sense to me now. We don't to my knowledge have any military events around the Fredericksburg area other than Civil War stuff and they didn't radio's then!!  I was for first time at Dayton this year and I did check in on the 51.00mhz frequency but I was using my Icom T90 as I just bought the PRC 6 and had no way to power it up.
I'll have to see if the Tank Farm up in Nokesville Va, has any PRC 6's that are operational. I know they have some BC 611's.
Mike

[KA3EKH]

Blue & Gray Military vehicle show in Hagerstown Maryland, I always set up there with my M151 and operate from that location. Hope to be at Tank Farm this year with the MUTT and the Vietnam Era road show and will have several radios there on 51.0 Usually the Gilbert MRCA show is on the same weekend so cant make Tank Farm but this year the schedule is good for both.

[Opcom]

Some low band VHF military radio comments:

RT-68 - 100KHz spacing*. using VFO. 38-54.9MHz. 30KHz deviation.
PRC-6 - 200KHz spacing*. 47-55.4MHz. 15-25KHz deviation.
PRC-6T - (modernized export versions also PRC-6/180) - 50KHz spacing. 47-55.4MHz. 10.5KHz deviation.
CPRC-26 -  200KHz spacing*. 47-51.4MHz. 15KHz deviation.
PRC-25 - 50KHz spacing. 15KHz deviation.
PRC-77 - 50KHz spacing. 15KHz deviation. (A later optional mod for 25KHz did not affect deviation.)
RT-246 - 50KHz spacing. 15KHz deviation.
RT-524 - 50KHz spacing. 15KHz deviation.
Racal TRA-967 - 36-75.975MHz. 25KHz spacing. 5KHz deviation.
RT-1439 - 25Khz spacing.

In addition to the deviation differences as discussed in the above posts, there is a 150Hz tone squelch on many of these, and this is a common cause of the narrow set not un-squelcing the wide set, and of older non-tone sets failing to be heard. On some tone-squelch sets, there is a carrier squelch provision with a selector switch or momentary switch, and some of those with the selector switch have an adjustable squelch level.

In any case, some of the radios are easier than others to modify for 'old' or carrier squelch.

In case of a narrow set wanting to be used with wider sets, it may be possible to increase the 150Hz tone level (deviation) to the point where it triggers the wider set's tone squelch recieve function, though it will then be clearly heard in the transmission to other narrowband sets. In several of the more modern of above radios, both the deviation of the tone, and its receieve sensitivity level, are set separately from the voice.

*spacing has to do with selectivity/interference on these crystal or VFO tuned units and has more meaning when several units are close physically and frequency-wise, as opposed to channelized frequencies offered in the specifications.