Crystals for VHF Low PT-500

[forjack842]

After locating a service manual for a MT-500 I see the MT and PT are very very close in schematics. THis is great! Thanks to all for that lead.
Now I need to verify the PT-500 uses a KNX6194AA Xmit and a KNX6193AA Rec. THe PT has HC49U style crystals. 1 rec and 1 xmit
ICM Part# 167575 and 167573 are the recommended crystalls for the MT-500. Question is that both are special order and probably expensive!
I'm moving the 2 PT-500s I have to 50.4 6mtr band and was wondering if I need the 5ppm or 10ppm called out by Motorola for their freq stability requirements or could a bit less tight crystals.
ICM has a HC49U with +-30ppm for alot less. I see that the Mt500 receiver has 3 crystal filters which should make the receive frequency tight.

Opinions please.
Thanks
Jack KG6INX

[abbylind]

You can take the TCXOs apart and change the Xtals. I used to do that a lot for MT500s and MXs. Ordering the Xtals alone is fairly cheap. $10-15 each if I remember correctly from International Crystal. (That was 20+ years ago)

Fowler

[WB6NVH]

Don't forget that the low band MT500 does not use channel elements, it uses exposed crystals, so he just needs to deal with that issue.

International gets $ 20.95 per crystal at the moment, based on a shipment received yesterday. Bomar seems to be cheaper and I have never had problems with their crystals, however they have a minimum order of something like $ 50. Not hard to meet that with two radios to retune, however.

Since the guts of the pack set are just MT500, the crystal types in the MT500 should be the same ones. Every two-way crystal International makes is a custom job. I would avoid some generic crystal even though it just happens to be on the right frequency. It may very well be impossible to warp it on frequency once installed. International should already have the data to make the crystals if you just tell them it's a PT500. In my experience with crystals, you get what you pay for.

Hopefully you do realize that these are not plug-and-play radios, you will need to go though an alignment of transmitter and receiver.
I haven't done one but it's my understanding that the MT500's have some issues as far as being tuned very far above 50 MHz. I don't know what mods are necessary, I have seen discussions of coil slug changes.

Why 50.4? That is not an "FM frequency" so to speak, and the only person you will be talking to is the other pack set, but maybe that's the idea. I normally put things on 52.525.

If these pack sets were other than low band, the consensus would probably be that they aren't worth the cost of crystals in this day and age. But being that there are no programmable low band pack sets (as far as I know) it's easier to justify the expense, especially where one channel is sufficient.

[Tom in D.C.]

Actually, 50.3 to 50.6 is listed for "all modes" in my 2006-2007 Repeater Directory.

I built (with help) two mint MT500s a few years ago, using Bomar crystals. To keep
the frequency excursion as low as possible we set them up on 50.525, not 52.525, again
in that 300 kHz "window" that's in the band plan for six meters. The radios are great to
work on, with wonderfully easy crystal access. We were able to crank the transmitters
on both units up to 8 watts, and could have gone higher, but we turned them 'way down
finally because they were intended for short-distance simplex use. Transmit tuning is
easy, but I had to get help to get the receivers tuned properly.

[WB6NVH]

I wasn't implying that there was some prohibition on using 50.4 with FM, just that the only person you'll be talking to is yourself or the other radio you tuned up on that frequency. It's the national AM calling frequency as well, although these days there isn't much AM activity there, if any.

It's hard even finding people on 52.525 out where I am.

[forjack842]

Hi all,
First the 50.4 selection was to be for local communications with another pt500 and 5 Czech RF-10's. Local group exploring with the PT's as base 1 & 2 and the RF-10 as manpacks. I thought of using the calling freq but then if were were rag chewing all day someone is going to get a bit upset. THere's only one freq in the PT's so no other options. I decided to move to the general use freq. Also the 50.4 is a slight hedge above the 50Mhz, the stated top end. Probably jus the end of the band....I believe the coils should accomodate this tweek (hope hope hope).

Yes, the PTs are VHF Low band and have the exposed crystals. I didn't see any pic of the MT-500 xtals in the manual and their reference numbers sent me to a page at ICM that flagged them as special order - also no pic. So the main reason for the initial question.

I've been outfitting all with the new LI-ION 3.7V 2.5A batts dropping the net weight of all radios a bunch just in batt weight alone. The 18650 batts are amazing! I put 4 in serial/parallal for the RF-10s giving 7.4@ 5a and I plan on running the PTs on 5x3.7 or about 18.5vdc @ 2.5A. Since there was 11 1.5vdc batts equaling 16.5vdc and there's a 7.5 regulator I'll have to do a little review of the power needs of the regulator/heat but there should be no problems.

I've got a HP8921A I plan on using for alignments on these PTs. The RF-10's tuned up real nice of the set. Excellent radio tool for all uses. Never regretted the grab off Ebay.

I was very surprised on the huge improvement between the 500's and the PT400/300s. I've got about 10 or 15 of these older 300/400 151Mhz puppies sitting here waiting for bench time. Everytime I open them up YIKES! What a Hodge Podge! Back they go under the bench!

I'll take a look at Bomar and International. I'm not hard over on resonable cost. I was taken back a bit at "special order". That usually has the "No Way Jose!" wallet dash. If reasonable I'll give one of them the order. I appreciate the feedback on the crystals. Have you used the 10ppm or is 30 decent in these radios. Since the xmit crystal is FCx3 I would guess 30ppm should be well within the limits. Opinions.

I also noticed that the current xmit xtal was 16066.66 x 3 for a 48199.98 FC for a 48.2Mhz actual FC on the radio stamp. I would guess this is to allow headroom for warping upwards. SO is this a rule of thumb to set the xtal fund = (FC / 3) - 20 Hz in the 40-50Mhz range?

Thansk
Jack KG6INX

[Tom in D.C.]

Jack,

That's all good and very interesting stuff about your project. The lithium-ion batteries sure do beat nicads and NiMH in many ways. About the crystals, as I recall they go in the MT500s with fine wire leads. As to frequency tolerance, what you've read about many zeroes to the right of the decimal point applies to commercially-used units; you definitely don't need that kind of high-spec crystals for hamming and you'll save a lot of money as well. Please let us all know how the PTs turn out.

Regards,