Can't Align UHF M120's Oscillator (Warp)

[KE4NYVJR]

I have a UHF M120 that was being used in a club repeater as the transmitter. It was running fine for about a year. I pulled it out to replace the controller, but when I went to key it up for a quick PM, the frequency was a good 40 kHz off. I attempted to "warp" the oscillator, but no matter what value I program, it won't budge. The test frequency is 452.945 and I see no power out, but if I tune my service monitor to about 452.905, I can see it.

Also, I tried to test receive, to see if it was any good as the receiver and it's just as far off, if not worse. I kind of assumed it would since I believe the oscillator is shared between the TX and RX sections.

Has anyone else seen this? Is this a logic problem or is my oscillator section toast?

Thanks!

[RFguy]

Inter-board connector. A common fault.

Take the boards apart, clean the inter-board pins and sockets and treat them with a suitable contact conditioner (Motorola used to recommend a product called Stabilant 22). You should find it comes back on frequency.

[KE4NYVJR]

Thanks! I'll give that a shot in the morning.

[Will]

Inter-board connector. A common fault.

Take the boards apart, clean the inter-board pins and sockets and treat them with a suitable contact conditioner (Motorola used to recommend a product called Stabilant 22). You should find it comes back on frequency.

Exactly, I use Craig Pro Gold applied with a Q Tip. Be very careful the feed thru caps on the pins break.

[KE4NYVJR]

Thanks for all of the tip! Going to work on it this morning. We have a launch coming up in a couple of days, so we can't really do any PMs or config changes until after. Leaves us with plenty of free time

[KE4NYVJR]

Pulled the boards on both sides. Cleaned the pin headers and the sockets on each board. Put it all back together and now I am about 27 kHz off frequency and osc warp still has no affect. Where is the VCO crystal located? I see one 49 style crystal that was under the pop on shield (I think this is the VCO section). I noticed that it was glued to the board around the base of it. When I move it, I don't see any change. Not sure if I am looking in the right place. I have no diagrams or schematics to go by, so I am flying blind on this one.

I was a Moto portable/site tech for five years. Didn't do a lot of mobile work (That's my excuse anyways!)

[Will]

Sounds like the feed thru header connector/capacitor assembly IS bad.

[KE4NYVJR]

Sounds like the feed thru header connector/capacitor assembly IS bad.

This morning I took out the old header. It IS the old part number and so far the only places I can find the new part wants about $35-37 for it. I have a GM300 that is working fine, so I decided to try a little swaptronics using the header out of it. The problem didn't go away. Out of fear that I damaged it pulling it out of the GM300, I put it back in the GM300 and the GM300 was working fine, so I think the header from the GM300 is working ok. So I'm back to square one and out of ideas.

Thanks for the tips and ideas so far!

[Will]

You can measure the Reference Frequency control voltage, pin 9, from the micro processor and set it at half scale in the Service section in the software. Then tweek the coil, L 151, for the Reference Crystal to 14.4000 mHz.

[Satelite]

Hello:
Im wondering if he might have a defective oscillator crystal Y52.
Yes id agree it could be Y151 14.4 reference crystal causing this but correct me if im wrong couldn't the Y52 44.645 2nd oscillator crystal cause what he is experiencing ?
That crystal has a huge known failure here anyway.
Don't want to step on any toes so im asking to verify my thinking and really do want to know if it could be at fault or if not why in this case its eliminated as a suspect.
For my knowledge base as well for better understanding too.
Also if your 40 kHz off on freq try reprogramming the radio with a freq entery 40 kHz off as you stated and see if by chance it shows up on your service monitor at the correct tx freq your looking for.
Its not my recommendation to leave it that way as itll show up when read via rss program wrong as 40 kHz off but should give you an idea if its one of the above mentioned crystals.
Hope im not steering you wrong so use this as info/testing till somebody confirms this.
Thank You
Satelite

[RFguy]

Hello:
Im wondering if he might have a defective oscillator crystal Y52.
Yes id agree it could be Y151 14.4 reference crystal causing this but correct me if im wrong couldn't the Y52 44.645 2nd oscillator crystal cause what he is experiencing ?

No, the second oscillator crystal is for the RX section only. It doesn't have an effect on the TX (or the first RX mixer) frequency.

[Satelite]

Hello:
Well what the hey.
Learn something everyday.
Too bad its on a old radio though.

Satelite

[JMINN]

The shield for the 16.8 osc coil is bad about growing `whiskers` which will detune the osc. remove the shield and clean out the `whiskers`, spray the inside of the can with clear lacquer and install. The interconnect sockets were also bad about growing `whiskers`(with a magnifying glass they can be seen, also will have a yellow tint on the metal connectors. Motorola use to sell replacement sockets and shield made from a different material that would not grow whiskers. The `whiskers` problem has been around for years. GE master II rx front end cavities were bad about detunning because of this.

Joe