Retune out-of-tune Pro-95 scanner? (off by .0125mHz)

[jaytango]

I've had my Pro-95 for 3 years now and have used it daily in my car, non-stop. Recently, my trunked banks stopped working. The scanner would not stop on any freq.... just scanned continuously. After switching to manual on each freq I noticed the there WERE transmissions, but very garbled. After much thought and many days of troubleshooting I decided to run the search feature between 850mHz and 860mHz... To my surprise, the scanner began stopping on freqs, with voices coming in loud and clear. After analyzing the freqs and writing them down (including the control freqs, which I've come to recognize their tones through the years) I realized that ALL the freqs which were being stopped on were EXACTLY .0125mHz (12.5kHz) higher than the normal ones. For example if a control channel was previously 858.9375, my scanner now heard it on 858.95. Once I realized this, I ran a little test and re-programmed all the voice channels (untrunked) into a bank and lo and behold, they all worked... at +.-0125. So I decided, hey, maybe if I reprogram my 4 control channels at +.0125 then I'll be back in business... Well, no go. The scanner DID trunk successfully at that point (it DID resolve the control freqs to voice freqs and the IDs began to show up) but the freqs they were resolving to were the original ones (which I now have to add .0125 in order to hear them).

So my question is...is there any way I can retune this band DOWN .0125mHz so I can be re-calibrated? Maybe like one of the small pots inside the unit? Any advice will be much appreciated. Thanks very much.

 

[DonS]

Except for the specific frequencies you listed (858.xxx), that sounds suspiciously like a Motorola 800MHz trunked system that has been 'rebanded'.

What system is this (state, county, "system name", etc.)?

 

[jaytango]

This is for Orange and Osceola Counties, Florida.

Motorola II Smartzone/Motorola II Smartnet respectively.

 

[fineshot1]

Except for the specific frequencies you listed (858.xxx), that sounds suspiciously like a Motorola 800MHz trunked system that has been 'rebanded'.

What system is this (state, county, "system name", etc.)?

Rebanded 800 Mhz systems are moved 15 Mhz down - not 12.5Khz down
so that is not the case here.

Clearly the scanner needs to be shipped back to RS's Ft. Worth repair
center and properly repaired by a tech familar with the RS product line.

 

[DonS]

Rebanded 800 Mhz systems are moved 15 Mhz down - not 12.5Khz down
so that is not the case here.

A rebanded Mot 800MHz system can be moved to any frequencies outside of the "reallocated" area, as long as there is a license for those destination frequencies. There's nothing magic about 15MHz. (Example: what if there's already another local licensee on such a "minus 15MHz frequency"?)

However, I did express a caveat in my post: Except for the specific frequencies listed. I said that because such a rebanded system probably wouldn't get a new frequency like 858.950MHz.

Clearly the scanner needs to be shipped back to RS's Ft. Worth repair
center and properly repaired by a tech familar with the RS product line.

It's extremely unlikely that a PRO-95 would be off by exactly 12.5kHz in only one part of its frequency range. There's nothing in the scanner to "repair" or "adjust" that would fix a 12.5kHz error that only appears in the 800MHz range.

Given the frequencies listed by the O.P., it's far more likely that the system is now using a different set of "trunking tables". If so, and if those tables are not part of the PRO-95's firmware, the O.P. is out of luck with that scanner for that system.

This is why I asked for the specific system & location. It's possible that the system, for some reason, is now using "splinter" tables (for the same "Motorola channel numbers", those would move by exactly 12.5kHz, and the frequencies described by the O.P. do "match up" to normal vs. splinter tables, though the up vs. down part is backwards).

 

[n1das]

You'll never notice a difference if your scanner is off-frequency by 0.0125 mHz.

0.0125 mHz = 12.5 uHz = 0.0000125 Hz

 

[DonS]

While you're at it, you may as well comment on "Mhz" and "Khz" (what's a "kelvin hour zepto"?)

 

[n1das]

It's extremely unlikely that a PRO-95 would be off by exactly 12.5kHz in only one part of its frequency range. There's nothing in the scanner to "repair" or "adjust" that would fix a 12.5kHz error that only appears in the 800MHz range.

An aging crystal in the reference oscillator in the PLL circuitry can cause this type of error. I've fixed this exact same problem before in an old RS Pro-2004 scanner around 20 years ago. I've also seen in it in some multiband ham transceivers due to the radio using a common PLL synthesizer system for all bands.

It is caused by small errors of a few PPM in the synthesizer's reference oscillator frequency showing up as large errors (a few kHz) in the final local oscillator frequency due to the number of times the synthesizer's frequency is multiplied up to to produce the final LO frequency for the 800MHz band.

There may be small trimmer capacitor in the PLL reference oscillator circuitry to trim the oscillator frequency. Giving this a slight tweak may bring it back in.

Good luck.

 

[jaytango]

DonS: I'm not certain if the 12.5kHz deviation is ONLY in the 800mHz band. The only other band I usually scan is airband, and being that THAT band is AM and I can actually pick up transmissions +/- one step either way I may have only noticed it in the 800mHz band. I guess a good test may be to check out the 70cm band and compare the values with my local repeater freqs. Perhaps that will show some deviation as well.

Also, I don't think the trunking tables have changed since when I reprogrammed the control freqs to +.0125 (so the scanner can actually "hear" them) the scanner DID trunk and every single station it stopped on is listed on the charts here on radioreference. It's just that when it stops on a freq, it's crackly and almost out of range.

n1das: What you mentioned is kind of what I wanted to try, though I will need some kind of schematic to know what pot to adjust. I'm not an electrical engineer but I would assume that each band might have it's own circuit and variable capacitor or something in order to capture such a wide range of frequencies.

Maybe instead of messing with the pots I could just replace the crystal. Maybe the Florida heat just took a toll on it.

P.S.- sorry my replies are taking so long. I'm a new member and my 1st three posts must be approved by a mod.

 

[DonS]

An aging crystal in the reference oscillator in the PLL circuitry can cause this type of error. I've fixed this exact same problem before in an old RS Pro-2004 scanner around 20 years ago. I've also seen in it in some multiband ham transceivers due to the radio using a common PLL synthesizer system for all bands.

It is caused by small errors of a few PPM in the synthesizer's reference oscillator frequency showing up as large errors (a few kHz) in the final local oscillator frequency due to the number of times the synthesizer's frequency is multiplied up to to produce the final LO frequency for the 800MHz band.

There may be small trimmer capacitor in the PLL reference oscillator circuitry to trim the oscillator frequency. Giving this a slight tweak may bring it back in.

Good luck.

IIRC: The PLL's reference oscillator in the PRO-95 is 12.75MHz, and is not adjustable or trimmable; there are two other oscillators in the scanner (380.8MHz and 21.4MHz), but errors in those would have pretty significant effects in other bands, not just 800MHz.

To me, it is especially significant that the O.P. has, so far, referred only to an 800MHz trunked system (Motorola, I presume), and has said that things have moved by "EXACTLY" 12.5kHz.

We really need more info from the O.P.:
* are there noticable "errors" in any other frequency bands or with other systems, or is it just this one band/system?
* what system is this, and where is it?

 

[fineshot1]

We really need more info from the O.P.:
* are there noticable "errors" in any other frequency bands or with other systems, or is it just this one band/system?
* what system is this, and where is it?

Good point - i should have asked for that right from the start.

An aging crystal in the reference oscillator in the PLL circuitry can cause this type of error. I've fixed this exact same problem before in an old RS Pro-2004 scanner around 20 years ago. I've also seen in it in some multiband ham transceivers due to the radio using a common PLL synthesizer system for all bands.

Yes - I too have seen this and sometimes all it takes is someone dropping the radio
and the crystal sustains slight damage but keeps working slightly off its original adjusted
frequency.

 

[DonS]

Good point - i should have asked for that right from the start.

I don't think it would've done much good. I asked for the info 5.6 hours ago; since then, the O.P. has visited this thread at least twice without electing to supply the info.

 

[pro92b]

I doubt that the original poster has the ability to determine that the radio is exactly 12.5 kHz off. The audio evidently sounds better when the radio is tuned two step sizes low, but the IF is wide, +/-10 kHz so there is some uncertainty in how much drift has occurred. It may be possible to better define the offset by entering frequencies in 6.25 kHz steps to see where the audio is best.

There is a trimmer capacitor in the 12.75 MHz oscillator called TC1 and it may be able to pull the oscillator frequency enough to get the radio back on frequency. Not much to lose in trying it at this point.

 

[n1das]

I doubt that the original poster has the ability to determine that the radio is exactly 12.5 kHz off. The audio evidently sounds better when the radio is tuned two step sizes low, but the IF is wide, +/-10 kHz so there is some uncertainty in how much drift has occurred. It may be possible to better define the offset by entering frequencies in 6.25 kHz steps to see where the audio is best.

There is a trimmer capacitor in the 12.75 MHz oscillator called TC1 and it may be able to pull the oscillator frequency enough to get the radio back on frequency. Not much to lose in trying it at this point.

Listening to a Moto trunked system control channel would be a good test signal to use. In a simulcast system they are dead-on in frequency and usually are locked together in frequency using a reference signal from a GPS receiver. Listen with the scanner in manual mode and have the control channel programmed as a conventional channel. Then give the reference oscillator frequency a small tweak to try to bring it in.

I've dealt with two versions of a scanner being off-frequency. One was exactly as the OP described where 800MHz appeared to be several kHz off-frequency and the lower bands were all OK. The reference oscillator for the PLL circuitry needed a slight tweak in frequency to bring it back in. The other case was due to a misaligned FM discriminator where frequencies on all bands were off by the same amount and the PLL ref oscillator was OK. It affected the window detector (aka "Zero-matic") circuit and caused the scanner to always stop one freq step away from the correct frequency during searching and stopping on an active frequency.

Good luck.

 

[jaytango]

Guys.... THANK YOU SO MUCH FOR YOUR HELP...

I don't know when this post will get approved (I started posting this morning and have 4 posts still pending) but I want to express my sincere gratitude for all your help.

I did as n1das explained... I opened the scanner, found the metallic box with the snap-on lid and opened it and inside,located the TC1 pot. I tuned the scanner to a control channel 858.9375 (which is currently heard on 858.95) and started turning counter clockwise. All it took was about one-half-millimeter's worth of a turn and VOILA... I heard the sweet sound of a control channel. I quickly put the scanner back together, re-initialized (just in case) re-programmed my 4 control channels, held my breath, pressed scan, and began hearing Osceola's finest coming through the speaker.... loud and clear.

I recommend anyone who has a scanner with similar symptoms to try this fix. It was SOOOO easy and worked so well. My scanner sounds crisper than ever. Too bad I had no patience and ordered a Pro-94 on ebay today.Oh well, now I have one for the house and one for the car. Thanks again guys....

Jay
KJ4OEA

 

[jaytango]

Guys, my posts finally got approved... so if you scroll back up, you'll see them. The thread will make more sense now... I think as a new member only the first 3 posts are moderated, so I should be all set from this point on.

This morning I drove into work in enjoyed listening to all of Osceola AND Orange Counties, so I think I'm good. One thing I thought about is maybe the scanner IS getting old and the drift that occurred - although I was able to fix it - might get progressively worse. I'll post it if that occurs.

 

[jaytango]

Just a follow up...over a month later and the scanner is still working great. Spot on!

 

[Caesar]

i've had to do this to a Pro-95 and Pro-2055, they seem to shift from bumpy riding over time....

 

[grstark]

Thanks for the post!

Thanks for posting the fix. I had the same problem with my Pro-95. I tried your solution and I'm now back in business. Thanks again!

 

[minasha]

Please be advised TC-1 is not a "pot" it is a "Trim Cap" hence a capacitor not a potentiometer.
Be very careful adjusting this, as it is very sensitive compared to a pot, making it harder to adjust.