VHF Maxtrac off freq. by 8 KHz

[hwstar]

Hi

I went to use my VHF Maxtrac and I noticed that I could not bring up any local repeaters, and the receive audio was severely distorted. After looking at the TX with a frequency counter, It was 8KHz too low!. This happened quite suddenly. I don't yet possess a service manual, so I though I'd put the question to this group. Before going through the RSS tuneup, is there anything I should look at with instrumentation first?

Steve
WA6ZFT.

[kc7gr]

Hi

I went to use my VHF Maxtrac and I noticed that I could not bring up any local repeaters, and the receive audio was severely distorted. After looking at the TX with a frequency counter, It was 8KHz too low!. This happened quite suddenly. I don't yet possess a service manual, so I though I'd put the question to this group. Before going through the RSS tuneup, is there anything I should look at with instrumentation first?

Steve
WA6ZFT.

The kicker here is that the alignment procedure in the RSS MUST be followed in exact sequence, with no omitted steps, and the procedure requires a lot more than a frequency counter. You will, in fact, require the capabilities of a service monitor (or, at the very least, a terminating wattmeter or thruline wattmeter and dummy load, a modulation monitor, and a spectrum 'scope to make sure you're not putting out anything other than an on-frequency signal).

The reference oscillator setting is purely RSS-driven. There is, as I recall, a warp coil, but I seem to recall that the service manual states not to touch it unless the reference oscillator itself has been repaired or replaced.

Good luck.

[hwstar]

The service monitor requirement isn't a problem, I have all the requisite equipment. What I'm really after is the fact the the radio went 8KHz off quite suddenly. This is usually indicative of a hardware failure of some sort. Before I go through the tuneup procedure I thought it would be prudent to see if anyone else has experienced the same issue, and if there's anything which needs to be looked at.

Steve
WA6ZFT

[n5tbu]

More than likely a poor connection at the interconnect pin connector,usual with a rotomola. Clean the pins and try again. There is a moto replacement pin and socket assembly..but it is almost as intermittant.
mod

[Nand]

I found that failed 14.4 MHz reference oscillator xtals were often the problem. Either a poor solder connection or a loose pin in the actual xtal holder. I believe Monty said that he had a working radio that was off frequency because one pin was actually broken.

Another item that affects the reference frequency is the 9.6 volt regulator. This is also the reason why the board interconnect causes problems at times, since the 9.6 volts is supplied though this connector at pin 1. The electronic warping signal also goes thought this connector as well.

While the calibration is done in sequence, you can go thought the reference xtal calibration procedure without doing the previous adjustments. All you need to do is select each previous adjustment and exit it without making any changes. This will automatically advance to the next adjustment. When you reach the reference oscillator adjustment, you enter the board and xtal data as found on both labels and also enter the measured 9.6x volts measured at pin 1.
I don’t believe this will be the solution to your problem since such a sudden change points to a component or connection failure.

Searching this board should give you many more items dealing with this problem.

Nand.

[jackhackett]

Unless you've messed with the warp settings, or opened up the radio and messed with the reference oscillator coil, anything much over 1KHz off frequency generally indicates a hardware problem, and a sudden 8KHz shift would definitely be a hardware failure. I think Nand covered most of the usual culprits, I'll just elaborate on the RSS tuning.

There are two different things that deal with frequency in the RSS, warp adjustment, and RF board replacement procedure.
Warp just adjusts the reference frequency, during normal aging of a radio it can shift off freq a small amount, usually less than 1KHz, the warp adjustment should take care of that sort of thing.

The RF board replacement procedure is used to set the temperature compensation, basically when they make the RF board, they test it and mark it with some numbers that depend on how the reference oscillator components react to temperature changes. The 14.4MHz crystals are also tested and marked with similar numbers. These numbers are entered into the RSS along with a voltage reading, and used by the radio's microprocessor to cancel out effects of temperature.
This procedure should only need to be done if either the RF board, the crystal, and maybe the logic board, has been replaced. Several points, never adjust the Ref. Osc. coil. You'll throw off the temperature compensation. Also, the numbers on the RF board are usually handwritten on a little sticker, they can be hard to read, or could be missing altogether. If you have to change the crystal and don't have the board numbers, I think you're pretty much S.O.L.

Try cleaning that connector, if that turns out to be the problem, you'll probably just have to tweak the warp a little, if it turns out you have to replace the crystal you'll have to do the board replacement stuff, and then the warp.

[Will]

One other possibility.. a bad code plug or one forced in from another radio would contain the refference oscillator compensation values as Jack reffered to. The incorrect values would throw the radio out in left feild.


When you clean the interconnect pins, use a cleaner specially made for that, I use Craig Pro Gold contact cleaner/protectent.

DO NOT use WD40 (or simmular corrosive soulutions) or you will have to change the pins, and both sockets on the two boards, not fun. And you thaught it was off freq. before!

Pins can cause noisey VCO, poor or low modulation, or low receive volume.

[jackhackett]

One other thing I just thought of, I've come across a couple of maxtracs with a broken contact on the female part of the interconnect. There was a hairline crack where the contact bends over the top, very hard to see unless you're looking for it.

[hwstar]

Thanks for all the specific and elaborate replies. This was exactly the kind of information I was looking for. I''m going to have a look at that connector first....

Steve
WA6ZFT

[hwstar]

Desoldering, removing, re-inserting, and resoldering the crystal put it back on channel. I'm still skeptical that this has been fixed. I'll have to watch it and see.

Steve
WA6ZFT

[kcbooboo]

Since you had to remove the logic board as well, I'd guess that the contacts between it and the RF board were dirty too, so you managed to "fix" two problems with one stone.

Bob M.

[wavetar]

Desoldering, removing, re-inserting, and resoldering the crystal put it back on channel. I'm still skeptical that this has been fixed. I'll have to watch it and see.

Steve
WA6ZFT

More than likely, the act of removing the RF board & re-assembling it after working on the crystal is what fixed your problem. Any oxidation/dirt on the interconnect pins could have been scraped clean just where it needed to be. Did you clean them while you had the radio apart? If not, you might need to take it apart sooner than later.

Todd

[jefflaur1]

Okay..my Desktrac was off frequency, and to get it spot on, it took a warp adjustment all the way to 148. (roughly 75% to the left on the slider in RSS) but it is all good now. All repeaters sound great and my transmissions are getting good reports. It is an old radio, but this fixed it for now. Is this too much of an internal WARP adjustment.. or is it okay to do this?... I will go and clean the pins, and reset it back to 101 if that's a better way to go.

[wavetar]

Okay..my Desktrac was off frequency, and to get it spot on, it took a warp adjustment all the way to 148. (roughly 75% to the left on the slider in RSS) but it is all good now. All repeaters sound great and my transmissions are getting good reports. It is an old radio, but this fixed it for now. Is this too much of an internal WARP adjustment.. or is it okay to do this?... I will go and clean the pins, and reset it back to 101 if that's a better way to go.

Resurrecting an 8 year old post, that might be a record. You didn't say how far off the frequency was, or when the last time the Desktrac alignment was checked. If it's been many years, the radio could very well have drifted a significant amount. Assuming the radio hadn't been messed with (LAB, codeplug forcing, etc), if the warp adjustment is able to bring it within spec and the radio remains stable afterwards, then it's probably just fine. Usually if it's dirty pins, the frequency error will still vary +/- a fair bit on every transmission, even after warp adjustment.