MCS2000 Model III VCO Adjustment

[kj6ko]

OK, I've been told NO NO NO it can't be done, but what can I say...I'M stubborn! I have a 900 Model III that won't RX below 927.3-927.4 depending on temperature. Sounds like VCO to me. The two VCO's on the bottom of the board look like they are tuneable via a laser etched tab, similar in the way a 900 Maxtrac is, but I'm told they are not??? Why? Why can't I add a little "material" to the tuning tab to lower the band edge to 927.000? The only thing is, where is the ref voltage to measure and which VCO is TA and which is TX/RX? One VCO has a flat "tab" similar to Maxtrac 900 tuning, the other has a "strip" that can be shortened or material added.
It is impossible to work on the VCO's while the unit is on since the board must be completely removed to work on it, so you will have to make a change, reassemble and test. But, what to test? Vref test point??? One VCO has a flat "tab" similar to Maxtrac 900 tuning, the other has a "strip" that can be shortened or material added.
I am curious as to why I have been told attempting this will destroy the radio?? I'm sure someone tried and killed it, but I wonder if they were working on the correct part of the board since they are completely hidden unless the radio is completely disassembled. Honestly, coming from someone that has built, tuned and tested MANY strip line tuned VCO's and microwave amplifiers, it doesn't look that hard!

Any help?...encouragement.....discouragement....steering voltage test point.....a really good explanation why not......someone that destroyed one and what they did to do it???


Thanks

[motorola_otaku]

Just to clarify, by "won't receive" do you mean the receive itself is deaf or nonexistent, or are you getting FAIL 001?

Retuning the VCO won't help deaf receive, even if it is possible. That's a "fault" of the receive preselector.

Also, I've played with a couple of 900 MCSes, and both of them not only received all of 927 MHz, but would also do 919 MHz. Maybe you're just unlucky? I did notice sometimes that I would get FAIL 001 when it got very cold (like been-in-the-fridge cold) but it always locked back up upon reaching room temperature.

[kj6ko]

It was the VCO. I never got a fail-001, but the frequency cutoff from RX vs NO RX wandered around depending on temperature. About 1 out of 20 MCS's won't RX all the way down into the 927 HAM band.
I was able to move the vce using similar techniques as with a Maxtrac. Info and pictures at http://users.innercite.com/kj6ko/page14.html