Batboard

I have a VHF MT2000 with a faulty RF board (RX is 5kHz high, TX is spot on...IF/Mixer fault)

I am going to use an RF board from another "identical"Jedi radio which is a European GP900 which looks like a HT1000 but does 5 Tone signalling.

I have noted down all the Alignment softpots which generally match BUT the GP900 does NOT have any TX or RX VCO Crossover data.

The logic board from the MT2000 will be looking for VCO Crossover data so I will use the original values (TX 161.5050 and RX 162.1750) Will this effect anything using the GP900 RF board? Presumably the GP900 uses some fixed VCO Crossover values written in firmware???

Does the MT2000 VCO Crossover values vary from radio to radio or is it generally a fixed value?

The only other difference in the softpots is the RX Rated Audio test frequency..the MT2000 uses 168.1250 Mhz and the GP900 uses 136.0250 MHz...but both values are almost identical - 201 vs 203

Also the MT2000 only uses 20KHz and 12.5Khz TX Deviation Limits Refs while the GP900 also has a 25/30khz softpot...but the 20/12.5Khz values are identical - 114 and 87.

Hope someone can help.

Thanks

I did some more searching of the archives and it seems that the TX VCO Crossover value is a function of the RF board ID which is linked to the year of manufacture.

Early VHF boards use a TX VCO Crossover value of 161.5050 and later boards use 164.8500 MHz. The service bulletin referring to the change is dated 15 Feb 1996. I don't know what the newer RX VCO Crossover value is..the early value from my MT2000 is 162.1750 MHz

My MT2000 was made in October 1993 and my GP900 was made in November 1995..so hopefully they both use the same early VCO Crossover values.

The service manual seems to imply that the VCO Crossover value is an incremental type of softpot...like the front end and SQ tuning where you make numerous small changes. BUT reading the archives it seems that there is only 2 values to select?? Either 161.5050 or 164.8500 MHz. Is this correct? If this is the case, then maybe that is why the GP900 service alignment does not include any VCO Crossover data..but what would happen if you replaced the RF board with a later version??

Well that was a complete waste of time!!!!!!!!!!!

The MT2000 RF board is connected to the control board with a soldered flex cable...whereas the GP900 RF board is connected with the usual push in flex cable.

I have only ever previously seen the push in flex cables..so the soldered type must have been on early models!!

A big disappointment all round. I guess I could carefully solder the replacement RF board to the control board by modding the push in flex??

Are there any bandwidth differences between the GP900 and the MT2000?

There are a few different VCO changeover frequencies that have values below your stated values, and I am wondering the actual range you require for proper operation.

Changing the flex to a pin configuration should be simple, but you may need to update the flex to make use of the pin jumper mod, as well as the socket on the RF board. Fine point soldering will be required, as the pin spacing is under .1", so you will need a fine tipped iron, and use a good, well lit magnifying lens to aid socket replacement.
I wouldn't go higher than a 25 watt iron, to prevent trace lifting.
I would also use a good desoldering pump, either hand vacuum, or the preferred electric type. You get a much cleaner surface, and it makes final cleanup a breeze when remounting a new socket. You can use a minute drop of superglue on the socket once you locate it on the board, so when you begin soldering the socket, it doesn't walk around, shorting pins as you go.

Look at the date on the RF board, the problem is locating where the data was printed. I have 4 boards here, and 3 are printed in a few different locations. Most should be on the top right side, below the RF switch assy. though, and near the outer edge, near a metal shield. But, YMMV here, of course.

AEC wrote:
Changing the flex to a pin configuration should be simple, but you may need to update the flex to make use of the pin jumper mod, as well as the socket on the RF board.

Back in the early 90's I had a friend do this for me. He worked at the factory in Plantation, & actually built Jedi's. All he used was a small soldering iron, & it worked fine. GARY

Bandwidth options are the same - 25/20/12.5 KHz

The radios are VHF with a usual range of 136 - 175 MHz, I think.

Is there any concept of "tweaking" the VCO Crossover value or is it just a matter of inputting the correct "default" value?

Yes, I read that there is a mod kit to update the flex on earlier models to the push in type..although I don't know whether its still available as a spare?

The date on the MT2000 RF board was October 1993 which matched the serial number manufacturing date.

20 years old..but the front end sensitivity is better than most of my modern VHF stuff. Sensitivity is also consistent between different VHF Jedi's sets. BUT my UHF Jedi's are a different story..a very wide difference in sensitivity so you might end up with a deaf one!!

20 years old..but the front end sensitivity is better than most of my modern VHF stuff. Sensitivity is also consistent between different VHF Jedi's sets. BUT my UHF Jedi's are a different story..a very wide difference in sensitivity so you might end up with a deaf one!!
g8tzl2004 Posts: 660Joined: Wed Sep 08, 2004 1:13 pm


This is a problem with converted radios, or more accurately, from say, a 450-520 range, to the 403-470 split, and many were never realigned to bring the performance in line with the change, as in front end filters, VCO changes, and squelch settings.
Even deviation levels change when the band ranges are altered, so a complete alignment is required.
This also includes the PA current draw, battery adjustment settings will affect battery life since changing the bandsplit also changes the current draw while the radio is operating in a different bandsplit, but with the same components designed for a higher (or lower) factory designed range.

My UHF Jedi's are all genuine unmodified sets and there is a big variation in sensitivity between otherwise identical radios..realignment makes no difference...in fact front end alignment is very broad with no obvious peak. I've plotted some of the Motorola values and it looks like they just input values which represent a straight line on an X Y graph..so I guess they just align the center freq and then apply adjusted default values. Careful realignment results in a slightly non-linear X Y graph but very little improvement in sensitivity. So I guess the variation is to do with component tolerances?

Reading the archives there are comments from Plantation workers that Jedi sensitivity can vary from 0.38 to 0.2uV ie about a 6db difference but still IN SPEC ..so if you get a 0.2uV radio you think the Jedi is the best ever but if you get a 0.38uV radio then you think it sucks!! Others also comment about sensitivity variations in Sabers and XTS's. Thankfully my XTS3000 is hot but others are deaf....but all are still within "spec". I think Motorola should tighten up the spec so that all radios met 0.2uV for 12dB SINAD or better. Having a "better than 0.35uV" spec allows substandard radios to be passed as good!!!!!!!!!

The OP asks about VCO crossover. In the 900 radio's controller there would not be any crossover setting or even a place for it in the firmware.

As the GP900 and MT2000 RF boards are basicaly identical, why does the GP900 RF board not need any VCO Crossover values?

Does anybody ever slightly tweak the VCO Crossover value to obtain correct operation? It appears that there are only 2 (maybe 3 or 4) default values ever used and these are a function of the year of manufacture of the RF boards - so no incremental tweaking is involved???