MCS2000 Transmitter Problem

[Glen57]

Gentlemen,

I recently acquired a used MCS2000, 900 MHz, 30W mobile radio "for cheap" and have noticed that there is a significant difference in the transmitter output power between the first three "CSQ" frequencies and the last three "CSQ" frequencies. The first three frequencies vary from 896-902 MHz. The last three frequencies vary from 935-940 MHz. The output power of 1CSQ-3CSQ varies from 14.5-18.5 watts. The output power of 4CSQ-6CSQ varies from 26-24watts. I've not found a similar problem described in this forum, but have only been looking for a few days. I have a service manual for the controller/control head circuits (Vol I), but nothing for the receiver/transmitter section (Vol II). Neither am I presently set up for checking the radio's programming. Does anybody have an idea about what could be causing this problem? Thank you in advance.

Glen

 

[Skypilot007]

Sounds like the "for cheap" radio may be a dud. It happens. I've been lucky so far with these radios. Of the 4 I've purchased 3 were good with 25-30 watts out across the ham portion of the band and 1 with 6 watts out. All had good receive. Testing with a Aeroflex IFR-1200S.

 

[Glen57]

Skypilot,

I'm not surprised that the radio requires maintenance. I'm thinking these MCS2000s are split band devices due to the 33 MHz difference between 3CSQ and 4CSQ frequencies. I haven't worked on radios since the early 80's, so I'm quite rusty. I'm hoping Vol II of the service manual will point me in the direction of the synthesizer, power control, or an LP filter circuit, but am open to any suggestions. Thanks for telling me about the Aeroflex service monitor, as that is something else I must investigate.

Glen

 

[cmdrwill]

Did you LOOK at the TX power in the programming and the TX values in the Tuner?

 

[Glen57]

cmdrwill,

I haven't the CPS or Tuner software for this radio, but do have a Radio Interface Box and the associated test cable. Neither do I presently have Vol II of the service manual. What I have is a microphone, a control head with cabling (apparently the radio's transceiver was previously mounted in the trunk of a public/utility vehicle), another cable that contains an ignition wire, a quarter-wave antenna with RG-59 cable and mini-UHF/PL259 connectors, and a 13.8 volt power supply. Thus far I've acquired a Bird Thruline wattmeter w/800-1000 MHz elements, 20 and 30 dB attentuators, a Sinadder S-101, and an old 100 watt, 50 ohm dummy load.

This is a 30 or 35 watt version of the MCS2000. After connecting the ignition wire, microphone, wattmeter, and antenna I turned up power, discovered that the Option button took me to the CSQ test frequencies, scrolled through the CSQ freqs with the Zone button, and briefly keyed the mic and checked the power output of each frequency. I expected small variations between each frequency and each band, but cannot believe that the radio was intentionally programmed with one band (1-3 CSQ) averaging 16 watts output and the other band (4-6 CSQ) averaging 25 watts output. I may be wrong, of course.

Having gone through most of the free information available on this forum I lean toward the idea that the radio's codeplug received corrupted information from the previous owner's tuning software. Or the information was corrupted by a RIB operating with too low a voltage (mine takes a 9 volt battery or a 12-14 volt DC input). Not having the software or Vol II of the service manual this is speculation of course. Plus there's likely a thousand other things I don't know.

I haven't been seriously involved with electronics bench repair for more than thirty years and these radios are unlike anything that I played with "back in the day." What I'm interested in is whether or not anybody on this forum has come across this problem before, and if so, then what are the possible problem areas? I would like to eventually repair this radio on my own.

 

[Skypilot007]

Since these radios are longer supported by Motorola the programming software is easily found on various other web sites free of charge. The last version released was R02.03.00. A quick Google search will lead you right to it. I would suggest you find that and read the radio and go from there.

 

[cmdrwill]

As Skypilot mentions, READ the radio FIRST and save that to a separate archive so you can always have it to fall back on.

The OP also mentioned RG59 coax cable, but that is NOT what is used on two-way radios. You may be getting errors in your measurements.

 

[petnrdx]

An 8 to 10 watt difference is not much with a 40 mhz freq change.
Could be the radio, could be the meter/cable/load.
In my experience with 800 and 900 radios that are used, its pretty common to find that discrepancy.
You really need to READ the radio with RSS/CPS/Tuner to find out what it is set at, and what the calibration/alignment is. Those OFTEN are all over the place.
Some people have the TX set to 15 watts or so on REPEAT. Keeps the heat down, might even be the licensed allowed ERP.
Who knows.
Then they often set things higher on TALKAROUND.
Maybe the radio is just more efficient at TA than RPT.
LOTS of MCS2000's and other radios that I have worked on exhibit the "issues" you describe.
You pretty much must check the radio into a termination wattmeter made for that freq range or into a service monitor. While doing so, you need to see what the current draw is. Not sure what you are using, but most VSWR and general use wattmeters are really only meant to be within 10 percent or so, and that is using correct setup.
If there is a SIGNIFICANT difference in the current draw per TX watt out, then there might be a problem.
If all is kinda in the 15 percent difference range, then its probably OK.
You need to get that checked out on a proper test set up to be sure.

 

[Glen57]

Since my last post I've acquired the CPS software and other items. Oscillator frequency error is within the +/-150 Hz tolerance. The sliders for adjusting the transmitted power at frequency points F1-F3 originally read 23, 24, 24 respectively. The sliders for adjusting the transmitted power at frequency points F4-F5 read 25, 25, and 26. I adjusted the sliders up and down for each band. Adjusting F4-F5 yielded changes from 12-29.51 watts. Adjusting F1-F3 yields changes from 12-18.63 watts. I had hoped to set power across both frequency bands to 24 watts. The transmitter current softspot was originally set to 19. Moving the slider up and down yielded changes in transmitter output power, but did nothing to eliminate the power difference between the split bands. The test and coax cables are good, the RIB is good, and all connectors are good. Have acquired Vol II of the service manual and have disassembled the radio looking for obvious problems. None were spotted other than antenna output connector, which I changed. The PIN diodes were good. The hybrid board connections to the mainboard are good. I believe the problem I'm describing is fairly common and cannot be disappeared through simple software adjustments. Thanks for all the suggestions. I'll buy another radio.