MCS2000 UHF Jedi - deaf or "within spec"???

[g8tzl2004]

I have recently acquired a UHF MCS2000 Type II 25W radio but find that receive sensitivity is well down on my other radios although maybe just within spec.

Sensitivity is around 116dBm (0.35uV) which is about 5-7dB down on some of my other MCS2000 radios.

In general I find that MCS2000 sensitivity to be excellent. In fact my Type I MCS2000 is one of my most sensitive UHF radios (123dBm) with my other UHF MCS2000 Type II radios not far behind ( around 121dBm). My VHF MCS2000's are also excellent performers.

I have experienced sensitivity variations with the UHF MTS2000 portables - some fantastic , other a bit deaf although NOT as poor as my lastest UHF MCS2000.

I have very carefully realigned the front end which resulted in a slight improvement. I've also aligned the radio so its spot on freq and backed off the squelch so it opens on very weak signals.

Is my UHF MCS2000 faulty? Has a front end pre-amp failed?

When I was reading the service manual, I saw that 110W MCS2000's have a pre-amp option which improves sensitivity from 0.3uv (117dBm) to 0.2uv (121dBm). However , all my other "low power" MCS2000's already have sensitivity around 0.2uV (121dBm) despite the quoted spec of 0.3uv(117dBm).

Anybody else experienced a big variation in MCS2000 sensitivity? Is there any fix for a deaf MCS2000 which is more or less within the "pessimistic" Motorola spec?

[N4KVE]

About 4 years ago I acquired 2 motorcycle UHF 438-470 Spectras. I had Will do the caps fix on the first one, as it was having problems. He mentioned the receive was "as hot as a pistol". I used it for 6 months, & was able to receive 2 repeaters in Miami, 60 miles south. I decided to use the 2nd radio, & while it had no problems, I sent it to Will for a similar caps fix. He didn't say anything about the receive on the 2nd radio. While it worked fine, the receive wasn't as good on those 2 Miami repeaters. I contacted Will, as I also realized UHF Spectras could have been ordered with a preamp. Will was able to install the preamp components on the second radio, & the receive was now equal to the first radio. Both radios by the way had serial numbers extremely close. I think while all radios are within spec, some turn out better than others due to component tolerances. By the way, I too now use a UHF 100 watt MCS2000, in the same car with the same antenna, & it receives those Miami repeaters as well as the 2 Spectras. GARY N4KVE

[fineshot1]

We have had some MCS2000 radios come in to the shop after being in the fleet for about
a little over 10 years(they had not been touched since they were first programed), and they
were way off frequency by about 3 or 4 Khz. I noticed while they were on the bench and before
warping them back on freq that the receivers were at about .3 or .35uV to open squelch but
after warping them back on freq they were back to about .25 so that helped out a little.
Due to narrow banding coming soon that I also had to do a squelch alignment and that also
helped a little and brought them down to about .22uV so every little bit helps.

[g8tzl2004]

Some of my other MCS2000's had originally been way off freq but the deaf MCS2000 was only 400Hz high...although its now spot on..although this resulted in no improvement in sensitivity. I find a radio need to be just over 1KHz off freq before you notice an effect ..although if you are 1KHz low and the other station is 1KHz high, then the effective 2KHz error is noticeable.

While you expect, say, a 1-2dB difference between identical radios , a 5-7dB difference with the UHF MCS2000 is very poor!! If I used a 5 element yagi with the deaf MCS2000, I only need to use a tiny 6 inch dipole with my other MCS2000 to have identical RX performance!!!

I'm still thinking that maybe I have a front end fault???...although you would perhaps expect the MCS2000 to be very deaf rather than just 5-7db down?

[WB6DGN]

I think the first thing I'd do is set the questionable radio and one on your "good" radios up on the bench side by side and, using the same procedure and connections as close as possible to each other, measure the gain through the IF stages. I'm not familiar with the MCS as my last ten years was spent in the portable shop, but I'm guessing that you'll see about the same difference there. Again, I don't know what type IF filters the MCS uses, but if its those little black Murata types, that is one place to look for this type of variation. If that looks OK, you might want to look at the RF amp (if there is one) or first mixer active device for variations.
Tom

[g8tzl2004]

Thanks Tom.

I'd be interested in the sensitivity variation you found with the portables you were working on. Was there a large difference between identical radios even though they were all "in spec". I just checked out a VHF HT750 and it has a very hot front end. All the VHF Moto stuff seems to be consistantly good - its the UHF Jedi stuff where I see lots of variation in sensitivity!!!

[WB6DGN]

I've been retired since 2005 so specifics have somewhat faded from memory but, generally speaking, the UHF Jedis were quite consistent from radio to radio. Interestingly, if there was one that varied from the "norm", it was most often more sensitive than normal as opposed to the other way around. A common problem with the Jedi portables was that antenna switch activated through the accessory connector and if the radio was "slightly deaf" that was the first (and usually the last) place to look. Also common was the reference oscillator being off frequency. Always low and very often out of spec. (greater than 0.8kc). Seems as though they may have been using crystals that weren't properly aged for a while. Another occasional problem was front end alignment; once in a while I would find a radio that was far enough out that it needed a complete realign., but that was rare. Many times the user never even knew it due to saturation coverage in their particular application. But all of the above amounted to a very small percentage of the Jedis that I would see over a period of time. Much higher percentage of problems with the Waris series but, at that time, they were a new model so somewhat to be expected. Hope this is useful, sorry I can't provide more detail.
Tom

[g8tzl2004]

Thanks Tom - a very useful insight. Its interesting that a few Jedi portables were above average sensitivity.

I've always wondered why Moto used such pessimistic Jedi "official spec" sensitivity figures - 0.35uV for 12dB SINAD - when typically sensitivity was much better..although maybe they did it to accomodate my deaf MCS2000!!! I've noticed that Moto now provide "typical" sensitivity figures of around 0.22uV for more modern stuff...although they rarely differentiate between 25KHz and 12KHz bandwidths (narrow bandwidth has 3dB extra sensitivity)...and when they have differentiated, they quote 12.5KHz bandwidth as being worse???!!

[Bill_G]

Motorola's specs have always been slightly pessimistic. But, they are taking into account two things - parts tolerances and customer test equipment. We have a customer right now that wants a new repeater replaced because it only puts out 96W on their wattmeter and cable. They purchased several. They are rated for 100W. Most put out 104W. This one puts out 96W. When I go through the alignment procedure with my equipment setting each for 100W, they all measure 95W on their wattmeter. If I closely couple their wattmeter with a NM-NM adaptor to the RFPA port, it almost reads 100W. If I use my rg142 jumper, it reads around 97W. If you lay their wattmeter flat, it reads slightly greater than standing up. I can realign all the stations but one to show 100W on their meter.

Obviously, the station is failed, not their meter. And the customer does know more about specs, test equipment, and radios in general than I do.

Unfortunately, this complete and utter loss in confidence in the former reliability of Motorola equipment necessitates that each new mobile and portable be bench tested before delivery. We are still negotiating. Thankfully, he does not own a 2975 or equivalent.

Specs give you room.

[motorola_otaku]

The only MCS radio I've noticed this in was one of the Indonesian 220 radios, which I could not get to go lower than .32. Everything else in the MCS line I've seen ran closer to .17 - .22 with or without tone squelch.