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I just wondered if anybody has compared the VHF RX sensitivity of a Spectra (with and without preamp option) to an MCS2000?

In my "collection" of commercial radios my VHF MCS2000 is probably the best performer for weak signal RX sensitivity..I even think its a bit ahead of my analog XTL2500 with pre-amp enabled. The MCS2000 also has a great SQ circuit (it can be set to open on the weakest of signals) and also handles over deviated signals with no issues. The XTL when set to 12.5 spacing (2.5KHz dev) cannot cope with even a slightly over deviated signal without massive clipping

I've read around and get the feeling that the Spectra's sensitivity is all over the place...with and without the pre-amp!!...some Spectra's have better RX sensitivity without the pre-amp compared to some with the pre-amp fitted...but maybe this is all due to the bad caps issue with the Spectra's?

So will a VHF Spectra without the pre-amp ever be as sensitive as a VHF MCS2000?

Both when properly tuned with the tuner app and a service monitor should be in the same range of sensativity

While I can not answer your question with technical spec's, I've used all the radios you've mentioned. In particular I had 2 motorcycle 438-470 Spectra's. The serial #'s were very close, so they were built within minutes of each other. I had recently moved 40 miles north of the repeater that I had been using for years. One Spectra worked fine, but the other was not as sensitive. Had I been 20 miles closer to the repeater, I'd have never noticed the difference. I had Will install the preamp on the offending radio. That radio was then as good as the radio w/o the preamp. Will had changed the caps on both radios. My MCS2000 was as good as both Spectra's. The repeater is now gone, so I can not test my XTL on it to compare, but it was auto tuned on a 3920. GARY

I've had both and would say they were very similar in preformance. I would argue the MCS has a better display and is easier to install (ie less wires).

My two cents

Thanks for the feedback. I read that you can significantly improve the Spectra's RX sensitivity just by changing the value of one resistor !!...I don't think you actually install a preamp - you simply enable what is already installed by changing a few parts..but the one resistor is the key part that needs to be changed. But RX sensitivity still appears to be variable between identical radios...so its all down to luck and whether you get a good one...although all radios will no doubt be "in spec".

On the UHF Spectra radios, the front end filtering is a tuned unit with plated ceramic. It can be modified to improve the frequency from a low government split to raise it into the UHF ham segment by grinding off some of the plating on the ceramic used to make the tuned circuits. Have done it a number of times. But you need to do it with care and test the sensitivity as you go with a signal generator.

The same goes with taking the mid split UHF front end filter and adding solder to get it to tune lower in frequency to make the sensitivity better in the ham segment. Again, I have done this a number of times with good results. But remember you need to keep checking the sensitivity as you work on the front end filter.

The big point to keep in mind is to do the same to each section of the front end filter in the Spectra receivers on UHF. The VHF models seem to do well moving into the VHF ham band.

You just need to pay attention to the VCO board in the UHF models. I have had very poor luck trying to raise the frequency in the low split UHF VCO boards. Have much better luck taking the mid split 450 to the 472 range and lowering them. This takes some careful soldering on the VCO substrate, but it works. just keep track of the VCO voltage as you walk them into the ham bands.

I tend to program the radios with 0.5 MHz. steps so I can record the VCO voltage with the changes I make. Think there is some information on the repeater builders site about doing all these changes.

Jim

He's my original work on this:

http://batboard.batlabs.com/viewtopic.php?f=4&t=97025

Unfortunately the image links are all dead so I'll need to try and find the originals and post again.

Thanks Jim and Astro Spectra.

Astro Spectra - is a 10 ohm resistor the optimum to maximize sensitivity? Will, say, a 5 ohm (or equivalent combination) give even higher sensitivity? - maybe too high (given selectivity issues) for commercial applications but maybe OK for weak signal ham radio applications?

Two out of the three "no preamp" Spectras that Astro Spectra measured had sensitivity of only -114/115dBm...I think thats around 8-10dB below my MCS2000...which is significant for weak signal ham applications...like using a dipole vs 6 element yagi!!!

I think Motorola radios outperform most other manufacturers, but I'm always disappointed with the RX sensitivity variability between identical radios...whether its Spectras, Sabers, UHF MTS2000's etc etc. They are always "in spec" but the official spec is very low..usually 0.3uV (-117dBm) for 12dB SINAD...although Moto now sometimes provide a "typical" figure of 0.22uV (-120dBm)for 12dB SINAD...but some models significantly outperform this...such as VHF (NOT UHF) MCS2000's and UHF XPR7550 in analog mode!!

Question: are these analog Spectras, or Astros?

Analog-only radios will always have slightly better receive sensitivity due to the direct FM demodulation versus digital radios doing it in DSP.

Analog Sabers and Spectras.

The XPR7550 is a digital DMR radio which also has an exceptionally sensitive analog front end..but I know what you mean with P25. If you open the fake SQ on a P25 radio in analog mode, you can hear the fake white noise switch to a real noisey signal....but an XTS5000 has a better analog front end than an XTS2500.

You're absolutely right. I benched a 4550 and a 5550 a couple of weeks ago and was very surprised at how hot their receivers were in analog.

Still though, nothing beats a Saber. Or a Maxtrac. I've had Maxtracs go as low as .11 microvolts.

XPR7550 portable analog front end is better than my XPR5550 mobile front end.

I have found that my VHF Motorola radios (MCS2000's, CDM's etc) have always had better sensitivity than the equivalent Motorola UHF radio...except for the UHF XPR7550 which is hot..but no easy external antenna connection.

The XPR5550 also only have 2 squelch settings...and the lowest setting is not low enough..so weak signals are missed. I am not aware of any way to actually adjust the SQ level on an XPR5550. On my MCS2000 and CDM stuff I can use service model to set the SQ so that it will open on the weakest of RX signals.

How variable is the RX sensitivity on Maxtracs...what % go as low as 0.11uV? I have 2 VHF Maxtracs and several 29 MHz Maxtracs...but all have wide 20/25KHz filtering....and sensitivity is poor compared to my MCS2000's and narrow filtered 10m stuff..although maybe 12.5KHz Maxtracs perform significantly better?

The MCS2000 on 900MHz must be the aberration. Its spec'd for 0.5uV and thats what I'm getting. Its the worst sensitive radio I own...

Check out the review of 900MHz radios on Batboard.

MCS2000 was the best of the bunch...0.19uV for 12dB SINAD at 927.5Mhz.

But there were issues with the SQ setting...you had to get it just right...2 unit down and SQ did not close..2 units up and SQ only opened at 0.4uV.

Is your radio actually deaf or is the SQ set too high? I think the MCS2000 reviewer completely opened up the SQ and relied on CTCSS.

Doesn't seem that anyone has answered the OP's original question about the VHF Astro or Analog spectra. They use the same receiver preselector.
I read an article on repeater builder that R3206 on the preselector needs to be bridged with a 15 ohm resistor. This combined with the existing 68 ohm resistor brings the total value down somewhere near 12 ohms. This will give you maximum sensitivity on the VHF preamp.
This information is also available on batlabs http://batboard.batlabs.com/viewtopic.php?f=4&t=97025

I was wondering if the official Motorola pre-amp 10 ohm "equivalent" value gives the absolute maximum RX sensitivity or whether, say, a 5 ohm "equivalent" value will provide even higher RX sensitivity. Maybe there might be a sensitivity/selectivity issue but is there a value which provides the maximum RX sensitivity?

[quote="radiodude123"]I read an article on repeater builder that R3206 on the preselector needs to be bridged with a 15 ohm resistor... [/quote]

I can't find that article, only the UHF parts addition mod. Can you please post a link?

Well My MCS2000 on 900 MHz is kinda deaf at ~0.5uV; While my Specta 900, is ~0.25uV approx...

syntor9k wrote:Well My MCS2000 on 900 MHz is kinda deaf at ~0.5uV; While my Specta 900, is ~0.25uV approx...

Is that in-band, or at 927 MHz? I've seen several MCS2000s that were deaf as a fencepost at 927 but great in the rated bandsplit.

The whole Jedi line has always been hit-or-miss for 900 ham. Some work fantastic, others just don't work at all. In my experience Spectras have been more consistent performers out of band on 900.

It made spec in-band at 0.4uV, so 0.45-0.5uV in the 927 MHz is to be expected, I guess...