spectra preselector preamp uhf

saberjerk · Post by **saberjerk** » Wed May 10, 2006 7:32 am

UHF spectra (astro) range 3 or whatever.
How much improvement can be expected for the UHF preselector
with the preamp components installed?

Almost all the parts to add a preamp to your preselector can be found
on an old range 1 preselector. Even one without the preamp.

jmr061 · Post by **jmr061** » Wed May 10, 2006 8:08 am

Have you had your radio checked on a service monitor to see where the receive opens up at?

Jason

kcbooboo · Post by **kcbooboo** » Wed May 10, 2006 8:19 am

The service manual says it improves 20dB quieting sensitivity from 0.50uV to 0.30uV, and 12dB SINAD from 0.35uV to 0.20uV. Neither of these is very much, on the order of 5dB or so. Selectivity is also reduced 5dB.

The 900 MHz Spectras don't have a preamp, and they're quite hot, at least as good as the UHF specs with a preamp.

Bob M.

saberjerk · Post by **saberjerk** » Wed May 10, 2006 8:21 am

No I really haven't Jason. I just know my mcx1000 can hear the signal but the spectra doesn't. I have tried 2 of the spectras with same results.
When I open the squelch on the spectras with the monitor button I can
hear the signal, it just isn't opening the squelch. That's with the squelch set at "1".

saberjerk · Post by **saberjerk** » Wed May 10, 2006 8:24 am

Thanks kcbooboo
That doesn't seem like much but it might be enough to pop open the squelch. Unfortunately I can't find a way to loosen the sqelch some.

kcbooboo · Post by **kcbooboo** » Wed May 10, 2006 8:38 am

If you can't open the squelch even at 1 or 0 (I don't recall if it goes that low), that could indicate more serious problems in the radio, like something else generating more noise than it should be, or the VCO being way off frequency or out of range, although that usually generates an error message. I don't think there's anything you can adjust in the front end, but I'm not the Spectra expert.

It could also be due to bad and/or leaky capacitors on the RF and command boards; this problem affects lots of things. They should be replaced if physically leaky before more damage is done. The electrolyte can actually eat traces on the circuit boards.

Bob M.

saberjerk · Post by **saberjerk** » Wed May 10, 2006 8:44 am

The squelch does open at "0" for both radios. And the signal I want to hear isn't strong enough at "1" to open it.
When I set it to 0 it opens and I can hear the signal. It's not full quieting
but good enough. Sqelch is to tight and I can't change it.

kcbooboo · Post by **kcbooboo** » Wed May 10, 2006 8:58 am

Sure sounds like something's wrong, maybe a bad RF amplifier or worse.

My 900 MHz Spectra hears so well at squelch threshold (3-4 if I recall corectly), that my sig gen can't go low enough for the radio NOT to hear it, and that's down at -135dBm (0.040uV). Of course, you can't recover anything audible, but at least the squelch will open down there.

You should see what the sensitivity really is. If it's off by about 20dB, I'd suspect the front end.

Bob M.

saberjerk · Post by **saberjerk** » Wed May 10, 2006 9:05 am

Yes, I would say you are right if it was only one radio. These two are astro spectras. They use the same front end though.
I also have a analog spectra same range equally numb. The analog spectra opens squelch about 3. I don't know how that number compares to the astro though.

kcbooboo · Post by **kcbooboo** » Wed May 10, 2006 9:24 am

Well, when all else fails... check the power supply (I must remember to do that more often).

Unlikely, but someone could have pulled or broken the RF cable going from the PA to the receiver.

Check the sensitivity with a known good signal generator with known good cables and connectors. Compare this with another known working radio. If it's happening to several, then it could be something in common, such as the test setup.

There has to be a good explanation for it, even operator error !

Bob M.

saberjerk · Post by **saberjerk** » Wed May 10, 2006 12:40 pm

Well there is nothing wrong with the cables. And they're getting power.
I don't think it's possible for 2 radios to fail at the same time.
If someone with a good generator would tell me at what input their
spectra opens squelch that would really be helpful.

kcbooboo · Post by **kcbooboo** » Wed May 10, 2006 4:20 pm

I asked a friend who has a few to see where the squelch opens and closes on his. Here's the reply:

My 900 spectra opens at 2 and closes at 3.
The 800 opens at 3 and closes at 5.
The VHF opens at 3 and closes at 4.
My UHF Astro Spectra will not open squelch on a digitally programmed channel. On an analog channel, it opens at zero and closes at 1.
I have never checked the sensitivity on the Astro but I do know that it is hot.

Bob M.

saberjerk · Post by **saberjerk** » Thu May 11, 2006 5:38 am

Thanks for the info. Sounds typical for analog and astro.
But what I really needed is the rf level required to open the squelch.

mancow · Post by **mancow** » Thu May 11, 2006 9:35 pm

Mine is a VHF but I hooked it to the IFR1900 and tried it anyway.

I set it mid band at 160.550.

Squelch opens at -122 dBm

Squelch closes at -124 dBm

Squelch setting was set at # 1.

For what it's worth.

saberjerk · Post by **saberjerk** » Fri May 12, 2006 5:54 am

Hey Mancow
Thanks for the effort. I don't know how the vhf compares to the UHF.
If I'm converting correctly -122dbm comes out pretty close to .5 microvolt.
I'm not sure on the specs for VHF but that's what UHF should be without
the preamp.
Isn't it interesting that the only model with the optional preamp is the UHF one? Tells me somethings up.

As soon as I can get my hands on a couple more radios I'm going to try to add preamps to them from the parts of leftover range one preselectors.
Thanks

kcbooboo · Post by **kcbooboo** » Fri May 12, 2006 7:10 am

-127dBm is 0.100uV. Add 6dB to double that to get 0.2uV.
-121dBm is 0.200uV. Add 6dB to double that to get 0.4uV.
-117dBm is 0.300uV. Add 6dB to double that to get 0.6uV.
-115dBm is 0.400uV. Add 6dB to double that to get 0.8uV.
-111dBm is 0.600uV.
-109dBm is 0.800uV.
-107dBm is 1.000uV.

So, your signal level of -122dBm is somewhere around 0.2uV

There's a handy PDF file that converts dBm to uV. It's mentioned in the article below:

http://www.repeater-builder.com/measuri ... ivity.html

Bob M.

jmr061 · Post by **jmr061** » Fri May 12, 2006 7:22 am

Both ranges of VHF also have an optional pre-amp. Its not just the UHF ranges.

Jason

saberjerk · Post by **saberjerk** » Fri May 12, 2006 7:29 am

Well that's something moto left out of the manual I have.
That's an interesting article on receiver sensitivity, thanks.

kcbooboo · Post by **kcbooboo** » Fri May 12, 2006 8:01 am

My "original" detailed service manual shows both VHF and UHF can have preamps.

Also, there's a sales brochure (PDF file) located at the end of this article:

http://www.repeater-builder.com/motorol ... index.html

It shows sensitivities with and without the optional preamp.

Bob M.

saberjerk · Post by **saberjerk** » Sat May 13, 2006 5:42 pm

Does anyone have a late copy of the astro spectra uhf range 3 preselector schematic? The one that includes the preamp. Mine seems to have several errors in it and I would like to compare.
Thanks

Josh · Post by **Josh** » Sat May 13, 2006 7:41 pm

Your radio is fine... the Astro Spectra's squelch settings are simply way too tight and there is nothing that can be done about it that I am aware of as I see the same problems in my VHF Astro Spectra that I built with parts from an Analog VHF Spectra radio.

If you need more sensitivity, then the Astro Spectra isn't what you need, but have to deal with if you need the digital aspects... if your interests are analog, get a CDM or something, they're fantastic- or perhaps an analog spectra if that's more suiting.

-Josh

saberjerk · Post by **saberjerk** » Sun May 14, 2006 6:12 am

Thanks to everybody for your helpful suggestions.

I did find one preselector with the preamp components installed.

It made all the differenct in the world. The squelch will now open on very small signals.

The problem is that these seem to be as rare as hens teeth.

From looking at the schematic I have I think most of the components needed to add the preamp can be found on otherwise useless range one preselectors.

Can someone give me a scanned copy of a late version of the astro spectra UHF Preamp schematic? I think my copy has errors in it.
Just PM me for my email address.
Thanks

saberjerk · Post by **saberjerk** » Wed May 17, 2006 6:06 am

Just called moto and verified that there are errors in the analog spectra schematic for the preselector preamp.
The schematic was corrected in the astro manual.
The correct values for C101 is 5.6pf and C103 is 91pf.

kcbooboo · Post by **kcbooboo** » Wed May 17, 2006 6:26 am

Thank you. "It has been noted and logged." Quite a difference from what the parts list shows; not even close. I wonder who decided to just throw in values like those.

Did you find out parts values for the Range 1 board?

Maybe it's worth having an Astro service manual as well. I wonder what else is different. They're still selling Astro products, so maybe the manuals will still be available too.

Bob M.