Tweaking/modifying a Spectra for max RX sensitivity?

Elroy Jetson · Post by **Elroy Jetson** » Mon Apr 02, 2007 2:40 pm

What can anyone here tell me about the prospects for tweaking or modding
a VHF high power Spectra (remote mount, T83 series) to push the receive
sensitivity up to the absolute max without destroying its selectivity?

I'm active on 2 meters and these days, the repeaters are all getting rusty
and covered with cobwebs. Everyone has gone to the simplex channels,
146.520 in particular. I figured (and rightly so) that I could benefit
greatly from getting a 110 watt Spectra and a 3 dB gain antenna, and my
talk range is simply smoking. But, I'm in a position where my quality
talk range is greater than my quality listening range, even though the
Spectra does meet its published specs, so I'd like to boost its sensitivity a
little more. I'll also be looking at my antenna options for possible boosts
there as well, but I'd really like to boost the sensitivity at 12 dB SINAD by
a respectable margin without having to take such drastic measures as
start reworking the receiver with higher spec, lower noise components or
add in a GaAsFET preamp.

But I may end up doing some of those things.

Tell me what you can about boosting its RX sensitivity, please.

Incidentally, I've received many positive comments on the TX audio
quality from the Spectra. I like that. There IS a difference between
Motorola's best and the typical amateur rig. Some other amateurs
I've heard have, frankly, crappy audio that almost sounds like a broken
microphone in action.

Elroy

Jim202 · Post by **Jim202** » Tue Apr 03, 2007 3:41 am

The normal receiver sensitivity in the VHF Spectras will walk
circles around most radios out there today. I find that I
have to tighten up the squelch on most radios to keep
them from popping open all the time on low level signals.

Many times when I am doing antenna work on a mobile,
with no antenna cable connected, the reciever is still
trying to hear local radio channels.

Just how much more ear do you expect the radio to
have. I suspect your at the point now where the local
noise floor is coming into play with these radios.

Jim

Elroy Jetson wrote:What can anyone here tell me about the prospects for tweaking or modding
a VHF high power Spectra (remote mount, T83 series) to push the receive
sensitivity up to the absolute max without destroying its selectivity?

I'm active on 2 meters and these days, the repeaters are all getting rusty
and covered with cobwebs. Everyone has gone to the simplex channels,
146.520 in particular. I figured (and rightly so) that I could benefit
greatly from getting a 110 watt Spectra and a 3 dB gain antenna, and my
talk range is simply smoking. But, I'm in a position where my quality
talk range is greater than my quality listening range, even though the
Spectra does meet its published specs, so I'd like to boost its sensitivity a
little more. I'll also be looking at my antenna options for possible boosts
there as well, but I'd really like to boost the sensitivity at 12 dB SINAD by
a respectable margin without having to take such drastic measures as
start reworking the receiver with higher spec, lower noise components or
add in a GaAsFET preamp.

But I may end up doing some of those things.

Tell me what you can about boosting its RX sensitivity, please.

Incidentally, I've received many positive comments on the TX audio
quality from the Spectra. I like that. There IS a difference between
Motorola's best and the typical amateur rig. Some other amateurs
I've heard have, frankly, crappy audio that almost sounds like a broken
microphone in action.

Elroy

Mike B · Post by **Mike B** » Tue Apr 03, 2007 10:58 am

I assume you have the detailed service manual, since you would need that to successfully tweak the front end. Do you have the optional Rx pre-amp? In the range 1 VHF the part number is HRD6011 and in the range 2 VHF it is HRD6012. If you look at the schematic you will see the only difference between the VHF regular and optional pre-amp version is some surface mount resistors and jumpers. The trouble with changing the parts is unsoldering some of the PC board shielding cages to get access to them can be difficult.

One problem with the Spectra is the small T/R switch PIN diode CR3903 on the bottom of the high power PA board. If your VHF antenna is very near any other high power radio antennas, this pin diode can die. Motorola documented this problem for the high power dual VHF/UHF HHCH radios. When this PIN diode dies your receive sensitivity will take a huge hit (3 microvolts just to open the squelch is not uncommon). I killed mine by putting the VHF Spectra and Syntor X 9000 low band antennas on opposite sides of the pickup truck cab sheet metal.

Receiver design is complex and adding too much gain to the Rx front end can cause other problems. Unless the other people you are communicating with also have high power radios and good antenna setups, reaching out farther than you can hear is kind of an expected problem with a 110 watt radio.

George · Post by **George** » Tue Apr 03, 2007 1:43 pm

The problem is, you will start hearing more noise than usable signal. Unless there is something wrong with it, you should have fine sensitivity as is.

In a mobile application, don't bother with a gasfet preamp or anything else like it. Tis just a waste of time.

If you are at .2 or so for 20 then you are hearing all you are going to hear without your noise floor turning to crap.

George

Elroy Jetson · Post by **Elroy Jetson** » Tue Apr 03, 2007 1:58 pm

I do need to characterize my actual receive sensitivity, first. It seems to be in the normal range, but frankly I expected a little better. Maybe that pin diode is damaged? I'll find out.

I'm having problems with my main signal generator right now but after it
warms up and I knock it around a bit, it eventually does come to its senses.
I'll use that to check the Spectra's sensitivity. The generator will go to
-140 dBM, lower than my spectrum analyzer can even detect, so I should
be able to challenge the receiver's sensitivity quite effectively.

I've got the detailed service manual coming. And I'm very able to handle
surface mount components, so if the preamp isn't in there, it WILL BE. I have access to any components I might need.

This evening I'll do the first tests.

Has anyone ever swapped out that crap mini UHF connector for something
DECENT instead, like a type N, maybe? Of course, that would require
some chassis mods.

Elroy

Will · Post by **Will** » Tue Apr 03, 2007 2:30 pm

Elroy,

The Spectra VHF receiver does not have any alignment. Every thing is aligned at the factory on the RF frontend filter module. "there are no tweeks"

As far as trying to get under the shield on the frontend assembley, forget that... BTDT won't try that again. It was not worth it. Also working on the cerramic substrate is very difacult if not imposible.

Let us know what the sensitivity is now.. Typically better than .3uV.?

George · Post by **George** » Wed Apr 04, 2007 3:46 am

Yes, actually I have physically seen some Spectra high power drawer units with a standard sized UHF connector. They were a SP for the DEA and Mother barely acknowledged their existance. But they do exist.

Don't ask me if there was a special casting made for them...I don't know.

George

Post by **wavetar** » Wed Apr 04, 2007 4:30 am

Elroy Jetson wrote:
Has anyone ever swapped out that crap mini UHF connector for something
DECENT instead, like a type N, maybe? Of course, that would require
some chassis mods.

I recall reading some threads in the past where people switched them out for BNC connectors. A search here didn't turn anything up, but I think it pre-dated this version of the board anyway. As I recall, it made a significant difference with the TX power readings. I do not know if it would make a large impact on RX, but you might gain a dB or so.

Todd

Elroy Jetson · Post by **Elroy Jetson** » Wed Apr 04, 2007 7:32 pm

I did the tests as I said I would, this evening.

Using a custom made 24 foot length of LMR-400 with an SO-239 on the radio end and a type N on the signal generator end, I connected it to the spectra via an SO-239 to mini UHF adapter. I would have lost less than 1/2 dB in the cable and maybe 1 dB to the adapter.

With the squelch on the Spectra set at minimum (closed squelch with no input signal) I was able to break squelch at .1 uV and hit what sounded to me to be about 20 dB quieting by the time I was up to .2 uV indicated. The generator I used, an HP 8660B, isn't calibrated for signal amplitude but I do believe that it's quite close. I will trust this measurement. Frequency wise, it's
dead on according to my freshly calibrated counter.

There's nothing wrong with this radio! It's basically at full quieting before
getting to .3 uV.

But that mini UHF connector on my antenna is pure crap. I've got a better
quality one to put on the antenna tomorrow and that should help. I could
tell that by wiggling the connector while listening to the radio. I didn't
know how bad it was and I'm glad the transmitter didn't take great
exception to that awful connector. The center pin was loose on the
connector. You could wiggle it so that it had NO contact with the center
conductor of the cable.

I've also got to sweep the antenna and see how it's tuned, anyway.

Elroy

Will · Post by **Will** » Thu Apr 05, 2007 1:48 pm

I have seen a few Spectra 'low power' chassis with the female TNC connector on them. TNC= threaded Nielson connector or miniture N)

The MiniUHF Connector (on the Spectra) is a 'bulkhead' type with the coax cable crimped to the connector and and going to the PA board. Yes the center pins in the female chassis/coax cable connectors do go 'bad' from the incorrect male miniUHF connectors and adaptors.. Too big center male pins on some connectors and adaptors.

So you need a bulkhead dual crimp coax female connector with about 4" of small primo coax.
Look at Amphenol part 31-2318 good to 11 giga hertz.....

Elroy Jetson · Post by **Elroy Jetson** » Thu Apr 05, 2007 2:02 pm

There's not much room in the RF connector recess on this chassis so for the moment, I'm happy with a FAR superior connector on my antenna cable which I put on this morning. I also shortened up the antenna cable to a
much more reasonable length to keep cable losses to an absolute minimum,
and with the new connector being tight without any rattle at all, things are
working noticeably better.

If I do choose to upgrade the connector, I'll probably end up with a
cable-through-the-bulkhead design that doesn't require me to modify the
frame of the radio and eliminates unneeded connector-to-connector interfaces.

I got the manual set on loan from another member here. (Thanks!) It's
really big and really detailed. I haven't gone through it all but I hope that
it does cover the high power chassis.

At the very minimum it gives me all the part numbers for the manuals and supplements that I may need. I don't foresee a lot of use for the 900 MHz model manuals, frankly.

Another tweak I did: Replaced the lame yellow/green backlight LEDs on the A9 control head with blue ones. It looks SO much cooler and more modern.

Elroy

tvsjr · Post by **tvsjr** » Thu Apr 05, 2007 3:29 pm

A BNC should be a drop-in replacement with no chassis rework required. The Euro version of many Motos have BNCs on them (GM1200 aka MCS2000, etc.) and work great. Were I more motivated, I'd swap mine out as well.

More details on the blue LED mod, please! Part numbers, etc. I'd love to do that to my W7s...

Elroy Jetson · Post by **Elroy Jetson** » Thu Apr 05, 2007 7:24 pm

Forget part numbers, etc.

Just go here and order this:

http://www.goldmine-elec-products.com/p ... ber=G13706

It takes 10 to do one A9 control head if you just do the backlights. Get a few
spares as they are easy to render non-functional...I know. I blew up two of them in the process of figuring out the best approach.

If I could find my camera I'd take a pic of the results and post it right now.

The best way to install these LEDs is to determine their polarity compared to the ones already in the control head first (voltmeter, diode check) and then remove the existing LEDs, carefully so you don't damage a solder pad on the board. Use solder wick and remove all excess solder from both pads and then apply a small dot of fresh solder to one pad. Place the LED over both pads, press it gently to the board using the tip of a small screwdriver or other suitable instrument, and heat the pad up with your soldering iron. The LED will settle to the pad very quickly. Remove the iron as soon as this happens. Allow a few seconds for that connection to cool. The one connection that's been made will hold the LED in place quite well, of course. Now do a fast touch of the iron and solder to the other connection and you're done with that one.

Check polarity again to be sure you got it right and from then on, you
align all the other LEDs with the indicator notch in one corner in the same
orientation.

Elroy

tvsjr · Post by **tvsjr** » Thu Apr 05, 2007 7:33 pm

Very cool. How bright is the modified head in comparison with a regular green backlight? Pimpadelic blue is cool... a small fireball between the seats, not so much.

Elroy Jetson · Post by **Elroy Jetson** » Thu Apr 05, 2007 8:04 pm

It's substantially brighter but not distractingly so.

You can see some trace of the blue backlighting in anything short of direct sunlight.

The camera's not quite accurate. The real color is a deeper blue with less green in it. But these are bright enough that the camera doesn't see it properly.

Elroy

Elroy Jetson · Post by **Elroy Jetson** » Sat Apr 07, 2007 4:14 am

I figured I'd bump this post up on the chance that some of you hadn't seen the new pimped-out control head.

"Pimp my commz, yo."

CJ

tvsjr · Post by **tvsjr** » Sat Apr 07, 2007 5:30 am

Very cool. I wonder how 6 W7s/A7s in a row would look in blue?

Elroy Jetson · Post by **Elroy Jetson** » Sat Apr 07, 2007 8:13 am

I'd do each one in a different color. You know, they even have PINK LEDs now.

I'd just be careful not to arrange them in a rainbow pattern. That would be gay.

CJ

mr.syntrx · Post by **mr.syntrx** » Sat Apr 07, 2007 11:06 am

No offence, but I hate blue LEDs. I'm hell sick of them. I like seeing green ones as a novelty for a change, cos everything you buy now has an ultra bright blue LED somewhere. They were cool when they were $15 each, but now...

Elroy Jetson · Post by **Elroy Jetson** » Sat Apr 07, 2007 12:14 pm

The original color was a nasty yellowish-green that you probably know about, and it was never more than functional.

Blue happens to be my favorite color, always has been, always will be, and this works just fine for me.

Purple is an option, though!

Elroy

tvsjr · Post by **tvsjr** » Sat Apr 07, 2007 4:49 pm

Eh, negative on the purple... my eyes always have trouble focusing on stuff that's close to UV.

Remember that I also have three orange Kenwoods, a green Uniden, a full-color MW800 (soon), etc... so I already have some variety.

Elroy Jetson · Post by **Elroy Jetson** » Sat Apr 07, 2007 7:28 pm

Very few people focus well on deep blue to violet. Ever noticed that when you encounter a sign that's blue, whether lit or painted, that you have to get closer to it to read it than if it's any other color?

But it does LOOK cool.

CJ

bellersley · Post by **bellersley** » Sat Apr 07, 2007 8:13 pm

tvsjr wrote:Eh, negative on the purple... my eyes always have trouble focusing on stuff that's close to UV.

That's just the old age creeping up on you.

Those blue LED's look SUPERBITCHIN Elroy, very good job! I've never really liked the Motorola yellow/green. In my old car, I changed all the interior lights to red (what prompted me was having a deck that was all red). I replaced the LED's in a ham rig I had at the time (RadioShack HTX242) with red and boy oh boy did it ever look sharp!

I don't think a Spectra would look too spiffy considering the VCD display, but on something like an MCS2000, woah that would be bad-ass.

4n6inv · Post by **4n6inv** » Sun Jun 16, 2013 8:33 pm

Diggin' up bones...

I found this on a Google search on modding my VHF Astro Spectra to add the preamp option.

Where I'm at is; I have a mid power VHF W-3 Astro Spectra VHF that was depoed in 07 and virtually everything was replaced. It was hot as a firecracker until about 6 months ago. I used to be able to receive the three agencys in the area close to 100 miles away with a rigid 1/4 antenna. Now; nadda when I get about 10 miles from the county line.

I replaced the mini-UHF connector (on the coax) with little to no difference. I read this thread and tonight; I removed that miserable little @#%@ mini-UHF on the radio and replaced it with a silver plated Amphenol BNC. No troubles. Currently using a mag mount 1/4 wave to test. So far, so good. Haven't had time to test distance.

I guess my question is: Are the mid-power drawers prone to the PIN diode going bad, as well? I have no other radios in this vehicle, due to space limitations, and I can't for my life, imagine any situation I may have encountered that may have caused it to snap. Any ideas?

Another odd thing - also relating to this thread is; I re-backlighted my W-3 with white 0805 SMD LED's and it is absolutely magnificient! I can read it at any angle - day or night. But; I just ordered a couple of YMN4016B HHCH's on "you know where" and modded one of them. They both seemed to work fine in an absolute plug-n-play fashion (Didn't test TX, though). After I modded one of them for the backlighting; I was testing TX tonight, and when I keyed up; my siren activated! My "bed with the chickens" neighbors loved that; I'm sure! So; yes the radio is working and following the commands on the soft buttons and assigned buttons, but when I key it up; whatever mode my siren DEK is in, the PTT activates momentarily! I feel like I'm living in some alternate universe!

Any ideas that don't involve locking myself in the bathroom with a loaded 9mm and doing the right thing?

Astro Spectra · Post by **Astro Spectra** » Mon Jun 17, 2013 10:24 am

The t/r diode sometimes blow if you power radio(s) with the antennas too close together, typically on a vehicle. On the preamp this might help: http://batboard.batlabs.com/viewtopic.php?f=4&t=97025

Suicide is never the right thing.

4n6inv · Post by **4n6inv** » Mon Jun 17, 2013 11:28 am

Thank you VERY much! I didn't expect that fast of a response - if one at all. jbdempsey@sbcglobal.net is my email addy, if you have the time to take this off the board for a few more questions. You seem to know quite a bit about this. I'd like to do a few more things, but would spend too much time and space on the board if you could find it in your heart to give me a few pointers / ideas. Mainly; I just don't want to screw anything up!

And, yes; the last comment was not very funny... Especially when you have two young children that depend on you and love you dearly. Thank you again for addressing that.

Will · Post by **Will** » Mon Jun 17, 2013 1:41 pm

Antennas to close, or bad SWR causing high reflected power. And that after blowing out he antenna switch diodes kills the RF amp in the receive. Some parts are NLA.

4n6inv · Post by **4n6inv** » Mon Jun 17, 2013 2:18 pm

Only one radio in the car - the mid-power VHF Astro Spectra, so too many antennas isn't the cause. And, I can't for my life imagine where I could have been exposed to a high RF level to blow it out. I'm about to swap out the original NMO mount with the white Teflon coax for one I made using mil-spec RG400. Bears watching. Another poster was kind enough to share some wisdom on modifying the radio for preamp operation. I rounded up all the parts up the street about an hour ago, and am about to do that, as well. I will keep you all posted.

FYI; the weird PTT issue is no longer present today. Ya got me on that one... Bears watching.

After this work; I'm going to drive as far as I can hear - up to 100 miles and see how my rx is after these changes.

Thank you all for your super fast responses and sharing your experiences with me. I haven't fiddled with radios in about 5 years, so I'm having to re-learn some of the principles.

Jim202 · Post by **Jim202** » Mon Jun 17, 2013 5:12 pm

4n6inv wrote:Only one radio in the car - the mid-power VHF Astro Spectra, so too many antennas isn't the cause. And, I can't for my life imagine where I could have been exposed to a high RF level to blow it out. I'm about to swap out the original NMO mount with the white Teflon coax for one I made using mil-spec RG400. Bears watching. Another poster was kind enough to share some wisdom on modifying the radio for preamp operation. I rounded up all the parts up the street about an hour ago, and am about to do that, as well. I will keep you all posted.

FYI; the weird PTT issue is no longer present today. Ya got me on that one... Bears watching.

After this work; I'm going to drive as far as I can hear - up to 100 miles and see how my rx is after these changes.

Thank you all for your super fast responses and sharing your experiences with me. I haven't fiddled with radios in about 5 years, so I'm having to re-learn some of the principles.

It has been mentioned that the mini uhf connector is not the best. Another issue with them is that the center female pin that is on the radio end tends to get spread. This causes a bad or poor connection some of the time. If you grab the cable and move it up and down, you might find you have a connection some of the time and not others. I have sort of found a maybe solution. I take a flat blade screwdriver and put the flat on top of the teflon ridge. Then take an object with a little weight and rap the handle end of the screwdriver. This will deform the inner part of the female pin and tighten up the connection. You might have to take a sharp object and pry up the outer section of the pin slightly to get the male coax connector to be able to slide in. Don't force it very hard. It will go in and you should find the connection now works better.

Those that work on the Astro Spectra radios will find that the antenna replacement cable will fit the Spectra radio in most cases. Might take a little soldering, but at least you have a new coax cable connector if you need to go that route.

Jim

4n6inv · Post by **4n6inv** » Mon Jun 17, 2013 5:47 pm

OK; the radio is BNC modded, and the complete antenna / coax and mount are replaced. Parts people gave me the wrong resistor value on the most important one for the preamp mod, so that's going to have to wait until tomorrow morning. Monday night isn't a good time to do a drive test in a small town, but short range (~15 miles) it's working fine. I keyed up on PD, SO, and FD but didn't want to keep driving out doing the Verizon test - "Can you hear me now?" I used Amphenol silver plated connectors for the BNC swap, so I should be good there.

One thing I did note as I was pulling the RG400 through the hidden parts of the interior: the old white teflon coax was INSIDE the seat belt tensioner on the "C" pillar and had some chaffing. Maybe, maybe not. I'll know soon enough - maybe REAL soon. We're under a tornado warning, so they may ask me to head out and check some of the more remote areas of the county if we get any really severe weather tonight. Still have my mag-mount in the car, if I need it.

As I mentioned earlier; I had the radio depoed in 07 - the end of the Astro Spectra support era - and virtually every board in the radio was replaced - including the UCM - and have 07 date codes on them. I seriously doubt that it's going to be a capacitor problem.

Thanks, again; and keep those ideas coming!

4n6inv · Post by **4n6inv** » Tue Jun 18, 2013 10:46 am

Replaced the crappy mini-UHF with a high quality BNC, replaced everything in line, and did the preamp mod this morning. The results were phenomenal. 55 miles away in Abilene, and it nearly blew me out of the car. My hunch is; regardless of the underlying problem; I now have one hot receiver again. Probably better... Time and distance will tell.

Thanks again for all of the fast replies that lead to a fantastic solution.

Jim

Tweaking/modifying a Spectra for max RX sensitivity?

Tweaking/modifying a Spectra for max RX sensitivity?

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Re: Tweaking/modifying a Spectra for max RX sensitivity?

Re: Tweaking/modifying a Spectra for max RX sensitivity?

Re: Tweaking/modifying a Spectra for max RX sensitivity?

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