Batboard

I have a UHF Spectra 450-482 that I am trying to move to the ham bands. I modified the VCO (by using a conductive pen on the strip) and can now transmit all the way down to 440 but Recieve stops at approx 445. Does any one know why the recieve won't work below 445? The VCO steering voltage at 445 is over 3 volts so I believe I should have enough range to go lower in freq.
Regards
LArry

Are you still getting a FAIL 001? If so, then you need to go further with the conductive pen. If not, then your receive prefilter cuts off at 445, and those can't be modified.

Agreed ! I try for about 2.5 v as a low limit at 440 just to be stable,
but MOTKU is right on that one. If the vco locks but rec stinks, then look
around for some preselectors that are a bit lower in pass band. Phrawg

I didn't have to fool with the VCO on my D44 Spectra, and it operates just fine down at 444.450 MHz simplex. I'm getting very close to rated sensitivity there, although it is a bit better between 450 and 480. That's where the filters should be working the best, but it's not like the front end just quits working down at 445. The sensitivity will drop off slightly after you get more than 15 MHz away from the center frequency of 465 MHz.

When you say it doesn't receive at 445, how so? Do you get the FAIL 001 message? Does it refuse to program down there? Is the sensitivity worse than 0.4uV?

Bob M.

Thanks for the comments.
I am gettting a Fail 001 indicating that the VCO is losing lock. As I move down freq from 450 I see the voltage inch incrementally lower as well. When I hit 445 it is still over 3 volts. 444 loses lock. I am moving in 1 mhz steps so I havn't tried freqs between 444 and 445 to see exacttly where the lower limit is . I would like optimally to hit 442 as the lower limit

Larry

Get a conductive pen that lays a silver bearing "ink" down.
Look at the stripline in the vco cavity very carefully. You will see laser cuts there. They are made to isolate pieces of the strip till the vco runs
in the right range. They always are too long and freq to low before the
laser cuts start. Clean the strip carefully with a q-tip and a touch of
alcohol then bridge the first cut. Re-test. If no go then electrically
lengthen the strip by adding another "ink" spot on the second cut,
etc,etc,etc untill the range is reached. I usually find several cuts
close together as the final adjustment cuts get closer. I usually
bridge the first 2-3 on the first pass and I come really close. If you
bridge too many a q-tip and alcohol or acetone will remove the
conductive ink alltogether then re do it and try again. This procedure
has always worked for me and I have done several dozen 450
range spectras. The conductive pens are not inexpensive but the
process works very well when tuning stripline circuits. The best way
to keep one fresh also is to just use it once a month and make a very
small mark somewhere with it so you keep the flow open in the nozzle
with minimum waste. It has silver in it. Store it tip up also.
Hope this may help. Phrawg

I used the conductive pen as described I extended the strip and the TX range moved nicely(over 5 megs) but the RX seemed to stay fixed. Any other reason anyone can think of that could allow the VCO on rx to lock lower in freq?

There's a much easier way to move the range. I'm going to have to look through my archives for the exact procedure, but it simply involves installing a 220K resistor between two pins in the VCO circuit. I've done this mode for three Spectra's (2 UHF and one 900) and they all work perfectly.

Send me an e-mail to nydxa "at" yahoo.com and I'll do my best to find the procedure and send it to you. It will take me a day or two to locate the photos I took. It's very easy if you can solder!

BTW, you might have to undo the mod you've already done.

Bob

BUT, A vco that runs naturally resonant is much more stable near
the design freq edges and has less phase noise that one that is
"forced" to run. The linearity of tracking steering voltage vs freq is
much truer if you actually tune the circuit instead of fooling it
into operation. Not saying that the resistor trick does not work,
and if not pushing extremes of frequency shift you will not notice
any problem, just saying that if possible, retuning the strip would
be the prefered method. Phrawg

You are correct, but after 3 years of continuous service, I've never had one bit of problems. Granted, I'm only shifting it down from 450 to 440. Instability might result if you try to pull it more.

The etite modification took me longerto open the radio than to install the resistor.

Instructions with photo were sent to you this evening. Let me know how it works!