RF damage to Uniden 996

[nydxa]

I'm wondering if anyone can offer some advice. I've got 2 radios installed in my vehicle: a Motorola UHF Spectra Plus (UHF 90W) and a Uniden 996 scanner. Shortly after installing the 996 I noticed the sensitivity of the 996 was rather poor. Sent it off to Uniden and sure enough the front end transistor was bad. I strobgly suspect that the damage was due to the high RF and close proximity to the 90W UHF radio.

Any suggestions?

[burner50]

separate the antennas... and kick the power down on the UHF.... why so much power?

[b7spectra]

I had two Spectra's (45 watts) and my 996 installed in my Sport Trac at the same time, the Spectra's being about 3 feet from the 996 and never had a problem. Each radio went to a separate antenna on the roof and even when I would key up, never even got bleed over.

[nydxa]

I'm beginning to wonder if the problem I had was pure coincidence...
Why the power.. well a few of the repeaters I use in Southern NJ are marginal if I'm running 30-40 watts.
The price on the radio was "the right price" so I'm not going to complain. Thanks to all who responded...

[w8jjr]

Not a coincidence. People blow the front ends all the time. Seperate those anttena by at least 3 foot.
The diodes and transistor will burn. Search this database and see it happens time after time.

[odontia32m]

Yep blew my pro-197 to crap. I was told by GRE that a minimum of 3 feet is needed between antennas.

[jparks29]

Observe proper distance.

At LEAST 1/4 wave distance for the lowest frequency.

So If you have one VHF and one UHF antenna, they should be spaced a MINIMUM of 26" apart... Not always practical in the real world, but it's the best bet.. (20" on VHF + 6" for UHF = 26" spacing)

That's just for proper groundplane, but is also a good guide for low/mid power transmitters... For higher power transmitters longer distances may be required.

[jim202]

People just don't bother to consider that receiver front ends are not made to handle high levels of RF.

Separating the antennas is a good idea, but 3 feet is just not much distance to attenuate high levels
of transmitting power.

It is much easier to do a combination of things. provide the maximum distance between the transmit
and the scanner antenna. Place a pair of hot carrier diodes across the gate input of the front end
transistor. Make sure you add a series resistor between the antenna coil and the diodes.

If you don't understand what I am talking about then you don't have a clue what your doing and
probably will blow the scanner again. Take some good advice from someone who has worked on
repairing public safety radios for over 40 years.

Jim

Originally Posted by nydxa

I'm wondering if anyone can offer some advice. I've got 2 radios installed in my vehicle: a Motorola UHF Spectra Plus (UHF 90W) and a Uniden 996 scanner. Shortly after installing the 996 I noticed the sensitivity of the 996 was rather poor. Sent it off to Uniden and sure enough the front end transistor was bad. I strobgly suspect that the damage was due to the high RF and close proximity to the 90W UHF radio.

Any suggestions?

[odontia32m]

Jim I know that my Pro-197 has anti tamper crap on the board and you can not do a resistor mod on it, I am building a box for the antenna line with Germanium diodes.

Bob

[w8jjr]

Originally Posted by jim202

People just don't bother to consider that receiver front ends are not made to handle high levels of RF.

Separating the antennas is a good idea, but 3 feet is just not much distance to attenuate high levels
of transmitting power.

It is much easier to do a combination of things. provide the maximum distance between the transmit
and the scanner antenna. Place a pair of hot carrier diodes across the gate input of the front end
transistor. Make sure you add a series resistor between the antenna coil and the diodes.

If you don't understand what I am talking about then you don't have a clue what your doing and
probably will blow the scanner again. Take some good advice from someone who has worked on
repairing public safety radios for over 40 years.

Jim

Jim

That works great on public safety radios (commercal).
The newer scanners (at least GRE's) wont allow for this mod as it detects it as a tamper attempt.
At least thats what GRE told me when I repaired one for a friend. Asked them why I couldnt do it
Perhaps a add-on box with the diodes would work

[w8jjr]

Ops Bob beat me to it

[mancow]

What do you mean "detects" it as a tamper?

I have completely disassembled and rebuilt a PSR-500 with no issues (removed the metal cans despite the black stuff).

[odontia32m]

Originally Posted by mancow

What do you mean "detects" it as a tamper?

I have completely disassembled and rebuilt a PSR-500 with no issues (removed the metal cans despite the black stuff).

If you try to remove the diodes it will destroy the board. The front end diodes on my 197 are covered by anti tamper junk. When you send it in the just replace the whole ass end of the scanner, not small parts.

Bob

[ofd8001]

Would I be correct in presuming if the scanner is powered off while the radio is transmitting, you don't have a worry with blowing out your scanner?

I've thinking about a homebrew deal that powers down the scanner whenever I remove the radio microphone. Usually I remember to manually turn the scanner off before transmitting, but I'm human and miss sometimes.

[jim202]

Power on or power off, the front end will still see the RF power and you will do the damage.

Not having been the owner of a scanner in my some 40 years playing with servicing radios, my
first thought would be to go back and tell GRE and the other scanner mfg to spend the 50 cents
and add the protection. They only look at the bottom line to make the max profit from each sale.

To my electronics and radio brain, they have designed an inferior product that doesn't stand up
to the normal use in a high RF environment. They are the ones that need to make the change.
If their management is smart, they would offer to make this mod for no charge to those that request
it. I guess you could wait until the warranty has expired and just go do it yourself.

Jim

Originally Posted by ofd8001

Would I be correct in presuming if the scanner is powered off while the radio is transmitting, you don't have a worry with blowing out your scanner?

I've thinking about a homebrew deal that powers down the scanner whenever I remove the radio microphone. Usually I remember to manually turn the scanner off before transmitting, but I'm human and miss sometimes.