RX Noise in Maxtrac

PY5FT · Post by **PY5FT** » Mon Sep 05, 2005 7:06 am

Hi!
Pls, anyone know this problem? Hi need to program a Maxtrac in
161.07 MHz but in this frequency exist a strong interferency from
the prescaler (MC12017) output harmonic frequency.

161.07 + 45.1 = 206.17MHz / 64 (prescaler) =3.221 * 14 (harmonic??) =
45.099 MHz (the IF...)

Thans a lot.
Antonio - PY5FT

k2hz · Post by **k2hz** » Tue Sep 06, 2005 4:09 am

I have noticed a very weak spurious signal in Maxtrac receivers on 161.07 but never "strong interference". The condition I have noticed can only be heard with the squelch open and does not cause any problem with a normal squelch setting. I first noticed it on some radios when scanning would stop on 161.07 or the busy indicator would light with no signal present. A normal squelch setting eliminated the problem. I never determined the source of this spurious signal but your theory is probably correct.

I believe you may have some defect with a bypass capacitor or circuit board grounding or shielding that is permitting this prescaler signal to be coupled to the mixer.

Dick, K2HZ

Will · Post by **Will** » Tue Sep 06, 2005 2:32 pm

Did you chaeck all the GND/mounting screws to make sure they make a solid ground. Also make sure the shields are tight and installed correctly.

Another thing, remove the RF board and clean the interconnect between the RF and Logic boards. We use Pro Gold cleanner.

WB2IID · Post by **WB2IID** » Wed Sep 07, 2005 9:23 am

I was going to try grounding, shielding, etc. but (having many years experience at fixing these types of problems for a living) I concluded that when all is said and done, the spur is still going to be there. You might get it down below the squelch threshold but it will still be on channel and degrade weak signals. If I remember correctly it was coming in at the RF, not through the IF.

One would have to completely shield the PLL circuit and/or the receiver front end in order to get some attenuation of this spur. My final fix was to switch to a Kenwood TK-730.

It is an unfortunate spur frequency since it lands directly on a popular northeastern RR frequency.

Joe

k2hz · Post by **k2hz** » Wed Sep 07, 2005 9:57 am

I have maintained several Maxtrac/Radius M200 series radios in railroad use on 161.070 with no serious problems as I mentioned in my previous response. This includes a Radius in my own truck for about 6 years.

In most radios I have seen, the spur causes about 3-4 db of quieting that is not even noticed with a properly set squelch. The worst case I encountered was about 8-10 db quieting and required a slightly tight squelch and some interference could be heard on very weak received signals.

One strange effect I noticed on this worst case radio is that the interference would be fairly strong on the bench with no antenna connected but it would drop as soon as an antenna or service monitor was connected. With the squelch set to open at about .35uv, it worked OK when it was installed in the vehicle but if you disconnected the antenna, it would unsquelch with an almost quieting signal.

If the interference is stronger that about 10-12 db quieting, I would suspect a defect that should be repairable in that particular radio.

It is unfortunate that 161.070 seems to be prone to "birdie" problems.
I had a similar situation on this frequency with GE Phoenix SX radios.

Dick, K2HZ