Maxtrac VHF poor receive

[wcu02mpa]

I have a 32 channel 25 watt 16-pin maxtrac 146-174 and I am experiencing poor RX sensitivity. I am using an IFR1000S service monitor and am only getting about a 2 (not a .2) uv RX squelch break. The TX is good and clear. I have heard of the PIN diode (s) going south on these and am wondering if they are the CR2145-215 and the CR207-207 diodes? Three of the diodes have continuity across, but the CR206 does not (when radio is on). If these are not the usual suspects, which one should I be focusing on. I have a full schematic.

Thanks,
Mike

[kcbooboo]

First thing to do is determine if the RF board has poor sensitivity by itself, or if it's poor when going through the antenna switching diodes in the PA module. The PIN diodes used on the RF board are very robust and usually don't go, but that's not to say they can't.

Try to inject a signal directly into the T-D jack on the RF board. Sensitivity should be around 0.3uV everywhere.

There is a protection diode right across the RX RF input. That often blows up when fed a lot of RF, and once it goes, the first RF amplifier transistor usually goes along with it. Both are mounted under the circuit board. Very common on GM300 radios but there's no reason it can't happen to a MaxTrac.

Bob M.

[wavetar]

Just my two cents worth...if you're getting 2 uV sensitivity, then the problem is likely the 2nd I.F. 455KHz ceramic filters. What I've found over the years is the worse the sensitivity, the closer the issue is to the antenna connector. Lower readings like 2 uV tend to be the ceramic filters, higher like 40-50 uV tend to be the 1st I.F. 44.55MHz filters. The diodes or first amplification stage tend to leave the radio either completely deaf, or needing a very high input in the milli-volts or higher. Of course, complete deafness can also occur with a dead 1st I.F. 45.1MHz crystal, which is a common problem, but not yours.

[RADIOMAN2002]

Every radio I even had with bad sense had bad diodes, caused by the bus company that had them ripping the antenna's off the radio and still transmitting.

[com501]

If you are injecting signal from the RF connector on the back, make SURE it is making good contact.

Have you separated the two boards from each other and put them back, tightening all the screws on both sides? These damn things seem to get intermittent after awhile and create all sorts of problems with grounding and cross connects between the two boards...