Batboard

I have a UHF GM300 that has poor RX sensitivity - about 10 microvolts. Squelch pot is set at threshold. It is on freq, going up or down 5 KHz makes it sound off freq and sensitivity gets worse. Local/DX set at DX, checked voltage at DX transistor switch and it is OK. Reseated RF board, no change. TX OK, full power and on freq.
Any common problems? Any ideas?

Thanks,
Ed

Have you tried unplugging the cable that goes from the PA to the RX input on the RF board and injecting the signal directly into the RF board there? That would tell you if the problem is on the RF board or in the PA.

If problem is in the PA it's probably the PIN diodes used for RX/TX switching.

On the RF board the most likely thing would be the RF front end, either the diodes across the input of the RF amp, the RF amp transistor, the AGC, or some combination of those. The diodes are the most likely.. not uncommon for those to get blasted and short out, especially during lightning season.

Usually 2nd X'tal 44.645MHz is damaged.
Inject 555KHz near IF test point, you will receive the signal.
Please remove X'tal and replace it.

smile@2006
"AZLAMDFD"

When the 2nd IF crystal goes you generally get no receive at all, weak receive usually points to the RF front end.
(also it's 455kHz, assuming that was a typo)

Front end protection diode(s) shorted.

RF amplifier transistor bad.

As said earlier, T/R diode switches on PA circuit board. If these go, it'll likely cause something on the RF board to go as well.

Bob M.

I tried bypassing the PA, only a very slight improvement.

Unfortunately, I don't have an RF voltmeter. At this point it is probably easiest to just replace the parts and eliminate some variables.
Are there favorite generic replacements for the diodes and transistors that I can get from Digikey or elsewhere or is this a Motorola purchase?

Thanks again for all your help,

Ed

Replace the RF overload diode on the base of the RF transistor first. The diode costs less and is usally what goes shorted.


Also check if there was a reduction in RX sensitivity when the LOCAL was turned on. If so, probally the overload diode.

These are surface mount parts, on the bottom of the RF board under a shield if I recall correctly, I won't have access to the manuals until monday, so I can't be more specific on their location. Once you locate the diode (I believe it is an SOT-23 dual diode) you can carefully remove it and recheck the sensitivity, if the sensitivity is okay without it in, the transistor is still okay.

Often you can see if an SOT diode or transistor is bad, there will be a small lump raised on it where the gasses vented when it burned up, also you can check them with the diode check function of most DMMs.

As for replacements, your best bet would be to get them from Moto. I'm pretty sure a generic replacement could be found, but you have to remember that you're dealing with UHF freqs.

You guys are good!
Removed CR2 & CR3, sensitivity is back
Any recommendations for generic replacements?
Thanks again,
Ed

Jack,
Ooooops. I posted my last message before I read yours. You are right. Moto is the best way to go.
Thanks again,
Ed

Does Motorola have an online site where I can order the diodes?
Thanks,
Ed

Emorth wrote:Does Motorola have an online site where I can order the diodes?
Thanks,
Ed

If you don't have a MOL account, it is less hassle to order from these guys:

http://www.myradiomall.com

Fast service and discount price.