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
GM300 UHF Deaf, 10 microvolts RX Sens.
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jackhackett
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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.
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.
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smile@2006
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jackhackett
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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
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
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jackhackett
- Posts: 1518
- Joined: Tue Jun 10, 2003 8:52 am
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.
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.
If you don't have a MOL account, it is less hassle to order from these guys:Emorth wrote:Does Motorola have an online site where I can order the diodes?
Thanks,
Ed
http://www.myradiomall.com
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