So here's the senario, I'm working on an msf for a guy that appears to have had the front end over loaded. It took out the injection amplifier which I have verified by taking apart one of my working spare repeaters and used my parts in its place. My question is do you think it would be possible (with modifications) to stick a micor pre-amp in its place? I've got a couple down ranged ones here that I maybe able to fit in it's place. I don't see where getting power to the pre-amp would be a problem as there are all kinds of options to grab it from.
Doug
UHF MSF receiver
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UHF MSF receiver
May the Schwarz be with you.
Re: UHF MSF receiver
The injection amp has to provide almost 1 watt of output signal (nearly +30dBm). The Micor preamp only provides about 6dB of gain with a very low noise figure. It won't provide 1 watt as it's meant to boost low-level signals into a receiver.
Odd that the injection amp got affected by overload. I'd have suspected the front end amp / mixer which is located on the front end casting, which must be dropped to access the circuit board.
I haven't checked the schematics so I don't know if they used different splits for the injection amp. It's basically untuned but they might have optimized the parts for range-1 and range-2. The mixer coil assembly is certainly range-specific. The amp is the same on both CLB and CXB stations, so a donor might be the most economical way, short of fixing the bad one.
Bob M.
Odd that the injection amp got affected by overload. I'd have suspected the front end amp / mixer which is located on the front end casting, which must be dropped to access the circuit board.
I haven't checked the schematics so I don't know if they used different splits for the injection amp. It's basically untuned but they might have optimized the parts for range-1 and range-2. The mixer coil assembly is certainly range-specific. The amp is the same on both CLB and CXB stations, so a donor might be the most economical way, short of fixing the bad one.
Bob M.
Re: UHF MSF receiver
Thanks Bob,
I've never really measured the output from either, 6db isn't enough to even mess with if re-engineering is going to be involved, this was kind of a reach in the dark. I guess back to square one. I'm still questioning why and how the component failed. The guy added a pre-amp and I believe it was more than what the station could handle. (who knows how efficient his duplexer tunning was)
Doug
I've never really measured the output from either, 6db isn't enough to even mess with if re-engineering is going to be involved, this was kind of a reach in the dark. I guess back to square one. I'm still questioning why and how the component failed. The guy added a pre-amp and I believe it was more than what the station could handle. (who knows how efficient his duplexer tunning was)
Doug
May the Schwarz be with you.
Re: UHF MSF receiver
I added a preamp to mine as well, an ARR model with 17dB of gain. I had to place a 10dB attenuator on its output because the MSF couldn't deal with that much more signal. It looked great on the meters but added noise along with the desired signal. Still that shouldn't have caused the injection amp to fail, even if the duplexer was feeding a hundred watts of power into the front end, unless it was on the input frequency by mistake.
I'm sure I've seen schematics of the injection amp, except for the 800/900 MHz stations, so it should be repairable.
Bob M.
I'm sure I've seen schematics of the injection amp, except for the 800/900 MHz stations, so it should be repairable.
Bob M.
Re: UHF MSF receiver
I did check a manual. There ARE two range splits for the UHF injection amp, with a handful of capacitors that are slightly different on each. I will correct an earlier statement and say that while the unit is not adjustable (tunable), it does have tuned circuits. Still not much to it: a pair of transistors and a 5V 3-terminal regulator configured for 10V.
Bob M.
Bob M.