Converting A Motorola MSF 5000 Power Amplifier

[krazybob]

As we consider options for our MSF's that are 30KHz wide on VHF I have heard that the PA's can be modified for use with, say, a mobile radio based repeater. The In/Out is easy but what of the molex connector? What has to be done? What amount of drive does it need? We have three of the 125 watt PA's that seem to loaf along with 100 watts output after the duplexer.

We're considering a much smaller, capable repeater built using two CDM750's with 85dB adjacent channel selectivity at 20KHz. We have an offending station's output just 15KHz from our input. Not to mix topics, but I figure on adding a helical resonator from Motorola or even a Hamtronics. Suggestions on this as well? MSF's seemed great at $350 complete. I was told that they were rock stars. Not in Los Angeles with a 15KHz band plan. The helical resonator should help tame the broadband front-end of the CDM750.

[Doug]

The molex connector is specific to the msf. It contains forward and reversing voltages and drives the exciter accordingly. You will not need to attach it to anything. The output will be dependent on the exciter drive. Somewhere around 2-2.5 watts of drive should take it to full output power.
I'm a bit confused, you have fabulous resource at repeater-builder and have been directed there several times over. The information is there with a bit of reading and a cup of coffee what you seek is there
Doug

[krazybob]

Hi Doug,

I read RB but I like the atmosphere better here. As a newer repeater trustee I have much to learn. I admit it. You have graciously answered my question. In my humble opinion RB isn't so much for new guys like me. I read a lot of arguments. But their research stuff is great if one knows what to look for. I once asked a question there that got many different answers and demands that I do this, do that. "Why didn't you run the tests I told you to run and report back to me?!@!!" No lie. He was ticked. I was stuffed in the middle of an argument. But here -- some of the same folks as over there but in a more laid back atmosphere. I sure appreciate it and have learned so much.

Not knowing what to type initially I just tried "repeater-builder MSF power amplifier" and got this: http://www.repeater-builder.com/motorol ... r-amp.html TADA!

Thanks, Doug. You fall into the nice guy category.

[Doug]

Just an out load thought, Is using a back to back radio configuration really your best option? Two things come to mind right off the top the first thing is your going to be using a radio as a transmitter/exciter. Duty cycle is foremost, this is a crap shoot I know of people who have tried and had good success however, most of them fall under the category of "garage repeater" If this is going to be a high profile machine I would rethink my approach. The next thing is the stability of the transmitter/exciter with some of the older equipment there used to be an unwritten rule not to reduce its output power to less than 75% of full capacity as I have not had much involvement with the CDM series I would be a bit leery of lowering its output to drive your amplifier. You could most likely over come this by using a pad on the transmitter. If my math is correct lets say you have a 25 watt cdm and you put a 9db pad on it that should lower the output to about 3 watts give or take which should give you adjustment room to work with the amp.
Doug

[krazybob]

Yes. If you lower the TX PA output the transmitter can get dirty and repeat trash. But if the CDM radio has a low power setting an attenuator on it could drop its level. But we're think more of just using the CDM for RX at this point. I am just exploring options. I have three MSF's that I'd hate to waste. But I did want to ask about the MSF PA's. Even if used on my radio at home.

[krazybob]

Your advice Doug of using the resources at RB was a good one and I've been there again. Unfortunately,

The file you clicked on does not exist or has been renamed or moved.
...
...

It is now at http://www.repeater-builder.com/motorol ... r-amp.html

I hope that this helps someone else in the future.

[PETNRDX]

If an MSF is not working for you, then, just my opinion, the CDM is very unlikely to work either.
Look at its specs.
The adjacent is -85 at 12.5 and -80 at 25. That doesn't sound too bad.
The intermod is only -75. That sounds (and is) bad to me.
The MSF is -90 at 25, intermod of -85.
I tested the CDM and several others using TIA-603 and while the CDM met the specs published, there were other radios that worked better.
Even the trusty GM300 worked better. That is what we stuck with by the way.
You have to go back to MICOR or MASTR II to get anything much better, and who wants to buy all those crystals.
If you are in LA, then likely your biggest challenge is going to be the intermod spec, the image a spurious spec, THEN maybe the adjacent. (Speaking specifically of the CDM in that statement).
Don't get me wrong, I LIKE the CDM, but as others have said, duty cycle on TX and the possibility of intermod on the RX would make me stick with the MSF.
I am pretty darn sure external filtering such as helical or pass cavity won't do much for someone that you say is 15 kHz away.
That is difficult no matter what.
I don't even know if a crystal filter would help.
This sounds like one of those cases where the actual, real at the site measurements and mathematics based on the measurements is the only thing that will tell you if your problem is solvable.
If the 15 kHz guy can be "nulled" by antenna pattern, and you can "afford" the little loss of signal that the extra filtering might cause, you *might* get it to work.
I hope you keep posting your efforts here.
Don't let too many of us opinionated folks scare you off.
Most of us really are nice, but some of us are sometimes a bit too abrupt. (I plead guilty)
Been at this a long while, but I learn something here every couple days.
For example, I recall using crystal filters (external) to solve this sort of problem long ago, but I forget exactly how close, and how much loss. I remember it was worth it for those systems. Not sure how much people use them anymore.

[krazybob]

Thank you Steve for seeming to understand what it is to try and learn over the internet. It would be far easier if I had a hands-on Elmer. But this thread is about using the MSF's PA and nothing more. Thank you nonetheless for your kind, polite and encouraging words.

[Doug]

Bob,
I did a setup for someone just recently with an MSF power amp. With the amplifier being non-linear it was interesting seeing the results. The setup was a standard 110 watt UHF amp. At 2.67 watts of drive it set the out put to 156 watts (no bueno) 2.53 watts drive set the output to 138 watts. The magic number for me turned out to be 2.37 watts of drive for the magic number of 103 watts. I'm guessing that ymmv from amp to amp. I thought I would throw this out for the collective should anybody else want to try.
Doug

[kcbooboo]

That's why there's such a high-gain and fast-responding power control circuit in the station. It takes a very slight adjustment in drive power to make a big change in PA output power. While running it by itself CAN be done, it's not at all an ideal situation.

Bob M.