Silly MSF5000 Internal Duplex Filter Question.

KD6MHO · Post by **KD6MHO** » Sat Mar 06, 2004 9:56 pm

I just acquired a UHF "analog" MSF 5000. It is the internally duplexed model with the phasing harness. I was "told" this was a functioning repeater removed from service to be replaced by a new Quantaur. I am planning on taking it to a Motorola Shop to get programmed since I obviously do not have the "know how" to properly set one up.

Below the power supply are TWO sets of filters. There is a small one on the left that contains L12, L13, and L14 that is shaped like a backwards "L". (#15D83188NO) THIS ONE IS NOT HOOKED UP TO ANYTHING.

To the right of it is a bank of coils containing L15 L16, L17 and L18. (#15E83187N)This one is connected to the PA output and then to the antenna phasing harness.

My question is what is the smaller set of coils in the "L" shaped housing for and why would they not be used in this repeater as configured?

Another question I had was how much power is this type of filter capable of handling without degradation in performance?

Thanks for any replies
Aaron

Post by **batdude** » Sat Mar 06, 2004 10:47 pm

but, since i own 2 of them...

the filter you are referring to is called the "pre-filter" that basically ensures that the input to the PA assembly is fed only "on channel" signals without broadband noise from the IPA (intermediate PA) on the RF tray.

the L15-L18 filter is the "post filter" that's used on the output of the PA, again, to ensure that a clean RF signal is presented to the diplex cable for repeater output.

basically - it's just a trade off and a way to duplex the station by filtering RF (and only amplifying clean signals) without using a traditional duplexer...

they probably are not rated much higher than the PA is capable of - maybe 150 watts or so ... the pre/post filter arrangement is also about twice as lossy as a regular duplexer... typically 2.5 - 3 db loss through the assembly regardless of how anal you are tuning it... 3 db = 1/2 power.... so sometimes the PA will be doing a buck 40 just to get 70 out.... now you know why the MSF PA weighs 40 lbs.

now, why in the heck your pre-filter was not cabled up, I don't know.... even in stations where "traditional" duplexers have replaced the MSF pre/post filter arrangement, i have yet to see one that did not utilize the pre-filter assembly before the input of the PA.... i am not going to judge the decision... it's just "NOT WHAT I WOULD DO"....

doug

Will · Post by **Will** » Sat Mar 06, 2004 11:46 pm

They are rated at 55 to 75 watts with the internal filtering. Most of the loss is in the triple circulators on the PA chassis, bypassing two or all three ( remove the circulator and jumper with High Temp coax) will help a whole lot.

Post by **xmo** » Sun Mar 07, 2004 7:49 am

The pre-filter may have been bypassed because it has a problem. Since the station will still run without it, someone may have taken the easy out. Some of these pre and post filter assemblies, particularly in the older CLB stations, have been known to develop problems like excessive loss, eratic operation, or duplex noise. Sometimes the castings 'grow hairs'. Often this can be corrected by disassembly and cleaning.

The loss of these filters is not significantly worse than a standalone bandpass duplexer. The factory curves book shows the prefilter loss at less than 2 dB and the post filter loss at less than 1.5 dB. The post filters I have measured have typically had a measured loss of 2.0 to 2.5 dB [on a network analyzer] whereas a bandpass only T1507 duplexer will also have at least 2 dB insertion loss.

Doug · Post by **Doug** » Sun Mar 07, 2004 10:03 am

As strange as it sounds I've run into a few low power stations that have the pre-filter by-passed. All were working repeaters.
Doug

Post by **batdude** » Sun Mar 07, 2004 10:18 am

that at one time i had (should have NEVER sold it) a Bp/Br TX/RX set of UHF cavities that measured at 0.6db loss

that's a heck of a lot better than any pre/post filter arrangement!

doug

ASTROMODAT · Post by **ASTROMODAT** » Sun Mar 07, 2004 11:55 am

You might want to consider having Motorola clean up and rebuild your filter, although I think the MSF5000 may be outside the window of support.

Larry

KD6MHO · Post by **KD6MHO** » Fri Mar 12, 2004 8:08 am

Thank you all so much for the replies! I will be taking it in soon and see what the "shop" says. I will let you guys know if I hear any other details when I take it to get tuned up. This board is awesome...keep up the good work!

73
Aaron

Will · Post by **Will** » Fri Mar 12, 2004 11:51 pm

BTW Aaron, I am in the L. A. aera and will be glad to align it for you. I also do repairs on the MSF's.

bernie · Post by **bernie** » Sun Mar 14, 2004 1:08 pm

My two bits worth:

I have seen equipment failures such as xmo describes, however there is also what I call "Technician failure"

Why did you bypass the filter?
"I didn't have the proper equipment and I couldn't figure out how to align the filter, especially after messed with it."
"I've never seen on one of these, and that is how I got it to work."
"Base stations don't have these so it is OK to remove it"

"The specs for the station are 110W and I was only getting 70, so I can meet spec with this filter removed, the station works, so so what?
If some power is good more must be better!"
(the duplex station with tripple circulator is rated at 55W.)

"I have absolutely no idea what side band noise, or desense is."

I used to teach a class on the MSF at Pacific Wireless, and also when we were Motorola area KW.

KD6MHO · Post by **KD6MHO** » Fri Mar 19, 2004 5:20 pm

Will,

I might take you up on that offer to go through the repeater for me. If you can e-mail me with details and the city where you are located that would be good.

73
Aaron

Silly MSF5000 Internal Duplex Filter Question.

Silly MSF5000 Internal Duplex Filter Question.

I AM NOT THE MSF EXPERT

i would add...