Batboard

Hey folks!

I've been having some RX issues with a MSF5000 "Limited" VHF 100W Repeater. I will try to keep this short, but hopefully supply you with enough information to steer me in the right direction. Our repeater is in a communications "shack" so to speak at the tower site. Antenna mounted 250' (3/4 way up) the tower.

We had a new hardline and Telewave VHF Antenna installed on our system last year because the old equipment was in horrible shape. Things worked alright till last fall when the repeater began to experiencing some RX issues. Funny thing is, this issue is intermittent. It'll RX fine for a few days and begin to do its thing on the next few. I'm starting to wonder if it's weather releated. I live in Northern Maine, so at this time we've been having very cold and somewhat humid weather.

The RX issues I describe are static "popping" noises while someone is talking over the repeater. The "popping" noise is most prevalent when a distant mobile radio or fairly close portable radio inside of a building is in use. This is not an issue when using our base stations since the antennas are line of sight with the repeater antenna and are operating at 50W.

Radio tech came by, climbed up the tower and re-weatherproofed the antenna connector & hardline. The results were great and the problem seemed to have subsided but now it's back again, and worse than ever. We've had a weeks worth of -30'F weather and now it has warmed up 0+ (yay!) and this is when it all began. Maybe it's coincidental but I figured I'd add it in anyway. Radio tech also checked TX output and duplexer and stated everything was fine.

Any thoughts and ideas would be appreciated! Thanks!

My first thought is that your on the right track in looking at the feedline. However, it might be
worth putting an SWR meter on the line and watching the reflected power while your hearing
the noise problem. My first thought is that the coax center pin, or the actual center conductor
is pulling back just enough to make a poor connection.

Second thought is that you have moisture in the connectors and or feedline. This may be
trying to arc over in the transmit condition. Again, you should see this with an SWR meter.

If you have access to one of the newer TDR / Frequency transmission test sets, take a look
at the feedline. Blow up the display and look for miss matching at any of the connectors.
You might even find a bad antenna that is making and breaking a connection in the cold with
the wind helping flex it. Unless you have an original saved sweep of the antenna system, you
might not pick out a bad antenna. The fiberglass sticks are very prone to this problem. It
normally shows up as loosing system coverage.

Jim

vb105 wrote:Hey folks!

I've been having some RX issues with a MSF5000 "Limited" VHF 100W Repeater. I will try to keep this short, but hopefully supply you with enough information to steer me in the right direction. Our repeater is in a communications "shack" so to speak at the tower site. Antenna mounted 250' (3/4 way up) the tower.

We had a new hardline and Telewave VHF Antenna installed on our system last year because the old equipment was in horrible shape. Things worked alright till last fall when the repeater began to experiencing some RX issues. Funny thing is, this issue is intermittent. It'll RX fine for a few days and begin to do its thing on the next few. I'm starting to wonder if it's weather releated. I live in Northern Maine, so at this time we've been having very cold and somewhat humid weather.

The RX issues I describe are static "popping" noises while someone is talking over the repeater. The "popping" noise is most prevalent when a distant mobile radio or fairly close portable radio inside of a building is in use. This is not an issue when using our base stations since the antennas are line of sight with the repeater antenna and are operating at 50W.

Radio tech came by, climbed up the tower and re-weatherproofed the antenna connector & hardline. The results were great and the problem seemed to have subsided but now it's back again, and worse than ever. We've had a weeks worth of -30'F weather and now it has warmed up 0+ (yay!) and this is when it all began. Maybe it's coincidental but I figured I'd add it in anyway. Radio tech also checked TX output and duplexer and stated everything was fine.

Any thoughts and ideas would be appreciated! Thanks!

vb105,
When all of this has been going on have you had the option to listen directly to the receiver with a test set?
Doug

Doug wrote:vb105,
When all of this has been going on have you had the option to listen directly to the receiver with a test set?
Doug

Thanks guys for your feedback! Doug, unfortunately, I have not. I am not doing the troubleshooting directly, we call our local radio maintenance people to check it out. It's more or less our combined efforts to try to get this repeater running smoothly. We're going to be troubleshooting it monday, I'll keep you guys posted.

The reason for me asking was I had a machine that acted very similar and the problem actually wasn't with the receiver section, it was a thermal issue in the pa. As strange as it sounds the omega strips between the power transistor boards were flakey which caused strange noises and would appear to wipe out weak signals in the receiver. I'm not saying that this is your problem but it wouldn't hurt to take a look at it.
Doug

Doug wrote:The reason for me asking was I had a machine that acted very similar and the problem actually wasn't with the receiver section, it was a thermal issue in the pa. As strange as it sounds the omega strips between the power transistor boards were flakey which caused strange noises and would appear to wipe out weak signals in the receiver. I'm not saying that this is your problem but it wouldn't hurt to take a look at it.
Doug

Thanks Doug, we'll definitely take a look at it!

It's also a known issue with these radios developing metal connections in the VCO and preselector sections of the RF deck.

It's not a bad idea to disassemble these assemblies and go through them with a brush or wooden stick and break loose any filaments that have grown, then reassemble and retune the station.

These things are known as "Tin Whiskers", and if you think I'm blowing smoke, read about it from NASA: http://nepp.nasa.gov/WHISKER/.

This problem is KNOWN and VERIFIED to occur in MSF 5000 and I believe GE MASTR II stations.

Not saying this is your issue, but it is most certainly a potential source of your problem.

In situations where disassembly is not an option, I suppose a few whacks on the preselector assembly with a rubber mallet would be better than nothing, though I can't condone it, I can't say I've never done it before. If you dent something, you're whacking too hard.

d119
VHF stations are a bit different in that they don't have adjustments in the vco circuit. In UHF, 800/900 MHz they are prone to the whiskers due to dis-similar metals. And here's a bit of MSF VCO trivia for the on lookers. Rumor has it that the MSF had two versions of VCOs an "A" version and a "B" (last letter stamped on the VCO) the "B" version they changed the metal compositions to rectify the whisker problems, if it solved their problem I'm not sure but that is what the intent was.
MSFs when they work they work great but when they start screwing up they become character builders.
Doug

Hi Doug,

OK. I wasn't aware that the issue mentioned didn't affect VHF stations. I'll remember that.

Thanks for the heads up!

A clarification: the whisker problem can affect ALL MSF5000 stations. They grow from the tin plating present inside and outside the VCOs, the mixer coil assembly, the front end assembly, and the internal filter/duplexer assembly. The shiny coating (or metal itself) is the tin plating that's great for containing RF and also great for whisker growth. The dull bare metal usually does not promote whiskers.

The fact that the VHF VCOs aren't user-tunable is what makes them different from their UHF/800/896 counterparts. They can still suffer from whisker growth, just not around the tuning slug.

Bob M.

Lets get back to basics,

Your on VHF located on a fairly high spot. If your using a fiberglass stick instead of a metal folded dipole then you may have static charge problems if you have poor grounding. The static buildup will cause the popping, crackling, or otherwise reduced performance on Rx while TX sounds normal.

The crackling can be determined on Tx or Rx by operating the station remotely. If it has tone control then key the station remotely to make sure when the radio is not repeating the Tx audio is clean, if so then its not Tx.
If its not a remoted station then standing at the radio when the problem may arise is going to be your other way to determine if its an Rx problem or Tx problem.

While in front as the problem is going on remove the antenna for the Rx side and then reattach to see if the problem goes away. This may tell you of a static problem as when you are holding the end of your coax you may discharge the static through your hand and then after reapplying the coax the noise is gone that may show you what is happening. This is just one thing you can try without any tools equipment or training. Also since this is very simple there is very little risk in damaging anything if you are unsure of what exactly is going on. (see note)

(If your unsure what cable goes where I hope you have a good radio shop because if I have to explain this you may not want to try and diagnose the problem as it gets complicated hereafter.)

CoM

CoM, I'll definitely try that out!

We hooked up meter and measured 75W output after the duplexer and less than 1/2W deflected - it was pretty much concluded that it wasn't a hardline or antenna issue. Regardless, we've temporarily switched to a another (known good working) VHF antenna on the tower. Unfortunately the issue still remains present. Next step is to hook up a service monitor to verify/tune the duplexer, there may be a slight chance that the duplexer may be off because it "was" mounted in such a location if someone weren't careful they could easily hit it by accident. There is another radio maintenance company that comes to service the other repeaters located in the same cabin, so I wouldn't be surprised.

I'll let you know what we find after we rule out the duplexer... I'm praying it's not the repeater!

Well,

If you have tried a different antenna then you have ruled out static.

CoM

The problem is a non-linear joint on the tower. The problem should go away when it rains. I would also check any conduits that run up the tower, try to get as far away as possible. It's not in your system, it's external.

This hasn't been proved one way or the other yet. There still could be coax cable, antenna or radio problems.
It is a little early to be pointing the finger in any direction yet.

Without a measurement by something like a "Site Master" to be able to look at the entire antenna system
when it was first installed and what it looks like now, would be real hard to point the finger. It wouldn't hurt
to take such a device and open up the scale to look very carefully at the entire coax cable run and the
antenna. You should be able to see almost every bend and connection between where you connect and the
antenna itself. You could have moisture migrating into the cable. You could have a bullet hole in the cable,
you could have a damaged antenna. Next time someone climbs the tower, have them check your coax cable
real close around the tower red lights. Hunters like to take target practice on the lights and the coax takes
the beating in the process.

I don't think any one has asked or you have provided just what type of coax feedline your using. The reason
I am asking is I hope your not using LMR series coax cable for the repeater. This style cable is prone to
generating noise in duplex operation after a while. You will never be able to clear up any noise or desense
with this cable being used. This is a wel documented problem that we don't need to go into again.

Bear with me as this thread has been on and off for a while and I can't remember all the details without
going back to read it again from the start.

Have you noticed if the problem is weather related, like it has to be windy for the problem to happen?

Does the problem get worse when it is damp, like with fog?

Does the problem go away when it rains?

Have you been able to get to the site while the problem is there and put the radio onto a dummy load?
Pull the antenna coax off and put the dummy load at that point. Use a service monitor to generate a
weak signal. Do you still have the noise?

Does the noise have any bearing on the room temperature of the room where the radio is located? Like
the noise is gone at night, but there during the middle of the day.

Does the noise just suddenly start or is it more of a gradual buildup when it occurs?

What kind of an antenna are you using? Is this a fiberglass pole or a metal folded dipole?

Is this a windy site location or just the run of the mill normal site? Reason I ask is that I have had some
real bad problems with some new, low cost fiberglass poles. One only lasted a month before we had to
replace it. Told the boss not to buy it, but he wouldn't listen. So he ended up buying a second antenna
and we used the second one like I recommended in the first place. To my knowledge it is still working
some 15 years later.

This should keep you busy trying to narrow down the source of the noise problem.

Jim

Beekeeper wrote:The problem is a non-linear joint on the tower. The problem should go away when it rains. I would also check any conduits that run up the tower, try to get as far away as possible. It's not in your system, it's external.