Batboard

Have a GE Master 3 in service and was just refreq for our new repeater channel.

The input is 156.150 and I can take the Rx Front End and tune the Rx sens all the way down to the best so far between 3 modules of 0.17mV. The other two have really only been tuned down to the 0.22mV area.

Now I would be happy with these levels if they would stay that way but when we put the repeater on the air for use we loose our Rx sens all the way back down to about 0.44mV to 0.56mV.

Now we of course are setting these Rx Sens right off the front of the repeater with a Motorola serivce monitor. We try to go back and retune the recieve and now we can get it to only go back down about 0.26mV or so. Then the same thing seems to happen all over again.

Now it gets down to a point where the recieve front end gets tuned so much that it will only go down to about 0.56 to 0.63 mV.

The only thought I can think of that might be the problem is the Tx freq which is 155.565Mhz is over loading the front end and killing the Rx slowly. We are running the repeater through a duplexer.

So does anyone work on GE Master 3 and ever run into this problem before???

[quote="n9upc"]Have a GE Master 3 in service and was just refreq for our new repeater channel.

The input is 156.150 and I can take the Rx Front End and tune the Rx sens all the way down to the best so far between 3 modules of 0.17mV. The other two have really only been tuned down to the 0.22mV area.

Now I would be happy with these levels if they would stay that way but when we put the repeater on the air for use we loose our Rx sens all the way back down to about 0.44mV to 0.56mV.

Now we of course are setting these Rx Sens right off the front of the repeater with a Motorola serivce monitor. We try to go back and retune the recieve and now we can get it to only go back down about 0.26mV or so. Then the same thing seems to happen all over again.

Now it gets down to a point where the recieve front end gets tuned so much that it will only go down to about 0.56 to 0.63 mV.

The only thought I can think of that might be the problem is the Tx freq which is 155.565Mhz is over loading the front end and killing the Rx slowly. We are running the repeater through a duplexer.

So does anyone work on GE Master 3 and ever run into this problem before???[/quote]

First of all, your not being real clear on just what is going on with the receiver sensitivity. Second the differences bewteen 0.22 and 0.26 are splitting hairs. You may have issues with the service equipment with these small differences. As for the 0.4 to 0.5 readings your talking about are still acceptable receiver sensitivity readings.

If your unhappy with that, then your in the wrong field. Many radio sites would kill to get a receiver that good. Today the RF levels at most radio sites make receivers with too hot a front end a bad thing to have. You may want to investigate just what other transmitters are at the site and see just how they may effect your receiver.

These RF levels at the radios sites cause the usable noise floor to raise above most receiver squelch thresholds. Some of this noise can be reduced with the duplexer, depending on what type is is. You may have to put a pass cavity on the receiver antenna cable after the duplexer and before the receiver to help.

Your test cables you use from the service monitor to the receiver will also effect your readings. I see it almost on a daily bassis, where the field tech is using cheap RG-58 type test cables at these radio sites. These cables don't have the shielding required to keep out stray signals. As a result they allow RF into the receiver front end when your trying to take the sensitivity measurements.

Unless your using something that is double shielded cable like RG-55, don't go looking for help. You need to clean up your test equipment before true testing can be done. If the shields are missing on the receiver in a repeater operation, don't go asking for help. You need to clean up the receiver first. If the shields are missing on your transmitter, don't go asking for help. These are plates that were installed by the design engineer to keep out unwanted stray RF.

Is the cabinet and feedlines grounded to a good low impedance grounding system? Don't go asking for help until you clean up the site and the probable grounding mess that is there. I am not talking a #14 wire, I am looking at a #6 or #2 ground wire system.

Have the cables on the duplexer been messed with and managed to get loose? Are the cables on the duplexer all using double shielded cables? Are the cables between the radio and the duplexer double shieded or of the "Heliax" type cable? Is the antenna cable coming into the duplexer of the "Heliax" type cable? By any chance are you running LMR type feedline? If so, I don't want to even hear about any problems there. It needs to go.

Go do your homework and then come back looking for help. Tell us what is there. Try pulling the antenna cable off the duplexer and injecting your receiver signal at that point. Then put the receive signal right into the receiver antenna port. You should see about a 1.5 Db loss through the duplexer. This loss will depend on what the duplexer is and how well it is tuned.

Jim

OK Jim by the way I have also done my home work!!!

Lets start from the tip of the antenna all the way back down to the transmitter.

Antenna : Commercial grade 3 dB gain antenna by maxrad. Model number escapes my head at the moment. Antenna was tuned and tested for 155.5650 Mhz and has an SWR of 1:1.

Antenna Feedline: 3/8" superflex with soldiered connectors. Feedline with connectors installed were test for shorting and found no shorts. All connectors are brand new and in good working order.

In addition swept the transmission line with a TDR and found that line had tested good with no problems found. Connected radio and SWR meter to line and found same reading of 1:1.

Duplexers : Tested and tuned duplexers the first time and have checked the duplexer each time this problem has happened. In addition had a person (retired from Motorola) double check my duplexer tuning and he verified that it is correct and properly tuned.

Cables: Checked the jumper cables from the duplexer to the PA, as well as the cable from the duplexer to the RF input on the front end reciever card. Both cables test good with no shorts and or loss.

Service Monitor: Using a Motorola R2600 which was just recalibrated not even 3 months ago.

RX Sens.: I have tuned and installed repeaters in the past. Everything from /\/\ to GE to even Kenwoods. In all cases with the repeaters even other Master 3's I have always been able to get the Rx sens all the way to at least the high teens. Such as 0.17mV to 0.19mV and some I have been only able to get down to 0.22 or 0.24mV.

However, maybe if you did not read my post or understand it correctly I am trying to figure out why I can get it down to 0.22 or 0.24mV and then after being in service for only a day or so I go back and recheck the RX sens and see that it is all the way up to 0.44 to 0.56mV. So are you telling me that 0.44mV is acceptable for Rx sens on a repeater??? If you are then I would ask that you do your home work.

Test Cables: They are double shielded cut to freq length test cables. Also I have test cables for all connectors. I do not use or believe in using adaptors (kit or solid) I believe using test cables with the correct connectors on each end.

Grounding: I use a #6 ground wire which is securly connected between the cabinet/radio equipment, and the grounding source.

The shields are not removed from any of the modules on the Master 3. All modules are in the same state (except for being tuned on the Rx sens side) that they were when they were purchased and recieved from GE MA/COM.

I have worked on Master 3's before and tuned them with freq splits like this size and have never found any to do what this one is doing.

So again I ask has anyone that has worked with Master 3's ever run into this problem before? In addition what was the fix for it?

The only thing I can think is for some reason that the TX is overloading the front end and causing the Rx sens to increase?

So have I done my homework, so that I can now ask my questions?

I am not familiar with the GE M3, but I assume since you can tune it, there must be adjustable ferrite inductor coils involved? It is possible one of them has a small crack in it. That can drive you nuts until it literally falls apart.

Another thing to try...the next time you go onsite to re-tune it, check to see if the rx sensitivity improves when you vary the R2600 generator frequency plus or minus 5KHz. I have seen a few instances (in various other equipment) of the 1st I.F. crystal filter drifting when heated, which then improves once you take the unit off-line & possibly open it up for tuning.

Failing that, what does it use for the 2nd I.F. filters? If it's those black/blue small square 450-455Khz ceramic filters, replace them & see what happens.

Todd

OK now that you have told us what you should have the first time around, Will has opened up a couple of thoughts.

Take a flashlight and look inside the cans that have the ferite slugs. You might or might not see a crack if there is one there. The core will change tuning with moisture.

Another thought is that you might just have a bad solder joint someplace in the receiver. Had a mastr II that drove me crazy for about 6 months. The TX would just quit. I would open it up and it would start working again. Finally had to leave the drop down drawer down and the cover off. Managed to get there one day while it was failing. Ever so gently I started moving the coil cans a little until it started working. Found the coil causing the trouble. Unsoldered the cover and found that GE had never soldered the wire connection to the tuned coil.

Sounds like you might just have a hard to find pesky trouble. You might just have to poke, push bend tug or what ever and see if you can change the problem.

By the way, don't get me wrong with asking all the detailed questions. Many of the people on here just don't have the technical background or expirence to get pointed in the right direction the first time. Just read some of the posts on here. You didn't give much to work on in your first post. Those of us that try to help have to make some judgement calls based on what is in the postings.

Jim

Forgot to make a comment about duplexer tuning. There are many people in the field that have tuned duplexers many times. The question that needs to be asked is did they do it the right way.

If you watch someone try to tune a duplexer with all the ports disconnected from everything, look and see if they terminated the unused ports. In order to have the tuning come out correctly, all three ports, the antenna, the RX and the TX ports must be maintained at the "50 Ohm" impedance. In order to do this you need to have at least a 6 db pad on all the ports at all times.

I start off by putting a pad on all ports and then connect my test equipment to the other side of the pads. This way your test cables that your moving around are not coming into play while you do the tuning.

Your going to say that if the duplexer is tuned this way, that when you connect it up to the radio, the tuning will change. Your absolutely correct. However, it is closer to the correct tuning point than doing it with the ports left open and trying to tune it.

If you have a pass reject duplexer, you can take and use a couple of portable radios that have both the TX and RX simplex frequencies. Connect them up to the ends of the cables that would normally go to your repeater. Put the wrong frequency into the RX port (TX) and listen for it at the antenna port. Fine tune the cavities for the best notch or minimum signal. It should only take some minor tweeking on the cavity rods to get there.

Then change the port to the TX side and use the RX frequency. Do the same thing and tune for the minimum signal on this side. Make sure you connect up the unused side back to the repeater TX or RX while your doing this.

Another thought just came to mind. If by some chance your using some old Decibel or Sinclare cavities, you might just have to rebuild them. They are prone to poor grounds on the center tuning rod pipes. If that is what you have, let us know. I will go into more details in another message. These show up by not being able to tune the cavities and have them stay there when your moving the tuning around.

Jim

Just wanted to post a little update and bring this thread to a close.

I rechecked and reverified the whole installation and set-up but still could not figure out what was causing all of the issues of the Rx losing it's sensitivity.

Well sadly and happily a strong storm with lighting came through and helped expose the problem and in turn cause another.

The SO which is near where our repeater is located took a direct hit and knocked out there consoles and phones and faxes and radios etc.....

In turn and upon finding out the news the next day I travel to the repeater and inspected our equipment. Well to make a long story short we have had to replace the hardline with LMR-195 temp. until we can get some more hardline and replace it.
But while I was up there I decided to check the repeater again and found now that the Rx sens has gone all the way up to about 1.25mV. So I said enough is enough let's retune this baby until it breaks or I go nuts.

Well for anyone that has worked with a GE Master III they know that on the Rx side there are two modules: Rx Front End and Rx IF. Well as I was pulling the card in and out to tune it as someone from the shop walked with my extender card I bumped the Rx IF module and all of the sudden boom she came back to life.

I was having a hell of a time trying to get some sort of quiting and not static and quieing down at 0.44mV. But when I bumped this card boom she went full quieting. So I started to resecure the modules and bumped the Rx IF module and boom she went down the tubes again.

So quickly I ran and got a different RX IF modules and installed it and boom right away she got her Rx sens back. While injecting a 1khz tone @ aprrox 3khz of dev. I can get the repeater to open sql @ 0.12mV right where it should be IMHO.

Since we have great Rx sens now the next task is to move to a better and higher location as we are pretty piss poor coverage wise to the Northwest due to terrain. Where our repeater location is at you can look from the antenna and about 3 miles to the north/northwest you are back at ground level.

This new location we are trying to secure would be at least 50 to 75 feet above the terrain at the highest spot in the county.

So thank you to all who helped me out.

Glad to hear you got it going. If the IF board has those 450/455KHz square plastic covered ceramic filters on it, I'd bet good money they're the problem as I mentioned above.

Todd

It was wierd as I never thought to check the Rx IF as it tuned up good before but then went down the tubes.

I guess the one other time I bumped the module and got it to tune right I then must have bumped the module again when I pushed the RX front end back in and that caused my down graded preformance.

I am sure happy as hell that this is up and running to the level it needs to be. We cover approx 210+ square miles and a week ago this past Saturday went live with our first 8 first responders (in the past we had EMT's and higher first respond but now we have actual 1st responders only!)

Now we were able to play with the budget to order pagers and portables but we needed a secondary channel to communicate with them and not tie up the main fire channel like every one else doe sin the county.

When I can get a chance I am going to take off the metal cover and look for those ceramic pots and go from there.

[quote="n9upc"]It was wierd as I never thought to check the Rx IF as it tuned up good before but then went down the tubes.

I guess the one other time I bumped the module and got it to tune right I then must have bumped the module again when I pushed the RX front end back in and that caused my down graded preformance.

[/quote]

Glad you found your intermitent mechanical problem. Pays to tap, twist, pull and push on the boards / parts once in a while.

Us old tube guys just don't know what we are talking about on how to locate troubles anymore. We seem to be ignored by all the younger "I KNOW IT ALL" radio techs.

Jim