Motorola Desk trac receiver issue

[FCM]

I am having an problem with my Desktrac repeater. A transmitting station will have a full quieting signal and after about 30 seconds the signal will become overridden with static. The static becomes so bad, often dropping out the transmitting station completely. We had a pre amp installed and have removed it thinking it may have been the issue but nope. There is no static sound on the tx from the repeater controller id voice synthesized voice, only when a received signal is repeated. I have checked all my cables, connectors etc.. they all are tight. I am thinking an issue obviously is with the receiver in the desktrac. Anyone have any ideas of what I can try before pulling it and taking it to the radio shop for service? Is it possible the preamp somehow caused a component to go bad in the receiver? Thanks for the help!!! I know I came to the right place!

Frank

[Bill_G]

Not without a service monitor. You need to check the rcvr with a sig gen to verify it has failed. Doubtful the preamp caused damage.

[Satelite]

Hello:
Havent had much experience with the desk track but is it possible your rx freq is a tad off freq ?
I was thinking the desk track was similar to the maxtrac and if so id check the rx 44.645 MHz oscillator crystal for proper operation and adjust if needed.
If the desk track does use the same crystal that the maxtrac uses then in the maxtrac repair manual Y52 is the crystal schematic number Y52.
Really think you need to put it on a service monitor and check the rx freqs tuning ect to know for sure.
Sounds like freq drift issues to me and there are other issues that can cause this but youd need to confirm this with a service monitor to know for sure.
Just my thoughts.
Satelite

[FCM]

Thanks for the help. Im looking for a decent radio shop I can send it to. It is a maxtrac basically. it uses a receiver and a transmitter seperate. A service monitor would be nice to have for sure. Thanks for the advice! It wouldn't hurt to have the whole thing checked anyway.

Frank

[Jim202]

You haven't provided much information to come up with any suggestions. Bill has the best starting point to make sure the RX is operating correctly.

Are you using a duplexer to split the antenna between the RX and TX?

What type of feedline are you using between the radio and the antenna?

What type of antenna are you using? Is it a used antenna or a new one?

What type of jumper coax cable are you using between the antenna feedline and the radio?

How much TX power are you trying to run?

Has this problem always been there or is it new?

With some answers to the questions, you might give us a chance to help you.

Jim

[FCM]

Problem just started to occur more often. It first showed itseld a month ago and seemed to go away but no. We are using a duplexer, the antenna is new running about 45 watts output. All of the feedline is heliax with n connectors. Even the jumpers are all heliax. The antenna is a diamond f23h 2 meter fiberglass antenna. Swr was checked a month ago and was 1.5. Whats odd is I can sit at home with a ht and hold a solid full quieting carrier with no audio. After about 30 seconds the static starts and becomes worse. The duplexer wes tuned when shipped to me. Its a telwave 100 db 6 can duplexer. We thought it may be due to the extreme temps we have been having here but it has since warmed up. Thanks for the help Jim!

[Bill_G]

Well, thinking about this some more, with not much to go on like Jim said, when Satellite wrote about drifting, I began to wonder if the Desktrac rfpa was going bad. It is a Desktrac after all. (Satellite - a Desktrac is a Maxtrac in a pretty box) Those PA sections are known for giving up the ghost. Kind of famous for it actually. The repeater rx's, xmitter keys, and for the first few moments it sounds great. Then things start to heat up, and the rfpa starts throwing off trash that gets into the rcvr because this repeater has a notch only duplexer. Within a minute it's unreadable, rx drops, the station dekeys, rinse, repeat.

Here's what you do since you don't seem keen on taking it to the doctor. Disconnect the transmitter from the duplexer, and run it into a dummy load. Then have your friend key up to see how it sounds. Obviously he won't hear anything coming back, but you will because you're right on top of it with your HT. I'm betting your friend will come through just fine, and stay great all the way through his test count. You might hear some sizzle or a little crackle, but it won't be like it was before. Ignore that for now. Then, reconnect the transmitter to the duplexer, and have him count again. Probably will sound bad again proving the rfpa has failed. Box the whole thing up, and take it to the shop.

While you're at it, might as well have the preamp reconnected to see if it actually does you any good. Sometimes they do, sometimes they don't, and sometimes they work against you. Preamps work well if you have a quiet site with a good antenna and a bandpass filter with tight skirts. If you can get -118db with this Desktrac as it stands, you probably will not be able to improve on that. If it comes in at -110db, you should have that looked at. But, if it's a normal -112 to -116db, you can try the preamp to see if it makes a difference. With a notch only duplexer, it can get swamped easily by other local transmitters essentially becoming deaf until they dekey. If you're the only repeater for miles around, and there isn't much chance of getting clobbered, and the bench tests show you'll get a couple db more sensitivity, then go for it.

[FCM]

Ok, thanks. The duplexer is a band pass/ band reject telwave trpd 1556 6 can. Good advice for sure. Im looking for a radio shop to take it to as we speak. Ill het back to you with the results..thanks again!
Frank

[PETNRDX]

I agree with Bill G.
Check the TX PA.
That was the weak part of the Maxtrac.
If you are running that thing at 45 watts, it is the most likely problem.
The solder actually melts with long TX sessions.
I would recommend reducing the power some to keep that heat down.
Often removing the old solder, reflowing new solder fixes it.

[Will]

Im looking for a radio shop to take it to as we speak. Ill het back to you with the results..thanks again!
Frank

So, Frank where are you located, might help for us to get you to a good reliable radio shop.

[FCM]

Located in western Md. I forgot to mention the tx was reduced to 6 watts to drive an external amp. 45 watts out of the duplexer to the antenna

[Will]

Located in western Md. I forgot to mention the tx was reduced to 6 watts to drive an external amp. 45 watts out of the duplexer to the antenna

Well there you go. De-tuning the transmitter that far will definitely cause spurs which will 'kill' the receiver.

Maxtracs and Radius mobiles do not work well below about two thirds transmit power. The transmitter power amplifier,
a class C amplifier, was never designed to operate at that power. So they become un stable not running class C.

[Jim202]

Located in western Md. I forgot to mention the tx was reduced to 6 watts to drive an external amp. 45 watts out of the duplexer to the antenna

Well there you go. De-tuning the transmitter that far will definitely cause spurs which will 'kill' the receiver.

Maxtracs and Radius mobiles do not work well below about two thirds transmit power. The transmitter power amplifier,
a class C amplifier, was never designed to operate at that power. So they become un stable not running class C.

As a suggestion, you could put an RF pad in line with the output feed from the radio to the external power amp. The size will depend on the power your trying to run. The amount of attenuation will also depend on the power your trying to get the radio down to.

Another suggestion would be to remove the external amp and just run the radio as it was intended without the external amp. You indicated that amp was giving you 45 watts. Not sure why your even trying to use the external amp. The radio by itself will just about do that power out.

Jim

[FCM]

The radio output was turned to 6 watts out to increase the duty cycle as repeater builder recommended. The amp is a commercial amp that will produce 100 watts out with 10 in. I am taking the desktrac to a radio shop to have it checked regardless. How does decreasing the output power affect the receive? Would increasing the output power to 10 watts help?

[Satelite]

Hello:
As Will stated the maxtracs and im sure the desktrac didn't perform well with the output wattage turned down too far.
They were designed to run at an output closer to the pas max.
So I always figured half of rated max was the lowest that could be used without troubles.
Now Will I believe stated 2/3 rds of max rating and that can be the case with some im sure.
Id try removing the 100 watt amp and turn the pa on the desktrac back up closer to 75% of the pas max output and see if by chance the problem goes away as a test.
Then if you find it works then get a 100 watt amp that takes more watts input drive to produce 100 watts out to stay within the desktracs specs.
Its been a good practice to turn down wattage output to help on heat issues but too far down and you get spurs and if I recall you still get heat issues as the pa produces a lot of heat even more so when too low due to inefeciency as its out of its designed specs causing more troubles.
Someone if you would please confirm if the above info is correct.
If the test without the 100 amp test works then you might consider modifying the amp to need 25 watts of drive for 100 w out and then see if the desktrac would need extra cooling.
Id think you would be surprised at how well itll work if the destrac isn't in tx mode for very long transmit times.
If it is then you may need a fan to help cooling ect.
Just out of curiousity what is the destracs wattage output ?
Destracs model number ?
Thankyou
Satelite

[FCM]

Thanks for all the help. We haven't noticed any issues for the past few days. Seems like it went away when the temperatures went above 0. Anyway, I went up today and upped the tx output to about 10 or 11 watts. The rated output is 25. I can't run anymore than 10 into the amplifier. I checked the swr also and it is about 2. Im going to see if this change keeps us out of trouble. Thanks

[jackhackett]

If it has Repeat Disable (Repeat/Base button maybe?) and an internal speaker you can just disable repeat and listen to the receive audio through the speaker. If it stays clear then the receiver is okay.