moto MoTRAC

[EdgeRadio34]

Hi

It has been a wile since I've been on here.

I recently acquired a Motorola MoTRAC UHF repeater with full set of manuals. The only thing missing was the pa.

When I first set it up i had limited transmit and receive. as if my antenna network was not hooked up, and only worked with the doors open.

I have since got a MOTO test set and a SWR/ power meter and a dummy load.
Now my unit has no transmit.

I'm thinking my channel element is ether not working or is not being selected. I have NO meter reading on any meter test points. (I do have reading on the receiver)

Some more question, I acquired a Micor 75w solid state power amplifier. will it work with my Motrac. The unit originally had a tubed amp. I do know that there is a 10 to 11 vDC hi that triggers the tubed amp will that work with the trigger on the Micor amp?

If any one has any info for me that would be awesome.

thanks Rich

[Bill_G]

The short answer is: No, the Micor rfpa will not work on the Motrac. The Motrac power supply will not handle it.

I personally would not put a lot of effort into the station since finding parts is a problem. Since you have the full set of manuals, you could go through the troubleshooting chart to determine why the transmitter is no longer functioning. It may require some rf test equipment like a service monitor to help. But, the meter panels and a vom will go a long ways towards answering your questions.

[EdgeRadio34]

Sorry I forgot to mention I was going to be powering the PA deck with a power supply scavenged from an 80 watt msr2000.
I was wondering if the PA was RF "keyd" or signaled "keyd" and is signaled keyd what kind of signal 10, 12 vDC hi or bring a high signal to ground??

Thanks for the info and your replay though. I hope I can get this running. I really liked it because its simple and simple to work on.

Thanks Rich

[Bill_G]

There is a keyed voltage present that drives the lower stages that in turn drives the final. In some models the keyed voltage is switched by a relay. In later versions they had transistor switching. You will have to trace out the ptt circuit to see what is going on. The manuals should have a detailed theory of operation. Familiarize yourself with it.

[RADIOMAN2002]

You can not use the Micor PA with the Motrac. The Micor PA requires 1/2 to 1 watt of input power. The Motrac puts out nearly 7 -10 watts. The other problem is you will have no PA power out control, since the SWR protection circuit for the Micor comes from the low level PA module. You would have to run the PA wide open and with no SWR protection. You would be better served by installing a TPL or similar manufacturer amp that has the proper RF input rating and a RF switch to detect the presence of RF when keyed. I have used the Motracs as RF links for years, they are bulletproof, the only limitation I ran into is they don't do DPL.
As far as no drive, make sure the card cage has all the right cards still in them. For the radio to select a channel element during transmit, you need either a tone key, DC key, or repeater control card installed. I have in the past jumped the channel element hot at all times in the station control module which is OK, but the oscillator running does then put out a low level signal close to the radio.

[EdgeRadio34]

HI thanks for all the info.

Some up dates,
For some reason my R-1033A test set with the RTK-4042A cable doesn't work on my transmitter other than to key it. i get no meter reading on anything. But it does work on my receiver. have no clue whats going on.

On another note I got my transmitter back by tuning it with a vom. not easy by your self.
But I still have no power out of it. or at least no readable power for the meter I got. (requires at least 1/2 watt.)
I tuned everything back to what my book said it should be.

So to recap I pick up the exciter with the cabinet doors open but not with them closed.

I've gone through all the rf connections between the Varactor and the harmonic filter, the harmonic filter in to the carbonate. made shore they were all clean and had good contact.

This one kinda has me stumped. I hope I don't have a transistor or something that took a crap.

Thanks for all the help

Rich

[RADIOMAN2002]

A common failure is the final transistor on the exciter board. You should get about 1/2 watt out. The frequency will be in the 70 mhz range. Just take your TX RF frequency and divide by 4. Another thing is it very easy to tune the stages to the wrong harmonic. If your tuning slugs are at either extreme, close to the board or all the way away from it, you are tuning to the wrong harmonic.
Those radios are basically a VHF transmitter with a Varactor Tripler. You should be able to see about 40 Watts VHF before the Varactor. The transistors for the amp stages are very finicky, you have to replace them in sets, or you can blow up the ones you just put in.

[EdgeRadio34]

Hi hope every one had a good holiday.

I final had a chance to go back out and play with the repeater agen.
I have about 1/2 watt coming out of my exciter board going into my power amplifying board. but nothing out of my power amplifying boards to my varactor.
I have since changed out all 6 of the transistors on the boards. and re tuned.
I still have nothing coming out.

On another note im using a VOM not a test set. have a test set model # r-1033a and made the appropriate test cable from a moto manual. The set set works great with the receiver but when I put it on the transmitter I get nothing. the only thing can do it transmit or transmit with a tone. I have no meter movement.

Im kinda at a loss right now.

O and im not shore but I think some one has added stuff to the power boards as far as test points. All the metering leads come together thrue a resistor diode thing then out to the the different thrue capacitors to the meter jack. I dont see any of that on the prints that i have. but everything else looks exactly the same.

And once agen thanks for all the help I really would like to get this thing working.

Rich

[Bill_G]

Maybe the test set is telling the truth - there's nothing there. Is the high voltage section working? It will require high voltage probes on your VOM to safely poke around. Do you have cathode voltage? That will be a low voltage to run the tube "heaters".

[EdgeRadio34]

Hi

The original tubed amp is not with the unit any more. My exciter should put out around 12 watts or so. After the exciter board there is a power doubling board with 3 transistors and then a small final power board with 3 transistors. After that it goes in to the "varactor" to get turned into UHF and then into a harmonic filter.

I have 1/2 watt coming from my exciter board going into my power doubling board. But after that I got nothing. I can tune my exciter using a VOM. Trying to get reading from the power doubling board I get erogenous readings. Looking at the board nothing apers to be burnt or have let the smoke out. I have checked all my power inputs and all read what they shuld. 13.5v. and according to the print Im getting the power to the places I need it. I don't have a RF volt probe though I wont to make one to check better.

As a reminder all transistors on the power doubling board and final boards have been replaced.
I get no out put with a watt meter.
And even on my exciter board I get no meter movements using the moto test set. But have reading with a VOM.

?? got even more questions.??

For the life of me I cant figure out how the transmitter gets keyed. as far as I can tell its always on and transmitting but thats not the case.

thanks for all the help.

[Bill_G]

Since you have the manual, read the theory of operation section about the ptt circuit, and the keyed voltages. It could be that your original guess is right - the channel element is bad. You'll need a service monitor for that.