UHF Mitrek tx power problem

[jistabout]

Hi again everyone,

I'm using a UHF 100w Mitrek in which the tx power is unstable. I can set it for 50-75w and it will work fine for awhile and then decrease in power down to 5-7w. I can then readjust the power control and limit pots and it comes back sometimes. Other times it will Increase in power. I've cleaned the pots & re-soldered everything in the PA and exciter power control circuits to no avail. I've heard of a common problem with these with a diode in the PA (D801?) that might cause this but haven't looked at it yet because it looks like its located underneath the PA board. I do have a manual and can do the job if needed. Has anyone got any tips on this? Do I need to remove the PA board to get to the diode or can I just remove the first RF module?. Could this diode be my problem? Any thoughts are certainly appreciated. Thank you....

- Darrell

[Will]

Well, seems you have done everything except fix it.
This IS a common problem on the UHF Mitrek/MSR2000 radios.

Tap on the exciter helical filters, L9, L10, L11, and L12. If the power comes and goes, then time to replace or rebuild the helical filters Same goes for the receiver side, L1-L8, all one assembly in the mobiles.

[n5tbu]

Common Mitrek problems are intermittant power control pot in the PA and the two pots in the exciter....I think your problem is an intermittant pot in the exciter. You could replace them or jump them wide open to see if your problem goes away.
mod

[kb0nly]

Common Mitrek problems are intermittant power control pot in the PA and the two pots in the exciter....I think your problem is an intermittant pot in the exciter. You could replace them or jump them wide open to see if your problem goes away.
mod

I will agree with that. 99.9% of the PA problems that i have experienced while converting Mitreks to repeaters is the trim pots. They usually tarnish, ones i have broken apart after replacing were entirely black with corrosion.

I would try replacing the pots first. If you need the actual part numbers or values post back and let me know. I will look in the service manual to get the part numbers, or otherwise just list the part id and value.

[kb0nly]

oops, sorry about the double post. It gave me an php fail or something inbetween reloading the page.

[jistabout]

Thanks guys . I'll replace the pots and go from there.

According to my schematic, R909 is 5k, R911 is 50k, and not sure about R813 in the PA but that one seems to be ok and I can measure it anyway if needed. Guess I've got me a date with a hot one! (Soldering iron, that is ).....

[57Shasta]

I give the nod to Will's reply. I have fixed plenty of uhf mitreks. My first thought when I see int tx power is the harmonic filter. Bang on the screw heads with a screwdriver butt while tx and watch power. Then pop the bottom and look at the coil solders to the filter assy. Any cracked? Common Mitrek prob. Takes mega-heat to reflow properly. I use a 60w pencil iron with backup from a propane torch on low (raised a few eyebrows from my customers on that one). On the flip side, rx sens comes and goes? Check those solders on preselector / injection section. I've seen sour pots too, but nothing clean spray with silicon would'nt settle down.

[Dan562]

I remember that the UHF Power Amplifiers also had Defective A+ Distribution boards, /\/\ engineering redesigned those PC boards back in 1990 and distributed a PSB about the problem. Oh it was a pain to carefully unsolder all of the A+ feed through points without damaging the copper substrates to get at the board.

Cracked or contaminated (copper in manufacturing) substrates were another common problem. And of course those Low Pass Harmonic Filters.

Dan

[kb0nly]

Those were some common failures. Also i will add one more. Though not likely your problem, for future reference.

The front interconnect board, the vertical board with the front cable connector on it, sometimes comes loose or gets corrosion where it makes connection to the main board. Pull the interconnect board out, just have to remove a screw or two depending on if you have a PL or DPL board across, and use an ordinary pencil eraser to buff the contacts back to a nice shine.

Then work the board in and out of the connector a few times and clean any residue that now shows up on the contacts you cleaned. They tend to be a problem now and then. When using them for repeaters i usually try to bypass that interconnect connector with jumpers for the signals needed directly to the cable connector. To many times i have had to go clean that bugger when something started acting up, or scratchiness in the receive or transmit audio, etc.