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Harris MW-1A Maintenance

Mike Phillips

Mike Phillips

An acquaintance has a Harris MW-1A transmitter. Some of the light bulbs inside the front window (failure indicators for the PA modules) are glowing faintly. My recollection is that a faint glow is normal and indicates a phasing issue between the modules. If a module fails, the bulbs glow brightly.

Further to my recollection, it is not advisable to attempt to tweak the phase on these modules in the field. Harris suggests returning them for alignment. The question, then, is at what point is it cost effective to return them? Should they be exchanged for other modules instead of repaired?

It's been a long time since I've worked on one of these. What is the current best practices for maintaining the MW-1?
 
I remember with the MW-1 (not "A" in my case) that I always had one module with a dim light. I finally nailed it somewhat by going through most every module and matching the transistors using my Sencore Cricket tester. It the gain wasn't really close on the pairs you'd get a glow. I remember it taking days (of course doing this after the daytimer AM signed off) to get things looking good.

It wasn't the worst box I've ever played with, but it could be a fussbucket when it comes to VSWR. I had tie-wraps that were holding the feedline for the STL dish crumble and when the lines moved around in the breeze the MW-1 would kick itself off the air. I had to run the backup until the tower climbers got there - an ol' RCA BTA-1R that did a decent job even though I never had the opporunity to put the update mods into it. And no, I would have climbed the tower myself if I had a climbing belt and the insurance from the station.
 
Best advice I have to is buy a used one for parts from one of my clients for $500 as is.... :)...contact me for details.
 
If the MW1 is making somewhat close to 1kW, and you have a faint indication on one of the bulbs...leave it alone. (Key phrase.."somewhat close to 1kW...)
There really is no use in trying to null out the few watts of imbalance that a faint indicator shows.....al least you know that one light bulb is working.

I have taken care of a few Mw1's over the years, and they STAYED on....even if the positive peak capability was not great after NRSC came along.

Keep a handfull of of the PA components around and a bunch of fuses, and it will generally play.
 
BobOnTheJob said:
Best advice I have to is buy a used one for parts from one of my clients for $500 as is.... :)...contact me for details.
Click to expand...

Even better advice is to get a new (NOT Harris) transmitter and push the one you have off in the Ohio River! Makes a great reef for the fishes.
 
Its buried in the book that there is a transistor that can cause the fault indicator to light even when there is nothing wrong with the module.

I agree with if it is still making power don't fool with it.

I worked on a 1A version at a station that I used to be at - biggest advice - keep LOTS of R-5 on hand to spray the relays inside the front door for the lower power levels. They are open type relays - if you see overmodulation at the lower power levels that is the first place to look. The overmod will cause some of the indicator lights to light sometimes.
 
If you don't need the full kW for your application you can run it on less modules and crank up the Vcc to get higher linearity. You then also have some spare modules to work with.

I never liked the audio input transformer and replaced it with a DC-coupled differential amplifier circuit using MA-332 op-amps. It was a lot easier to get positive peaks out of the box.

Fuses do come in handy with that transmitter and I also agree with the Cramolin comment. Cramolin was a godsend back in the 70s when I first started using it.
 
Mike P - I would agree with the other posts, as long as the modules don't glow, or glow very dimly, that is normal. The glow may also change with power level-- you may not see it at all at 500 watts, etc. But it's not worth messing with.

The good news is the modules are VERY field-repairable. In fact, I've repaired dozens of modules over the years and get the majority of my parts from Digi-Key. Some parts, such as large carbon-comp resistors are still available from Harris.

As long as your voltage/current readings and multimeter readings are in the recommended ranges, it will be a very reliable box.

Feel free to contact me off list if you need help troubleshooting. ;D

Murphys Dad
 
sparks794 said:
Its buried in the book that there is a transistor that can cause the fault indicator to light even when there is nothing wrong with the module.

I agree with if it is still making power don't fool with it.

I worked on a 1A version at a station that I used to be at - biggest advice - keep LOTS of R-5 on hand to spray the relays inside the front door for the lower power levels. They are open type relays - if you see overmodulation at the lower power levels that is the first place to look. The overmod will cause some of the indicator lights to light sometimes.
Click to expand...

Relay contacts in a metering circuit at low levels may indeed cause lighting of imbalance indication when all modules are working correctly.
Doncha love De-Oxit?
 
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