palomar tx 75

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palomar tx 75

Post by goffiefarmer » Wednesday 12th Jul 2006, 12:00

need help powers on relays trip
rf out light dont light up when radio is keyed

put on swr/ porew meter only showed a increse of about 3 wats when linear it turned on aney ideas would be grate full for aney help

my email is


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Post by nomadradio » Friday 14th Jul 2006, 9:26

Hi Max,
That thing sure is old. They used two types of relay. First was a flat, opaque-plastic relay made by Potter and Brumfield (P&B) marked "R50-E2-Y2-12VDC". This part was used for both the main and the preamp relay on early models.

After a while they figured out that the "R50" part wasn't big enough for the main (T-R) relay, and would install a larger clear-plastic enclosed relay. Because this type was too tall, it would be turned on its side, with wire jumpers to the circuit board holes. This allowed the top cover to fit without bumping into the taller "R10"-type relay. They continued to use the R50 relay for the preamp, in all models.

Trouble is, the R50 relay just goes bad. No amount of cleaning or adjusting will restore one that goes bad.

From the sound of your description, the relays are not the entire problem. The RF power-amplifier transistors are roughly 30 years old, if they are the original parts. Palomar marked them "TX-75", but I'm told that they were reallly Motorola type MRF-454. Not sure they are still made.

Odds are that someone has either used a radio that is too large to drive it, or keyed it into a coax cable/antenna system that was defective. A lot can happen in 30 years, and I'm guessing that you have never seen this unit function properly since you obtained it.

Hope you didn't pay too much for it.

The R50 relay on the preamp side is best removed and bypassed with a wire jumper. The preamp is not all that useful in the first place. Since your receiver signal must pass through that one, AND youre 'barefoot' transmit power goes throuh it on standby mode, that relay can cause trouble even if you never turn the preamp on, even once.

The main relay can clobber the RF power transistors, if it cuts in and out hwile you're keyed. Maybe the relay created this problem, maybe a bad coax cable blew it out? No telling, unless you were there as a witness when it failed in the first place.

Odds are that the two RF power transistors are blown. Even if only one of them is damaged, you'll never find a new, single part that will match that one correctly. It really needs two parts that are fairly well-matched to each other for this model to work right. This generally means replacing them as a pair, even when only one of them checks blown. Two transistors with the same "batch" number below the part number are usually a safe enough bet on matching. Paying extra for a "matched pair" is cheap insurance.

That type RF power transistor has undergone some improvement in the engineering and manufacturing standards in 30 years. Newer parts tend to be more forgiving of high SWR and such hazards. A 1976 or 1977 transistor tends to be more fragile than modern-day equivalents. In a nutshell, the old ones just failed more frequently than they do now, 30 years later.

Sounds more like a "restoration" project than a "repair". Remember, it's not just the years, it's the miles, too.


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Post by kc8mob » Wednesday 18th Apr 2007, 6:40

Well, if you are seeing any amplification. Like 3 in 7 out or waht ever you said, chances are you're good.

Are there any tunable capacitors. White things with silver looking flat head screws on them. If so, try tunning the input and output.

Tune the output first, side with big coil (TX) and then the input, small TX

Try this if you see anything out the other side your amp is not completely screwed

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Re: palomar tx 75

Post by KC7NOA » Friday 9th Dec 2011, 14:01

This is an old thread ...

But just bought one with a pair of CD2545'S installed ... looks original...

Two watt drive keys 45watts ....

Probably replace the output transformer with something longer and a pair of mrf454's ...

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