[R-390] URM-25 replacement

[Bob Camp]

Hi

According to the manual on Bama the magic resistor is 59 ohms. It is DC 
coupled to the output jack. If you hit the output jack with 12 volts 
then E^2/R gets you up above 2 watts. Most 1/4 watt metal film 
resistors aren't going to take that for a real long time ....

The interesting thing is that it's always the final resistor that 
blows. The 65.6 ohm parts in the rest of the attenuator aren't all that 
less likely to blow out.

With R-390 type stuff I would suspect the whole attenuator goes if you 
have B+ on the antenna connector. Not to mention a bit of a surprise to 
who ever is working on the radio.

Sounds like a great way to rig the generator to get everything to pass 
....

	Take Care

		Bob Camp
		KB8TQ

On Feb 26, 2005, at 1:38 PM, Jack Antonio wrote:

> Hi all
>
> Bob's post reminded of something I saw in my URM-25s.
>
> The URM-25 generator uses a switched series of resistors in
> its output attenuator, the last being a 50 ohm-ish  to ground.
> (It's not 50 ohm but either 49.9 or 50.1 or something like
> that, can't remember which).
>
> In both of the URM-25s I have owned, that 50 ohm resistor was open.
> Unless you actually measured the output of the generator, you
> probably wouldn't notice the problem.
>
> Then, a couple of years ago, Fair was selling the attenuator
> assembly as a separate part, and I bought one. Guess what?
> That resistor in that was open as well.
>
> Another thing to check on your URM-25s
>
> Jack
>
> Jack Antonio WA7DIA
> scr287 at sbcglobal.net
>
>> If you have a generator that uses a "stacked attenuator" rather than 
>> the old sliding coax attenuator it's a good idea to check the 
>> attenuator.
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