[R-390] Servicing Advice on Signal Generator URM-25D

[Bob Camp]

Hi

The basic experience on an R390 seems to apply to a lot of older tube 
based gear. Pretty much:

1) Replace the electrolytic capacitors, especially the ones in the 
power supply.
2) Replace any of the black or brown body tubular paper and foil 
capacitors.
3) Look for any high wattage carbon comp resistors and check them. 
Replace if they are more than 30% out of value.

Different pieces of gear run at different temperatures. That seems to 
make some of them more prone to parts failure than others. I still 
claim that it is just a matter of time for some of the parts. The parts 
listed above show up on lists for just about every piece of gear out 
there.

I'm not sure what took out your choke. It may have been a wear out 
mechanism or it could have been some kind of line transient. One of the 
interesting failure mechanisms on a choke is a short on the output that 
heats the poor thing up. Heat attacks the insulation on the winding. 
Insulation breaks down next time power is applied. Result is a short to 
ground. Makes for a good argument to fuse the B+ lead. In an R390 it 
makes for a good argument to be sure the B+ fuses (we all do the B+ 
fuse mod don't we ...) are the correct size and type.

I would replace the tube rectifier with a pair of silicon diodes and a 
47 ohm 5W resistor. The rectifier tubes just aren't worth tracking down 
any more. The same comment goes for just about any supply in any of 
these older pieces of gear.

Now for the nasty part.

Once you get the URM back to life you have to calibrate it. Like all 
military gear there's a manual somewhere that tells you more than you 
would ever want to know about how to do it. The same thing applies to 
stuff like TV-7 tube testers. We all (my self *very* much included) 
seem to overlook this little chore all to often ....

	Take Care!

		Bob Camp
		KB8TQ


On Jan 8, 2005, at 10:19 PM, <pdulaff at earthlink.net> wrote:

> BlankL117 has failed on my signal generator. The choke shorted to 
> ground and
> also took out the 6X4 power supply rectifier. While I am waiting for 
> the
> parts to arrive to complete this fix, what else in the unit should I 
> change
> as a preventative maintenance item ?
>
> Paul - WB2NMI
>
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