[Milsurplus] URM-25D Questions

[Bruce Gentry]

Condensors labeled "MICAMOLD" are actually paper and usually bad because 
the paper was poor quality and the case developes tiny cracks that let 
in moisture. Other manufactureres also made some that look like mica 
condensors but really aren't.  The metal cased condensors with the white 
or grey sealed glass ends are excellent quality and rarely fail.  The 4 
MFD.  filter condensors are oil filled with paper dielectric, they 
should be good unless they are leaking oil. Try to test them  for 
leakage with a 500-600 volt source- a "Megger" is ideal. Once they 
charge up, they should read in the hundreds of megohms. I think 600 volt 
was chosen because it was a common value, the power supply won't 
develope much over 380 volts under any circumstances, and the voltage is 
probably around 300 at the most in operation. If one of the oil filled 
condensors is bad, you must use a considerably larger- about 4 times as 
large- capacitance electrolytic to get the same filtering action because 
electrolytics have a higher internal resistance. It is far better to 
find a 4 MFD. plastic film condensor at 400 volts or another good oil 
condensor to replace a bad one.      A URM-25 is a challenge to work on 
to find  and replace bad   condensors, but it's worth it when you are 
done. I've had many different signal generators over the years, and  
think  the big Hewlett Packard 606 or  Marconi signal generators are the 
only reasonanbly priced, well featured, and easily available non-digital 
ones that are better than a URM-25.

        Bruce Gentry, KA2IVY



On 5/27/17 5:48 PM, Ian Wilson wrote:
> I had a unit where several of the brown (Micamold?) caps had become 
> leaky.
> The instance that led me to look into this was the screen decoupling 
> capacitor
> on one of the pentodes (6AG7 maybe?).
>
> 73, ian K3IMW
>
> On Sat, May 27, 2017 at 10:22 AM, Joe Connor via Milsurplus 
> <milsurplus at mailman.qth.net <mailto:milsurplus at mailman.qth.net>> wrote:
>
>         I just picked up a URM-25D signal generator at a hamfest
>     today, and I have a few questions:
>
>         1. The three filter caps are 4 mfd. at 600 WVDC. Is 600V
>     necessary or is it excessive headroom? The closest I can find from
>     the usual parts sources is 8 or 10 mfd at 600V.
>
>         2. How reliable are the other caps in these units? Should I
>     replace them as a matter of course?
>
>         3. What's the name of the connector to hook up to the output
>     jack? (This may be where I'm going to really miss Radio Shack and
>     their supply of connectors).
>
>         4. The old Hollow State Newsletter had a few articles on these
>     units. Is there an on-line archive for this publication?
>
>         5. Any other tips or advice?
>
>         As always, thank you for your help.
>
>
>                     Joe Connor
>
>
>
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