[Milsurplus] URM-25D Questions

[WA5CAB at cs.com]

Joe,

In a message dated 05/27/2017 12:26:24 PM Central Daylight Time, 
milsurplus at mailman.qth.net writes: 
>     I just picked up a URM-25D signal generator at a hamfest today, and I 
> have a few questions:

First, in some parts of the circuit, it is important to know which model of 
URM-25 you really have.  Some people are still confused by the nomenclature 
assignment rules of the AN system.  There are two significant components to 
the AN/URM-25, the signal generator and the power supply.  The signal 
generator used in AN/URM-25D, G, H, J and K is SG-85 through SG-85D/URM-25D.  
Power supplies are PP-977and PP-977A/URM-25D, PP-6106/URM-25J and 
PP-6205/URM-25K.  So make sure of what you actually have before trusting any one 
schematic.  The AN/URM-25 through 25D used SG-55(*)/URM-25.  The E and F models were 
again different.  The F in particular is sorta like the BC-348-JNQ.  It's 
prime criteria was price reduction.  How well it worked was secondary.

> 
> 
> 
>     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.

It is usually best for a number of reasons not to treat filter capacitors 
on the basis of if some's good, more's better.  The second and third filter 
capacitors in the PP-977 and 977A are paralled.  So replacing them with a 
single 8ufd unit would be preferable.  Unfortunately, I would judge a 400 VDC 
rated unit to be marginal at C201 (the first filter cap).  So given the 
choices of an 8ufd/600VDC or a 4ufd/400VDC, I would go with another 8ufd.

> 
> 
> 
>     2. How reliable are the other caps in these units? Should I replace 
> them as a matter of course?

Contrary to the philosophy of some here, my answer has to be "it all 
depends".  A few years ago, my SG-85A developed a modulation problem.  So I went 
through the set and tested most of the capacitors.  About half of the 
capacitors in the audio box were leaky.  So I replaced all of them.  No bad ones 
were found in the rest of the set.  So my recommendation would be this.  First 
replace the filter caps that you already know are bad.  Pull the (I'll 
assume SG-85(*)) part way out of the cabinet, prop it up level, and sniff for 
the smell of burned insulation or resistors.  If you don't detect any, turn 
the set on, let it warm up for a few minutes and test all functions and all 
bands. If everything works OK (disregard minor calibration errors at this 
point), let it run for a few hous and repeat the tests.  If OK, slide the SG-85 
back into the cabinet and let it run for a few more hours, and repeat the 
tests.  If you find any problems as you go along, of course stop and fix them.


> 
> 
> 
>     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).

BNC

> 
> 
>                 Joe Connor
> 
> 
> 
> 
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20170527/c392b4a8/attachment-0001.html>