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

Mon Jan 10 09:08:04 EST 2005

Sure.  Let me prerface my remarks by saying that much of what is said about preventive maintenance on the reflector is baloney.  

Take, for example, the "resistor replacers."  You can't measure most resistors accurately in circuit unless you "lift at least one end," and if you do that, you might as well go ahead and replace it.  Ditto for the "capacitor replacers."  Unless there is some strong reason to replace a component (it is burnt dark brown or black, cracked case, oil leaking out, hot enough to boil water, etc.), leave it alone.  Just because the case of a capacitor is black or brown does not mean it should be replaced.  To illustrate how silly that is, I could say (tongue in cheek) replace all capacitors with a yellow case.  Would you do that?

There are, of course, a few cases where you should replace capacitors.  The mechanical filter blocking capacitor of an R-390A is an example of such a case.  Now I don't really believe the story that the R-390A filter blocking capacitor failed in some R-390A many moons ago, and the operator switched through all the filters "killing" all the filters before he discovered what had happened.  But because I don't want to tempt fate, I have replaced that cap with a disc ceramic cap having a much higher voltage rating in all of my R-390A's.  And any black case cap in an SP-600 should be replaced immediately before you even turn it on (but you can usually see the cracks in the cases of the SP-600 "black beauties").

As for the URM-25D, every one I have seen has tan cased MicaMold oil filled capacitors, and at least one of those is leaking oil.  Some of them are tough to get to, but every one should be removed and replaced.  I like the "yellow wraps" available from AES for replacements.

Personally, I would never convert any tube rectifier power supply to solid state, if only for aesthetic reasons.  Tube rectifier power supplies "come up gently."  If you convert to solid state, every time you turn your gear on is like hitting it  

with a big hammer.  And anyway, what is the point of removing tubes from tube gear?  Too cheap to buy the appropriate rectifier tubes?  If we are going to remove the rectifier tubes, let's remove all the tubes!!!  And then why buy tube gear and convert it?  Let's just buy stuff that is already all solid state!!!!

And electrolytics...  dare I bring up that topic again.?  I think not because the doctor is probably lurking and I am not in the mood right now for a tussle.  But here is a war story I don't think anyone can object to.  A European friend of mine sent me his Telefunken E 1501 to fix.  While fixing it I noticed that the power supply was quite noisy (loud mechanical hum).  The E 1501 is modular, so I removed the power supply module to give it a look-see.  While I was examining the power supply I noticed that a previous owner replaced two of the electrolytics in a misguided attempt to reduce the mechanical hum.  Why have I concluded this?  Because the "zipper crew" did not remove the old elctrolytics, but merely "scabbed in" the replacements.  So I measured the resistsances of the old electrolytics with my DVM.  They seemed fine (no shorts, no leaks, reasonably high resistance after charging for a while).  Later I will actually power up the electros with a DC supply and check them for leakage under operating voltage.  If they pass that test, I will remove the replacements and restore the originals.  I gave seen this kind of thing before in a Hammarulnd HQ-180A.  Fortunately, that zipper crew also left the old electrolytic can in place, and I restored it.

Have fun,

Dallas

  ----- Original Message ----- 
  From: pdulaff at earthlink.net 
  To: dallas at bayou.com 
  Sent: Sunday, January 09, 2005 10:42 PM
  Subject: Fw: [R-390] Servicing Advice on Signal Generator URM-25D

  Dallas

  Can you supply me any advice on preventive maintenance items for my URM-25D ?

  Thanks

  Paul - WB2NMI

  ----- Original Message ----- 
  From: Joe Foley 
  To: pdulaff at earthlink.net 
  Sent: Sunday, January 09, 2005 10:20 PM
  Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D

  Ask Dallas Lankford for his notes on that unit,
  there's some valuable information in there.

  Joe

  --- pdulaff at earthlink.net wrote:

  > Bob
  > 
  > Thanks for the input. I have it on the bench now. It
  > is very well built. 
  > There aren't any black beauty caps in this thing.
  > They use metal tubular 
  > decoupling caps that is flange mounted with a glass
  > seal for the terminals. 
  > Resistors look like new but I will check them for
  > increase in value. The RF 
  > choke that shorted to ground looks like the plastic
  > terminal that holds the 
  > end of the wire had cold flowed over the last 40 +
  > years and changed shape 
  > enough that the terminal contacted the ferrite core
  > of the choke and shorted 
  > that side of the choke to ground. I have the manual
  > for the unit and was 
  > studying it since I figured I had better check both
  > frequency and signal 
  > output calibration after I get it back together.
  > Another thing to consider 
  > on these units is lubricating the worm gear for the
  > turret drive . Works OK 
  > but the grease is all dried out.
  > 
  > Paul - WB2NMI
  > 
  >
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