URM-25D Problem amd Fix[via LSMTP - see www.lsoft.com]

[Bill Abate]

Hello to the group.  I've had a problem with my 25D signal generator for
years but I never got the ambition to fix it until now.  Above say
25MHZ, the carrier generated was not clean.  It had a raspiness to it.
Not FM.  A little like a parasitic.  My armchair diagnosis was a bad
bypass cap or filer cap.  About 4 hours later, alter replacing good caps
and substituting another power supply, I still had the problem.  BTW,
the second harmonic of the generator was clean.

I changed the oscillator tube but no difference.  If I pulled the buffer
tube, I still had the problem, so I figured it had to be the
oscillator.  The tube is a 6AH6 operated as a triode with the plate and
screen bypassed.  Then I noticed that the side of the tube socket had
the word Sylvania stenciled on it.  Strange, that was the only socket so
marked.  The manual didn't say anything like that.  I know sometimes one
tube manufacturer is preferred in some equipment.  I had a Sylvania 6AH6
so I tried it.  Clean signal!  Hard to believe.  But that was not the
end.

I reassembled the generator (no small feat if you ever have tried it)
and the raspiness was back.  Took it apart again but could not find the
cause.  Then I found it by chance.  The Set Carrier Control had an
effect.  It varied the output of the oscillator by acting as a voltage
divider for the plate and screen supply.  Not the way I would have done
it but I ain't a designer.  It seems that below some voltage, the tube
misbehaves.  The trick is that voltage threshold varies with the
oscillator frequency.  At 5 MHZ, it was very low but at 30 MHz you can't
set it the way the manual instructs you.  Instead of setting the
Microvolts knob fully CW, set it at 45 degrees, then use the Carrier
knob to set the meter at 10.  The carrier sounds nice and clean with
this higher voltage.

I'm not sure whether I swept the problem under the rug or covered up a
design flaw but it worked.  So if your 25D acts up, use a Sylvania
oscillator tube.  Then experiment with the Set Carrier Control.  BTW,
this does not affect the accuracy of the meter reading.  You just get
less resolution with the Microvolts control.  The 25D is a nice
generator.  I like my HP 608 better but then the 608 stops at 10MHZ.

Hope I have helped someone.

73,

Bill Abate, K3PGB