[sdiy] HP130C scope issue

[Bob Weigel]

Gotta love me still using a 1962 HP scope...but..This is so strange.  My 
scope I'm so used to I refuse to ditch it..lol... besides it's full of 
telefunken EC88 tubes.  ... I have had it out of cal for a while and a 
JITTER in the trigger.  I mean you can trigger if you get right on the 
edge but anywhere between it's bouncing around from some 60hz 
modulation.  Oh and I opened it because an electrolytic was 
smelling...found that and replaced it..the final one that has the 250 
and 100V filters to ground on it.  Replaced with an 80uF since I had a 
can NOS in a box.

ANyway the jitter was still there sadly.  But I found with another scope 
that there is signal on the 250V that rises WITH the jitter..(Initially 
there is no perceivable jitter..but after a few minutes it's quite bad.  
Typically seeing three waveforms offset by a few mm when it's in it's 
warmed up mode)

The 250V comes right off a diode from a xformer winding... with a 150uF 
cap between it and the regulator feed for the 100V  (There are two 
transformer windings stacked with a regulator for the 100V between..then 
ground..then another similar circuit for -100V below that)...and 
initially it was more like almost 300V!  And again..no adjustment for 
that.  I probably should have just made a regulator and the issue might 
be gone.

Instead I assume it was the other cap. I replaced the 150 with a 470uF 
.  THE MAGNITUDE dropped..but otherwise it basically went through the 
same routine.  So strange... with the simple nature of the circuit I 
just can't see another contributor.  The waveform is certainly smoother 
now... but it's got a rounded saw wave that's near what it was before 
when it warms up a few minutes!

Then I put in a 10W zener at 33V with a 10Ohm inrush limit to drop the 
voltage down to near spec at 261V.  Now I've got some jitter all the 
time probably from the 10ohm resistor though I thought it probably 
wouldn't introduce enough ripple..I'm too tired to think.  If anyone has 
another idea who's maybe seen this kind of thing in the past.... I can't 
believe I spent that much time on this relic.  I sure must love it 
dearly.  lol

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