Re: 2235 with 360 kHz 100mV P-P ringing on +100V PSU rail


Tom Miller <tmiller@...>
 

Hi Tom,
 
I think you may be on to something. There is a wire jumper (W-955) that could be lifted and a series impedance with a cap to ground would clean up the +100Va buss. Maybe a small choke and a 10 uF 168 volt cap? Or just add a larger low ESR cap across the 33 uF cap for a test. The +100Vb1 has a 10 ohm resistor and a 0.1 uF cap for decoupling.
 
Vauga is in a good position to work this out. If he had an isolation transformer, that would make things much easier. May he lives near someone on the list that has one?
 
 
Regards,
Tom M
 
 
 

----- Original Message -----
From: tom jobe
Sent: Monday, May 23, 2011 4:53 PM
Subject: Re: [TekScopes] Re: 2235 with 360 kHz 100mV P-P ringing on +100V PSU rail

 

Hi Vauga,
I was studying the service manual for the 2235 military version, which is
what it looks like you have. The military 2235 seems to be a slightly higher
quality 22xx scope in my experience, even though all of what I think of as
the "Beaverton design" 22xx's are quite nice and quite similar.
The +100 volt supply is used on several circuits that go to the CRT such as
the Horizontal Deflection Plates, the Grid Bias, the Z axis ,and the
Geometry circuits. The +100 volt supply is used very directly to these
circuits with a very small capacitor right at the point of each use.
Look at the far right side of schematic <6> and notice how many places the
+100 volt supply is used just before those horizontal leads that go up to
the CRT. It is no surprise that this seemingly small ripple might be causing
so much grief.
Since this problem seems so common in the 22xx scopes, I'm thinking that you
are not looking for a defective component, I think we are looking for a
modification of some kind.
I wonder what kind of a modification it would take to eliminate the 100 mV
ripple from a +100 volt supply? Maybe we will luck out and the brighter and
more experienced folks on Tekscopes will offer some suggestions on this
matter.
"Thank You!" in advance to anyone who might have any thoughts on this
problem or on my speculations.
tom jobe...

----- Original Message -----
From: "vaugha69" <vaugha_brewchuk@...>
To: <TekScopes@...>
Sent: Monday, May 23, 2011 10:37 AM
Subject: [TekScopes] Re: 2235 with 360 kHz 100mV P-P ringing on +100V PSU
rail

Hello Tom,

Thank you for your quick response! Yes, I replaced C940, but the closest I
could source was a 1200uF Nichicon PW, which in the worst case scenario
could fall outside of the maximum specified tolerance of 1000uF + 30%. Mind
you, I had to do pretty much the same for most of the other capacitors,
except all the other replacements fall within the original tolerance bands:
C954 47uF vs. 33uF; C956 390uF vs. 270uF; C960 etc. 1200uF vs. 840uF; C942 &
C943 15uF vs. 10uF.

I also have at the ready a United Chemi-Con 100uF, 400V (could not source
450V) KXG series capacitor, so I can easily replace C906. I got a bit of a
cold feet syndrome since the KXG is 1/4 the size of the Sprague cap, and I
also read elsewhere that the Sprague telephone grade caps last "forever". I
found that last bit hard to believe mind you, since it is an electrolytic
after all. I could not find the ripple current rating of the Sprague
capacitor, but I'm guessing that the KXG will work just fine. Maybe I
should jus go ahead and replace C906?

I was wondering whether replacing the original 0.01uF 3000V caps with
polypropylene could have disrupted the power supply somehow? Or perhaps it
could be the overall collection of slightly higher than nominal capacitance
on all the output filter caps?

I realize that I am shotgunning the whole issue, but I really did not mind
replacing all the electrolytics. I have been doing this for most of my
other equipment at around 15 to 20 years of use as preventative maintenance.
Unfortunately I do not have access to an isolation transformer.

Thank you again,
Vaugha.

--- In TekScopes@..., "Tom Miller" wrote:
>
> Hello Vaugha,
>
> I would replace C-906 as it is the main filter for the supply.
>
> You can't really check the ripple on that cap without an isolation
transformer.
>
> Did you replace C-940? Without an isolation transformer you are just
shotgunning for the main problem.
>
>
>
> Tom
>
>
>
> ----- Original Message -----
> From: Vaugha Brewchuk
> To: TekScopes@...
> Sent: Monday, May 23, 2011 11:05 AM
> Subject: [TekScopes] 2235 with 360 kHz 100mV P-P ringing on +100V PSU
rail [1 Attachment]
>
>
>
> [Attachment(s) from Vaugha Brewchuk included below]
>
> Good day,
>
> This is my first post about my first scope that I purchased recently. I
am also not an electronics technician, so please be gentle with me :-).
>
> The scope is in excellent condition and has been recently calibrated.
However, when using the scope I noticed occasional small ripples in the
traces. I opened the scope and tested the power supplies and determined that
the ripple on all the rails was 2 to 3 times larger than the limit quoted in
the service manual. Consequently I replaced all electrolytic capacitors in
the power supply, except the C906 input filter capacitor, with Nichicon PW
series capacitors. I used the exact same capacity components where the
originals were +/-20% and the next larger capacity where the originals
were -10% to +30, +50 or +100%. I also replaced the four 0.01uF 3000V
capacitors with CDE 940C30S1K-F polypropylene capacitors.
>
> Unfortunately the occasional ripples have not completely disappeared
after the overhaul even though the traces seem to be a bit sharper. The
ripples on all the power supply rails have significantly reduced, with the
exception of the +100V rail. Here the actual low frequency ripple seems to
be in check at approximately 30mV P-P, but every second ripple peak exhibits
damped high frequency ringing at approximately 350 - 360kHz with 100mV P-P
amplitude. I attached an image showing the ripple and the ringing on the
+100V rail.
>

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