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


tom jobe <tomjobe@...>
 

Hi Vauga,
Your initial question is one of my ongoing 22xx mysteries too.
The 22xx's all seem to have the fine ripple in the trace, that comes and
goes as you use the scope. Some 22xx's have it worse than others but they
all seem to have it to some degree.
I checked a 2215A I have been working on and the trace at the plus lead of
C954 was the same as the picture you posted, with the exception that the
overall peak to peak voltage was 70 mV instead of the 95 mV you show (C954
is a new capacitor). The ringing was almost exactly the same, and the period
from ring to ring was about the same. This 2215A has many new capacitors in
it, but C904 and C906 are the original caps that check out fine with the ESR
meter. Now that you mention it, I don't think I have ever seen C906 be bad
on the ESR test, so I don't think I have ever replaced a C906 capacitor, or
a C904 for that matter, but then I have only worked on maybe 30 of these
22xx's.
It is great to see that Tom Miller has taken an interest in your question,
he really knows his stuff!
I'm like you, I barely know what I'm doing around these electronic marvels.
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" <tmiller@...> 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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