2235 2mV vertical range noise and surprising fixes


 

The 2mV vertical range on the 2235 is a triumph of marketing over technology, as it really pushes the performance of the scope to a place where it doesn't willingly want to go. virtually all 2235's exhibit increasing noise on this range as they age, and sometimes the displayed noise is huge (when the psu caps deteriorate in the secondary of the switching supply).

I have had to fix many of these, and generally follow this plan:
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1. replace all LV psu secondary caps with modern low ESR versions (820uf becomes 1000uf).
check for ripple after the change at the psu test points, and replace any bad switching rectifiers if needed.
2. tighten all ground hardware all over the scope, especially around the vertical assembly. make sure all case ground screws (2) are there too.

Recently I had one stubborn unit that just wouldn't behave even after re-capping (which it desperately needed), and I traced the fault to a very unusual area. all of the switching supplies are slaved the the adjustment of the -8.6V level. my unit measured -8.42V, which I initially ignored as it seemed to be within spec. BUT, the manual specifies a tighter adjustment range, and incredibly, as I raised the voltage to just over -8.6V with the rear edge trimpot on the motherboard, the displayed ripple almost vanished. I have never seen this interaction before, so I thought I would pass it on, as it may prove useful to anybody re-capping one of these nice little 100MHz scopes.

the difference between good and bad ripple display is a very narrow -8.6V adjustment range, about 150mV, which I found amazing, as I cannot see how this has such a dramatic effect, but it does. no adjustment or parts replacement can totally remove ALL ripple from the 2mV range display, but you can certainly improve it down to about one minor division. bad scopes with poor caps can be 5-10 minor divisions, making it unusable on the 2, 5 and 10mV ranges.

I hope this proves useful to others working on 2235's.
all the best,
walter (@walter2)
sphere research corp.
https://www.sphere.bc.ca/test/index.html

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