On 15/12/2020 15:54, Grumps wrote:
On 15/12/2020 09:18, Dave Liquorice wrote:
On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...
That would tend to produce step changes in the time base rather than,
asĀ* how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.
It's not a step change. It's smooth.
It can be steady, then fall in frequency, and the next moment it can
rise in frequency - all within the space of several seconds.
HV stability? It may just be a high resistance HV connection near to the
front of the tube. All scopes I've worked on have a square test wave
output. Connect this to the input, select the appropriate timebase and
vertical scaling. Does the vertical scaling also change in the same way
as the timebase variation. If so suspect the HV supply. However if the
HV is varying I would expect the trace brightness to vary as well,
possibly more noticeable if the brightness is turned down.
Warning: bleed resistors across a scopes HV supply have a habit of going
open circuit so just make sure that your hand is not going to hit
anything when you remove it at speed after getting the shock
Once whilst working on an old valve based Tektronic scope I rested my
arm across a valve base. I ended up with two cauterised holes as the
voltage on one pin discharged to another through my flesh.
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