[GreenKeys] OT: Need help to T-Shoot this Tek 485 scope

[drlegendre .]

@All

We're in full agreement as regards sending all 'helpful' replies to the
list. But being that this post is OT, at least in my opinion, I was simply
trying to reduce what I presumed to be OT list traffic. I'm sure you can
all understand why that might be..

But if this O-scope thread is acceptably topical, then by all means, let's
share the results!

So in that spirit, here's some background. This scope fell into my hands
along with the Tek Scopemobile cart; it came from a freecycle list member.
The PO claimed it was in working order, and was surprised to hear it
wasn't. As arrived, the PSU wouldn't start up - the inverter was cycling at
about 3Hz, which indicates an overload condition / automatic shutdown
fault. Overload can be a short on one of the supplies or excessive beam
current.

In this case, it was a short on the +15V (reg'd) line, from a - you know it
- tantalum 15uf / 25V cap. Replaced it with a non-tant (all I had..) and it
was just fine. I need to order some tant parts.

Then I ran a shakedown test, just let it trace away at low intensity for
several hours. Then I heard clicking again, and saw that the traces were
blinding bright.. it was again cycling the inverter, but this time due to
excess beam current. Increasing sweep speed reduced peak beam currents to a
level that the prot. wasn't triggering, but the trace intensity was still
uncontrollable. However, it does seem that the focus is controllable..

And that's where I am.

On Wed, May 10, 2017 at 6:02 PM, Bruce Phillips <bruce at ecsindustries.com>
wrote:

>
> Hi drlegendre,
>
> This link shows photo's of the boards / interconnects:
> https://sites.google.com/site/linuxdigitallab/home/
> tektronix-485-cathode-ray-interconnection
>
> It looks like A15 is below A11, and vertical at the back. (see attached
> photo)
>
> Hope that is some help.
>
> Cheers,
> Bruce
>
>
>
>
>
>
>
> On 10/05/2017 11:15 AM, drlegendre . wrote:
>
> Paul,
>
> Yes, I know exactly what you're talking about and that's a very good
> suggestion. If you have the Tek 485 manual they're R1603,04,05,06,08,09. I
> think they use multiple parts as to avoid exceeding the Emax of the
> resistors, which is typically around 600V. One thing so far - I can find
> the parts on the drawing, but not on the machine. Tek says they are on
> "transformer board A15" but no such board exists, at least in the docs!
>
> But either way it's a very high-Z circuit and any kind of leakage or other
> trouble will spoil it..
>
> Now what I +really+ need to know, is what voltage should the g1 have vs.
> cathode? IIRC, I read -9V on g1, so at least it's proper in the sense that
> it is negative - but what should the actual value be? And what would I
> expect to measure with a typical (11M Zin) DVM? I mean, the overall accel.
> potential in that tube is like 21kV (18kV anode, -3kV cathode) so I'd
> expect to see a whole lot more than -9V bias, eh - or am I way off base in
> my assessment?
>
>
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