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CRT scope high voltage supply problem?

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Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 10:42 am
by Adam Inglis
When I turn it on, I get about 2800 volts on the PDA line, and about -1200 volts on the -1kV line. After some minutes these voltages climb to 4800 and -1700!

Reviewing the advice from this forum, I notice I never replaced C80 and it's associated diode D18 at the secondary winding that gives the extra 80 volts. I think they might be next.....

Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 12:06 pm
by Folderol
This is not good. Something is changing value as it warms up.

Things worth changing (if you haven't already).
C 72
C 73
R 122
Also check the DC supplies to the oscillator are stable - especially the +18V.

Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 12:09 pm
by Adam Inglis
Page 3 ?
What a mission!
Schematic here
Description here but note there does seem to be some errors, e.g. I can't find D15 anywhere!

I was just going to add...
isn't it good practice to put a protection diode across the base and emitter of transistors that are involved with inductive circuits? Something about the EMF kick on switch-off?

Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 12:12 pm
by Adam Inglis
Folderol wrote:This is not good. Something is changing value as it warms up.

Things worth changing (if you haven't already).
C 72
C 73
R 122
Also check the DC supplies to the oscillator are stable - especially the +18V.

Thanks Will - I believe I've changed those caps early on, but I will check, and yes, I had better check all the low voltage DC levels again on the primary side.

Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 12:21 pm
by Wonks
The section referring to D15 does reference drawing 1065, whilst the drawing you've linked to is 681. Probably why you can't find it!

Re: CRT scope high voltage supply problem?

PostPosted: Wed Jan 16, 2019 12:35 pm
by Folderol
Adam Inglis wrote:Page 3 ?
What a mission!
Schematic here
Description here but note there does seem to be some errors, e.g. I can't find D15 anywhere!
Ah! Forgot about that resistor chain starting with RV12. That control and the 2.7M further down are favourite suspects and RV14B/20B.
In fact, I'd also change the 330k and 820k - might as well do the whole chain.
RV14B/20B could be a problem as the bigger presets with a decent power rating are almost impossible to get these days, and you need the size to get a decent voltage rating. My suggestion would be to find a way to mount a pair of these with cut down shafts.
I was just going to add...
isn't it good practice to put a protection diode across the base and emitter of transistors that are involved with inductive circuits? Something about the EMF kick on switch-off?
Not in this situation. The back emf is actually essential in this application!

Re: CRT scope high voltage supply problem?

PostPosted: Thu Jan 17, 2019 2:35 am
by Adam Inglis
Well spotted Wonks! I missed that.
I managed to track down drawing 1065, dated 1976 (mine is from around 1968 or so) so it seems to describe a much later version of that model scope.
On that drawing, the resistor divider chain matches the description with the extra R123 and R124 (each 2M7 in series with RV12 and the 330k, and in parallel with C90). D15 is still missing however!!!

Mine just has RV12 and the 330k, in series with C90 in that section between +100v and -1000v.

So, I need to find some 1 watt resistors (going by the size). Then I'll take some voltage measurements on both sides of the coil once again. RV12 looks quite worn, but I haven't measured it yet.

Note RV14b and RV20b don't have the little "preset" symbol next to them - I think these refer to the front panel "Focus" pots for each beam (which seem to be doing their job).

Re: CRT scope high voltage supply problem?

PostPosted: Thu Jan 17, 2019 9:53 am
by Folderol
Adam Inglis wrote:Well spotted Wonks! I missed that.
Note RV14b and RV20b don't have the little "preset" symbol next to them - I think these refer to the front panel "Focus" pots for each beam (which seem to be doing their job).
It's not whether they work that's the potential issue but that their overall value might be changing as they warm up - thus changing the reference voltage for the oscillator - the same for everything in that chain. Old high value carbon resistors are notorious for that.

Re: CRT scope high voltage supply problem?

PostPosted: Mon Jan 21, 2019 1:41 am
by Adam Inglis
Having a bit of trouble finding a replacement for C80 - the electrolytic in the 80 volt secondary winding, quoted as 12.5uF at 150 volts.
My understanding is that this is simply a filter cap to minimise ripple, so more is better in terms of farads, and keep it above but close to the 80 volt rating, yes?

Re: CRT scope high voltage supply problem?

PostPosted: Mon Jan 21, 2019 2:32 am
by James Perrett
I don't know the circuit but, assuming it is nothing too unusual I'd look for 15uF 160V replacements - Farnell have them in stock.

Re: CRT scope high voltage supply problem?

PostPosted: Fri Feb 01, 2019 11:17 am
by Adam Inglis
Thanks James, good call, I'm sure that will do.

Re: CRT scope high voltage supply problem?

PostPosted: Sat Feb 02, 2019 3:44 am
by Adam Inglis
Right, so that resistor chain has been replaced, along with the 1000pF disc cap (C90).
Also, the diode and cap (C80) in the 80 volt secondary loop have been replaced.

For good measure I thought hang it, I'll also replace the last 2 caps on the EHT circuit, C78 and 79, the 0.033uF high voltage ones (are they part of the doubler circuit?), and it was then I may have discovered the original problem. Where C78 joins ground is a rather inaccessible area of the board due to the double coil arrangement. After some difficulty in replacing the cap, I found a track lifted adjacent to the joint, so that there was no connection to this end. Could this have been the original problem? Who knows! (Perhaps I lifted it in the repair). There certainly seem to have been several areas out-of-spec in the course of this saga. Really though, it wasn't until I replaced the big rectifiers that I started getting somewhere.

Anyway I'm happy to report that after all these measures, all voltages are pretty spot on and stable, except the phosphor display line which seems content to sit at +2500 volts , rather than 3000 as specified. However, the unit is behaving generally very well, calibrates nicely eliminating pin-cushion etc, and even a few timebase anomalies seem to have sorted themselves out! I celebrated by making twin Lissajous displays dancing off the mains frequency for a while..

It's going to be nice having an analogue scope once again - there is something so immediate and responsive about them, that my Rigol can't match. And the fact that the aesthetics are from a 1950's US submarine are just an added bonus!

Many thanks team (especially Will)!

Re: CRT scope high voltage supply problem?

PostPosted: Sat Feb 02, 2019 10:16 am
by Folderol
Well done!
Delighted to hear this. That damaged track is very likely highly significant - if not the root problem.