Most DMMs have 10Meg input impedance, so you can put a 10meg resistor in-series with your meter, and measure double the voltage. It wont be exact, but pretty close. If you really need an exact measurement, then measure a stable lower voltage without the resistor, then again with it. Dividing those 2 measurements will give you a ratio close to 2.0, and you can use that when measuring your high voltage.
On Tuesday, October 26, 2021 at 8:40:54 PM UTC-7 GEK wrote: > Thanks everyone for the hints, I'll look more closely at the HV - I'm a > bit shy about measuring as I don't have a meter that will go to -1300v for > the bright. > > - Gary > > On Tuesday, October 26, 2021 at 3:17:49 PM UTC-4 Bartek Ogryz wrote: > >> I'm not familiar with that project and that circuit, but based on my >> experience in CRT TV repairs, if you have a proper blanking signal, >> something must be wrong with HV as Tristan said. >> Grid1 - negative voltage on this grid allows electrons to flow (brighter >> picture), 0V blocks them (darkness). This grid can be driven by the >> blanking signal, >> Grid2 - is the screen grid, high voltage accelerates the electrons and >> brightens the screen. If that voltage is too high, it can "force" electrons >> to flow, regardless of the Grid1 voltage. >> Chceck the main power supply first - if it's voltage is too high, all HV >> voltages will also be too high. >> wtorek, 26 października 2021 o 05:02:21 UTC+2 Tristan napisał(a): >> >>> The brightness pot needing to be turned right up still suggests that you >>> have a problem with grid voltage control. I'd expect to see some higher >>> frequencies much higher than 60Hz on the BLNK- signal as well though. I'd >>> be surprised if the problem was in the supply sections for the cathode or >>> anode given that it is mostly working in your photo but there could be an >>> issue with the specific section of the supply dedicated to the grid and it >>> is referenced to the cathode. Although you have now replaced most of the >>> potentially offending components unless you have an open winding on the >>> transformer. You could check the resistance across pins 4,5 and 6 of the >>> transformer. I'd expect them all to read fairly low. >>> On Tuesday, October 26, 2021 at 10:20:26 AM UTC+10 GEK wrote: >>> >>>> Ok, so I'm stumped (again)... >>>> >>>> I have replaced D35-38, C52, C53, C54, C55, Q1, C51 and U17. >>>> R67, F68, R69, R70 and R71 measure correctly in circuit. >>>> I've confirmed pulsing on the BLNK- signal at 60hz >>>> I've also swapped U5 (74HC74) and U4 (74HC151). >>>> I've swapped the CRT. >>>> >>>> Curiously I need to have the Bright potentiometer (R11) at maximum to >>>> get an image. >>>> Could something in the High Voltage Supply be causing problems with the >>>> blanking? >>>> >>>> On Thursday, October 21, 2021 at 3:05:10 PM UTC-4 Paul Andrews wrote: >>>> >>>>> Hah yes. I have one of its brethren that is even more 'eclectic'. >>>>> >>>>> On Wednesday, October 20, 2021 at 8:04:03 PM UTC-4 Terry S wrote: >>>>> >>>>>> >>>>>> That may just be one of the worst PCB layouts I've ever seen. >>>>>> On Wednesday, October 20, 2021 at 8:33:41 AM UTC-5 martin martin >>>>>> wrote: >>>>>> >>>>>>> Here's a simple one >>>>>>> >>>>>>> https://www.tindie.com/products/howchon/osc45-oscilloscope-clock-kit-for-many-crt-types/ >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> ~ >>>>>>> *mcve...@gmail.com* >>>>>>> >>>>>>> >>>>>>> On Wed, Oct 20, 2021 at 4:16 AM GEK <geka...@gmail.com> wrote: >>>>>>> >>>>>>>> Will do, Hopefully parts will be here by Friday. I may try and >>>>>>>> shotgun D35-38 and C52-53 before then as I should have spares around >>>>>>>> here. >>>>>>>> >>>>>>>> Finger's crossed! >>>>>>>> >>>>>>>> - Gary >>>>>>>> >>>>>>>> On Wednesday, October 20, 2021 at 1:19:58 AM UTC-4 Tristan wrote: >>>>>>>> >>>>>>>>> Hi Gary, >>>>>>>>> >>>>>>>>> That's a bit odd. While I'm no EE I would have expected that >>>>>>>>> removing U17 would have blanked the display if the rest of the >>>>>>>>> circuit was >>>>>>>>> working correctly. It looks Q13 should be conducting without U17 >>>>>>>>> fitted and >>>>>>>>> the grid should become more negative with respect to the cathode to >>>>>>>>> ensure >>>>>>>>> cutoff. I think you may need to look at the D35-38 and C52 & C53. >>>>>>>>> >>>>>>>>> - Tristan >>>>>>>>> >>>>>>>>> On Wednesday, October 20, 2021 at 12:42:53 PM UTC+10 GEK wrote: >>>>>>>>> >>>>>>>>>> Tristan - >>>>>>>>>> >>>>>>>>>> Thanks for those hints. the BLNK signal is pulsing at 60hz, and >>>>>>>>>> I swapped Q13 without any change. I also socketed U17 (6N137) while >>>>>>>>>> waiting for a replacement to arrive. Removing U17 from the circuit >>>>>>>>>> leaves >>>>>>>>>> the display unchanged, so it does appear that the blanking circuit >>>>>>>>>> is the >>>>>>>>>> culprit. >>>>>>>>>> >>>>>>>>>> I'll swap U17 once it arrives - and if that doesn't fix things >>>>>>>>>> start shotgunning the passives. >>>>>>>>>> >>>>>>>>>> Thanks again for the guidance, very appreciated! >>>>>>>>>> >>>>>>>>>> - Gary >>>>>>>>>> >>>>>>>>>> On Tuesday, October 19, 2021 at 10:04:24 PM UTC-4 Tristan wrote: >>>>>>>>>> >>>>>>>>>>> Also the voltage between the cathode and the blanking section >>>>>>>>>>> should be checked (I.e. the voltage across C51) and R65 and R67 >>>>>>>>>>> should be >>>>>>>>>>> checked (or potentially replaced along with C51 if shotgunning it). >>>>>>>>>>> The >>>>>>>>>>> rest of the high voltage supply appears to be working. >>>>>>>>>>> >>>>>>>>>>> - Tristan >>>>>>>>>>> On Wednesday, October 20, 2021 at 11:50:56 AM UTC+10 Tristan >>>>>>>>>>> wrote: >>>>>>>>>>> >>>>>>>>>>>> I agree. The blanking is not working and as a result the beam >>>>>>>>>>>> is also faintly visible as it moves quickly between the characters >>>>>>>>>>>> as well >>>>>>>>>>>> drawing the arcs. It would be a good idea to check the BLNK signal >>>>>>>>>>>> on a >>>>>>>>>>>> scope and at least see that is is toggling but my money would be >>>>>>>>>>>> on the >>>>>>>>>>>> analogue blanking section. Given the other side of the >>>>>>>>>>>> optoisolator in the >>>>>>>>>>>> blanking section is at a fairly high negative voltage potential >>>>>>>>>>>> (as this >>>>>>>>>>>> reduces the need for the deflection amps to handle higher >>>>>>>>>>>> voltages) you >>>>>>>>>>>> will need to be careful when probing around there but I would >>>>>>>>>>>> start by >>>>>>>>>>>> checking the voltages across C54 and C55 (as long as you are >>>>>>>>>>>> confident >>>>>>>>>>>> doing so safely with the HV potential to ground). The correct >>>>>>>>>>>> voltages >>>>>>>>>>>> aren't specified on the schematic but they will be based on the >>>>>>>>>>>> grid cutoff >>>>>>>>>>>> voltage of the tube. If you wanted to avoid probing while the >>>>>>>>>>>> clock is on >>>>>>>>>>>> I'd be inclined to shotgun replace that section starting with the >>>>>>>>>>>> power >>>>>>>>>>>> supply diodes, caps and Q13 and then U17 and finally the passives >>>>>>>>>>>> and see >>>>>>>>>>>> how you go. >>>>>>>>>>>> >>>>>>>>>>>> - Tristan >>>>>>>>>>>> On Tuesday, October 19, 2021 at 4:24:11 PM UTC+10 iavine wrote: >>>>>>>>>>>> >>>>>>>>>>>>> From what I remember: >>>>>>>>>>>>> the characters are generated from circles, ellipses or lines >>>>>>>>>>>>> which in turn are created using sun or cosine waveforms >>>>>>>>>>>>> To make a character the beam is blanked for certain parts eg C >>>>>>>>>>>>> is a circle with a bit cut out. >>>>>>>>>>>>> >>>>>>>>>>>>> Looks like the blanking is not working as you’ve got a lot of >>>>>>>>>>>>> circles and ellipses there. >>>>>>>>>>>>> >>>>>>>>>>>>> Looks cool though >>>>>>>>>>>>> >>>>>>>>>>>>> Cheers >>>>>>>>>>>>> Ian V >>>>>>>>>>>>> >>>>>>>>>>>>> On 19 Oct 2021, at 06:08, Mac Doktor <themac...@gmail.com> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> On Oct 18, 2021, at 7:53 PM, Gary Kaufman <geka...@gmail.com> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>> I've now got a stable, but scrambled display (see pic's). I >>>>>>>>>>>>> think I have a problem with the shape generators but would >>>>>>>>>>>>> appreciate any >>>>>>>>>>>>> help or suggestions you could offer. >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> Nah, that's just Cyrillic Neu Sans. >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> Terry Bowman, KA4HJH >>>>>>>>>>>>> "The Mac Doctor" >>>>>>>>>>>>> >>>>>>>>>>>>> "If only you could see what I've seen with your eyes."—Roy >>>>>>>>>>>>> Batty, *Blade Runner* >>>>>>>>>>>>> >>>>>>>>>>>>> -- >>>>>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>>>>> Google Groups "neonixie-l" group. >>>>>>>>>>>>> >>>>>>>>>>>>> To unsubscribe from this group and stop receiving emails from >>>>>>>>>>>>> it, send an email to neonixie-l+...@googlegroups.com. >>>>>>>>>>>>> To view this discussion on the web, visit >>>>>>>>>>>>> https://groups.google.com/d/msgid/neonixie-l/3FBEDB26-E97C-44E2-B088-BDA7DAE4A020%40gmail.com >>>>>>>>>>>>> >>>>>>>>>>>>> <https://groups.google.com/d/msgid/neonixie-l/3FBEDB26-E97C-44E2-B088-BDA7DAE4A020%40gmail.com?utm_medium=email&utm_source=footer> >>>>>>>>>>>>> . >>>>>>>>>>>>> >>>>>>>>>>>>> -- >>>>>>>> You received this message because you are subscribed to the Google >>>>>>>> Groups "neonixie-l" group. >>>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>>> send an email to neonixie-l+...@googlegroups.com. >>>>>>>> >>>>>>> To view this discussion on the web, visit >>>>>>>> https://groups.google.com/d/msgid/neonixie-l/00c69936-4f04-4a88-8343-f2f7d860135fn%40googlegroups.com >>>>>>>> >>>>>>>> <https://groups.google.com/d/msgid/neonixie-l/00c69936-4f04-4a88-8343-f2f7d860135fn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>>>> . >>>>>>>> >>>>>>> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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