23rd Mar 2014, 10:33 pm | #141 |
Heptode
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
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Re: Tektronix 465B Scope - Some problems.
Hi Bill,
Didn't think about that one. I did use the Advance SG63 HT rail and by selecting the stabilizer valves got it to 215V after it had warmed up nicely. I used a string of 5 x 10Meg and as your suggestion a 1Meg as the resistor in parallel to the DMM. Using 2 DMM's (Both 10Meg at the input) I used the Fluke to read the attenuated voltage, whilst the other monitored the HT rail. I ended up with a conversion factor of 215/3.74 that I worked backwards to get the value that would be expected on the DMM for the 2450V. 2450/215 x 3.74 = 42.61V at the DMM 42.61 x 215/3.74 = 2449.50V Just as interest that says that to obtain a maximum reading of 1000V on the DMM it would need 23KV. NO, I wont be trying it. The thing that comes to mind now is that the voltage across each 10Meg resistor is way above the specification of the resistors at 500V each. I think I will make it a string of 10 resistors at least it will half the voltage across each of them. I will add the 5 resistors, see what it then gives, and then look at making it up into a usable probe. Dave. |
23rd Mar 2014, 10:57 pm | #142 |
Rest in Peace
Join Date: May 2011
Location: Preston, Lancashire UK
Posts: 955
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Re: Tektronix 465B Scope - Some problems.
Dave, its overkill. Normal resistors will be fine for short periods, its usually if there is any damp or you are using over a prolonged period that they will break down. For a minute or so you will be fine with that setup. If I were you, I wouldn't try to measure the multiplier output to the final anode, the noise will put you off ! Just set the cathode volts to spec and that should do, the final anode voltage should be ok if no components have been changed. 1 watt or 2W resistors generally have a higher voltage handling capability if you want some piece of mind.
Bill |
24th Mar 2014, 3:53 pm | #143 |
Heptode
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
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Re: Tektronix 465B Scope - Some problems.
Hi Bill,
I have now made the probe and done a final check on it and all is OK. I did add the extra resistors, I just felt happier about doing that and I had room available to allow for it. The additional resistor produced a multiplication factor of DMM Reading x 216/1.9 I used the empty canister from a large bullet point marker with the inside removed it was ideal. Some pictures added. Now I will make a check on the cathode voltage and see what difference that may make. Dave |
25th Mar 2014, 12:28 am | #144 |
Heptode
Join Date: Feb 2011
Location: Sheerness, Kent, UK.
Posts: 936
|
Re: Tektronix 465B Scope - Some problems.
I have now checked the cathode voltage and it is within the spec limits at 2458.98V
3 pictures have been added showing details of the spot. The original pictures were quite large so I copied out the center section so that the details of the spot can be seen. The brightness may seem as though it is a bit high but actually is not, it is set at a level that if a high speed trace is switched in you cant even see it. Picture 1 is the spot with best focus I can obtain. 2 is the focus turned right, and 3 turned left. All in all I still think it has a problem, maybe the replacement tube is not much better than the original?? Dave. |