Submitted for the first time into the show the monitor died during pre-show burn in :(
The Hantarex MTC-90 lost main HV and was arcing at the tripler in the focus output. I suspected a tripler failure but had no spares :(
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Having previously finished working on the MTC-90 in the Sea Scare and discovering that the focus voltage arcs when it's open I checked out the wiring and discrete components in the focus divider on this one. Both 5M Ohm resistors in the focus divider were short circuit (a very unusual failure mode). Having no spares left from the scrap chassis I needed to find some replacements for these, clearly a common failure point on this chassis :(
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It turned out to be a bit of a challenge to find appropriate replacement resistors as 5M Ohm is not a common value in power resistors. The MTC-90 manual has them marked as 1W but the size of the ones in there look more like 3W. Common 1W resistors are only rated at 250V that's not enough for this usage. I managed to find some Tesla 5M Ohm 1W 1.5KV resistors designed for HV use on Ebay that looked like they'd fit the bill. Fitting a new pair fixed the monitor :)
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With the monitor fixed I decided to try out a HV probe I bought a while back and
take some readings for future use as a reference:
| U~ tripler input | 1.16KV |
| Focus, tripler output | 7.35KV |
| Focus, top of glass 8MOhm | 6.00KV |
| Focus, top of R7 | 1.51KV |
| Focus, top of R8 | 0.75KV |
| EHT, suction cup | 21.3KV |
| glass | 8.12MOhm |
| R5 | 1.00KOhm |
| R6 | 2.17MOhm |
| R8 & R9 | 5.01MOhm |