| Products > Test Equipment |
| Tektronix 2235 repair thread |
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| bd139:
The deflection changes as well. If you stick it on X-Y mode it loses focus and shifts slightly. In sweep mode, the sweep length drops. I made a very hooky looking high voltage AC probe with some left over VR37 resistors and 5x 1n4007's in series and stuffed it across the ground / T948 p23 junction and there was no significant amplitude change going into the multiplier when this event was occurring. I even disconnected the multiplier and checked the transformer unloaded and that was fine. The -2KV is rock solid out of the multiplier as well so I think this is the 6x chain in there. I have run it without the PDA and it's perfectly stable there but as you say half deflection and brightness. Ergo I concluded the multiplier. It could be the CRT anode cap leaking but that usually leaves deposits around the cap and it's totally clean. Good news however. I managed to get my hands on an entirely dead 2235. This one's mains filter exploded and the attenuators are shot but the thing comes up on DC apparently fine. Also the channel switch is good if I have to hook that out. The cost? One £12 crate of strongbow and waiting until mid Jan. Seeing if I can get a multiplier before then as well. I really want this to work as for an analogue scope it weighs a lot less than a 465! |
| mnementh:
You bastard... you and your stick-to-it-iveness had me up at 2AM calculating the charge level I'd need to put to 3 LiPo batteries to get a stable 42-43V. |O mnem Where is the "Kick that guy in the junk!!!" button on my keyboard?!? :-DD |
| bd139:
:-DD Trick is you don’t need exactly 43V. Anything around that but less than the trip voltage works nicely :) In fact it seems to run around 35V fine. |
| bd139:
Got another multiplier here for ref as I'm too impatient to wait until Jan: https://www.ebay.co.uk/itm/283298267147 ... I found the guy looking through Google and asked if he had any more so has fished them out of some cannibal scopes he has on demand :-+ Also has transformers: https://www.ebay.co.uk/itm/283292726524 |
| David Hess:
--- Quote from: bd139 on December 10, 2018, 07:52:27 am ---Trick is you don’t need exactly 43V. Anything around that but less than the trip voltage works nicely :) In fact it seems to run around 35V fine. --- End quote --- The inverter output transistors operate in their linear region when turned on to regulate the output voltage so the voltage from the switching preregulator needs to be high enough to prevent them from saturating or the voltages on the secondary will drop. Later versions of this circuit in the 4-channel 22xx series and 24xx series did away with linear regulation in the inverter which may have been a reliability issue and relied instead on the preregulator to produce the exact voltage needed. |
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