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RustySparks
Are you sure this is not the typical leaky HV transformer winding problem that seems to affect practically all the 453 series scopes and other similar models simply due to the transformer HV winding becoming leaky because of chemical changes in the epoxy used in transformer assembly over the years?
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My 453A has pretty much the same problem with HV, initially it would stop oscillating after being on for 30 minutes or so, got progressively worse after a year or so, eventually wouldn't start at all. It was through this forum that I learned what the problem was. At least one member here has corrected his by rewinding the HV transformer secondary. Pretty amazing project! -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of william_b_noble Sent: Monday, June 11, 2018 1:51 AM To: [email protected] Subject: [TekScopes] let's bring this back to life Re: Tektronix 453 high voltage problem I decided to dig into an early 453. Interestingly it does not match the schematics for the 453 or the 453A - some boards seem closer to the A, some to the earlier one - for example the Z axis board is clearly dated (C) 1965, but its layout matches the board found in the A manual (unless I'm just loosing my mind). This scope sat around for several years after I got it and never plugged it in - when I did plug it in there was initially no light on the CRT (except the scale illuminator) - as I messed with it for about an hour trying to make sure what was wrong, it fired up and showed a spot and vertical deflection, I could center it, but there was no trace. then it stopped. The HV circuit is one of those that will stop oscillating if there is an overload, so it's hard to troubleshoot, and harder without an HV probe. But it's clear that something is overloading the circuit. If I disconnect the cathode and grid bias circuits, it makes HV and powers the CRT filament just fine. I've tried all sorts of things, one at a time, and disconnecting the .015uf 3Kv capacitors by itself doesn't work, disconnecting the diode that drives the grid string does let it work, and the list goes on. I've come to the conclusion without a good way to verify it that it is most likely that at least one of those 3KV ceramic caps is leaky, and if one is bad, the others are probably not far behind, so I've ordered some. The HV diodes are also suspect, though they both seem to act like diodes at 115V (with a 10 W lightbulb as the load), I ordered some microwave oven diodes just to be on the safe side. I don't think my 1M CRT Grid Bias control is bad like was found upthread, when the power supply works, I can adjust the grid bias. But now, if the supply oscillates the current can rise to about 2.9 amps (it's fused at 2 amps) with all the loads connected. The transformer isn't potted, so I think it's probably not the issue. anyone care to offer some advise on something I could have missed? I'd like to get this scope working just to say that I did it - I have a couple of newer ones to look at next, I figured this one first, it's all discretes, how hard could that be >>> |
