Re: 465B Horizontal Trace Issues - Help requested


Tom Lee
 

All I can convey is what I've learned from decades of fixing scopes (I own almost 200 of them, and perhaps a dozen or so 465/465B and related models). By far the most common sources of trouble are the two that I've mentioned: Flaky contacts (which afflict pretty much all vintage electronics), and tantalum capacitors. Sure, other things go wrong, too, but those two dominate by far, so I spend a little time exploring those first before moving on down the list.

Tek seems to have purchased a bad (and large) batch of tantalum caps in that era, so when I see a failure on the output side of the PS, I no longer spend a lot of time checking. It's much faster to replace the tants on the afflicted rail(s). There are not that many, thank goodness, and they are easy to access and replace. If I see a problem on the input side, it's easy enough to see if there are failed rectifiers or a bad primary cap. The latter are a bit of a PITA to replace (certainly compared to tants), but still readily doable. But you aren't having a problem with the primary side of the PS, and it isn't even clear that you have a PS problem of any kind.

I share your disdain for shotgunning. However the problem here is that you appear to lack the necessary instrumentation to perform more detailed investigations, and you are also reluctant to disassemble things much. That severely limits what you can accomplish. It's hard to advise you how to proceed, given the overly constrained set of steps you are willing/able to take. So, I'll limit my comments to what I would do, and you are free to ignore my advice, whether for taste, or lack of tools.

Put the scope in x-y mode (to bypass all the sweep generator circuits) and drive both x and y with the calibration source on the front panel (it's just a handy signal source). If you still see width compression there, the problem is downstream of the sweep generator. If you no longer see width compression, then the problem lies before the deflection amplifier.

Wherever the problem lies, I would use Deoxit (don't flood; a little will do a lot) on all connectors to/from the horizontal circuitry, and the pushbutton switches (these have often been a source of maddening problems for me). If the first step revealed a problem with the sweep generator, note that I have also sprayed Deoxit on the contact fingers of the sweep speed knob, despite warnings that it may be a bad idea (I have never done so on the vertical attenuator fingers, though, where the danger of affecting the transient response is much higher). Like you, the prospect of disassembling the whole thing to do it the factory-recommended way (sliding strips of paper soaked in IPA between each contact pair) seems unattractive. I do it anyway, and have not had any problems. Of course, I have the luxury of many spare scopes, so you may have a different tolerance for gambling. Proceed at your own risk.

I would reseat all ICs on the relevant boards, and carefully inspect all pins and socket contacts in doing so. Depending on the environment in which the scope was used/stored, weird corrosion can occur. Carefully clean any contacts that look dodgy.

If none of the foregoing fixes things, I would monitor the power supply voltages and watch for dips (problems from spikes are rare). I long ago jimmied up a simple comparator-and-latch gizmo that does the watching for me. Set the lower voltage limit (e.g., 20% below nominal; you're not looking for tiny effects here), hook it up to the rail in question, and go watch Netflix. If the supply ever dips below the set threshold, an LED illuminates (and stays on), letting you know that that rail is flaky. Replace tantalum. Repeat. You can monitor several rails simultaneously; the monitoring circuit is simple to build.

Or just replace the tants on the board(s) in question. It won't take much time.

Good luck. The 465B is a nice scope, and well worth fixing up.

--Cheers,
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/26/2020 13:00, gary@pincus4.com wrote:
Moving or jiggling the time base switch (Time/Div and Delay Time switch) has no effect on the problem. I have not cleaned it as it is difficult to get to, even after lifting the DM44 module.

Regarding the tantalum caps, I have read that these can be problematic, but not ready to start replacing parts that may not be related to the problem. However, I do restoration of 30-50 year old audio gear, and replacement of electrolytic caps is pretty much the starting point, so I understand replacing caps that simply due to age, are likely to be bad, marginal, or will go bad soon. However due to the complexity of the 465B, I am reluctant to take this apart unless there is a goal (finding my problem - something specific to check and either identify or rule out) -- perhaps that is too much to expect.

However, the reason I am asking for your time and tapping the experience and wisdom on this group, is to proceed based on that wisdom, rather than blindly. If that experience indicates that I should start by replacing certain caps (e.g. the electrolytics in the PS, and some/all tantalums), then I will gather up the gumption to do so - and will need to ask more questions. But taking on this task and then finding I still have the compression problem, while giving me a more "stable" scope, does not really fix the problem. I would rather find/fix the problem, and then with the knowledge that my scope does not have a major deficiency, address the (potential) issue of the caps. Is this thinking backwards?

Thanks to all for your patience and advice.
Gary



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