Re: What replacement for 22uF tantalum beads?

Tom Gardner

On 19/03/16 16:45, analogaddict013@... [TekScopes] wrote:

I had no warning of failure, the 485 had been on foe an hour
and when I came back half an hour later the +13V line was dead.
Now 13<15, but not by as large a margin as I would have liked.
One tant was a short, one 86R6ohm (!) resistor was toasting itself,
and fortunately the 741 has current-limited outputs :)
Currently I'm using 10uF 15V replacements I had to hand, but
they are relatively easy to replace should I feel the urge.
(My s/n is 702507, but I can't date that)
I'm confused..
I suppose you are aware that there is no "+13V" rail in the 485?
There is, if you know where to look, and that isn't the power inverter :)

On the Power Distribution schematic 17 (p163), there is U960 used as a shunt regulator. Its job is to clean up the 2V that is dropped across 86r6 resistor R965, in conjunction with tants-to-ground C965, C911, C861. In my case C911 was shorted, which caused R965 to have 15V across it instead of 2V, leading a 0.25W resistor to dissipate 2W, ahem. In addition, the U960 741 was vainly trying to hold the line at 13V through a 10ohm resistor

The result of 15V-2V=13V is a line shown on that schematic that goes to schematics 7, 9, 11, 16 and, if you trace things through, schematic 10. It visits boards A10, A12 and some other places. Mostly it is a pseudo voltage reference for the top end of potentiometers, but in a few places it doubles as a conventional power supply.

Unless there is another rail offsetting the 2V difference, U1910 (power supply regulator) should have kicked off the inverter when the +15V rail went 2V low.

After the short, was there an audible clicking from the inverter? U1910's balance node should be sensing the current from the short and stopping the inverter.
All the inverter's PSU outputs were in spec; the fault only caused an extra 150mA to be drawn from the +15V rail. My luck was in finding the calibrator output non-functional, starting there, and noticing a toasting resistor a couple of inches away.

I wouldn't have heard a clicking in any circumstances short of someone dropping a CRT :(

I've gotten into the habit of checking every dipped tantalum for shorts and excessively high ESR in each scope before bothering to do much troubleshooting. Is this 485 a scope you've had for a while and been using, or a project scope you recently started working on?
I've had it a couple of weeks. I was picking up a CRT for a dead 465, and I couldn't resist the seller's offer - which is a little worrying :)

It would take 60s to start, unless it had previously been "warmed up". That was C1848 (almost certainly) in the inverter being a deceptive little b*gg*r. Hence I had already recapped all the ali electrolytics in that area, and this was an unpleasant surprise.

I suspect the 485 will become my favourite scope, rather than the 465 or my HP1740A

I've dismantled the broken 465 CRT to make a pretty shelf ornament (sad I know), and when finished I'll publicise my photos of it.

And to think only a month ago I knew zilch about CRTs other than what I had been taught at school. (I can't remember being taught about them at university!)

I don't know enough about Tek's SN info or manufacturing to date them by that - if it's possible. The way I date my scopes is by seeing what the most recurring, earliest date codes off of components are.
One PCB is copyright 1970, but the 74 series logic appears to be 1982.

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