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Inrush limiting question - Tek 603 refurb


toby@...
 

Hi list,

I've been working on a Tek 603 for a few months, starting with recapping
and replacing a transistor to resolve a Z amp problem.

Today I was able to complete X and Y gain adjustment and the display
should be basically usable.

However, on a previous power on, after sitting on the bench for a couple
of months, it blew the 215V fuse F830. I replaced the fuse and brought
it up on Variac and it hasn't had any problems since. So now I'm
wondering if it's a good idea to fit a thermistor as an inrush limiter
to be more gentle on the 50 year old supplies.

Is this a nutty idea? I'm thinking it should be easy to do taking
advantage of existing AC jumpers on the rectifier board.

--Toby


Ed Breya
 

An NTC inrush limiting thermistor should work just fine. There are a lot of kinds, so choose properly. It should stay hot with the normal load, so be sure to provide good clearance around it.

Ed


toby@...
 

On 2020-11-10 9:54 p.m., Ed Breya via groups.io wrote:
An NTC inrush limiting thermistor should work just fine. There are a lot of kinds, so choose properly. It should stay hot with the normal load, so be sure to provide good clearance around it.

Ed
Hi Ed,

Thanks. I did a bit of investigation into what the 603 should need. I
don't have any way to measure the actual current draw at the moment, but
I've used the following:

* The smoothing caps (as replaced) across the four main windings are
charged by about 11.3 J
* The overall system fuse is 1.6A slow blow -- I am estimating the
operating current is probably not greater than 1A
* I've chosen a 120Ω 2A thermistor, e.g. Ametherm SL12 12102 -
https://www.ametherm.com/inrush-current/inrush-current-limiters-full-line

I've never spec'd one of these before so let me know if anything is
obviously wrong.

--Toby