Products > Test Equipment
Stunning 3000A, 2000A, 3000T power supply discovery - MUST READ IF YOU HAVE ONE
(1/6) > >>
EE-digger:
After spending some time on the repair thread for these scope power supplies, I did further research and also prepped a Murata supply which I had on hand, to replace the original Cherokee/GE CCH125 in the scope.

So today, I took the factory supply out of my pristine, dare I say virgin, DSOX3104T.  The scope is super clean inside and out.  Hadn't really looked at the supply much because ... it's a supply.

Also, haven't used the scope in weeks but it's plugged in a day at a time.  First thing I noticed was that the ac mains cap was hot as hell.  (thermo and IR camera in the days to come)

Back of the board looks like crap.  Not blackened but clearly looked like it was sitting in an oven at high temp. 

I'll cut this short with more to come later.

I've designed SMPS from low level to high level modulars into many medical products but do not profess to be an expert in topologies.

So, need some help here.  The 420v mains cap is sitting at 397 volts dc   :wtf: :wtf: :wtf: :scared:  This is on a 120v mains.

This is why these scopes are cooking.  Might this supply be using doubling up front?  You just can't do it with these bleed resistors.  They will be running red hot, and right below the cap with thermal coupling via the cap leads.  With very little thermal decoupling, these 1W resistors are running at 0.4W, and the thermal rise is increased due to their close proximity.

The new Murata is sitting on the bench, most parts cool to touch and the voltage across the cap sitting at ..... drumroll ..... 168vdc.  I could hear it saying "take me", "take me", "take me".

Ok little Murata, as soon as the connectors arrive.

It seems like these scopes are a ticking bomb, almost guaranteed to blow on European and other 230v mains, although I don't know what their design would do there.

perdrix:
It's quite normal to run 120V setting through a doubler (for switchers that convert mains input to DC as the first step).   When set for 240V input the doubler isn't used.

In the absence of other faults, I would suspect that the cap in question has gone high ESR, and therefore gets seriously toasty.

If there are bleed resistors across the cap, you could consider increasing their value - its not uncommon for the value of the bleed resistor to be unnecessarily low.
EE-digger:
Thanks for that. 

I'm going to refurb several of these with a higher value bleed resistor and an 8000 hour cap of identical size.  I just realized that in the powered off scope, this supply has aluminum chassis on one side which will conduct heat if close enough (not so sure it is) and the bright plated shield on the other side, which may be closing the "oven door" so to speak.

Open on the bench, the resistors get up to 100C and the cap to 60+C.  I may emulate or just put it back in the scope to test the "oven" effect.  But that +397v cuts it too close to the 420v rating for my taste.

And as they say, the proof is in the pudding.  For a very low hours scope, this supply looks "cooked", just short of board blackening / burning.

But for my scopes, this baby is gone
TheSteve:
You've got me curious so I will open my scope this weekend and have a peak. It has been connected to power 100% of the time for many years.
EE-digger:
Think of it less as a scope and more as a crypt.   :-DD   Can you post a pic of the board bottom?

I've installed new cap and resistors.  Excellent improvement and thermal images posted below.

It's a design flaw.  Someone missed the +397 volts sitting across the series Rs.

Resistors were at 100C, now at 64C, capacitor pins are about 10C below that  (new R value = 68K each,  bleed time to 50V = 24s)
There are also hot resistors lower in the image but they are only pushing 70C+

--- End quote ---


Capacitor top was pushing 70C, now at 46C  (what looks like a hot ring around the capacitor is the emissivity of the shrink wrap tricking the camera)

--- End quote ---

Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod