Info for searchers: PSU for old Cricut

[eccerr0r]

I just got my hands on an old Cricut machine for cheap, as it was dead. The PSU is a 18V@2A brick, and I noticed its LED stayed off... It must be a dead PSU at least at the first order of repair.

I got it home and cracked the plastic case open with a hammer and a bench vise. It was a fairly typical flyback design with large heat sinks along the sides... so perhaps it wasn't that great an idea to use the vise, alas it was necessary. It uses the venerable 3843 SMPS chip, surface mounted on the bottom.

Using an ESR meter, I quickly found that the 3843 power filter, a 100uF 25V capacitor wedged next to one of the heatsinks near the optoisolator was at 4 ohms ESR. I pulled it and measured a whopping 23uF and leaked 4mA at 25V.

Swapping it for another cap restored operation of the PSU. One of the output caps was bloated, may replace that at a later time since its ESR seems OK for now. But perhaps need to swap it before figuring out how to re-seal the plastic brick PSU case...

Pictures later...

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

Running it 'off' for a few hours, the PSU gets warm... terrible efficiency at low load.
I suppose the inefficiency heat it generates is what killed that capacitor.

The cap is branded "Rifeking" ... The PSU does give a warm feeling due to its inefficiency, but not a fuzzy one.

The bad cap is that little one near the big main/primary 82uF 400V capacitor.

[sam_sam_sam]

Re: Info for searchers: PSU for old Cricut

I have seen this myself that some switching power supply that do not have much load on them run a lot warm than they should

In fact I have seen this layout before I think I have a couple of these power supply myself

[PeteS in CA]

Re: Info for searchers: PSU for old Cricut

That little start-up cap is a critical part for most 384x family circuits. Bad cap, no start-up.

Output caps in discontinuous flyback circuits are very stressed, should be a No Crap-Cap Zone.

Discontinuous flyback circuits are not super high efficiency. But more to the point, running a discontinuous flyback circuit no load means much/most of the charge stored into the primary during the switch on time has to be dumped into the primary side clamp (instead of into the load). So that clamp resistor is hotter at no or very light load than it is at heavier load.

A 3843 is "venerable"? Come a little closer ... so I can whack you with my walker!

[stj]

Re: Info for searchers: PSU for old Cricut

a lot of early designs, when the startup cap fails the psu goes bang.
literally!

i used to repair sat recievers,
i had large stocks of topswitches, 3842's and fets etc.

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

Indeed the Unitrode and clone 384x SMPS chips have been around for a long time, they're everywhere. And work...until that cap dies.

The MC34063 probably also deserves the same fame though it's used more for dc-dc converters...

Speaking of the 3842 and 3843, they differ in turn on and sustain voltages, the seeming more popular 3842 looks like it's more susceptible to low frequency cycling due to the huge range. The 3843 is much smaller.

I also wonder why I don't see many 3844 and especially 3845s...

[stj]

Re: Info for searchers: PSU for old Cricut

price?

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

I'd think that the 3842 3843 3844 3845 would all be similar to each other in price, or are they that radically different?

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

I just found my first 3845 switcher. My HTPC custom microATX PSU uses a 3845, so now I've finally seen a PSU with each of the four of the Unitrodes...

I had to open it as it too sort of failed. Well, it works, but I found that the 3v3 and 5v0 line had 300mV of switching noise on it. Temporarily swapping for an external full sized PSU seems to make it work better (it was belching hard drive errors with the original.)

More OST caps to stick in my dead parts jar.

[stj]

Re: Info for searchers: PSU for old Cricut

i know one thing often not shared,
the clones are not 100% compatable.

many years ago i had a table of different resistor values needed for different makes of 3842 when substituting one for another!!

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

wow, crap... Is it due to the internal reference differing between them or input leakage current? How many percent different were they from clone to clone?

Most of the ones I've seen recently are ST, and a few Unitrode...

Speaking of accuracy between manufacturers, though I haven't needed to swap any 384x, I have swapped a TL494/DBL494/KA7500/... Perhaps I got lucky but it turned pretty close. These chips are also kind of popular though the 384x designs seem more abundant...

[stj]

Re: Info for searchers: PSU for old Cricut

cant remember the detail,
it was in a book of common faults>fixes for analog sat receivers.
i also remember it recomending replacing the old green mylar caps with polypropylene or polyester ones because of film breakdown!!!

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

BTW, here's the ST 3845B design:

https://www.badcaps.net/forum/showpo...postcount=2715

Curious what people thought of the power rating of this PSU...

[stj]

Re: Info for searchers: PSU for old Cricut

looking at the heatsinks i think maybe 120-150w, with a good fan - it's a bit packed at the output end.

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

The rated wattage of that microatx is ... higher ... than your guess. Whether it really can get that wattage it claims is questionable indeed.

I have another PSU used in a 1U that equally has a "surprising" (or overrated) wattage.
It has the form factor of a 30W open frame but claims 70-80W, so that it can power a P3 and peripherals...

These PSUs had premature failure so perhaps they indeed are overrated.

[Behemot]

Re: Info for searchers: PSU for old Cricut

The MC34063 probably also deserves the same fame though it's used more for dc-dc converters...

Indeed. I've finally went to rework the stockpile of SOHO switches and routers and it's in about 4/5 of them. Which is great as with mostly FR 470/35s I put on input it can work with input voltage up to 34 V. It usually makes noises if you use something else than the voltage it's balanced for, but it works…

[stj]

Re: Info for searchers: PSU for old Cricut

so do 34063's self distruct if they get stuck in a startup mode from poor capacitance??
because i'm pretty sure they are in the jaguar game console - i had to replace one once.

[eccerr0r]

Re: Info for searchers: PSU for old Cricut

The 34063's are powered directly from the input power (hence limited voltage input) and not bootstrapped from input power like the 384x. Most 34063 designs use their internal switches directly (it goes up to 1.5A) much like the TNY* chips...

I haven't seen too many 34063 failures, seems fairly reliable. Just commenting about its popularity through the years which unfortunately the 384x simply croak...