Re: Old Tatung SMPS design, expert comment needed ...

I replaced the entire power supply over 5 years ago with one fit for purpose.

Re: Old Tatung SMPS design, expert comment needed ...

That's the inrush current limiting resistor, which is not getting too hot anyway since the current passing through at 230VAC is roughly half that at 110VAC.

I have a small collection of about 10 old computers, dating from 1983 to 1997, all in very tidy condition. Silicon Graphics, SUN, IBM, HP and Apple. These are kept mostly for display only, but I insist that they all work correctly and test them at least yearly. Obviously they all have switch-mode power supplies.

The very few power supply failures to date have been EMI suppression caps and 5V smoothing caps, not primary smoothing caps or the multitude of other small caps found throughout. The EMI caps started failing only after I moved to a 230 VAC country a decade ago, no surprise...

Re: Old Tatung SMPS design, expert comment needed ...


Yeah, but there's no low voltage available in this monitor to run a DC fan, I don't want to use an external power pack and an AC fan ties me to one mains voltage. Easier just to continue to run it at 110 VAC off a stepdown transformer, or do nothing. Don't need much time using CGA these days


Unless I can do that using differential mode on a 'scope (400V isolation to ground) I don't have the kit to determine that. In any case, I know it's easily twice as high.


Per pic, R805...

Re: Old Tatung SMPS design, expert comment needed ...

Not entirely. The drive transistor is rated to 800V, the transformer 400V p-p and the nearest cap 1.6kV. I have a photo from a later revision of this power supply (one that is specified to run on 100-250VAC) that shows the same transformer part number, while R805 (mentioned first post) is twice the value I have. Other than that, it's fingers crossed.

I've run it a total of about 2 hours without issue (other than heat) but I realize that doesn't mean it's working within its design envelope. I'm hoping to get more...

Re: Old Tatung SMPS design, expert comment needed ...

Yes, thanks, I've done that (10w 15 ohm at right) and reassembled the monitor until I come up with some better ideas. It's still getting a bit toasty inside from the 5w 180 ohm as well as there is only passive cooling via small vent holes. I may try leaving the cover off (as shown in pic) internal to the monitor. Don't think that would hurt anything ...


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The PS in question is from an IBM 5153 CGA monitor which was made by Tatung in 1985. The PS is a separate internal assembly that produces 115 VDC @ 0.5 A for the CRT monitor chassis. It's housed in a metal enclosure with ventilation holes.

The monitor is labeled as "120 VAC" (only) but I live in a 230 VAC country and would prefer to run it directly off the line without a stepdown transformer.

Looking at the design (SAMs schematic,) since the primary DC rail runs fixed in full-wave mode and the smoothing caps are rated at 400 WVDC but only see 153 VDC at...

Re: New Members - please post your introductions here

Hi all from New Zealand. I've got a small collection of vintage computers ranging from 1982 through 1997, many of which I procured in the US and brought here to NZ. Clearly "bad caps" are one of the many difficulties in keeping these running, especially given that they now need to run on 230 VAC power.

I also have a mechanical engineering degree, decades of hobby-level electronics experience and am semi-retired.

This week I've had a number of issues plague me including a "Y"...