Hello group,
I discovered I have a Dynacomp PSU thst will change the +5/-5 rails drastically after 15-20 minutes of being powered. They hold a good 5.03/-5.12 for 15-20 minutes then I see them (dropping on the +5 and increasing in the -5). Will change from 5.03/-5.12, minute later 5.02/-5.11, minute later 5.01/-5.10 it will keep going. I left the room for a few minutes and when I came back they were at 3.45/-3.75. I shut it down. After a full recap it did the same thing. After I shut it off for 5 seconds and power back up it holds fine for the same 15-20 minutes before changing again. I’ve read several posts here in PSU issues and I have a feeling this problem is deeper. First time I’ve seen this happen with a PSU.
Any input would be appreciated.
Thanks
It sounds like there's a problem in the feedback of the switcher. Without a schematic it's hard to say, but for example, if it uses a TL431 or optocoupler for the feedback, those may need replacement.
Thanks for the reply,
I couldn’t find a schematic for this Dynacomp. I remember looking months ago when I was recapping PSUs. Astec schematic yes Dynacomp no soap. I would like to repair it if possible before throwing in the towel.
Also unable to locate a schematic for the DynaComp `606-5001`, I went ahead and reverse engineered it:
KiCad and PDF schematic can be found here:
https://github.com/fredsa/apple-ii-dyna-comp-606-5001-power-supply
The usual accepted repair procedure for DynaComp power supplies is as follows:
Gut the case and throw the board in the trash.
You can install a Universal Power Supply Replacement kit from ReActive Micro or use a really inexpensive MeanWell RPT 60B with a small modification described here: https://www.applefritter.com/content/12v-power-supply-conversion-using-mean-well-rpt-60b
Sorry, but I don't know of any accurate schematics for the DynaComp and as you discovered, recapping these power supplies is a waste of time.
However, for what it's worth, most of these failures will probably be rooted in the optoisolators that serve as feedback devices.
If it helps I can sell you a replacement DynaComp power supply board but I can't guarantee it will be reliable.
I'm with baldrick as far as this goes. If it isn't something really obvious, even for an Astec made power supply I just swap in a Mean Well. I did a few with the ReActive Micro boards also and they're a great product and super easy but the Mean Well conversion isn't much more work and it is 1/2 the price.
Here's another thread using a slightly different model Mean Well power supply board, the PT-65B.
https://www.applefritter.com/content/putting-pt-65b-switchmode-power-module-apple-iie
The last two Apple branded power supplies I've done I used a slightly modified version of that process, using a piece of acrylic cut to the size of the original power supply board and drilled to fit the existing mounting holes. I then mount the PT-65B board to that peice of acrylic along with an LM7905 regulator and heat sink for the -5V the Mean Well doesn't deliver, splitting it off from the -12V. That still leaves plenty of power for both -12V and -5V, more than the stock Apple supply does for sure. It's pretty simple and all-in including the acrylic, connectors and PT-65B board it is under $40 shipped from Amazon, Mouser or Digi-Key.
In the end you essentially get a brand new, higher output supply that's pretty much guaranteed to work easily and be reliable for many years, vs. a mostly 30+ year old power supply that may require lots of work to get going and could fail again in a different way at any time. It's just almost not worth the effort to deal with trying to repair the original Astec and especially Dynacomp boards.