I had a pair of rack end pieces left over from a previous rack project. And I also had a power supply left over from that same project. All I needed was a pair of rails, which I got from Detroit Modular. The rails and threaded nut strips are made by Eurorack Hardware.
Re-capping the power supply
This old power supply was pretty crappy. A while back I updated the +12V regulator with a Recom unit, but that had a problem starting up and I was told it was a bad substitute. This time I replaced it with a good old LM7812.
The PC board of this supply is badly designed. Traces carrying power are thin. Solder lands in the ground plane have no thermal relief, what a pain! The 1000uf capacitors were rated at 16V, but the incoming voltage from the diode rectifiers is 18V. The input and output pins of the regulators were bypassed by 1uf electrolytic caps. I replaced the large caps with 1000uf 25V new ones, replaced the 1uf caps on the regulator inputs with 0.1uf ceramics and the 1uf on the outputs with 10uf electrolytic caps. (Those are soldered to the regulator pins on the back side.)
The AC Wall Wart
I have an adapter cable I made up to go between a 2.1mm wall wart barrel and a Radio Shack power connector. I normally use this for my power harness for DIY Ciat-Lonbarde gear, but it works fine here, too.
The power traces on that board are ~10 mil. They should be soldered over point to point with bus wire. Also you can get more margin by replacing the simple regulators with LDOs.
Yeah, I thought about using bus wire to beef up the power traces. I’ll do it. Would you explain LDO?
Thanks,
Richard
It is a half wave rectified system, not full wave. The caps are insufficient for filtering to the rated current. IIRC, the power supply was “rated” at 800 mA, but only put out 200-300 mA before exposing huge AC ripple (2-3 volts) at the DC outputs. To fix that you add the biggest bulk caps possible, though at 25V as you suggested not 16V. I was able to fit 1500 uF / 25 V in there. Normal linear voltage regulators need about 3 Volts of margin. LDOs need about 1V, so it gives about 2V extra margin before AC ripple kills it. I was able to get about 600 mA clean with the fixes. With the parts you have in there it is going to be in the 200-300 mA range.
I looked up this LDO +12V 1A regulator on Mouser: 755-BAJ2CC0T
For the -12V this: 926-LM2990T-12/NOPB (1.8A)
That plus the bus wire would be plenty of improvement for this use. Even as it is, I doubt I’ll be testing modules with a combined draw of even 200 ma. At what point would heat sinks be needed, too?
Thanks for the suggestions, John.
While you are at it you want to add heatsinks for the regulators.
I found my digikey parts order. I can email it to you.
That would be great, John. Thank you!