Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

[ckrtech]

Hey guys -

Finally got around to taking apart one of the PSUs I plan on using in a 486DX2-66 build.

This is a Besttec BPS-2004-4U AT power supply that was pulled from a Pentium 90 machine.

I have honestly run this guy quite a bit as I have built the system, but I haven't really cracked it until now. I was getting close to screwing everything down, and I can't remove the power supply without removing the front panel. Front panel was off, so it was time to pull the PSU.

Dusty. Super loud fan. Recommendations for a replacement fan?

Photos enclosed. Nicon caps in the middle and on output. KA7500B IC (sorry about the wire).

I suppose i could wire it back up for a load and hook up the oscilloscope. Guess I got a little teardown happy when I got home today.

[kaboom]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

Looks decent!

Recap with Panasonic FC/FR, or Nichicon PW/HE. Except maybe those two Panasonic filter/doubler caps...

If you can get to them with small needlenose pliers, make sure the heatsink screws are tight. Blow it out with an air compressor too.

I've used Arctic Cooling F8 fans (not F8 PWM) which are 80mm, quiet and still move a decent amount of air. Wire directly to +12. Another tip is to remove the stamped grille and replace with a wire one. Measure first- sometimes those older cases don't give enough clearance in their PSU/fan cutouts to accomodate the higher-sitting wire grille.

I always scope with resistors and/or 12V lamps. Scoping while connected to a system can sometimes be misleading, esp if poorly filtered devices backfeed their ripple (if they've got marginal/bad caps) into the supply.

Sounds like you're building a "retroputer." Have fun- no need for windows "rage."

[ckrtech]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

kaboom!

I told my wife that you (a forum user named "kaboom") responded to a thread I started, and I think she took it as a bad omen (she can solder, too). Guess I had better triple check my polarity this time around!

I've got my cap list just about done and ready for the grocery cart. Thanks for the fan recommendation. Also thanks for the note on the wire grille - I probably would have just purchased one without thinking about external fit should I decide to dremel-off the cage-like grille that - now that you brought my attention to it - would probably impede airflow a bit.

I might as well pick up some resistors for testing the lines (and for maiden voyage power-up?) while I am at it. What do you recommend (large wattage resistor with X resistance)? And also if I might ask - what is your procedure for walking your resistor/lamp and checking your voltages? Despite all the replacing of caps, resistors, building of circuits, etc. over the years, I have never created a dummy load.

I've recapped a monitor power supply before and just fired it right up. Don't mind playing this one safe with this vintage equipment. (Although I don't really know what I have to worry about so long as I get reliable, appropriate replacement caps and put them in correctly.)

[PeteS in CA]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

Since that's ~23 years old, definitely re-cap the output caps. I'd even suggest Nichicon PJ or PS, or UCC LXV. FC and PW (and UCC LXZ) might be "too good" and cause stability issues, and FR and HE might be be even more "too good".

For test loads, go for 10%-20% load on the main outputs and 5%-20% on the -12V and -5V. For your +5V and +12V load resistors, only go up to 50%-75% of the power rating, unless you need a night light. And cool them (even 20W total, 10% load, is a fair amount of heat).

NMB, Panasonic (Panaflo), Sanyo (San-Ace), and Delta make good fans. Try to find the CFM rating for the original fan and match that with the new fan.

[ckrtech]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

Interesting. What factors into the possibility of caps being "too good?" Something designed for a little more wiggle room on cap rating/ESR?

Sanity check - If I were to examine +5V (as an example) at 10% of the max load of 20A, I would see:

5V = 2A * R
R = 2.5 ohms
5V * 2A = 10 watts

So if I want to do a 10% load test on +5V, I should pick up a 2.5 ohm resistor rated at better than 10 watts and cool it with a heatsink?

The fan was fortunately easy to find - http://www.mouser.com/ProductDetail/...4fzQstqw%3D%3D

31.4 CFM
28.3 dBA

So it is slightly above the Arctic F8 (31 CFM) in terms of what it can move per minute. Not sure if the difference of 0.4 is noteworthy or not. The drop to ~22.5 dBA sounds nice. I can take a look at some of those other fans, too I suppose.

[PeteS in CA]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

Interesting. What factors into the possibility of caps being "too good?" Something designed for a little more wiggle room on cap rating/ESR?
...

I would not consider a 1% drop in fan CFM to be significant.

The frequency response of the feedback loop is adjusted to compensate for the effects of various circuit elements. The purpose is to have fast loop response without oscillation. One of the elements the circuit compensates for is the ESR of the output capacitor. One need not precisely match the ESR of replacement caps to the ESR of the originals, but low impedance capacitor technology has improved a lot in the past 20+ years. As good as HE or FR or KY series parts are , they may be so much better than what was originally installed that stability could be come marginal or bad. Nichicon PJ, PS (and PM) and UCC LXV are older series and closer to the impedance of the parts originally installed in the PSU. Nichicon PW, Panasonic FC, and UCC LXZ are very similar and good enough that I'd be nervous about stability.

[ckrtech]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

Recapped with new fan.

All caps are Nichicon UPJ series (name change for PJ with updated RoHS compliance?) except for the 1uf, which are Rubycon YXJ.

Results attached. +12V is high... although this was one Pentium 90 motherboard + a single PCI video card and nothing else. So I suppose the only +12V load was the fan inside the PSU.

Clipped the grounding lead to the metal bracket on the PCI video card. Measurements were from an empty ISA slot. ...dunno if I am shooting for the most accurate measurements. Ha!

[PeteS in CA]

Re: Bestec 200W AT PSU (BPS-2004-4U) - How they used to make them. Your take

PSU noise measurements are not easy. Your ground lead, at high frequencies, is an antenna. And if your grounding point isn't close to the ground for where you are measuring, that also lets in noise. With the scope ground lead clipped at the same place, try measuring the noise at that point. Then, try measuring the noise at the ground of that ISA slot. I think you'll see what I mean about noise pick up.