I'm not particularly picky, even no-name caps will probably last a decade or more and there is no guarantee that another irreplacable or irrepairable part of the device won't fail by then. If you can invest in and source japanese caps, go for it, but I am ok with buying no-name chinese caps if it will get a non-functional card working again. 😒

The only exception to this is my Amiga. Amiga gets quality caps, no matter what. 😎

appiah4 wrote:

I'm not particularly picky, even no-name caps will probably last a decade or more and there is no guarantee that another irreplacable or irrepairable part of the device won't fail by then. If you can invest in and source japanese caps, go for it, but I am ok with buying no-name chinese caps if it will get a non-functional card working again. 😒

The only exception to this is my Amiga. Amiga gets quality caps, no matter what. 😎

Yeah for sure.
It's only $15 for all the caps I mentioned, so I may as well get top quality ones as I have a bunch of stuff I need from Digikey anyway, so there won't be shipping cost.
Chinese junk caps will take weeks to arrive haha

Also, I wasn't asking about brands, more, I was asking about types of cap. You can get 470uF electrolytics that fit on your fingertip, and you can get ones that fit in your palm. They aren't equivalent for some usage scenarios.

I replace them with the same size caps. AFAIK cap size is irrelevant aside from aesthetics.

appiah4 wrote:

I replace them with the same size caps. AFAIK cap size is irrelevant aside from aesthetics.

From my reading, larger caps generally are rated for much higher ripple, and can withstand more power dissipation.

Always a good idea to replace very old electrolytic caps. However, the symptoms you described in your first post seem to point to a short-circuit or other fault from which the supply tries to protect itself...

Just make sure not to use low-impedance caps. While this works wonders for your Pentium 3 or higher motherboard, they can change the way the circuit design behaves in your PSU. Rule of thumb for PSU's, especially, vintage is General Purpose, but from a great brand. I'd go ahead and op for long life, 105-degree. The mains caps are generally, considered non-critical to begin with so if finding a 105 in equivalent size of the original 85 is tough, just get an 85. I used to use Panasonic TS-ED's, which were high ripple 105's IIRC, but I think they stopped making them. 🙁

Anyway, looks like a fun project! Got me giddy about restoring some more PSU's.

Edit: and I'm not familiar enough with these to say, but many an old 8086-286 PSU would not fire up without minimum load, and has mislead many to think they have bad supplies. Did you hook up everything or just try to breadboard the motherboard?

Logistics wrote:

Just make sure not to use low-impedance caps. While this works wonders for your Pentium 3 or higher motherboard, they can change the way the circuit design behaves in your PSU. Rule of thumb for PSU's, especially, vintage is General Purpose, but from a great brand. I'd go ahead and op for long life, 105-degree. The mains caps are generally, considered non-critical to begin with so if finding a 105 in equivalent size of the original 85 is tough, just get an 85. I used to use Panasonic TS-ED's, which were high ripple 105's IIRC, but I think they stopped making them. 🙁

Anyway, looks like a fun project! Got me giddy about restoring some more PSU's.

Edit: and I'm not familiar enough with these to say, but many an old 8086-286 PSU would not fire up without minimum load, and has mislead many to think they have bad supplies. Did you hook up everything or just try to breadboard the motherboard?

Don't use low impedance caps? Now I don't know what caps to get... Low impedance tends to come with high ripple from what I see on Digikey. Hmm...
Also, one component had a tantalum shorting 12V to 0V. Could that damage any PSU parts other than caps?

Update!

The motherboard works!
Keyboard works except a couple keys. Will fix when I take it apart to clean and replace crumbling cable.

Tested with ATX PSU, and card-edge connector desoldered from faulty PSU board. I can solder it back if I decide to repair it.

99% sure the CGA card is faulty. (VDU Controller GA2 - ASSY 000160)
I get 1 long, 2 short beeps (video error) with it installed. Same result when jumpered for composite mode enabled.
With the CRT connected with its ribbon cable, I can hear it turn on with the usual CRT high pitched sound. With the ribbon cable disconnected, it doesn't turn on. Never anything on screen. Sounds normal I think.
Did have to replace one shorted tantalum on 12V line on the CGA card. It had 10-15 written. I assume 10uF 15V. Replaced with a 10uF electrolytic as that's all I had. Wonder if that's not adequate...
I am able to desolder most of the logic ICs and test them. I can't test the 81416-12 RAMs, or the large chips (such as the huge, 64-pin, half-pitch DIP-64, with ambiguous Compaq model number), or the Compaq OKI M3864-04 ROM chip.
There are 5 other tantalums of the same value on different rails. They aren't shorted. I wonder if they need replacing anyway.

I am watching some compatible cards on eBay. Will see how I go with that. I would love to just confirm the CRT works before investing more into the project.

I'd say replace all of the caps since they can go out at anytime or cause a spark. Happened to a YouTuber when he was jump starting the internal PSU.

Replaced all 6 10uF tantalums on the video card with multilayer ceramic capacitors. Then it worked, but only in a random XT I have with the composite output. Would just get blinking cursor in the Compaq.

I then replaced all 16 10uF tantalums on the motherboard with MLCCs too. Now everything works!

Still a lot to do, including fixing the original power supply, but the unknowns are out of the way! Nothing else can be broken that isn't fixable.
Happy right now!

Great work getting that working again, I especially like the ATX test PSU idea. Seeing your diagnostic work encouraged me to have another look at a broken AT power supply I'd been avoiding, which should now be fixed (broken PCB = broken traces), thanks for the inspiration.

I hope that the Compaq is all smooth sailing from here for you 😁

I have a 386 board with tantalums on it that's completely dead, I should probably try replacing its capacitors at some point as well.

Thermalwrong wrote:

Great work getting that working again, I especially like the ATX test PSU idea. Seeing your diagnostic work encouraged me to have another look at a broken AT power supply I'd been avoiding, which should now be fixed (broken PCB = broken traces), thanks for the inspiration.

I hope that the Compaq is all smooth sailing from here for you 😁

I have a 386 board with tantalums on it that's completely dead, I should probably try replacing its capacitors at some point as well.

Awesome stuff!
Yea replace those tantalums with multilayer ceramics, or maybe low-ESR aluminium electrolytics.

And yup I've gotten even further on this.
I've set the system options and date/time, and they're holding with the CR2032 cell holder I installed. That's much smaller than the AA-sized 3.6V lithium it's meant to have; let's see if it lasts more than a few months.
I don't think I'm understanding floppy controllers correctly. The original MultiIO card is working fine with a 1.44MB drive, even though the controller chip is a "double density" one: NEC D765AC.

The ROM version I had was "G". This doesn't support 1.44MB floppies, even though the Compaq Setup v8.00 lets me set 1.44MB as the option. It'd halt at "Starting MS-DOS...".
I found the P.1 ROM version online, erased the two Intel 27128 chips, and programmed them. Success! No longer need an HD floppy BIOS extension.
The latest ROM is S.1. I can't find it online and don't know what it adds.
But I now have G and P.1 here, and some ROMs from other systems. I need to make some sort of repository for them.

The original MultiIO card's "WINCH" header is just standard IDE. I planned on installing XT-IDE Universal BIOS into the two free ROM sockets (likely EVEN/ODD like the System ROM), but I ended up not needing to.
The built-in Compaq drive type "47" (61.8MB) worked fine with a 64MB CF card. The geometry is wrong, so I needed to start fresh with FDISK. But after that, it seems to work perfectly. SCANDISK surface test passes with no issues. I can even put the card in a USB card reader and transfer files to it from Windows 10, without messing anything up.
64MB is a suitable size for a machine this age. There are drive types over 500MB in the BIOS though, and XT-IDE would allow any type.

The Miniscribe 20MB MFM drive (with MFM>IDE converter board) doesn't spin up. It doesn't draw any 12V. Oh well.

Still to do:

• Fix the 360KB push-eject slim-5.25: floppy drive. Probably just needs a head clean and lubricate the eject/close mechanism.
  Order caps from Digikey and fix the power supply.
  Find new leather handle.
  Figure out how to mount a Gotek in the panel where the HDD used to be.
  Mount a cabled CF>IDE adapter somewhere.
  Clean everything.
  Open the keyboard and replace the crumbling coiled cable with a new one, and fix the numpad "2" key, and clean all around. And fix the clip things that hold the keyboard to the system when packed up.
  Find a suitable screwdriver to adjust the CRT potentiometers.
  Install a suitable sound card. One with a PC_Speaker header and amplified output. Connect the internal speaker to the sound card's amplified output (internally). I like these things self-contained.
  Give the motherboard, video card, and multiIO a painstaking dust clean.
  Replace the brightness potentiometer. It's temperamental.
  Glue the rails for the two PSU and I/O plastic slide covers. They probably need some grease too. Lithium grease?
  Replace PSU fan if it is noisy. We'll see.

Unrelated interesting thing:
The Miniscribe drive has surface-mount multilayer ceramic capacitors, in glass tubes, converting them to through-hole parts. Never seen that before.

Hey nztdm.
I'm not sure I'm following forum rules or whatever.
Just a quick question.
I see you're a veteran on SLT/286 psu.
Following three burnt capacitors on it, I'm currently on my fourth. For the first time I'm stumbled cause I'm not sure which capacitor it was before it blew beyond recognition.
I'm adding photos of the blown capacitor in it's location...
Could you please point me in to the right capacitor?
Thanks! Tal.

talglazer82 wrote:

Hey nztdm. I'm not sure I'm following forum rules or whatever. Just a quick question. I see you're a veteran on SLT/286 psu. Fol […]
Show full quote

Hey nztdm.
I'm not sure I'm following forum rules or whatever.
Just a quick question.
I see you're a veteran on SLT/286 psu.
Following three burnt capacitors on it, I'm currently on my fourth. For the first time I'm stumbled cause I'm not sure which capacitor it was before it blew beyond recognition.
I'm adding photos of the blown capacitor in it's location...
Could you please point me in to the right capacitor?
Thanks! Tal.

IMG_20190511_183456.jpg

IMG_20190511_183513.jpg

Hello

I am not quite understanding.
Are you saying the capacitor keeps popping after you replace it?
It's a tantalum. This is happening for one of three reasons:
- you are installing it backwards (measure voltage there to see if the + marking on PCB is indeed correct)
- you are installing a tantalum with a voltage rating too low
- there is a fault in the PSU causing voltage there that is higher than the tantalum can handle

Hey nztdm.

Sorry to reply to such an old thread - I’m glad I found it though because I’m facing down the exact same issue you described with your PSU.

I got my CP2 about 5 years ago, and installed a hdd and a 3.5” floppy drive. Made a config disk and installed DOS 6.2 or something. Had a blast with it - and it held up between multiple moves within the past few years.

Sadly, I ended up letting it sit for a long while after getting my hands on a VT103. Long story short, I went to boot it up this morning and could immediately tell the PSU had given up.I tried some basic cleaning and reseating, but the same persistent beeping and flickering happens. I found this thread and was so relieved to see someone else already encountered this.

So, my plan is to get my hands on a small-ish, modern PSU and retrofit. I have already desoldered (more or less) the edge connector from the PSU - I have no plans to try and fix the faulty OEM power supply as that is not my wheelhouse (software engineer 😉 ). I do however think I can handle mapping the pinouts from a newer mini-atx PSU, and buttoning everything up nicely.

Anyway - I was mainly just wondering if you ended up fixing your stock PSU or replacing. I also would love some tips on connecting an ATX psu, as I noticed you had the motherboard powered without all of the pins connected.

Thanks!

Beefybread wrote on 2024-02-25, 07:38:

Hey nztdm. […]
Show full quote

Hey nztdm.

Sorry to reply to such an old thread - I’m glad I found it though because I’m facing down the exact same issue you described with your PSU.

I got my CP2 about 5 years ago, and installed a hdd and a 3.5” floppy drive. Made a config disk and installed DOS 6.2 or something. Had a blast with it - and it held up between multiple moves within the past few years.

Sadly, I ended up letting it sit for a long while after getting my hands on a VT103. Long story short, I went to boot it up this morning and could immediately tell the PSU had given up.I tried some basic cleaning and reseating, but the same persistent beeping and flickering happens. I found this thread and was so relieved to see someone else already encountered this.

So, my plan is to get my hands on a small-ish, modern PSU and retrofit. I have already desoldered (more or less) the edge connector from the PSU - I have no plans to try and fix the faulty OEM power supply as that is not my wheelhouse (software engineer 😉 ). I do however think I can handle mapping the pinouts from a newer mini-atx PSU, and buttoning everything up nicely.

Anyway - I was mainly just wondering if you ended up fixing your stock PSU or replacing. I also would love some tips on connecting an ATX psu, as I noticed you had the motherboard powered without all of the pins connected.

Thanks!

Hello

I just connected a few wires (double ups for 5V and GND). I'd have done that a bit neater these days.

I still have this machine and it still works.
CR2032 only lasts a couple years compared to the 10 years of the lithium thionyl chloride 3.6V it's meant to take. Those cost more tho.

Just wire up the ATX PSU as per my pinout and you should be good.
ATX PS_On (green wire) should connect to GND to make it always on (you control power with the original rocker switch).
If you use an ISA card that needs -5V, you can add a 7905 regulator with a couple caps to the edge connector like I did.

The original IEC power input socket had a mains input filter module attached. I cut this open and replaced the paper film X2 caps with modern plastic film X2 caps. Not sure if they all have paper film caps - other types don't need to be replaced.

Cheers
JD