Author Topic: EEVblog #1364 - Compaq Portable PSU REPAIR  (Read 964 times)

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Offline EEVblog

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EEVblog #1364 - Compaq Portable PSU REPAIR
« on: January 10, 2021, 09:51:25 pm »
Dave repairs the Compaq Portable Plus power supply.
And takes a look at the schematic and an unusual old documentation system.

The following users thanked this post: txNgineer

Offline woodge

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #1 on: January 11, 2021, 03:07:01 am »
As of five hours ago, this blog post was made, during which the new BM786 multimeter was featured; Dave also noted that he had fresh stock of this multimeter in the store.

Sadly, when you go to the store, the meter is now shown as being "out of stock."

That sure didn't take long.

Offline txNgineer

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #2 on: January 11, 2021, 03:17:57 am »
When you power it on, watch the CRT for the animated Compaq Logo. It was "the talk of the town" at the time. Nothing like it existed then.

About Christmas 1984 I got a brand new, dual floppy Compaq portable. A few months later they released the Plus model that included a "giant" 10 MB hard drive. I got the parts to upgrade mine to have the hard drive. Within a short time the power supply cratered. I looked it over and found it to be the strange mess yours is. I bought a replacement from Compaq (they were in Houston then, as I was) and it too died within a week. That caused me to call Compaq and plead until I got through to a Technical Engineer and asked for a schematic so I could repair the very expensive PSU. He sheepishly admitted to me that their original design was not robust enough for the hard drive upgrade that they sold me and said he would send me a replacement (next version?), but couldn't share the schematic. I was totally shocked when the package arrived and contained 3 new PSUs. I guess he didn't trust them either, ha ha. A few years ago I came across the two old New In Box PSUs in my barn. The sad part is they are not the oldest electronics stuff I have in there.  ;D

About Sam's Photofacts -- they were an absolute necessity when TV & Radio repair was still a thing. Every repair shop had 5 or more four drawer file cabinets full of them. They were bought by annual subscription and it was priced so only companies could afford them. They started in 1946 and ran up to about 1991. However  ITT bought them out in 1967. There was no Google or internet to get the data from, so you HAD to have the hard copy. Remember that even the plain paper copier was not introduced until 1963. I was always suspicious that Sam's Photofact saw that as writing on the wall and that is why they sold out within the next 4 years.

The pictures and annotations were to be used like a map to help you link the schematic to the board you had in front of you. Most of the folders also included alignment and calibration procedures, dial string diagrams for radios, model variants and what was different between them, etc. The original ITT Sams division went through a bunch of sales and splits after 1985 and I suspect that the current "owners of the corpse", Simon & Schuster, might still try to have some copyright claim. But most antique radio and TV enthusiasts are generally comfortable with any of the folders published before the the ITT buy out.

I love the vintage equipment repair, especially test equipment. A buddy of mine once commented that my lab was half cutting edge and half like a time warp to a 1987 research lab.  ;D But really, some of that equipment is still very useful if you understand how it works and how to use it. Nearly all of my test equipment is older items I bought and repaired. Thank again for all your time and effort creating this "watering hole" for us!

« Last Edit: January 11, 2021, 03:20:59 am by txNgineer »

Online TurboTom

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #3 on: January 11, 2021, 01:52:15 pm »
Dave, sorry to argue, but you're in error about the 5V rail regulation mechanism. The 5.6V Zener that you mentioned as part of the regulation (CR26) is actually a sacrificial crowbar, i.e. if the 5V line rises due to a malfunction of other circuitry above 5.6V, CR26 clamps the voltage and converts it into heat until it fails by shorting out the rail or the other protection mechanism (via photocoupler U1) kicks in (whichever comes first...). These 5.6V (or rather probably 6V) overvoltage will be tolerated by the digitial circuitry from the 1980s momentarily without damage.

The 5V regulation is arranged by modulating the primary of the PSU via U9, referenced by the TL431 just below, and then Q2 driving the photocoupler U2.

Since the coupling of the transformer windings is not close enough for a direct regulation of several outputs (like it's done today on the cheaper low power SMPS's), the lower current lines are stabilized with linear post-regulators.

Hope this clarifies the control scheme for those who are interested in the details. Anyway, thanks for this very nice "retro" repair clip  :D

« Last Edit: January 11, 2021, 04:14:32 pm by TurboTom »

Offline CJay

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #4 on: January 11, 2021, 03:20:08 pm »
Hah, 'old format' opto-couplers, they're still available from Mouser.


Brought back memories, I must have repaired dozens of those, they were rarely as simple a fix as that :)

I think I've still got a bag of LM338K chips too...

Offline Cyberdragon

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #5 on: January 11, 2021, 06:30:23 pm »
Sam's is way older than 1958, it predates transistors. They would have huge volumes by year of all consumer electronics (dating back to AFAIK the 20s) service manuals. If you watch many vintage electronics restoration channels they often mention "The Sams", even if they are just referring to the schematic or service manual outside of the actual Sams guide (as Sams usually had it anyway).
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Offline I.T. Manager

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Re: EEVblog #1364 - Compaq Portable PSU REPAIR
« Reply #6 on: January 12, 2021, 06:22:49 am »

Brought back memories, I must have repaired dozens of those, they were rarely as simple a fix as that :)

I am surprised that the Compaq power supply didn't self destruct when it was first switched on after all those years !

I repaired many computer SMPS, it was always a dried or shorted capacitor that took out the power transistor and / or other components, NEVER a loose wire, even though that was what the customer suggested it probably was.

Repaired one Antec 450W (Greenpower ?) power supply that faulty (bloated) caps on the +12 rail that were dragging it down to around 10v causing the computer behave a bit erratic.
The poor power supply was struggling to work with a massive load on the +12V rail !, no magic smoke, no other damaged components !.
It had been running in a file server 24/7 for over 10 years without missing a beat !.
I replaced the offending caps with Panasonics and all was good.
Good one Antec !.
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