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MindWalker

Mac 512K analog board (with presumably 220V fed into a 120V board)

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Hi! New here :) Was going to post this here first, but as my activation email didn't get thrue until today, I posted this already in Reddit /r/vintageMac.

 

I came across a local ad for a Mac 512K keyboard and mouse. Reading the ad, I noticed that it actually noted "a non-working Mac 512K can also be included". This was interesting as I rarely see Macs of this vintage locally, and this was only a short distance away from me. From the photos I could see that the machine was a 120V which was interesting (living in a 220V country). Ok, I went to bought the set for less than 100$.


What I have observed so far:

  • Serial number dates the unit to a late 1985
  • A battery was still inside, but it had leaked only a very little and after cleaning and scraping the battery contacts they look Ok. There was no damage to the PCB where the holder sits.
  • The floppy drive is a Sony MP-F51W, meaning that it has been upgraded (800k)
  • ROMs are 23512-1010 and 23512-1007 (ie. "old ROMs" if I am not mistaken?)
  • The analog board is marked 630-0102-J (ie. US, 110V non-switchable?)
  • The digital board looks pristine and all the solder joints look original as well.
  • Keyboard (M0110, 1985 per serial) and mouse (M0100, 1985) look intact and are reasonably clean. Keyboard cable is missing (seller said he might still have it, I might get it later).
  • Externally the computer is in average condition, only major issue being a burnt spot in the plastic at the top left (covering part of the vent and also a bit of the front panel's top). The two short T15 screws were missing, perhaps a repair-attempt was made but they never get the case open due to the two recessed screws in the handle-space :)

 

I have not tried to power up the unit yet. There was no backstory of how it failed, but being a 120V set in a 220V country, it's most likely that it was plugged in without step-down transfomer and was struck with overvoltage mains.

 

Looking at the analog board, I see very little obvious damage. The fuse and resistor R51 next to the mains input have obviously burnt. On the bottom of the board resistors R46, R47 and R49 have slight discoloration and all three had their values gone very high. I have also done some measurements according to the books "Macintosh_Repair_&_Upgrade_Secrets_1990" and "The_Dead_Mac_Scrolls_1992" (there are plenty of in-circuit resistance measurements to go by).

 

6p0j9ehcg3751.thumb.jpg.76fcb52c2b2d91d9cdbf35237b462dc2.jpg

 

At the moment I am thinking about replacing the obviously burnt (/gone-high value) resistors, keep measuring around to find anything else obvious, and then power up (with a proper step-down converter). Adding to a larger component order (need to do this during the weekend), I'll also buy some spares of the nearby components (next to the burnt resistors in the schematics) like Q11, Q9 and perhaps the opto-coupler U3 too? Also some electrolytics, but I am not bothing a full re-cap yet. At first I'd like to see if replacing the obvious faults would give me any signs of life.

 

Now my main question is; has anyone experienced a similar 120/220V over-voltage repair? How much was damaged etc. Any other pointers on what to keep an eye on?

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Ok, I have now replaced the burnt resistors, and also Q9 (2N3906) and Q11 (2SC2810) which I found shorted.

 

Now powering the machine up with a step-down transformer, I get a hiccuping sound (I guess this is the "flup flup flup" as described in The Dead Mac Scrolls). I quickly powered down.

 

Following now the troubleshooting guide of the Dead Mac Scrolls; I am having trouble sourcing a replacement (or testing) the Q10 silicon controlled rectifier (CR400Y). With the CR400Y part number I can found pretty much nothing, and the parts list in Macintosh_Repair_&_Upgrade_Secrets_1990.pdf doesn't help either, the part numbers bring no search results. The description of "SCR,1200 V,550 A" sounds strange (really, 550A?). What could I use as a replacement there?

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This part is, from what I gather, a right pain to replace: there was a thread on suitable replacements (or the lack thereof) on the forum a while ago, if you search you ought to be able to find it. 

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Just a thought, where you testing with a logicboard attached? From what I understand a 512k analogue board will not regulate correctly with no load and will make exactly the sound you described

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@max1zzz: yes, I had the logic board attached. @cheesestraws: ok, saw that just now.

(Thanks to both of you).

 

In the listings the only given non-OEM part number is Philips ECG5589, and that is a huge bolt-like (ST-75 package) 550A part (according to Philips datasheet). I think that must be where the description in the manual comes (and must be a mistake). The original parts is a tiny package and it's current rating must be much, much smaller. Hmm, must have a look at the component itself when I have the board out next time.

 

I am not sure that's my real problem, but other than that I am not sure what else to try :(

 

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Hello @MindWalker

I recently fixed a 120V Macintosh that I had been given who's previous owner had plugged it into a European 240V outlet. It went bang.

 

I managed to fix it so perhaps the failed components that were on my effected board will be the same! Firstly, the fuse had blow so I dropped in a replacement 2.5A glass cartridge fuse.

After asking around and doing some testing with my multimeter I found that CR23, CR25, R47 and R49 had failed. I replaced CR23 and CR25 with new 1N4007 rectifier diodes and R47 and R49 with 1/4W 1.5 Ohm carbon film resistors.

 

And that was it! It worked again - being sure to use a step-down voltage converter though! I recapped the entire board for good measure as well.

Luckily all the components are readily available and inexpensive.

 

Good luck. Let me know how you get on.

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Hi @Will Cheyney, thanks for writing! Good to know that you got yours working afer the issue, that does give me hope!

 

Those resistors R47, R49 were indeed bad in my board, too. My board already has had more damage (Q9, Q11) but hopefully I can still figure this out. I am not sure if I have yet checked those CR23 and -25 (looking at my photos I think those diodes are partly under the glue holding the adjucent big cap on my board) - but I do have a plenty of those 1N4007s at stock if needed.

 

I am waiting for some more components to arrive (tomorrow or the day after), then I can get back to working on the 512k.

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Still no dice :sad:

 

I have noticed one bigger issue. In the Dead Mac Scrolls (Troubleshooting, flup-flup- issues, page 24) there is a description that one should be able to measure a lowish 31/30 ohm resistance over the CR20 rectifier diode. And 52/53 ohm across CR21. If I try to measure that on my board, I briefly get about the correct value but then it starts rising, and ends up somewhere over 2-2,5 kOhm. I can't quite figure out why, as I have replaced both CR20 and CR21, and the electrolytics around them. 

 

Also adjusting R56 (+5V adj) fully counterclockwise (page 27) does not help the issue.

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