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Original Macintosh 512 no longer boots, need help diagnosing

Posted by: Verault on 2018-04-02 14:27:49 I have an original macintosh 512K model which wouldn't not boot most of the time. What I mean by that is 4 out of 5 times when you hit the power button the crt would glow but the unit would not boot; no boot sound. I have replaced all caps on the analog board and the logic board. Now it will not post or beep at all, instead All I get is one broken line horizontally across the crt. See image: analog board model is 820-0082-d and 630-0102 Standing by for some guidance, thanks guys. Oh yea, tried another known good MAC CRT on the system and it displayed the same symptoms. Dont have another matching analog board.

Posted by: MOS8_030 on 2018-04-02 14:34:34 Howdy! Welcome! 🙂 Apparently there's more going there than bad caps. I recently repaired my 512K Mac and in addition to the caps I had to replace Q3 and the flyback transformer. Have you checked these references yet? http://www.maccaps.com/MacCaps/Repair_books_files/Macintosh Repair and Upgrade Secrets.pdf and http://www.ccadams.org/se/classicmac2.pdf

Posted by: dcr on 2018-04-02 14:41:48 FWIW, that's similar to the line the CRT on my Mac SE displayed until I swapped out the power supply.

Posted by: Verault on 2018-04-02 14:41:54 Thanks! I probably should have joined by now but no time like the present right? I have one flyback transformer from an analog board off a 1990 mac classic which I assume works. But I really hoped it wasn't that! Whats a typical sympton if the flyback transformer fails? No I have not checked those links but will right now!

Posted by: Verault on 2018-04-02 14:49:44 Well here is a question. The machine is not booting, I hear no Boot sound. Would the flyback transformer stop the machine from booting? I assume it would simply cause the CRT not to function.

Posted by: Verault on 2018-04-02 14:50:09 By the way, Yes I have read over both those PDF's in the past. Thanks for sharing them.

Posted by: Verault on 2018-04-02 16:22:26 Well I decided to try the flyback transformer from my macintosh classic. Still getting a horizontal line, this time much brighter and solid. Also the electric hum is much louder! attached is a photo of the old flyback transformer

Posted by: techknight on 2018-04-02 17:17:07 that is a vertical output/vertical drive failure.  Nothing to do with the power supply or flyback.

Posted by: Verault on 2018-04-02 18:25:10 Ok Great! Vertical output/Drive!  How do I fix that? What component(s) controls vertical drive?

Posted by: bibilit on 2018-04-03 00:17:30 What are the voltage outputs ?

Posted by: Verault on 2018-04-03 05:39:00 Bibilit I am getting 3.9Vdc on pin 6 pin 7 and 8 are 9.9Vdc and pin 5 is -10Vdc Also! We are back to the original single broken horizontal Line. There was a bad solder joint on the last pin of the flyback transformer when I swapped it last night. I should have checked every pin but It was late and I was tired. So the spare flyback transformer is behaving the same way as the original at this point.

Posted by: bibilit on 2018-04-03 06:00:41 Don't remember pin related, but the Plus is probably expecting 5 and 12 volts (and also -12 v) so probably first problem is here.

Posted by: Verault on 2018-04-03 06:14:59 those are the outputs on the external floppy connector. YEs they are looking for the voltages you mentioned,  and what is coming out is obviously wrong voltages.. But what is causing it? Where do I determine the cause of these voltage issues?

Posted by: bibilit on 2018-04-03 07:00:27 Hard to say, i have repaired a lot of those and Capacitors are not an issue usually. The Optocoupler can be a possibility, located near the brightness knob in the Analog Board (position U3 if i remember clearly) if you have another Classic of the same type (128 / 512 / Plus) you can swap Logic Boards for a test, or even swap the Optocoupler, part can be removed easily.

Posted by: Verault on 2018-04-03 07:12:46 Funny you should mention u3, I was just looking at it and some surrounding diodes. I only own a working SE and a mac classic for parts (alot of which are bad) neither has a matching analog board. the mac classic analog board (1990) has a phillips cny17g-3 which looks to be compatible with u3, ill try swapping it.

Posted by: Verault on 2018-04-03 07:59:43 well swapped out u3 and getting the same horizontal broken line.. so that was a nogo.

Posted by: Verault on 2018-04-03 15:23:33 Anyone have any other points to test?

Posted by: apm on 2018-04-03 16:58:59 It's definitely a vertical sweep problem, so as @techknight says, nothing to do with the flyback. What you don't know is whether the problem is on the analog board (vertical drive circuitry) or the logic board (no vertical sync signal). A scope is the right way to test that, but if you have a multimeter with a frequency measure, put it on J4 pin 5. You should get a 60Hz signal. Given the other voltages it does sound like the problem might be in the power supply. U1 (op-amp LM324) is shared between power supply and vertical circuits. It's also a cheap and common part, easy to replace. Since both vertical and power supply circuits seem amiss, you might try swapping it.

Posted by: Verault on 2018-04-04 17:15:39 ok, give me a couple days and I will give it a try as I got busy with work.

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