Macintosh repair adventure

[andy2k68kMLA]

Ever heard the phrase no good deed ever goes unpunished? Anyways, I own a not so long ago working Macintosh Plus 1 MB. I had recapped it with all new electrolytics. After I recapped it. I powered it up, it chimed and seemed like it was working, then about 8 seconds in started flupping (I'm assuming it's what it is, sounds similar to an analog telephone that's been off the hook too long, only sped up). I also removed solder, cleaned with alcohol, fluxed and soldered nice new shiny connections for flyback transformer, J1, J2 and J3.

Did some voltage checks, the 12 volt line was low at 11.88 volts when I adjusted voltage for 5 volts. The 12 volt zener measured low voltage. I've since replaced CR18 - CR21, with upgraded parts (Schottky TO220). Now the voltage looks good on both 5 and 12 volt lines that stay in spec but it still is flupping.

Display shows full screen of solid black and white horizontal stripes. Relatively fine lines.

-5v regulator on logic board at -4.99v and looks good. If I unplug logic board flupping stops and just clicks and no display.

 

[andy2k68kMLA]

Oh and replaced Q8 SCR crowbar TO220. Also, I've measured just about every diode, resistor, capacitor, inductor on the AB. None of the diodes are shorted or resistors open. All the new caps (and old) seem to be okay and measure high resistance. Q8 never gets hot indicating something is truly in an overvoltaged state.

I forgot to mention I had also recapped the logic board.

 

[Elfen]

Flubbing is usually the circuit around the Flyblack, including the flyback transformer. C1 C2, Q1, D1, L1 (needs to be resoldered, not replaced, and by resoldering - desolder, clean it and put in new fresh solder and flux! Reflowing the old solder will crack and cause issues weeks later). Also check the connection jacks and resolder the whole thing if one of the poles looks iffy. (Again - like before - desolder, clean up and resolder with fresh solder and fiux).

Caps will always measure with high resistance no matter the setting you put the meter too on Resistance. You need a meter that measures Capacitance to check out a cap. Personally, I picked up one up at Home Depot for under $20. It does a descent job on measuring caps. And to measure a cap, you need to remove it from the board or you will be picking up capacitance from the other caps in the board.

 

[andy2k68kMLA]

Okay I can give that a try. How do you know when U3 the optocoupler is bad? I've heard it can cause the flubbing. I will try posting a video of it to make sure we're on the same page. Thanks for your help! Any other tests or voltage measurements etc that I can do on J1? Will flubbing usually stop when you disconnect the logic board from the AB? Mine does.

 

[bibilit]

 Will flubbing usually stop when you disconnect the logic board from the AB? Mine does. 

yes it will

How do you know when U3 the optocoupler is bad?

voltage unstable, going from too low to too high.

usually the optocoupler is socketed on those boards, so easy replacement.

 

[andy2k68kMLA]

Hey guys,

I had a break through (I think/hope). I bought another analog board but it had the same exact problem when I swapped it out with a different good AB. Tightly spaced horizontal lines and everything. So I started looking at the logic board. I measured all the caps and found C6 a 33 uf wasn't measuring right on my meter. Upon further scrutiny, I found that the 5 volt line measures 280 ohms to ground. This doesn't seem right, especially because the -12 and 12 volt lines measure in the 300K+ I'm going to check the -5 volt regulator first the the TSM chip per Pina instructions. Any other known logic board failures? Would someone mind measuring their analog board 5v line to ground? (not attached to AB) thanks guys for your help. Macintosh Plus logic board 820-0174-C

 

[tjjq44]

I just pulled my Plus logicboard out and measured the +5v to GND resistance: 120 Ohm!! I also measured the 12v lines but via the floppy connector since I've already put all back in place.

For the -12v: 450 kOhm but on the +12v I get only 30 Ohm!! And my Plus is running fine... 

 

[andy2k68kMLA]

Thanks so much for checking that for me, seems odd but normal. Any other ideas as to what might be wrong with the logic board. I've heard the TSM chip and -5 volt regulator fail on the logic boards. The -5 regulator seems good. Anyway to test the TSM DIP IC? Any other logic board testing procedures/documents anyone know of that I could try?

 

[techknight]

low 5V resistance to ground is normal folks. Your reading through EVERY chip in the entire unit. usually the PALs are the cause of that, and of course the CPU. 

 

[andy2k68kMLA]

Thanks TechKnight. Do you have any other ideas / suggestions for troubleshooting the logic board? I have access to a nice oscilloscope? I've tried removing all memory expect one slot. Removed and cleaned all socketed IC's. Any other ideas?

 

[techknight]

I would like a picture of what the display is showing. you probably have an IC failure of some sorts. 

 

[andy2k68kMLA]

Thanks TK I was thinking the same exact thing. I'll try to upload a video of what it's doing when I get off of work. Do you (or anyone else) know of a document that explains the logic board circuits. Something similar to the well written manuals made by Lee and Pina?