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Classic not booting after a full recap. Missing battery?

benanderson89

benanderson89

6502
I've done a recap of both the analogue and logic board in a Macintosh Classic. Triple checked all the capacitor values and orientations and they're fine.

First time switching it on I heard the speaker crackle into life, but there was no chime. The speaker after a few seconds tried to make noise, like the first fraction of a second of the boot chime being repeated, and then the monitor rapidly started turning off and on with the speaker noise, but I turned it off quickly as it kept on centring the beam after each attempt to create a dot on screen, and I don't want to damage the phosphor.

Any subsequent attempts you hear the speaker crackle and the fan spin up, but nothing else.

I'm assuming the weird behaviour is because there is no PRAM battery installed. Google hasn't been much help but it seems like erratic behaviour/no-boot is linked directly to the lack of battery. If this is the case, what value and size do I need?

Just to double check for the sake of through trouble shooting:

1) no battery is installed

2) no hard disk is installed

3) no keyboard or mouse is connected

4) floppy drive is installed but there is no disk inserted

5) daughter ram board is inserted but I'm unsure if it's working

Any help would be fantastic! Thank You. :)

 
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benanderson89 said:
I'm assuming the weird behaviour is because there is no PRAM battery installed.
Click to expand...
I have no PRAM batteries in any of my 9" macs and all boot fine (sans accurate time :) - only Mac I know of that is picky about the PRAM battery and booting is the 6100 series.

It sounds more like unstable voltage at first glance - do you have a multi meter you can check it with? If you tested the analog board without load it may be unstable with - so check it at the floppy port with load.

 
Found some wire and measured from the floppy port. Stable voltage (from what I can tell as it was hard to get good purchase on the wires) The 5v was only outputting about 4v. The 12v was slightly under 10v. Massively under-voltage. I hope the freak-out when I first plugged it in was just a fluke, else I might've blown something up!

Is there an adjustment pot somewhere on the board? Maybe it just needs a tweek to go with the new caps.

 
benanderson89 said:
Is there an adjustment pot somewhere on the board?
Click to expand...
Yes, my classic isn't open right now but it's right in the middle of the board - use a plastic or wooden tool to adjust. I had to do this as well on mine after recapping. I think the one of the old books has a photo/location, let me look.

 
Thank you! I'll head to my parent's place tomorrow and steal my dad's electricians screwdrivers to do the adjustment. All of mine are metal AND magnetic so that's a bit too unsafe for my liking. :D

 
Adjusting PP1 is not safe. 

You can have a try, but if voltages are not close from 5 and 12 volts, probably something else is wrong. 

 
Anything else I could potentially look at before trying PP1?

EDIT: I found a Reddit post with similar super low voltage issues. For him it turns out it was a diode located at DP6. Others suggested he check resistors as well before he found the bad diode. I dug out the Engrish manual to my cheap multimeter and it has a diode test mode, so I'll give that a look tomorrow. I'll take a look at the resistors on the power input side of things, too.

 
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Basically you need to go through all of chapters 8 and 9 of the previously linked book. It will give a rundown of what to check and what to replace if certain conditions are met.

 
PP1 can be used for fine adjustment only. 

Been there many times, PP1 will crank one rail safely, but not the other. 

You will end with 13 volts or above trying to reach the 5 volt figure. 

The Optoisolator is known to go bad on this situation. 

 
My meter has a proper diode test mode according to the Engrish manual. Diode 7 has died. Shows a read out of 040 and the continuity beep sounds. In contrast, Diode 7 on my spare Rev A board reads 335.

So I guess I'm buying a new diode! May as well buy a new optoisolator, too, regardless if the one currently on-board works or not.

What's an appropriate diode value?

 
That's fantastic! I'll get that ordered ASAP. Mouser and Digikey want £12,00 postage fees if my order is below £33, so I'll be going on eBay instead. Haha :lol:

 
  • Tested CNY17G-3 and that appears to be working perfectly: for those that don't know, diode test mode across pins one and two, with one being the anode. Then 20kO resistance test across four and five.
  • With TDA4605, all pins (except pin 2, which I believe is by design) have resistance to ground (pin 4), so I'm confident I don't need a new switching chip, either.
  • DP3 and DP4 also register correctly. I'm glad I have my dead Rev A board here because the cathode of DP3 is under a mountain of hot glue and I needed to find a spot to test without having to remove the board from the chassis and it's vinyl insulation pad -- Pin 5 of OP1 is what you need.

So it looks like the only component I'm having issues with (that is a known, common problem) is Diode 7. I'll head over to eBay and see about ordering a couple of those now. I was going to use the diode off of my spare Rev A board, but once removed I saw how dirty, bent and corroded the legs were (the battery explosion in that machine really did kill everything). Plus, I don't really want to be replacing a dead 30 year old diode with a probably-dying 30 year old diode. :lol:

EDIT: Meh, went to mouser anyway. Extortionate shipping for a tiny box of caps (£12,00!) but I need new caps for my IIci anyway, so I may as well buy them all in one on go, plus a spare optoisolator to keep in a parts stash.

 
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