Macintosh classic AB woes

[dragonpunchu]

I'm trying to fix my Classic 240v AB but things went worst than expected. I was having some know power delivery issues and decided to recap the AB.

I've replaced the usual caps, cleaned the board with IPA (some corrosion but not that much) but when I turned it on: nothing. 5v was low, 12v was 0.

Reinspected the caps and noticed I had put the 470uf 50v backwards...

Put a new one in, with the right polarity and now I get a wobbly screen and 11.8v on the 12v and 4.67v on the 5v.

I tried adjusting the PP1 pot, I can increase the 12v no problem, but can't get 5v (got to 4.75v at 12.6v). I'm afraid to take the 12v too much out of spec while trying to increase 5v.

Did I burn something else when I put the 50v cap backwards?

I've read that DP3, DP4 and the optocoupler can also go wrong, should I change them?

 

[davidg5678]

Welcome to the 68kmla forum!

I've read that DP3, DP4 and the optocoupler can also go wrong, should I change them?

Recapping the Analog board was definitely the right first step to take, and I think replacing these parts is the next logical thing to do. From what I've seen here over the last few years, these parts seem to be the most commonly failing ones. (After the electrolytic capacitors, of course) --I would also recommend turning the PP1 potentiometer back down again before replacing these components. The new components should fix the voltage problems you are having, and if PP1 is turned up, the voltage may initially be too high. (Generally, I think it is better to go too low than too high, and then correct the voltage later)

Some people also report that they need to change the controller chip: a TDA4605. This part might be a little harder to source than the optocoupler and diodes, so I'd probably still just swap those parts first.

Did I burn something else when I put the 50v cap backwards?

I'm not sure whether or not the backwards capacitor would fry anything, but if you need to order some extra components anyway, I think it would be smart to replace this part with a new one. I have heard about people experiencing similar voltage problems with their Classic Analog boards, regardless of the capacitor problem you had, so this could still be okay.

 

[CharlesT]

Here's an Ebay listing for the TDA4605s.  https://www.ebay.com/itm/291471780717

I just received an order from them. It seems less likely these would be at fault in your case since a bad one would most likely result in your supply not switching at all, meaning no output.  

 

[dragonpunchu]

Thank you guys. I've ordered the diodes and the optocoupler and will try those.

Do I have to socket the optocoupler because of solder iron heat?

 

[AndiS]

I just changed this part yesterday. No socket and it works. - See this thread. Try not to heat it up more than necessary, but this should be standard practice anyway.

BTW, I used a CNY17-3 instead of a CNY17G-3 because it was readily available.

As you can see, I'm following the same general direction as you do. Next, I'll try to adjust the voltage.

 

[dragonpunchu]

I've seen in a youtube video that someone was having issues with the AB after a recap and that cleaning the board again fixed it.

I had cleaned mine with IPA after the recap but decided to give it another go. I've took the board out and soaked the pcb with my IPA spray and let it run and dry. I did this a couple of times and some black stuff came out indeed.

Put it back together and turned it on. I get good voltages now just shy of 12v and 5v and the mac boots fine. 

Unfortunately the screen brightness was all over the place. Going up and down, from very bright to very dim. I turned it off and waited a few hours. Tried again seemed more stable for a while and then it went dim and dimmer and now doesn't turn on at all. When I turn it on in the dark I can't see any arcing, but I can see a faint light in the back of the crt where the board is attached.

I'm at a loss now, the low voltages are fixed now, but the high voltages seem to have a problem.

Any ideas?

 

[dragonpunchu]

It seems to be fine now. Maybe the board was not completly dry and shorting?

Well I will keep testing it for the next few days.

 

[AndiS]

Congratulations! You saved another one

And as I wrote in the other thread - I'm still trying to figure out some things with my Classic's AB, altough the Machine itself is not working as it should!

 

[dragonpunchu]

Now I have another issue... I have a bright horizontal line across the screen.

• I've reflowed the yoke pins, the flyback pins, all of the logic board connection pins (I have continuity on all of these pins).
  
• Tested the logic board and it is fine in another macintosh chassis.
  
• I've ordered a TEA2037A in case mine has died.
  
• Tried to test RF13 resistor, I can't really read the rings so I don't know what it is. I get continuity if I measure both leads, does this mean it is dead?
  

Also I've only changed the usual capacitors, not all of them, they look ok.

Capacitors? RF13? TEA2037A? What do you think?

 

[dragonpunchu]

I've replaced all capacitors and no change.

But changing the TEA2037 fixed it!

So for future reference if you get horizontal line across the screen, this IC could be the culprit.

 

[cheesestraws]

Thankyou for posting what worked for you