Mac Classic: Swap analog board with Classic II

[joshc]

I wonder, could it have anything to do with the LB? Would it be worth recapping or trying a different LB?

Most likely not an LB problem just yet (the LB could be bad too, but you need to test with a good AB). You've got low voltages which are caused by a fault on the AB.

 

[ClassicMacintosh]

Did you change the TDA4605?

Yes, it has been changed. Didn't make a difference.

Not sure replacing the TDA will sort your issue.

Main purpose of the TDA is to trigger the Mosfet, your issue is probably not in the regulation side but more in the secondary side.

Can be wrong though.

I still have the following parts to replace:

DP3 and DP4, CNY17G, IRFBC40

Also have 22k ohm 3W resistor but struggling to work out where it is / why ordered

 

[ClassicMacintosh]

Most likely not an LB problem just yet (the LB could be bad too, but you need to test with a good AB). You've got low voltages which are caused by a fault on the AB.

Okay so I replaced DP3, DP4, CNY17G, IRFBC40, and the 22k Resistor. Same problem, wavy screen lines. However, I now have:

5v: 4.74v
12v: 12.11v

 

[dochilli]

Did you try to turn pp1? 12 V can be up to 12.7 V. 5 V should be 4.85 or higher.

 

[ClassicMacintosh]

Did you try to turn pp1? 12 V can be up to 12.7 V. 5 V should be 4.85 or higher.

Yes, turning it up to 5v now produces 12.56v on +12v. Doesn't appear to affect screen. Also tried turning it all the way both ways all the way, and taking it off for isopropyl bath.

 

[xeiter]

Did you end up fixing it?

 

[chrisjenkynw]

Also interested ^

 

[ClassicMacintosh]

Hi all, thanks for the interest. Yes, after shelving it for 6 months, twice. Ended up reflowing and double-checking work everywhere, as well as replacing a couple more things, and changing some caps for non low-ESR. It finally booted last month and voltages appear good. What a head-scratcher!