Macintosh Classic blown fuse after QP1 and diodes replacement

[Fiat131Mirafiori]

Hello everyone!
I've got myself a nice Macintosh Classic with leaking caps but hopefully no battery leakage and in very good conditions. It was a nice deal!
I've started with replacing the caps indicated by this link, both AB and logic board. Since the RIFA capacitors were cracked, I went ahead and replaced those too. A general alcohol scrub of the boards was of course needed.
The startup case up until now was a minute of slowly rising voltage in the 5V floppy port pin (starting from 4.70V) with horizontal bars and wobbly screen on CRT, up until 4.85V where the computer manages to boot and from there it's perfectly usable (minus the constant screen wobbling).
I've went ahead and replaced DP4 and DP3 with 1N4148 diodes, QP1 with CNY17-2X (I've been told by shop staff that it's a comparable replacement) and reflowed all the connections from the AB to the CRT and logic board (flyback included as it had a small crack on the solder).
At my startup attempt, the main fuse got blow the moment I flipped the switch. There are no visible shorts on board (tested for continuity between connections with multimeter just to make sure) and all components I've replaced in this second phase are correctly oriented.
I've taken a look at the forum and it might be the case that the TDA4605 chip and the IRFBC40 MOSFET are now liable for a replacement, but I am afraid that something else might be the cause of this. Maybe CNY17-2X is not so comparable after all, but could an optoisolator blow a fuse just like that? Could the TDA IC and linked MOSFET die so easily between one boot and another?
Thanks for your replies!

 

[Byrd]

Welcome!

The CNY17-2X is similar, according to this doc - and if not functional will usually result in no power, not fuse blown.

https://www.vishay.com/docs/81863/cny17.pdf

Noting your rising voltages from cold start, I would replace TDA4605 and IRFBC40, and check the board itself for further leakage from the old caps. I've gone through a few Classic analog boards recently, and the gap goo can be like honey.

Check the flyback too - for arcing, noting the solder crack. Did you ever notice the CRT glow from the rear when repairing?

 

[Fiat131Mirafiori]

Welcome!

The CNY17-2X is similar, according to this doc - and if not functional will usually result in no power, not fuse blown.

https://www.vishay.com/docs/81863/cny17.pdf

Noting your rising voltages from cold start, I would replace TDA4605 and IRFBC40, and check the board itself for further leakage from the old caps. I've gone through a few Classic analog boards recently, and the gap goo can be like honey.

Check the flyback too - for arcing, noting the solder crack. Did you ever notice the CRT glow from the rear when repairing?

Thank you. I will order those two components, hopefully they will be here in no time. I didn't notice any strange glow in operation, except for the reversed picture on the CRT if looking from the back/sides of course. In fact, I never put the back plastic shell on while testing, so that would have been quite easy to observe I believe.
I will also completely lift the caps to clean the board then, just to make sure it's clean beneath them. I will keep you posted!

 

[Fiat131Mirafiori]

Here I am with some promised updates.
I've changed the two voltage components and gave another alcohol scrub: below the remaining caps there wasn't much gunk to be honest, but I cleaned there too.
That was the fix: it now works straight after a cold boot and the image is wobble-free! I can claim this as a win.
A small question though: I've noticed that the overall image is very slightly leaning towards the top right corner as you can see from the attachments. Is this normal? Is there an image regulation to make?

 

[bibilit]

Probably an adjustment to be done with the upper magnet in the yoke

 

[Fiat131Mirafiori]

Thank you, I am a little bit afraid of that kind of hot CRT regulations!
I've read in this forum that this kind of artifacts can appear depending on the latitude of where the system is located and which setting the CRT got from the factory (speaking of the late Classic models). Since I bought it in a northern European country and moved back to the south of Europe (some thousand kilometers south), could it make sense that this is a "natural" glitch?

 

[dochilli]

Perhaps you should read this, before you do something with the magnets.

What I learned fiddling with CRT magnets...

https://getyarn.io/yarn-clip/c17c5371-38c9-453c-a496-a442ed15a2d5

68kmla.org

 

[Fiat131Mirafiori]

Perhaps you should read this, before you do something with the magnets.

What I learned fiddling with CRT magnets...

https://getyarn.io/yarn-clip/c17c5371-38c9-453c-a496-a442ed15a2d5

68kmla.org

Thank you, so that's about it for my Classic then!

 

[Fiat131Mirafiori]

Sorry for reviving this post, but I have some updates.
After about a week of fine behaviour, my Classic started to take about 30 seconds of wobbly screen again in a cold boot, only to later stabilise itself later on.
I've done some surfing in the net and in this forum and I've changed capacitors CP4, CP5, CP20, CP21, CP40. I've double checked the polarities of the first two, and the ratings of the latter three and they match Bomarc's schematics. Now the behaviour on a cold boot took a step back from before, as I get 4.6V from the floppy port and therefore the computer does not boot and the screen is significantly wobblier than before. Am I missing something else?