CRT Not working on Mac 512

[chrisrueckert]

I recapped the analog board of a Mac 512. All worked fine. After it was stored for a couple of months, I started the Mac again But this time, the CRT would not turn on. There was a solid boot chime/beep though. Voltage on the 5V and 12V/-12V rails were normal.

Troubleshooting done so far:
- moved the brightness know back and forth, waited a few minutes for the CRT to warm up
- Re-checked the solder joints of the flyback transformer and the CRT cable
- Checked the Q2 resistor (on some boards this seems to be the Q3 resistor). I could not measure a short. This was done following a tip in "A Dead Mac Scrolls" from this PDF.

Any other tips where to check next?

 

[bibilit]

Hi, hard to say from the picture, but i can maybe see some cracked solder joints.

Have a look at those areas.

 

[superjer2000]

Can you tell if the CRT is working, ie any static or similar noises? I would take a look at the analog board to logic board cable plugs and resolder them at the analog and logic boards. I'd also check each wire on the cable for continuity as well.

 

[Juror22]

Also, the T2 (transformer) terminals can look brown and be ok, but on the photo here they appear nearly black. Look closely for signs that the windings or attachments could be burned or separated.

 

[wskjinfen]

This is a typical problem of loose connectors. It is recommended to re-solder all connectors and large components.

 

[chrisrueckert]

It works again. After putting the transistor back in Q3/2, refloweing the J4 joints and one of W6 that looked odd, the CRT is back to live. Thanks all for sharing your thoughts!

 

[chrisrueckert]

Update: The 512k Mac screen stopped working shortly after I had put it together again. I then decided to replace the BU406 with a new one and voilà, it's working again.

 

[wskjinfen]

Update: The 512k Mac screen stopped working shortly after I had put it together again. I then decided to replace the BU406 with a new one and voilà, it's working again.

If the transistor is indeed broken, it is best to carefully check whether there is leakage in the flyback transformer. Excessive leakage will cause the transistor to overload. The insulating glue on the top of the flyback transformer in the picture has aged and turned from yellow to brown.

 

[chrisrueckert]

Another update: The CRT went dark again. I investigated further and reflowwed all connectors. This must have solved the issue more reliably. The Mac runs now reliably.

 

[chrisrueckert]

And another update: The CRT went dark again. I noticed I could reproduce the issue by turning the brightness knob. It's now been removed, taken apart and cleaned with Deoxid. Seems to work now reliably. Let's see...