Macintosh Classic II Brightness Issue

The Problem

My Macintosh Classic II suffers from an issue where the display CRT goes dim after a few minutes on a cold boot. Every time it happens, I need to crank up the cut-off and brightness pot behind the machine to show the image.

The Analyze

At first, I thought it must be the brightness issue, so I searched for the Macintosh Classic analog board schematic to check the brightness line. I found the schematic from Bomarc, and the schematic is suitable with my board, since there are two analog board versions. 

I started to suspect the resistors and potentiometer on the brightness line. This Macintosh version has software-controlled brightness, so the analog board will get a 22.25 kHz square wave PBM signal from the logic board to control the brightness. I checked and replaced some resistors and cleaned the potentiometer, but nothing seems to work. The signal is there, and no sign of degradation that makes the screen dim after a few minutes. 

Brightness Line

So I suspect another line, which is the cut-off line. The display "cut-off" refers to a voltage applied to the grid (G1) that causes the electron beam to disappear, creating a black image on the screen. Some CRTs has the cut-off potentiometer located on the flyback, but this Macintosh has a cut-off potentiometer on the back of the machine. 

Cut-off and Focus line

The voltage on the HV6 is around 320V DC, and when the screen goes dim, the voltage also drops. The cut-off and focus voltage come from the flyback transformer. I hope it was not the flyback, since the flyback is quite rare nowadays. I check the voltage on the CL4, and it is quite stable. So I suspect that either the CL4 caps, DL3, or DL6 are the problem. 

CL4 is the blue caps next to the flyback transformer

So I check the CL4 first, desolder, and check using my LCR meter. The value of the capacitor is 10nF / 2kV. But I only get around 7nF at normal room temperature. I tried to spray it with cold spray to drop down the temperature and the value back to 10nF. So I suspect it was the issue. Since the location is near the flyback transformer, when the temperature gets hot, the value of the CL4 drops, and the screen gets very dim. 

Old caps

So I replaced the capacitor and let it turn on for hours, and the issue is gone. 

The Reference

• Macintosh Classic II Schematic (Bomarc)