Hello Cheesestraws,
I want to thank you for drawing my attention to what turned out to be the crucial, dead component for me; the 2N3904
As seems often the case with analog boards, the journey was not without missteps and red herrings…
Previously, this Macintosh 512 had low voltage/screen wobbles. After resoldering the connector for the logic board cable, I traced the fault to the optocoupler - replacing this made it possible to obtain a nice steady 5V and BANG! Hissss… the mains filter caps exploded. Replaced those and all was well for a few minutes until Pop! went the fuse - one rectifier diode was shorted. Replaced all four.
The display lacked a little height, so following the Dead Mac Scrolls, I replaced the 6.2V Zener diode beside the height control - which fixed that.
There was a random brightness flicker which seemed to go away when things warmed up, until suddenly one day on a cold start, there was no video at all, but still a startup bong and normal disk activity.
What threw me was that turning up the cutoff control (next to focus) produced NO grey background or scan lines. This turned out to be a red herring.
There was still a static crackle when turning off, and there appeared to be a glow from the CRT heater, but not as much as I would have expected. I fixed lots of oxidised solder joints and tested all corroded (black) traces.
Fortunately, I had another analog board to compare. I tested all diodes and found them the same. I discovered a different resistance across the middle pins of the flyback transformer, so I swapped that. No change. I replaced the electrolytic capacitor connected to one of those pins.
Wondering if the flyback was running correctly, I replaced the BU406 horizontal output transistor with a new BU406G. No change. That turned out to be a dumb move, but I had wondered whether the wrong flyback resistance I measured was caused by the BU406.
I even swapped the CRT after cleaning all pins and connections, with no change.
Finally I swapped the cutoff potentiometer and that made a difference - now, I could obtain a narrow grey 1cm horizontal band in the middle, with retrace lines.
As vertical deflection was clearly inadequate, I replaced the Zener diode again - no change. I swapped the LM324 in that circuit and straightaway had a full-height grey screen (with the cutoff control turned up). I don’t know why the LM324 happened to fail at this time while there was no video, but I guess given the history of this board, nothing surprises me.
Still no video though, so it seemed at last I was onto the real problem. I was thinking of swapping the 74LS38 but wondered whether I had the right symptoms - and that’s when I found your post.
I removed the 2N3904 and found it faulty. I replaced it with a BC548, as that’s what I had on hand. I was greeted with almost a correct picture. It was off-centre with very slight foldover on the left, so I refitted the old BU406 and got a correctly-centered display. After adjusting the focus, the single-pixel parts of text seem a little smeared. I suspect this may be due to a longer response time of the incorrect BC548. Or, the CRT I substituted is worn out. I’ll find out when I get some new 2N3904s.
Most importantly, the brightness ‘random’ flicker is no longer present from a cold start. As the connections were all dealt with before with no improvement to the flickering, I reckon that was caused by that transistor as well.
Thank you again for identifying the video amplifier transistor as a culprit
I have a problem on three Mac Plus analog boards that is possibly related (I don’t know yet). When the brightness control (under the front, technically the contrast) is set to maximum, there is no fault, but when it is set to anything less than maximum, the whole display rhythmically pulses (not a random flicker) and it is not possible to darken the display very far. When the board(s) are warmed up, the contrast control responds normally. I wonder if that is another symptom of the amplifier transistor misbehaving? I’m clutching at straws on that one, as I’ve already tried swapping the potentiometer, 1M resistors, a capacitor, and anything else related to the contrast circuit that I can see.
