Mac Plus seems to power cycle, short beep

[unity]

This is more of a documentary attempt to revive a Mac Plus 1Mb. It arrived today and physically is in excellent shape - less horrific smell of cigarettes! First thing was to fix solder points on analog and logic boards. As expected, many had cracks. I did them all though since sometimes the cracks can be microscopic.

But she is a no-go to boot. She beeps but is cut off short of the full tone. The floppy just cycles. If I power it off for a short time, it will beep again then cycle the floppy. I have lots of test parts so I can narrow it down to the component pretty quickly. I have a feeling this will be my first venture into analyzing an analog board!

I will update as I go along.

 

[techknight]

low or cycling voltage.

 

[unity]

Well I found one problem I must have over-looked. Usually I catch things like this before a power up. The diode seems to have been damaged. Not sure if it was by a person or some other issue. Either way, I will replace. Another issue I noticed when I first had it apart was that someone has played with the yoke. I did find some damage to the analog where the yoke plugs in. I bet someone tried to fix things but missed the obvious burn marks. Although I am not sure how adjusting centering would have helped.

 

[techknight]

Who cares about the yoke at this point, but that diode definitely need replaced.

But before you go and fry another diode, You need to check Q3 for shorts. CR1 is the supply feed for your horizontal output transistor. Without that, you will have no raster, or no B supply for your CRT gun.

if Q3 is shorted, its possible your flyback is wasted.

 

[unity]

Who cares about the yoke at this point, but that diode definitely need replaced.
But before you go and fry another diode, You need to check Q3 for shorts. CR1 is the supply feed for your horizontal output transistor. Without that, you will have no raster, or no B supply for your CRT gun.

if Q3 is shorted, its possible your flyback is wasted.

I will pull Q3 and test. Not sure if I have to, but its not testing good while on the board. To be certain my testing was in order, I benched tested a good Bu406. But the one on the analog is showing different results.

 

[techknight]

its probably bad. Check the rest of the secondary diodes off the flyback, if they are ok, the flyback probably had failed.

on a transistor, you diode check it. You should get roughly .5XX from B to C, and slightly higher from B to E as it measures the R prime E resistance on the emitter junction. obviously nothing between C to E, and nothing in reverse.

if you get .000 its bad.