Macintosh Plus "ticking" sound after RAM test through speaker/analog board

[rtappmeyer]

I'm looking to get some more insights from people who might be familiar with this issue:

I have restored a Macintosh Plus, and as part of this process, I have recapped the Analog board. I noticed that after I was done with that, I could tell a faint ticking sound coming from the analog board side of the Mac. In fact it sounded like it was coming from the speaker. It would appear right after the RAM test completes and the Disk with the question mark shows up.

I took the case off and I could confirm the loudest part of this ticking was coming from the speaker. (Video IMG_9744.mov)
Then I disconnected the speaker from the analog board, and tried again. The ticking was a lot less loud, but it was still there. (Video IMG_9747.mov)

It sounded like it was coming from the analog board rather than the CRT. I also tested it in a dark room and I could not see any sparks or flicker in the CRT or yoke.
The CRT in fact does not show any breathing or static reflecting the ticking. It is rock solid and crystal clear.
I can't figure out where it is coming from or what is causing it. I have reflowed all connectors on the Analog board and also on the logic board side. I have reflowed the connectors on the flyback as well.
The only thing I noticed is that one small prong of the suction cup is broken off, but the suction cup has a great fit and seal, and again there is no issues with the picture.
Any advice or guidance would be much appreciated.

 

[Builder68]

If the ticking starts only after the RAM test, it coincides with the logic board waking up and starting to draw a more varied current.

Check the voltages on the +5V, +12V, and -12V rails while the ticking is occurring. If they are fluctuating even slightly in sync with the tick, the feedback loop (optoisolator area) is the prime suspect. It could be the optoisolator itself or any surrounding components of the feedback loop circuit.

Have you checked if the ticking frequency changes if you adjust the Brightness knob?

 

[rtappmeyer]

If the ticking starts only after the RAM test, it coincides with the logic board waking up and starting to draw a more varied current.

Check the voltages on the +5V, +12V, and -12V rails while the ticking is occurring. If they are fluctuating even slightly in sync with the tick, the feedback loop (optoisolator area) is the prime suspect. It could be the optoisolator itself or any surrounding components of the feedback loop circuit.

Have you checked if the ticking frequency changes if you adjust the Brightness knob?

Thank you for the quick response.
I have measured the voltages: the +5V is at 4.96V, the +12V at 12.18V, and the -12V at -12.06V. They stayed consistent, I did not see them vary at all.
The brightness knob also did not change the ticking frequency.