Sanyo VM4509 - adjustment questions

[desertrout]

I have a Sanyo VM4509 monitor that doesn't meet the vertical height and linearity preliminary / baseline described on page 4 of the repair manual: http://www.vintagecomputer.ca/files/Sanyo/VM4509_SM_SANYO_EN.pdf

In particular, those settings suggest the v-hold should be set somewhere in the middle, and the screen should flip when the dial is turned either way... this one needs to be set all the way to the right in order to hold. Contrast and brightness set according to those instructions (fully clockwise) causes everything to 'blow out'...

I haven't opened it up yet to assess caps etc, and I do know the adjustment pots on the front need some cleaning, but I'm wondering if anyone has an idea of what might be a possible cause here, particularly with v-hold. Are there common suspects?

 

[Hugo Holden]

I have a Sanyo VM4509 monitor that doesn't meet the vertical height and linearity preliminary / baseline described on page 4 of the repair manual: http://www.vintagecomputer.ca/files/Sanyo/VM4509_SM_SANYO_EN.pdf

In particular, those settings suggest the v-hold should be set somewhere in the middle, and the screen should flip when the dial is turned either way... this one needs to be set all the way to the right in order to hold. Contrast and brightness set according to those instructions (fully clockwise) causes everything to 'blow out'...

I haven't opened it up yet to assess caps etc, and I do know the adjustment pots on the front need some cleaning, but I'm wondering if anyone has an idea of what might be a possible cause here, particularly with v-hold. Are there common suspects?

It is a very straightforward monochrome monitor.

Assuming the power supply was normal, and it appears the V hold control is off center it would be worth checking the values of C503, C504 and C511.

The correct setting for the vertical hold control is, in the moment before it locks, the vertical blanking bar is seen to be drifting upward toward the top of the screen. The corresponds to the vertical oscillator running just a tad slower than the incoming V sync.

Once it has locked though, if the control does not cause it to unlock when you rotate it further in the same direction, this is not necessarily a defect, because the control is merely pushing it to a lower free running frequency and if there is a strong V sync pulse injection voltage, the oscillator, for the next cycle, will start at the correct time and it will remain in lock.

The only way you can really know if the V hold control is setting the V osc frequency to the correct value near the center of its mechanical range is to disconnect the V sync pulse (disconnect one leg of C503) and watch the picture float by vertically.

 

[desertrout]

Amazing info Hugo Holden - Thank you so much! I'll take it apart this weekend. Aside from this behaviour, the monitor appears to function well.