[sebhc] Question about testing the Z-89-37 controller

Mark Garlanger garlanger at gmail.com
Sun Jul 23 02:04:10 CDT 2006

I'm not sure where the controller's ground is, but I used the Black on
connect P516 on the CPU board. It seems good since it's the same
ground I used to test the VCO Bias Voltage - got 1.418 V, manual wants
1.42 V. Now with the negative lead on ground, the DMM shows 0.000 Hz.
For the heck of it I measured the voltage on Test Point 2, and saw
2.013 V. Anyone have any ideas why the VCO center frequency would be


On 7/23/06, William Elkins <bill at elkcomm.com> wrote:
> Typically measurements are made referenced to chassis ground. One lead
> should go to ground point of the power feed on the card and the other
> lead goes to the test point.
> The 15.84KHz is the horizontal scan on the crt. As this freq is also
> used to generate the hi voltage for the crt there is a lot it
> available to be detected.
> Bill Elkins
> On 7/23/06, Mark Garlanger <garlanger at gmail.com> wrote:
> > I bought up a DMM that has a frequency measurement (supports 10 Hz -
> > 10 MHz). The Z-89-37 manual states that you should "connect the
> > frequency counter to test point 2 on the board." Do I just put the
> > positive lead on test point 2 and leave the negative lead floating? It
> > doesn't state where the negative lead should be placed. I tested a
> > power outlet and with only the positive lead inserted it read
> > 59.97-60.02 but was jumping around (that small range) alot more than
> > if I inserted the negative lead also.
> >
> > So if it's valid to only place the positive lead, then my reading is
> > WAY off. It should be 2 MHz, but the DMM is reading 15.84 KHz. I
> > haven't yet tried to turn/adjust R17 because I can not figure out how
> > I'm supposed to get to it and wanted to make sure I was measuring it
> > properly before adjusting. A screwdriver of ANY length will be blocked
> > by the CRT, and I don't think I should have my hand that close to the
> > CRT. Now that I think about it, I do remember my dad having long
> > narrow plastic tool that would probably work good for this.
> >
> > Mark
> > --
> > Delivered by the SEBHC Mailing List
> >
> --
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