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

Lee Hart leeahart at earthlink.net
Sun Jul 23 10:17:17 CDT 2006

Mark Garlanger 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.

Just like making a voltage measurment, you need to connect both the 
positive and negative leads to the circuit being measured. Actually, I'm 
  impressed that connecting it to the AC power line didn't smoke the 
meter. The frequency scale is intended for low-level signals like 5v 
logic; not 120vac!

> 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.

Try it with the negative meter lead on ground. The disk controller 
circuit can't generate 15 KHz, but the video board certainly can! With 
the ground wire floating, you're probably measuring the radiated noise 
from the flyback and yoke on the video board. The noise from the video 
board is so strong that you may have to twist the meter leads together 
and connect the ground very close to the test point for measuring the 2 
MHz to get a good reading.

> 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.

The black coating on the outside of the CRT is supposed to be grounded 
by the long wire and springs that run over it. So, it is normally safe 
to touch that part of the CRT.

The yoke on the neck of the CRT, the flyback transformer, and the 
heavily insulated lead that goes to the left side of the CRT are *not* 
safe to touch! You can get a very bad "bite" from them indeed!

When I need to work on an H89 CPU board or anything on it, I remove the 
CRT board, and re-mount it horizontally, above the top of the CRT, so 
all the accessory boards stick straight up. The existing cables are all 
long enough as-is, though you have to re-arrange them around things. Put 
some insulating pad across the top of the CRT, so the CPU board traces 
don't short to the metal band around the CRT. Use the two screws at the 
top left and right corners of the CPU to hold it to the vertical 
supports as usual. Now you can "get at" everything while it is still 
connected and working.
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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