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

[Mark Garlanger]

On 7/23/06, Lee Hart <leeahart at earthlink.net> wrote:
> 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!
>

The probes are across the connectors that state "600V max" and I
didn't see any other warnings in the book, so 120V should be fine.
Infact, the specs seem only concerned about min not max, see my next
comment.

> > 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.
>

With the negative grounded, I see 0.000 Hz.  But your earlier comment
cause me to look closer at the specs and I think I see the problem I'm
having this DMM: For the frequency section, it states:
   "sensitivity: 0.8V rms min @ 20-80% duty cycle and <100 kHz;
       5V rms min @ 20-80% duty cycle and >100 kHz."

Since the VCO bias voltage is only 1.42 V and the frequency is well
above 100 kHz, the DMM is not sensitive enough to pick up the signal.
I'm going to go back to Fry's to return this one and see if they have
another reasonably priced DMM that able to measure this.

> > 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!
>

Yea, trying to turn R17 would put my hand VERY close to the yoke at
the back of the neck of the CRT.

> 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.

That sounds like a good plan.
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