Re: 7603 weirdo inverted Z axis

Marian B
 

On 01/22/2016 10:11 PM, aodiversen@... [TekScopes] wrote:
Hi Marian, do I understand you correctly if I think that the Z amplifier
itself is working correctly but that the CRT gets brighter when the Z
output at B gets lower (less positive)? In that case I have no idea
what's going on, but then you should also "complain" that the Readout is
shown as negative image.
If the Readout is correct then the logic board is suspect, for instance
a faulty U99.

Albert
Albert, yes this is what I am thinking. If I remove all inputs to the Z
axis amplifier it outputs about 20 V, which should blank the beam - but
doesn't. Shorting the input to ground, i.e. full input current, produces
approx. 90 V at the output (pin B), which should produce full intensity
- instead it blanks the beam.

On 01/22/2016 10:11 PM, aodiversen@... [TekScopes] wrote:
In that case I have no idea
what's going on, but then you should also "complain" that the Readout
is shown as negative image.
I don't think it would work this way, since the readout uses vectors to
display the text, fiddling with the Z axis would only change the beam
current, while the beam still traces the same characters.

On 01/23/2016 06:41 AM, David @DWH [TekScopes] wrote:
1. Does the readout intensity control also operate backwards?
The readout board in this unit was shot (1 simple ASIC missing, at least
2 ROMs dead), I replaced it by a known-working board yesterday. Readout
works just fine, but like the other intensity control the readout
intensity control works backwards.

3. Is the leading edge unblanking working correctly when a fast pulse
edge is observed?
If I adjust intensity so that I can see the beam it is never blanked.
When retrace / wait for sweep occurs the Z axis logic produces a low
current (high voltage, turning Q1107 off), which leads to ~20 V from the
Z axis amplifier output, which should blank the beam, but instead the
beam turns all the way up.

On 01/23/2016 06:41 AM, David @DWH [TekScopes] wrote:
So instead of checking to see if the intensity potentiometer is
connected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.
Yes, I measure this correctly at logic board pin ES and see the expected
effect in Z axis amplifire output as described above.

-

I tried to probe the grid-cathode voltage yesterday with my homebrew
"HV" "probe", but it's just to bulky and doesn't stay in place. I
suspect at this point that one of the original caps in the DC restorer
was failed, either open or leaky, and that one of the replacement caps
has the same or a similar problem. I'll buy another round of HV caps and
see if that changes anything.

Cheers,
Marian

Join TekScopes@groups.io to automatically receive all group messages.