Re: 7603 weirdo inverted Z axis


On 23 Jan 2016 10:45:22 -0800, you wrote:

Hi David,

The sweep gate rises at the base of U99B to redirect the current from
the intensity control through the emitter of U99B to the collector.
The intensity is controlled by this current. If the -15 volt
connection to the intensity potentiometer is open, then the current
would *increase* as it is turned counterclockwise lowering the
resistance between the wiper and ground reversing its operation. The
intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted.
Though it looks like this, I think that the CRT will simply remain dark. With the wiper at 0 V the CRT should definitively be dark. An extra resistance to 0 V will decrease the current to U99B and reduce the intensity which was already at dark. IIRC there are other scopes in which the wiper actively pulls a voltage or current higher and lower (and the current to the wiper changes direction). In that case the effect of an open connection can be different.
You're going to make me break out my Clipboard, aren't you?

One thing I wondered about the z-axis logic shown in the 7603
schematics is if the schematics were drawn wrong but I checked the
other 76xx series service manuals and they are all identical.

The gate signal is generated by an active pull-up from the horizontal
plug-in and its level is about +5 volts. That produces about 1.25
volts at the base of U99B and 0.6 volts at the emitter of U99B which
will be sufficient to turn U99B completely on and U99A completely off.
The voltages shown in the 76xx schematics are present when the gate is

With -15 volts missing at the 5 kilohm intensity potentiometer, the
current through the emitter of U99B will vary from 0.6V/3k to 0.6V/8k
or 200uA to 75uA and increase as the intensity control is turned
counter clockwise.

Under normal conditions, the current will vary from 0.6V/3k to (0.6V +
15V)/8k or 200uA to 2.0mA and increase as the intensity control is
turned clockwise. That range of 1:10 had to be a deliberate design

Is 200uA to 75uA enough to unblank the display and cause an adjustable
change in intensity? I did some of the calculations for the current
at the emitter of U99C and it looks to me like the intensity
adjustment will definitely be active and working from 200uA to 75uA if
the grid bias is adjusted appropriately.

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.

Sure that's the first thing to check anyway. I thought this would have been done already as part of checking for a cabling faults.

Public only stated that cabling was not an issue unless I missed
something and if the potentiometer was open between the -15 volt lead
and the element, then the only way to verify it would be to measure
the wiper voltage.

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