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I have been working on a 2245 for a while that has multiple issues but
this particular problem is stumping me as it's become clear I don't really
understand how the Z-axis/Focus circuits in this unit actually work.

I started with no horizontal and traced that to Q805 being shot. Those
FETs are pretty rare so I replaced with a "best I could find" substitute
that got the horizontal running. I have since found a source for
the FETs and can replace with correct replacement once I finish the
other debug.

With the horizontal working, it became clear that the scope seems to be
running at max intensity and the controls on the front have little to
no effect on the intensity level.

It's so bright that it is "blooming" and defocusing what might otherwise
be a sharp display. The FOCUS control has some effect but only to
defocus it more. There is no in-focus point that the control can reach.

I've done many of the measurements on sheet 7 and most are pretty good
and within tolerance except one,

At the emitter of Q2708, I measure -0.18v rather than the spec'd +0.18v
so that is suspicious.

However, I do not understand how the circuit ahead of that-- including
the testpoint 7B is supposed to work. I can actually disconnect this
circuit, by opening W2701, and with that disconnected, there is no change
in the display. Everything is still at max brightness and everything
is still visible on the display-- trace and readouts.

I've tried putting some voltage into the external Z-axis input, also
without getting any change in the display.

I've measured the outputs of the op-amps U1001, which the front panel
INTEN pots control and there is full range of swing on the outputs of these
op-amps. So, the knobs and the followers after them appear to be working.

So, I'm looking for some guidance on how the circuit just ahead of
testpoint 7B is supposed to work? eg, what purpose do Q1001 ... Q1004
serve? Some kind of bias maybe?

Or, the Z-axis circuit in general as my problem might be around Q2708
or Q2712/Q2713 ...

I put a photo of the display here,

/g/TekScopes2/photo/302416/3915732

You can see what I think is the retrace of the readout "drawing" since the
intensity is so high or there is lack of blanking.

Thanks for any input!

Chris
--
Chris Elmquist N?JCF

Hi Chris,
This may be the common DC restorer diode failure. You can search this group or other Tekscopes group for many lack of intensity posts for DC restorer issues (search for "DC restorer"). If node 7G signal is correct, and grid bias trimpot can't control intensity then most likely one or multiple of CR2701-CR2704 are bad. You can also observe node 7I, note that other side of C1712 is high voltage -2.7kV.

Q2708 collector (I assume you meant collector, not emitter) voltage might be OK based on test conditions. I don't think voltage difference is the root cause of maximum brightness. I wouldn't touch it before confirming grid bias restorer.

Q1001-Q1004 form a current summer, collector current is sum of all the branches at the emitter. Gating signals turn off the intensity paths when that path should be disabled (for example turn off A/B intensity paths when readout is active). I don't expect this section to be bad if waveform 7G was correct.

Ozan

On Mon, May 12, 2025 at 10:04 AM, Chris Elmquist wrote:

I have been working on a 2245 for a while that has multiple issues but
this particular problem is stumping me as it's become clear I don't really

understand how the Z-axis/Focus circuits in this unit actually work.

I started with no horizontal and traced that to Q805 being shot. Those
FETs are pretty rare so I replaced with a "best I could find" substitute
that got the horizontal running. I have since found a source for
the FETs and can replace with correct replacement once I finish the
other debug.

With the horizontal working, it became clear that the scope seems to be
running at max intensity and the controls on the front have little to
no effect on the intensity level.

It's so bright that it is "blooming" and defocusing what might otherwise
be a sharp display. The FOCUS control has some effect but only to
defocus it more. There is no in-focus point that the control can reach.

I've done many of the measurements on sheet 7 and most are pretty good
and within tolerance except one,

At the emitter of Q2708, I measure -0.18v rather than the spec'd +0.18v
so that is suspicious.

However, I do not understand how the circuit ahead of that-- including
the testpoint 7B is supposed to work. I can actually disconnect this
circuit, by opening W2701, and with that disconnected, there is no change
in the display. Everything is still at max brightness and everything
is still visible on the display-- trace and readouts.

I've tried putting some voltage into the external Z-axis input, also
without getting any change in the display.

I've measured the outputs of the op-amps U1001, which the front panel
INTEN pots control and there is full range of swing on the outputs of
these
op-amps. So, the knobs and the followers after them appear to be working.

So, I'm looking for some guidance on how the circuit just ahead of
testpoint 7B is supposed to work? eg, what purpose do Q1001 ... Q1004
serve? Some kind of bias maybe?

Or, the Z-axis circuit in general as my problem might be around Q2708
or Q2712/Q2713 ...

I put a photo of the display here,

/g/TekScopes2/photo/302416/3915732

You can see what I think is the retrace of the readout "drawing" since the

intensity is so high or there is lack of blanking.

Thanks for any input!

Chris
--
Chris Elmquist N?JCF