Re: Newbie with a 466 that won't sweep


Harvey White
 

The more things you have to debug at once, the more difficult it can be.  I'd go to XY mode and debug using that.  I'd hope you have a second oscilloscope, because otherwise things get difficult.

Your symptoms suggest that the horizontal amplifier has problems.  If the dot (in XY mode) cannot go to the left (or right) of the screen, then it suggests that half of the amplifier is bad.  Fortunately, the amplifier is symmetrical, so the collectors of the + part should have the same voltage

The horizontal amplifier takes a single (almost a current into a base of a transistor) input.  That is part of a differential pair that copies the signal, and also produces an out of phase signal. Those two signals drive an amplifier that has cascaded transistors to produce a + signal and a - signal, each of which drive one deflection plate.

If, for instance, the collectors of the mirror stage don't match the collectors of the other stage (one stage each deflection plate), then start looking there.

Harvey

On 10/20/2021 10:32 AM, Richard Halkyard wrote:
Hi all,

I've recently become the proud owner of an early 466 (IC date codes
are all '74) that unfortunately is a bit poorly. I've done bits of
hobbyist electronics repair in the past, but I've never tackled
something quite so complicated as this before. I've been studying the
service manual and the Troubleshooting Your Oscilloscope document, but
I'd greatly appreciate any wisdom anyone would have to share.

My symptoms are:

* No horizontal sweep on any timebase setting
* X-Y mode operates as it should, but the trace is off-screen
horizontally (to the right), and can't be fully centered even with the
horizontal control all the way to the left
* Beam finder shows a bright dot on the screen, offset to the right,
but visible with the horizontal control centered
* Vertical deflection of both channels (with beam finder depressed)
looks correct in all ranges, and is more-or-less centered with the
vertical position controls at midpoint
* Depressing the Trigger View and Beam Finder buttons together shows
reasonable vertical deflection on all A trigger sources, and responds
to trigger level control.
* With a square wave input, the Triggered and/or Ready indicators
never light in any of the trigger modes (Auto, Norm, Single)
* The trigger gate outputs on the back panel are low all the time

The scope exhibited these symptoms when I first powered it on. I moved
it to my workbench and it started working for a few hours (although
with a significant horizontal offset to the trace), but after I
powered it off and put it away for the night, the problems came back
and I haven't seen a sweep since.

Power rail voltages are all within spec, although with about 20mV of
ripple, which I gather is a bit high. I haven't checked the HV and
don't have the equipment to do so safely.

Probing the pins of the sweep IC (U980) suggests that indeed, not much
is happening in sweep-land; the outputs of pins 10 (low) and 17 (2
volts) suggest that it might be stuck in holdoff, but I'm seeing DC on
the holdoff timing terminal. The IC is also noticeably hot to the
touch (although not scorching).

Additionally, the service manual suggests that the READY light
(controlled by U980 pin 11) should illuminate when the single-sweep
reset button is pressed (input on pins 2, 3), which seems to be a
fairly straightforward function of the U980's inputs and outputs.
Since this doesn't seem to happen, should I suspect U980 to be bad, or
is this likely to just be a symptom of it being stuck in holdoff for
another reason?

I realise that the sweep section isn't my only problem here, but my
rationale is that I'll have a much easier time debugging the
horizontal section if I have a working sweep, and while the power
supply ripple is out of spec, it's not drastically bad and shouldn't
be affecting the sweep section, right? If this is the wrong order in
which to tackle things, please feel free to set me straight.

Any and all help and wisdom would be appreciated, even the really
elementary stuff (I won't be offended!). I realise it's probably a bit
of an ambitious project for a newcomer, but I've always wanted a
vintage Tek scope on my workbench, and I'm quite determined to see
this through.

Thanks in advance,
Richard Halkyard




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