Re: 468 sweep offset issue


 

On Thu, 25 Feb 2016 16:10:21 +0000, you wrote:

Hey there,

do you have a name we can call you by?

I took a look at the sweep generator schematics, and in your shoes I'd
start looking at the quiescent DC levels of the sweep generators. R970
trims the B sweep start voltage, and Q958/Q960 are the differential
amplifier that set the quiescent (start) voltage of the B sweep. You should
see the same voltage on the base (&emitter) of both transistors, and you
should be able to trim it with the trimmer. The schematic I got from
koo4b's site is pretty poor, but it looks like the base of Q958 is
annotated with +13.0V or +13.6V.
Somewhere in there (I didn't find it right quick) there's also a comparator
between the A sweep ramp and the delay pot's output voltage.
It's worth checking the quiescent state of the A sweep as well, as if
that's off, you may be looking at a stretched sweep that's out of range of
the delay pot at the start end.

... time passes ...

Interestingly it looks like the "B sweep start" trim sets the start voltage
for both sweeps, so if they're significantly off, there'll have to be a bad
component someplace. So, perhaps start by measuring the quiescent voltage
of the A/B sweeps - they ought to be at the same level.
Looking closer, according to figure 67, the A sweep ramp has ~11V range
(this is for - what - 12-15divs of sweep normally?), so a division off
smells like a diode drop :).
There is only one adjustment for the A and B sweep start voltage. The
comparator circuit they use for each one could have 10s to 100s of
millivolts of offset so potentially the start voltage could be offset
by that much. With a sensitivity of 1 sweep volt per division, a
sweep offset of more than a minor division (200 millivolts) is
possible.

The A and B sweep stop voltages are fixed at the same level although
independently referenced to the +5 volt supply and will have even more
offset. The stop points are not important as long as they are greater
than 10 divisions.

In summary, check the quiescent DC levels of the sweeps, they should be at
the same voltage. If not, look at the start voltage diff amp for the sweep
that's off.
If the sweeps are at the same quiescent voltage, you probably need to look
at the B sweep start comparator.

The other problem commonly seen in these sweep generators is leaky or bad
FET. If you measure the voltage over R866, you should measure precisely
zero volts, as there should be no current (to mention) going through the
gate of Q867. The symptoms to a bad FET are usually more obvious, as in the
sweep hangs, goes backwards or is very non-linear, but it's easy to check
for this.
The sweep start voltage are independent of any offset in the sweep
integrators unless they are so broken that the start voltage cannot be
attained.

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