David,
I suspect the tunnel diodes were ok, since replacing them didn't change any symptoms or offsettings needed to compensate. I really don't want to put the originals back in to find out since this 475 is lousy with sockets for semis including the tunnel diodes. Every insert, and removal brings a socket (aka minisert as you call them) closer to its end-of-life. I'll leave well enough alone for now.
Stan
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--- In TekScopes@..., David <davidwhess@...> wrote:
I am a little surprised that the A trigger centering adjustment could
not calibrated that offset out but with Q522 bad, the trigger
sensitivity could have been screwed up anyway.
Do you have any idea if the tunnel diodes you replaced were bad as
well? I have an idea for how to replace the tunnel diodes used in the
465/475 trigger circuits with 3 transistors each but will need to pick
up a broken oscilloscope to try it out.
I have a nice copy now of the 475 schematics in my notebook. I just
need to find some 465 schematics I can clean up. I noticed that the
465 schematics have more diagnostic notations like voltages and
waveforms than the 475 schematics.
On Mon, 25 Feb 2013 01:21:44 -0000, "stan_katz" <stan_katz@...>
wrote:
SOLVED...while pin 3 of U520 had a good copy of the waveform being triggered on, the DC voltage on pin 3 was 1.6v. I found the U520 pin voltages posted by Tom Miller and noted that proper voltage for pin 3 should be .7v. In addition, the base of Q526 had almost 1 volt on it. Not right! I don't have a curve tracer but a simple DMM probing of the diff amp JFETS proved Q522 was blown. Ordered another matched pair and my 475 is back on the air. Now...with all my troubleshooting cal. tweaks behind me, this scope can really use a good calibration. Whew!
Thanks - Tom, David, Max, Albert, et.al
Stan
--- In TekScopes@..., "raymonddompfrank" <r.domp.frank@> wrote:
OK, glad to hear that!
Raymond
--- In TekScopes@..., "stan_katz" <stan_katz@> wrote:
I purchased original NOS Tektronix p/n tunnel diodes listed in the 475 manual.
--- In TekScopes@..., "raymonddompfrank" <r.domp.frank@> wrote:
I haven't followed this thread but noticed that you purchased new TD's and as David says, your adjustments seem to be (quite far) off.
Did you purchase the right TD's? They're not all the same!
Peak currents vary from 1mA to more than 10mA. They need appropriate biase adjustments!
Raymond
--- In TekScopes@..., David <davidwhess@> wrote:
If those low profile socket elements are the ones I am thinking of,
they are called Miniserts. I have had at least two incidents now
where a Minisert was open and reseating the lead at least temporarily
fixed the problem.
There is a datasheet for them in the files area:
Q532 (and Q526) are operating as emitter followers. The emitter
voltages for each are low impedance and should be about 0.6 volts
higher than their base voltages at all times. They are just used as
simple voltage buffers.
Trigger B is completely independent from trigger A and is only used if
a sweep mode other than A internal is selected.
From your description I suspect everything is working but that the A
trigger circuit needs to be calibrated:
R534 is used to center the trigger point vertically on the CRT when
R530 is in the middle of its rotation.
R673 adjusts the trigger view vertical positioning on the CRT.
R547 adjusts the trigger level so that it does not change when the
slope is changed.
R565 adjusts the tunnel diode threshold for maximum sensitivity.
