I found a couple of much better and complete 475 service manuals. You
may want to go through the trigger adjustment procedures on page 5-18
before risking any tunnel diodes. Do not let the equipment
requirements or setup dissuade you as you can certainly complete it if
you can generate sine or triangle waveforms from 0.35 to 2 divisions.
I am kind of tempted to grab a 465 or similar with tunnel diode
problems just to see if I can fabricate replacements using a Schmitt
comparator circuit. I have been studying the 475/465 design and doing
some schematic doodles and believe I could replace a tunnel diode with
2 transistors plus external biasing.
Tektronix has the 475 service manual available here although you need
to register with them:
The KO4BB manual is excellent also but larger in size:
On Tue, 29 Jan 2013 02:59:40 -0000, "stan_katz" <stan_katz@...>
wrote:
I've noticed a more than 10 fold improvement in triggering since adjusting R565. Scope now triggers on a sine wave with attenuator at .5v/div. and trigger slope cranked all the way over clockwise. My Army 475 manual doesn't seem to have "A trigger sensitivity" or "A trigger symmetry" calibration instructions. However I didn't notice any improvement adjusting R547 from nominal. The CR566 waveform is no longer as previously reported. It is still not a clean signal, as CR556 is. CR566 is still suspect although not as a failed TD, but rather degraded and out of spec part...still working up the courage to lift a lead.
Stan
--- In TekScopes@..., "stan_katz" wrote:
R565 is right, after all. I had scope in "auto", should have been in "norm". Both diodes start comutating at the same time when advancing R565.
Stan
--- In TekScopes@..., "stan_katz" wrote:
Yes, Max, you're right. I moved to the second channel of my student scope and could slow down the free running trace (this little scope never could trigger below 1v sig.) and note that these are positive 500mv pulses on CR556, and now I see a different situation than first reported on CR566. CR566 has a positive DC offset of about 200mv on top of which are riding 200mv pulses. These pulses vary in width depending on the attenuation. As attenuation is increased these riding pulses decrease in duration until the point (.1v/div) at which time the signal vanishes and trig lamp extinguishes. Pulse duration remains constant on CR556. I was able to stablize student scope channel 1 to duplicate this picture, so I'm now confident this is the real picture. I used a 2v p-p 1khz triangle wave input signal, as Dave suggested, this time around.
As for observing anodes while adjusting R565, CR556 kept commutating, and CR566 remained dead. Are you sure it's R565 I was to play with?
Max, I hear you on the need for a better scope to diagnose my 475. Can you recommend a used scope I can find on Ebay that I can get for under, say $350.00? In the meantime, it seems that my next move is to pull out the cathode lead of CR566 and report on anode signals, as the change in signal level should be detectable on the student scope, and answer the tunnel diode question once and for all. Right?
Stan
--- In TekScopes@..., "Albert" wrote:
Could CR566 start to oscillate for some reason? If CR566 is shortcut or passive restive or open then the 300 mV signal at CR566 can't be explained. When the 6 mA supplied to CR566 after the switch of C556 does not produce a measurable signal at CR566, then an additional current switch in Q566 (less than 10 mA) will not produce 300 mV across CR566.
Albert
--- In TekScopes@..., "Max Mazza" wrote:
I've checked the schematic, "A' trigger generator".
The anode of CR556 should steps to about + 500mV. It's thearming diode.
Once it switches in high voltage state, then CR566 can commutate.
CR566 also steps to + 500mV
From your measurements, I conclude that CR556 is surely good, but CR566 not. The TD commutation is of the type you observe on CR556: clean, steep rising to +500 mV
Remember that CR566 switches only AFTER CR556 switches.
If a pulse reaches CR566 with CR556 still in low voltage state, CR566 doesn't commutates.
I hope that, since you have written the voltages with the - (minus) sign, you simply have made a mistake....
The "A trig sensitivity" simply regulates the quiescent point for both diodes. In low voltage state, it should be presumably around a few tens of mV.
You can play with R565 (with NO signal applied to scope inputs) to observe that increasing the voltage (rotating (slowly!) the pot toward the +5V TDs should commutate in their high voltage state, while rotating toward the -8V should restore their levels to +5/80 mV or so.
You have the 475 schematics, I presume. See schematic page 5 for more details.
CR566 can be supposed open, not only shorted. In the open case, you can equally observe a dirty pulse like your, because is the TD that makes (generates) the steep voltage swing of +0.5V. This is precisely the reason for which TDs are used.
Max
--- In TekScopes@..., "stan_katz" wrote:
Max,
Remember, these measurements are made with a student scope that has its own problems and can't measure below 100mv/div. So with 10x probe its hard to read. Amplitude and pulse widths given are very approximate.
I'm still working up the courage to pull out a TD lead. Anyway, with both TDs in circuit:
2v p-p square wave in
.5v/div ... auto ... trigger source lamp extinguished
CR566 -- nothing
CR556 -- approx. -500mv nice commutation approx. 300us pulse width
2v p-p square wave in
20mv/div...auto...trigger source lamp lit
CR566...crummy,noisy elongated commutation pulses approx -300mv approx 400us width.
CR556 -- approx. -500mv nice commutation approx. 300us pulse width
Isn't this experiment sufficient to prove CR566 is shorted?
Should I still pull a lead. If so. On CR566 or CR556? And what/where do I measure? I'm confused about the post pull procedure.
stan
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