I purchased original NOS Tektronix p/n tunnel diodes listed in the 475 manual.
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--- 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.
On Wed, 20 Feb 2013 02:17:18 -0000, "stan_katz" <stan_katz@>
wrote:
I know I've let this thread go cold, but I assure members it wasn't out of neglect. For one thing, I was advised to read the manual, and I have been doing so. In the meantime, I had been monitoring the trigger diodes by clipping scope probes on their anodes. I intended on recording waveforms with my digital camera. Alas, I found the camera was fried. Nevertheless, I decided to do some more examination of the Trigger A circuits. I took hold of one of the probes attached to a tunnel diode, intending to disengage it. The instrument was powered on. The probe came off the board, WITH THE TUNNEL DIODE STILL ATTACHED to the probe! The diode had no sign of being soldered in. I turned off the instrument, and pulled on the other tunnel diode, and it came out. No solder. Now it sunk in. Those phosphor/bronze stakes weren't hiding solder pads underneath, they were all sockets! I must have missed that in some FAQ or manual addendum. The only semiconductors soldered in my 475 are small signal
diodes. Who knows if my probes were intermittently yanking out the anode ends of the TDs. I certainly wasn't going to waste members' time by trying to diagnose TD damage. I bit my lip, and purchased two NOS tunnel diodes for $OUCH.00. As least this message would start with good tunnel diodes.
Following David's advise to do a modified 5-40 trigger A calibration from just Channel 1 input lead me nowhere. The scope wouldn't trigger on the 12mv p-p 350khz sine wave I was using to obtain a .32 division height on the CRT. Adjusting R565 had no effect. Lowering the attenuator and filling the screen with the signal also had no effect. I then returned the trigger level to full + ( clockwise ), attenuator down to 50mv, and with the waveform filling the screen, I got trigger lock. I did note, that during the modified 5-40 calibration attempt, there was no sign of the trigger signal with TRIG VIEW depressed. However, with the intentional misadjustments reported above, I could just see some light at the top of the screen in TRIG VIEW. Although I was advised not to start messing with anything but R565, I adjusted R534 in the "misadjusted" state using TRIG VIEW. The waveform came into view, and I could center it. With this set of adjustments, I could now trigger on the 12mv waveform
with the attenuator turned up to .1v so that I could meet the .32 divisions performance called out in 5-40. I thought I was on to something regarding the trigger centering. I esr'ed all the electrolytics in the triggering section. All checked out. A separate check with a VOM didn't show shorted electrolytics. I thought there may be a fault with U520, and given both it, and U720 were both socketed, I swapped them. No change. I haven't been able to find any shorted bypass caps..yet. I replaced Q532, and two resistors that were more than 50% out of spec. No go. One thing that bothers me is that, on the schematic, the emitter of Q532, is pulled up through R535 to +8.2v. I only measure +5.0v. I can't find any errata that verifies my measurement. The 8.2v source is functioning as it is available everywhere else noted on partial A8 schematic with trigger circuit. Trigger level at pin 14 of U520 varies from 1.7v (full + level) to 200mv (full - level). Is this OK? TP526 has a copy of the 12mv
waveform as expected. I'm fixated on the centering circuit, but can go no further. With HORIZ DISPLAY on A LOCK, there shouldn't be any interaction with TRIG B? Right? I suppose trimmer 534 could be bad, but checking that out requires flipping A8. Yeesh! I haven't messed with R673 yet, as my troubleshooting has been with AC coupling only. Should I go there?
