Tek465 Auto Mode Analysis

2018/12/26

Knowing how Auto Mode works is so important to understanding how Oscilloscope works.  There is a mastery i always want to know but never did.

Recently I fixed one of my rear version Tektronix  465,  it's necessary go though  checking Auto Mode's signal path several times. I gonna take this chance digging into it and read  the manual carefully. 

Auto Mode and related circuits is basically the most complex part of the CRO, include almost every CRO module. 

• the Tektronix service manual said that the long tail Gate pair on Fig.8, on any given moment,  only one transistor conducted . but weird thing is the long tail itself could  not latch itself at all.

So somewhere, must enforce the latch logic. For a long time I believe It's the U870 logic IC (Fig.8).  From the service manual,   it seems the tunnel diode pulse and sweep un-blanking pulse could not latch the long tail gate.  Because it's pulse seems so short, it's not last that long as the Z-amp needed to keep the trace visible/or invisible. Another strong reason why it's not U870 locking the Long Tail Gates is that while in B delay mode,  the U870 almost have nothing to do with it, but B sweep obviously works well.

After days reading the circuits, It hit me, why not just testing the Tektronix 465 to confirm how it works? The manual's signal picture not sync up with each other, the time sequence is missing. There are several 465 on my benches!

First of all, checking the tunnel diode. A & B sweep have it's own Triggering circuits there are 2 tunnel diode for each sweep logic. one of them is arming diode which  enforce the "Hold Off" logic: after seep end, this diode is locked by hold-off control for a while before it's could response to another triggering signal.

The fire tunnel Diode lag about one Div (0.5ms, half cycle of the calibrate signal) than the Arm diode for exert to it's high voltage state, which is  0.5V.

Because Q650 designed not to provide enough current to exert the FIRING Tunnel diode CR650 to high voltage state. But Q652 could provide the current(around 4mA quiescently, fix me ) to triggering the ARMING diode So, first signal raising edge triggering the ARMING tunnel diode to it's high voltage state.

After ARMING Diode jump to high voltage state,  it  only need 1 mA  to stay in high voltage state (high voltage low current), so now there is 3mA more current to trigger FIRING tunnel diode at next raising edge, and next raising edge is just the half of the cycle, This trigger amplifier is  differential.  

Time Sequence on Real Hardware

Here we are,  connecting tunnel diode and Z-amplifier to CH1 and CH2 of same oscilloscope, then we could capture the timing information. 

Obviously, the tunnel diode's 2 voltage status synchronized with Z-amplifier,  This indicate the tunnel diode actually responsible locking that bunch  of long tail Gates on A&B Sweep Logical Circuits.

This explained why B sweep almost have nothing to do with the U870. In Normal mode or in the Auto Mode but there is triggering signal the whole working flow is:

• Signal triggering the tunnel diode, and FIRING tunnel diode step up to high voltage state and stay there, at same time the pulse sent  to Logical board A sweep gate.

• Logical board A sweep Gate flipped state,  then sent a pulse to Sweep circuits and pull Z-ampler(un-blanking)

• Sweep begin generating the ramp signal, at the end of sweep a pulse sent out to A sweep's Z Gate to black the screen

• Another signal, sweep reset generated by sweep circuits sent to U870, U870 pull down the tunnel diode, reset it to lower voltage status.

That all happens to the beam on screen!

Go through Circuits

• while Q1014 closed, Q1012 turned on at same time, this signal is Sweep Reset, connected to U870 pin 16.

• U870 Pull up Pin 3, close the Q864, turn on Q862, this Gate been reset to it's quiescent state. 

• then A-sweep Z-gate reset, the Q1304 turned on, Q1306 turned Off,  Z-amplifier been locked to retrace state.

• Both Sweep  pulse from Q1014 and U870 take affected the Z-amplifier,  but sweep un-blanking pulse dis-pear before the whole retrace end,  because on retrace, the sweep voltage going back above 2V again, but the U870  turn off Q1306, locked Z-amplifier on retrace state.

B sweep is also affected by U870. B-hold off signal controlled by U870, B-latch Gate  reset signal come from U870 . 

To make life easy, i commented the control logic schematic, highlighted the ON state transistor on quiescent state.

Quiescent state defined as:

• triggering signal not come yet.

• sweep retrace done

• Hold-off ended.