Tektronix 465B B sweep Fix

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How B sweep works

I do have several extra Tektronix scope, I have too many of them, I guess.  This 465B have a defect, B sweep stop working anymore.

I have several article to learn how the trigger works: basily, how  the scope works. Trigger is on the center of all analog scope logic.  

Only 2 things will make the scope trace show up:  

But for B sweep, the only way make the B trace show is trigger the TD, if no signal, and A  is Auto mode, B is on 'delay after A', the control loop is the most complex:


B Sweep

So, I seem know well about B sweep, I feed confidence to fix it. Start from sweep sawtooth generator, of cause there is no sawtooth.   Test Point 73 is no trigger signal at all, in any combine mode.  So, came to the upstream of sawtooth circuit.

Trigger TD

B trigger could not fired in any combination: trigger mode, B mode(alt, intense or whatever). Test point 53, 52 should be 10.6V. It isn't! Bingo. The most difficulty to understand this piece of circuit is how the DC level been setup. 

To analysis the DC voltage, especially involved so many transistors, one key point is very essential:

For TD triggering, thinking the current is the more convenient way to analysis. Each TD need  ~ 4.7mA to trigger it jump to high voltage(+0.5V) mode. 

As design, Q1743 could not provide 4.7mA to ARM TD CR7148, only  ( 55V-11.2V)/R7150/2 ~= 1.3mA provide(there is extra 0.2mA from R7143). but while in hold status,  test point 54 is held to zero voltage, it's not possible trigger the  TD(CR7148).

CR7149,  held the ARM TD to zero volt. But how it hold the voltage?  Since Q7155 is OFF. R1757 needs (-8v+0.5V)/0.845 = 8.9 mA to held voltage to Zero.   R7149 provide 3.2mA, CR7148 provide it's bias current 1.5mA, still missing 5.5mA current !

Wow, this is because, TD could work reversely as a very low voltage drop diode ( less than ~0.1V, to tens mA!), this is to say, TD held the voltage on test Point 54.  With TD installed,  the point 54 helded to zero or 0.5V! no other possible voltage. While hold off, TD current is reservely flow to TD, ensure not possible triggered! 

A&B trigger and trigger logic board

While, now, 53,52 voltage abnormal, checking 54, 55 voltage is on track.  but Q7142/Q7143's collector voltage is wrong.  Collector's voltage is set by other stuff, here, it's the VR7145/VR7047.  checking voltage on  VR7145/VR7047, they was not right!  replace both VR. we got right 53/52 DC voltage.

Q7155 Base

Things it not that easy, B sweep still does not work. Q7155 Base waveform seems not right.  Point 8 -0.5V, checking the comment below CR7149, this voltage means B sweep is under hold on status.  Point 7 is 5.2V, Q7155 turned off.  Test point 61 is exactly 1.2V. all seems fine.

Understand the Comparator

Because Q7155 is turned off, so B TD is held on.  Who held the Q7155 off?  Q7155 collector voltage is always keep on -0.5V, while Q7155 base got the comparator input(seems strange, though).  

Everything around  Q7155 is seems quite right. The possible culprits is B comparator. Blind checking Q7074 Q7075 Q7067, no evidence it's broken, every transistor had two 0.7V diode.  Checking the resistor, all of them have a value no more than mark value: there is no  valuable information, because all circuits tied together, value less than mark value is quite normal.

Now considering the voltage.  Point 1, DC voltage vary with 'B delay POT', PASS.  Point 3, sweep A's sawtooth, PASS.   Point 5,2, PASS.  Waveform on point 4, seems OK.  Waveform on point 6,  not sure, it's strange, refer to Q7155 base waveform.  

Figure out how the comparator works is essential.  The Q7075 base, marked 15.9Volt, there is around 13V instead. Not sure what this means.  R7164 and R7065 set the Q7075 base voltage.  Current flow from right Q7067 is about:  (8-0.6V)/2.26 k ~= 3.3mA maximum.  If right transistor of  Q7067 is Off, base Q7075 will around  18V(suppose Q7075 base voltage is X, solve the equation),   and if  Q7067 is On, extra several mA flow throw  R7164, it's the marked voltage 15.9.  

Another hint about this calculation, right there ,near the Q7074,  there is 2 marked voltage, 16.5V and 18.5V. This is a very important information.  If everything works, Q7075 base never could lower than 15V. now it's 13V.  The only possibility to achieve this,  R7164 must be OPEN, current provided by  15V2.

This may not be lucky, this analysis base on solid understanding this cir uts.  replace this resistor, B sweep back!

Q7155 Base: Abnormal waveform

Q7155 Base: correct waveform

TD waveform

Q7155 Collector waveform

Design Consideration of Comparators

Several questions:

The answer exist in the second one:

Yes. The saw tooth swing from 13V to 2V.   15V obviously not possible provide too much design space for that. Plus we want a constant currents.

Yes. the point 60 need swing 13V to 2V, we want a constant current too, without -8V, Q7076 have no enough operate voltage .

Q7075 collector provide output voltage, the following stage only have 5V supply: point 61.  without 15V2 680 ohm,  the load resistor is  R7164 , it's way too much big, the Q7067 's 3mA working current cause a 15V voltage swing!  With  R7065 the DC change limited to 2V!