Tektronix 468 B sweep unblank or enable issue

[Addicted2AnalogTek]

I am currently working on calibrating a Tek 468 that I fully restored and have run across an issue that I haven't yet been able to resolve.  The B sweep isn't unblanking at the same point as the A sweep (as seen in photos) and no settings or adjustments have any effect on it.  I tried all the timebase settings and triggering settings and believe I have isolated it to the horizontal sweep logic. 

I have been going through and attempting to see if all of the A/B sweep waveforms from the manual are lining up with each other correctly with the clock pulses, and haven't yet found anything unusual, yet....

I have another 468 that isn't fully restored but is functioning correctly in reference to this manner which I am using to compare against. 

In the pictures, I am running a 1ms time mark pulse from a Tek 184 into both scopes via 50ohm terminator and tee.  Both externally triggered (not that it matters for this).

Any ideas??

http://www.ebaman.com/index.php/remository/ELECTRONICS/Test-Equipment/Tektronix/Scopes/4XX-series/Tektronix-468-Vol2/
http://elektrotanya.com/tektronix_468_volume2.pdf/download.html

[Addicted2AnalogTek]

Re-reading the theory of operation section is giving me some fresh ideas on where to look and what to look for.   Forgive my lack of understanding and knowledge about circuit functions as I am still trying to learn much of the operation of various circuits..  it's quite a journey so far, but once it all "clicks" in my mind and I become able to visualize everything, things will become clear and fairly simple.

[MrSlack]

Had similar issues on my first 465B. There was a bad diode in the delay sweep if I remember (may be wrong, it was 8 years ago) which left it sort of half working. Took me a month to find it!

[Addicted2AnalogTek]

Had similar issues on my first 465B. There was a bad diode in the delay sweep if I remember (may be wrong, it was 8 years ago) which left it sort of half working. Took me a month to find it!

Fantastic!  You have just confirmed my suspicions!  After reading the trigger, sweep, and z-axis sections of the theory of operation I had the idea that maybe there is a diode that is not switching properly.  Considering the circuit actually functions and this is the only problem, I felt that it is unlikely to be a transistor or the sweep IC.    Now to find the bugger....

[MrSlack]

Good luck

[Addicted2AnalogTek]

It's one of these guys in between test point 66 and test point 67.  (just as a quick update and for anyone wanting to take a guess....)

[MrSlack]

Looks about right. Check that FET as well. Every damn Tek I've had eats them.

[Addicted2AnalogTek]

There is a 800us delay between the charging of the A timing capacitor and the B timing capacitor.   

Unfortunately all of the sockets have been removed and the components soldered into place on this specific scope, so I am going to attempt to completely isolate the issue as best as possible before swapping parts.

[MrSlack]

That's a dick move by whoever removed the sockets. Might be an ex military one though;  they tend to be 'replace entire subassembly' mentality when something goes pop and require slightly better mechanical stability.

[Addicted2AnalogTek]

If I remember correctly, this one came out of a GE lab.  It's in far too good of condition to be ex-military, though.

[Addicted2AnalogTek]

Do you think that it's possible the problem lies within the A sweep circuitry?  I have tested/swapped the fet, Q967, Q864, and all of the diodes in the "problem area"... however, my only indication I have found to go on is the 800uS delay between the start of the timing capacitor charging.   I wish I understood circuitry better already!!

[MrSlack]

Is there any difference if you select horizontal mode of A only, turn both channels on and onto GND and use horiz position to set the left of both channels mid screen? Leave triggering on NORM across the board. Also when in X-Y mode does the dot stay still if both channels are grounded?

[Addicted2AnalogTek]

The issue is only with B sweep.  CH1 and CH2 traces both have the same start point if I have only the A sweep selected.   XY mode functions normally and the dot stays in position when switching CH1/CH2 between input coupling settings & ground.   It's only when I display the B sweep that anything becomes apparently wrong.  No matter what the settings on anything are, the B sweep starts one minor division after the A sweep.

What I was thinking about in my other post regarding the possibility of the issue being in the A sweep circuitry, is that if A sweep was actually starting too soon and B sweep was the correct one. 

[Addicted2AnalogTek]

Also, I am mostly using the manual specified settings for the logic waveform comparisons that I am making.  every once in a while I will input a signal and compare the two scopes against each other, and also the A and B sweeps against each other.      I will eventually track this turd down................

[Addicted2AnalogTek]

I'm going to take a look into the B sweep / Z axis unblanking signal area and see if I can find any discrepancies..

[Addicted2AnalogTek]

I may be correct in thinking that it is an issue with the A sweep and not the B sweep.   When comparing the two 468s in horizontal ALT mode, DTP=0, the A sweep on the problem scope starts 1 minor division before DTP=0  ( there is 1 minor division of lower intensity at the beginning of A sweep with the delay time position control at 0 )   This is not the case on the other scope, where bringing the DTP control to 0 completely eliminates all of the un-intensified portion of the trace.

[MrSlack]

Ok I can't shed any more light on that here. Blanking didn't work at all on my second 465b but that was a relatively easy to find out of tolerance resistor.

[Addicted2AnalogTek]

from what I am gathering thus far, it appears to be the timing capacitors or something else on the timing board. All of the test point signals line up with the horizonal alt sync pulse on both A and B sweep circuits only before the signal passes through the timing board. After the timing board, both A and B sweep signals are out of sync with the alt sync pulse.

[MrSlack]

Has to be something around the unblanking diodes. Can only thing that you're going to have to trace it component by component to where it barfs

[Addicted2AnalogTek]

Well.... while poking around test points to see what matches up and has the correct waveforms and what does not, I managed to find a very unhappy B Trig Holdoff/Reset signal.  1V/div with baseline at 2nd from bottom.  Compared to A sweep fast blanking signal.   I know it's unrelated to the issue at hand, but it's something that the trigger IC must not be very happy about as well as something that should not be.
Correct signal should be -0.9V to +1.2V fairly clean square wave.

[Addicted2AnalogTek]

im about to break down and just buy the volume 2 manual or at the very least the artek cd manuals.  the part numbers on these printouts are total junk.  I suppose I have no choice but to deal for now though.

[MrSlack]

the Tek 465B AFAIK is the same on the analog side of things. Service manual is here: http://www.opweb.de/en/model.php?id=2650

Much better quality than the other copies and is actually complete.

[Addicted2AnalogTek]

Awesome, thanks!   As for my last post regarding that odd B trig holdoff signal.. it's the same on the other scope.   I have to stop letting myself get sidetracked. I'm sure it's irritating for anyone following along and especially you attempting to help out. So, sorry for that!

[MrSlack]

Awesome, thanks!   As for my last post regarding that odd B trig holdoff signal.. it's the same on the other scope.   I have to stop letting myself get sidetracked. I'm sure it's irritating for anyone following along and especially you attempting to help out. So, sorry for that!

I do that so many times. I do software as a day job and I'll spend all day fixing something that isn't wrong. This is part and parcel of debugging and fixing things. It's always valuable though as you get to understand what you're fixing more every time

[Addicted2AnalogTek]

When comparing test points against the A sweep fast blanking signal, the only one that does not match up is test point 58, which is the same as test point 68 from the sweep generator schematic. Right back into the area that you stated "seems about right"... so time to stray from the test points to really get it isolated....

[Addicted2AnalogTek]

probably would help to post this, too.

[Addicted2AnalogTek]

There are multiple out of tolerance resistors on the timebase board, all of which coincide with the settings one would expect to see used most frequently.   I am considering putting in the timebase board from my other 468 until I get new resistors for both, but may not since it is one revision earlier than the one that belongs in this scope. 

I am wondering if there may also be an issue with the A sweep start comparator (Q987A/B 151-0354-00) since the A sweep waveform from the scope under test is 400uS longer than the other scope's A sweep waveform on most timebase settings.  This would coincide with A sweep starting too early, which seems to be what I am dealing with here instead of the B sweep not unblanking correctly. 

For now I will just order and replace the out of tolerance resistors that I've found and  go from there.  I also intend to check the timing capacitors as best I can with the meters I have.   On another note, the trigger level adjust on the 465 I use is no longer working, so I now feel the need to get one of these repair pile scopes done so I have one to troubleshoot/cal my 465 with.

[tautech]

On another note, the trigger level adjust on the 465 I use is no longer working, so I now feel the need to get one of these repair pile scopes done so I have one to troubleshoot/cal my 465 with.

Yep, know that feeling from the past....fingers crossed for you. 
 

[Addicted2AnalogTek]

Interesting.... replacing the A sweep start comparator has 'cleaned up' the end of the A sweep.  Previously, when in ALT mode, there was a period approx half of a division at the end of A sweep that coincided with the intensity of the B sweep (as the beginning of A sweep does).  This is now gone entirely.   

Of course upon removing it, I checked it with DMM diode check and hFE and compared it against a NOS one and ones out of a 365 I use for parts.  It appeared to be rather degraded but not entirely bad, but apparently not good enough for this circuit.

In order to determine whether or not my remaining sweep issues lie in the timing circuitry, I have decided to swap timing boards since there is only minor difference between the 00 and 01 revisions. Specifically in the A sweep high speed timing caps, which I have the parts to change over if I decide to keep it in.

[Addicted2AnalogTek]

I have swapped out the A13 timebase board and A12 trigger gen/sweep logic boards with my other 468 (one at a time of course) and now know that it is something on the A15 main interface board.  Now to isolate it further...