Tektronix 2210 Channel Bleeding Over

[mbarszcz]

I picked up a Tektronix 2210 (which seems to be basically a 2225 but with an extra digital board) with some strange behavior on channel 2.  Basically Channel 2 seems to be partially bleeding over (and summing) into Channel 1.

Here are both traces on the screen, sine wave  feeding into channel 1, and you can see channel 2 is a flat trace as it should be.


Here is the sine wave feeding into channel 2 and you can see it bleeding over into channel 1.


Even when just channel 1 is being displayed, channel 2 still bleeds over.  If ch2 is set to gnd it does not.

I also checked with my digital scope and probed Channel 1, and the signal is not bleeding over to the input jack, so it must be happening deeper into the circuitry. 

The scope did have some scratchy controls and switches, but i gave them some deoxit and exercised them back and forth.  While that got rid of the scratchy controls, it did not solve this problem.

Does anyone familiar with the analog Tek scopes have any idea what may be going on here?

[Jeff eelcr]

Try the tek scope sites first but my 2225 had 47uf capacitors leaking.
Jeff

[David Hess]

There is a circuit which selects which of the analog inputs to display called the channel switch.  Either the channel switch is damaged, or more likely the logic levels which drive the channel switch to select which channel to display are wrong.

I do not have a 2210 schematic so I am not sure how it differs from the 2225.

Does the bleed over happen in digital mode?

[mbarszcz]

I can't attach the service manual because it is too large here, so here's a link to it on my google drive: https://drive.google.com/file/d/1Dj5t5OpL0ronNlo76dUOpZA3ByNOvy4P/view?usp=sharing

The channel selection portion of the schematic seems to be in the middle of the schematic on page 211.

The theory of operation surrounding the Channel switching starts on page 48, there is also another block diagram, but this really assumes a lot.

The block diagram on page 199 shows the bigger picture around the channel switch logic U537B, U540B, U600B,C,D,E.

I really can't make sense of what's going on with the digital logic happening (there is more on page 217) before it gets to the double hex inverters.  There are so many different things all feeding this whole arrangement, the channel selections, alt, the chop pulses, trigger lines, sync signals, etc etc,  I'm really not sure what I'm even looking at.

[David Hess]

The channel switch is made up of U130 and U180.  Each of these forwards the respective channel to either the CRT or the digitizer.  The outputs to the CRT are in parallel so when they both send to the CRT, that is how ADD mode is implemented.

Channel 2 is switched by U180.  Either U180 is damaged, or the digital signals to control U180 are not of the proper level.  That signal comes from U600 (74LS05) and the -STORE signal from schematic 9 which comes from either the store switch producing GND or VCC, or U3001F (74HC04), on schematic 20.

I would measure the levels at the outputs of U600E and U600D, and then level of -STORE at R602, R603, or R653.

Maybe one of the resistors leading into pins 4 and 8, or pins 1 and 6, which control U180 is open.

You could also check the levels at pins 4 and 8, and pins 1 and 6 of U180 and compare them to the levels at U130.

Check the Chop Switch Balance as down on page 5-10.

[mbarszcz]

Thanks David for taking the time to go through that and give me a place to start looking.  I had some time to sit down with this scope on the bench tonight after work and here's what I found.

The store signal coming in at R602 is nice and strong, 0V/5.1V, -0.3V/5V on the collector, so I don't think there's any issue there.

Checking the resistors that drive U130 and U180 all check out the same in circuit, everything looks pretty reasonable there.  I was measuring in-circuit values, but nothing obviously shorted.  However, following along on the schematic "vertical preamplifier. and channel switch <2>":

The output from U600D and U600E switches only between 0V/3V depending on the Ch selection switch.   The inputs/outputs of the flip flop U540 and U600B/C all flip between a nice strong 0V and 5.1V.  Is that right?  I guess it is driving various loads including the bases of two transistors instead of other TTL ICs, so its not really "TTL" anymore past that point.  The voltages read the same on both Ch1 and Ch2. 

At the top around U130 there are two markings that indicate +3V on and +2.5V off.  For U130, both of these values check out when flipping back and forth between channel 1 and channel 2.
At the bottom around U180, they do not measure correctly, they flip back and forth between 2.54V / 2.57V on the top one, and 2.48/2.67V on the bottom.  Not a nice strong 3V like U130.  Would this be channel 2 being stuck "on" at ~2.5V all the time and bleeding over into channel 1?  Seems like it certainly might be suspect.  Pin 1 of U130 measures -0.59V / 0.59V and Pin 1 of U180 measures -0.57 / 0.62V, so it would appear they are being driven about the same, but the output varies between ICs.


What would you make of that?  A bad U180 perhaps?  A CA3102E.

After focusing in on that chip, I found this similar post here on the EEVBlog forum: https://www.eevblog.com/forum/repair/tektronix-2225-very-weird-vertical-position-fault/

I didn't realize it was an issue, but I have that same vertical position fault, looks like the labels are even the same on the 2225, U130 and U180.  A new CA3102E fixed his issue, now time to find some on ebay.

[David Hess]

It does seem like a bad U180.  Unfortunately the CA3102 seems to be a common failure.

[mbarszcz]

I received two replacement CA3102s in the mail today, socketed the board and swapped the new ones in. 

The problem is 90% better, but it isn't gone yet.

Channel 1 now ever so slightly bleeds into channel 2, and channel 2 ever so slightly bleeds into channel 1.  Nothing like it was before, but it still seems the channels aren't turning off completely.

The voltage on pin 14 of U130 and U180 switches between 2.48 and 2.97, just slightly under the specified 2.5V and 3.0V.  That seems pretty close to me, but it still short of 3V.

I noticed that digital STORE mode only seems to work now on channel 1.  I'm unsure if it was working before.  If the scope is showing both channels in non-store, and I switch to store, channel 2 disappears.

[David Hess]

There is a specification in the manual for channel isolation.  A 5 volt 10 MHz sine wave should show less than 0.1 division at 0.5V/div in the other channel.

Do the adjustment procedure for the Chop Switch Balance on page 5-10; I think it might have an effect.

[mbarszcz]

There is a specification in the manual for channel isolation.  A 5 volt 10 MHz sine wave should show less than 0.1 division at 0.5V/div in the other channel.

Do the adjustment procedure for the Chop Switch Balance on page 5-10; I think it might have an effect.

I followed along with the instructions on page 5-10, sections 10 and 11.  In section 10, I had two slow traces moving across the screen.  In section 11, I set up the scope in normal mode, and it was waiting for a trigger.  I adjusted the trigger knob, about half way it triggered a single sweep across the screen for both channels.  Moving the knob back and forth would cause a single trigger at this point.  The adjustment at R140 did nothing to change this.  Adjusting the trigger knob would still cause a single trigger near the mid point.  I then proceeded to section 15, check channel isolation.  These tests failed.  I am supposed to have less than .1 division between channels, i have ~.5.  It doesn't have any adjustments though in this section to do anything about it though. 

I pulled R140 out of circuit and measured it at is 4.18kOhm, the adjustment seems smooth and linear.  The two resistors around it R1349 and R189 are spot on, 1K and 3.9K.

After soldering R140 back it, it seems to now have a much larger effect, from the point where channel 1 just displays wiggly noise, then starts to have an impact on the amplitude of the channel 1 signal.  When R140 is adjusted so that it is at the proper amplitude, the channel bleed is now just about back where I started, .4 divisions of channel 2's input when channel 2 is set to a 10

I did this test as the manual suggested at 10MHz, 5V PP.

I'm noticing that the Channel 2 vertical position also seems to have a has a huge impact on what channel 1 is displaying in terms of bleed over, which was what that one post with a similar issue mentioned.   Demonstration here:

[David Hess]

I thought of something else to check.  The channel switches depend on both of their outputs from each channel switch being properly terminated for them to operate correctly.  If channel 2 is not working in storage mode, then maybe the signal going out through connector W1002 is not connected.  Check the ribbon cable between J1012 and J1002.

When these oscilloscopes are operated without the storage board, a special termination must be attached to connectors J1012 and J1011.  Page 6-6 just mentions extending the connections to the storage board.

You might also try swapping the cables between J1011 and J1012 to see if the problem swaps between channels.

[mbarszcz]

You hit the nail on the head with channel 2's ribbon cable being disconnected causing the digital section not to work on channel 2.  CH2_ACQ+ was in it's connector a bit sideways.

Unfortunately that doesn't seem to have helped the issue with the channel bleeding over.

Edit: now that I got the digital section sorted out and adjusted, I noticed there is in fact no channel bleed over in store mode, only in analog non-store mode.

[David Hess]

That is progress of a sort.

I would double check that the channel isolation does not meet the specifications.  Maybe there is no problem.  The test is on page 4-6 of the service manual.

Check the common mode rejection shown on the same page.  Maybe do all of the vertical tests and calibrations that you can to see if something else is going on.  Gain adjustments can be skipped unless there is an obvious gain problem.

I would compare the levels at the bases of the transistor switches, pins 1, 4, 8, and 11, between the two channel switches, U130 and U180, to see if they match.  Excessive crosstalk could be caused by a logic level not being sufficient to completely turn off one of the channel switches.

Check the levels at pins 3 and 9 of each channel switch.

Check that pins 5 and 12 are being held at -8.6 volts.  This is the substrate connection to U130 and U180 and they will misbehave if this voltage is higher.

[mbarszcz]

The channel isolation does seem to be out of spec as per the test.  Running through the vertical calibrations (and horizontal, because hey I'm in this far, might as well adjust it back into spec) did not seem to improve the channel isolation.

I would compare the levels at the bases of the transistor switches, pins 1, 4, 8, and 11, between the two channel switches, U130 and U180, to see if they match.  Excessive crosstalk could be caused by a logic level not being sufficient to completely turn off one of the channel switches.

This differs between the two channels, not sure which one is "correct" ch1 or ch2.     Should the base of channel 1 be ~0V or +0.5v?  It seems like it takes very little to turn the digital transistors on/off (pins 4,8) and they are working ok.

U130 (Ch 1 position /  Ch 2 Position)
Pins 1,11: 0.104v / -0.648v (before changing the chips, it was 0.59v / -0.59v, more like channel 2).
Pins 4,8 : -0.002v / -0.108v  (store mode: 0.68V/0.68V)

U180 (Ch 1 position / Ch 2 Position
Pins 1,11: -0.536v / 0.619v
Pins 4,8 : -0.077v / 0.001v  (store mode: 0.676V/0.6776V)

Check the levels at pins 3 and 9 of each channel switch.

On both U130 and U180 these are both a bit high, -5.4V instead of the indicated -5.6V, but they are both the same.

Check that pins 5 and 12 are being held at -8.6 volts.  This is the substrate connection to U130 and U180 and they will misbehave if this voltage is higher.

This is spot on, -8.59V both channels.


Originally Channel 1 did not bleed into Channel 2, it was only the other way around.  However, now both channels seem to bleed over into each other equally, but only in analog mode.

[David Hess]

I would compare the levels at the bases of the transistor switches, pins 1, 4, 8, and 11, between the two channel switches, U130 and U180, to see if they match.  Excessive crosstalk could be caused by a logic level not being sufficient to completely turn off one of the channel switches.

This differs between the two channels, not sure which one is "correct" ch1 or ch2.     Should the base of channel 1 be ~0V or +0.5v?  It seems like it takes very little to turn the digital transistors on/off (pins 4,8) and they are working ok.

U130 (Ch 1 position /  Ch 2 Position)
Pins 1,11: 0.104v / -0.648v (before changing the chips, it was 0.59v / -0.59v, more like channel 2).
Pins 4,8 : -0.002v / -0.108v  (store mode: 0.68V/0.68V)

U180 (Ch 1 position / Ch 2 Position)
Pins 1,11: -0.536v / 0.619v
Pins 4,8 : -0.077v / 0.001v  (store mode: 0.676V/0.6776V)

That sure looks like a problem with pins 1 and 11 of U130 that could cause poor isolation.  The anti-parallel diodes to ground, CR133 and CR139, imply that it should reach -0.5 volts, and it does on U180.

Check the voltages at the outputs of the 74LS05 inverters at pins 8 and 10 of U600.

Does the CHOP SWITCH BALANCE potentiometer, R140, do anything?  Maybe the potentiometer is open.

Check the value of R139.

[mbarszcz]

Pot R140, and resistors R139, R189 were checked out of circuit:

I pulled R140 out of circuit and measured it at is 4.18kOhm, the adjustment seems smooth and linear.  The two resistors around it R1349 R139 and R189 are spot on, 1K and 3.9K.

I tried socketing and swapping the 74LS05 for a 74LS06, which if anything should be able to supply more current than the 74LS05, but there was no change.

Twiddling R140 just reduces the Ch1 amplitude down to 0 on one end, and ends up with a normal waveform around the midpoint, and then no change after that.  Watching channel 2 while adjusting R140 makes no change to the crosstalk.

I re-ran the check channel isolation procedure #15 on 5-11 exactly as indicated, and I have 0.2 divisions of crosstalk, twice the maximum 0.1.

[David Hess]

The puzzling part is that when pins 1 and 11 of U130 are not pulled high enough, then that channel does not turn fully on which would not cause the observed loss of isolation.  So there is a problem, but it does not explain what is happening.  The voltage at pins 1 and 11, or pins 4 and 8, will be low if the transistor current gain is low causing excessive base current.

I am left to think both U130 and U180 are bad, but with different faults.