BK Precision 2160A repair

[ZaUcY]

Hello,
First time posting, and I am new to the blog.  I searched for the BK 2160A but there didn't seem to be much about repair, so sorry if this post is redundant.

So I bought a BK 2160A, for $50, because I just graduated and need to start building up my lab equipment collection.  As shown in the picture, the oscilloscope does power on, the CRT still works and is still very bright, the controls seems to be responsive, and there is some wave displayed.  I figured since the scope seems to be mostly working, fixing it should not be that hard, but I'm having a little trouble.  As you can see in the picture, the cal signal is displayed sort of curved square wave missing the falling edge.  I am borrowing an "Analog Discovery" from a friend to test the scopes cal signal, but I need to buy a micro-USB cable.

condition of the scope:
Very nice condition! it looks like someone bought this scope in 1997 and stored it in the original packaging(which it came with) until now.  The inside of the scope is equally as nice as the outside with minimal dust (I will post inside pictures later).

What I have done:
I have measured the low voltage rails, 5-16V, and all seem nominal.  There are some mid-range rails, 140-200V, that I will measure later because my DMM/probes are supposed to handle up to 1000V.  I am having trouble confidently probing around the scope due to two rails at -1.9kV and 12kV!!!

There is nothing visibly wrong with any components, but I still need to start testing the R/C/D values.  With all that said, I guess my main questions are:

1)  Is there an obvious culprit for waves looking like this?  Where should I mainly focus my debugging?

2)  When is it safe to probe around the scope when all the HV circuitry is near?  Is simply disconnecting the power from the scope and cycling the power sufficient to drain any residual energy?

(When I have access again, I will be posting detailed internal pictures of the scope and the entire scopes schematics)
Thanks

[Purevector]

Maybe its just me, but it seems you simply need to change your vertical resolution.  You are set to 5 mv/div if I am reading that right.  Try going to 1 or 5 v/div.  The test signal is probably 1Vp-p or something along those lines.

[tautech]

It is unwise to display a waveform on a CRT scope in such a manner that ALL the vertical waveform is not on the screen.
This condition is stressing the vertical output amplifiers.

Start with a sufficiently high input attenuator setting and then adjust for close to FSD.

The falling edge is there, you just can't see it with those settings.
Try a falling edge trigger setting and you should see it.

[What_NZ]

Maybe its just me, but it seems you simply need to change your vertical resolution.  You are set to 5 mv/div if I am reading that right.  Try going to 1 or 5 v/div.  The test signal is probably 1Vp-p or something along those lines.

The CAL terminal is 2Vp-p so turn the CH1 VOLT/DIV Knob to the 0.5V or 1V position. Also make sure the VOLT/DIV Knob is pushed in.

[ZaUcY]

Hey,

Response:
So I know it looks like I just have my V/div turned down too low, but I am 100% certain that is not the issue (I have just revisited adjusting the knob settings).  If necessary, I can upload a video of myself adjusting the scope settings while measuring the cal signal?

Update:
So I have uploaded some close up pictures of the internals.  Sorry if they don't show much, it's hard to get good internal shots will all the wires in the way and board mounted awkwardly.  I have also uploaded scans of the original schematics for the 2160A model.  However, the board layouts seem to be slightly out of sync with my scopes boards.  As you can see, the schematics show versions D,F, etc., where my boards seems to be on revision A.

In one picture I have uploaded, you can see that I am testing the cal signal from my Oscope (2Vp-p, 1kHz) with both the Oscope and a USB "Analog Discovery."  The Oscope's cal signal seems to be spot on, but I am still getting a bad reading on my Oscope.  One thing I noticed is that the trigger level seems to do absolutely nothing to the measured signal on my Oscope, that doesn't seem right!

I probed around some more for all the sub-HV rails, and all the voltage rails seems to be fine.  However the two two 16V differential pairs coming off the main transformer were both reading out a stable 20V, is this something to worry about?  Every other rail was within +/-1% of the designed value.

Questions:
1) Is there somewhere I can probe around to test for my trigger signal?  I have two rear BNCs labeled "Z-axis input" and "CH-B(Y) Signal Output."

2) With all the High Voltage circuitry present, when is all the residual energy dissipated and safe for me to probe around?  Some of these boards are buried in the chassis and I would like to remove them for easier examination safely.

3) What else should I be doing to further debug my Oscope?  I am feeling kind of lost right now :/

Thanks!
 

!!!Here are the Schematics to the scope!!!
https://drive.google.com/file/d/0Bzao2JBCAMY0YWYtbFFJLW9xcFk/edit?usp=sharing



Solder side of PSU board


Solder side of vertical amplification circuit (I believe)


Component side of CRT control, trigger, and possibly position and time pos. circuit



Component side of PSU board:


Component side of PSU board on left side of CRT:



Time/div and Time/delay circuit:

[macboy]

Hey,

Response:
So I know it looks like I just have my V/div turned down too low, but I am 100% certain that is not the issue (I have just revisited adjusting the knob settings).  If necessary, I can upload a video of myself adjusting the scope settings while measuring the cal signal?
...

Add my voice to those saying, adjust that vertical knob!
If you can't see the top and bottom of the square wave cal signal on screen, then you do not have it adjusted properly. You are effectively looking at a very zoomed in view of the top of the wave only.
Also note that you will not see vertical edges on very fast rising waveforms, on an analog scope.

[tautech]

Ok we can see by your photos there is no problem with the CAL output.

But first I say again get the waveform within the vertical limits of the screen . 

Once done we can then use the graticules, dependent on attenuator setting and probe setting(1:1,10:1) to measure the value your scope is displaying.

This we need to know.

It is possible a X 10 multiplier is selected on the scope controls that has given you an incorrect waveform amplitude or even the setting on the probe.

We must exhaust all possibilities before adjusting anything inside the scope. 

[What_NZ]

Adding another voice too......

I don't need to see a video. I'm 100% certain that I just need to see - A picture that shows the display centred, with the VOLTS/DIV knob turned to 0.5 V/DIV and confirmation that the VOLTS/DIV Knob has been pushed in (if it has been pulled out there is a x5 gain)

From the Instruction Manual -
  VERTICAL FEATURES
  High Sensitivity
      5 mV/div sensitivity for full bandwidth. High-sensitivity
      1 mV/div and 2 mV/div using PULL X5 gain
      control.

For me that would be enough and we can move on..........

[ZaUcY]

Alright guys,

I still (respectfully) disagree and think there is something internally wrong with my scope.  Here is a video of me trying to display a square wave and sine wave on the scope:


If everyone still believes that there is a "settings" problem, then I guess I will need to do some more research on analog scopes.

If someone could answer my main question I would appreciate it.
1) When is all the High Voltage circuitry safely drained of it's energy for me to probe around further in the scope???

Thanks

[What_NZ]

Thanks for the video but honestly a picture would have done the same. Nevermind.......

Your scope does have a problem. In the 7.16 minutes of watching the video I was hoping you would have tried Channel 2.

Is Channel 2 working ok?

[tautech]

If someone could answer my main question I would appreciate it.
1) When is all the High Voltage circuitry safely drained of it's energy for me to probe around further in the scope???

The HV -1.9 KV will be gone seconds after power off.

The 12 KV PDA supply is a different matter.
The HV multiplier, by virtue of the way it works, it can store a nasty bite.
In your case, it is working, so no need to disturb it.

What_NZ's question of Ch 2 is valid, in case there is a problem on only Ch 1's attenuator

[What_NZ]

The posted circuit diagram is pretty much useless as there is no "Vertical Pre-Amp" section. Which is likely to be where the problem is,  unless CH 2 is faulty too.

Have you checked the +12 and -12 volt rails especially where they connect to the big PCB (IMG_0571) on the bottom?
Most likely they will eventually go to the front of the PCB (front of the oscilloscope) try and trace them. You will need to remove the metal shield cover.
Step 1/ They probably branch out and go to many places on the PCB.
When they branch out check for any low value Resistors (probably around 10 to 20 Ohms) they will be in series.
The other side of the resistor (output) will go to some part of the circuit.
Visually check and measure (multimeter ohms range) these resistors in case they are high resistance or open circuit.
If the resistor is ok go back to Step 1/ and continue to trace the main branch of the voltage rail until you find another resistor. Continue and repeat.

Hopefully the values of the resistors are indicated with different coloured bands.
Google "Resistor Colour Code" for how to read each coloured band to ascertain the correct value of the resistor.

Over to you...........

[ZaUcY]

Thanks for the video but honestly a picture would have done the same. Nevermind.......

Eh! I figured a video would help answer any further questions you guys may have, hope it was more enlightening than a picture.  If there's anything else I can add, via a picture or video, let me know.  Trying to get every bit of information posted to help find the bug.

Is Channel 2 working ok?

Channel 2 has identical behavior as channel 1, unfortunately.

If someone could answer my main question I would appreciate it.
1) When is all the High Voltage circuitry safely drained of it's energy for me to probe around further in the scope???

The HV -1.9 KV will be gone seconds after power off.

The 12 KV PDA supply is a different matter.
The HV multiplier, by virtue of the way it works, it can store a nasty bite.
In your case, it is working, so no need to disturb it.

What_NZ's question of Ch 2 is valid, in case there is a problem on only Ch 1's attenuator

Thanks, I don't plan on probing around the HV circuitry, I just don't want to accidentally bump it when moving boards or wires around.  This statement makes me feel safer about tinkering with the scope.

The posted circuit diagram is pretty much useless as there is no "Vertical Pre-Amp" section. Which is likely to be where the problem is,  unless CH 2 is faulty too.

There is a pre-amp schematic page in my booklet, but does not claim to be for the 2160A model.  I have added this to the posted schematics found here, https://drive.google.com/file/d/0Bzao2JBCAMY0YWYtbFFJLW9xcFk/edit?usp=sharing

Have you checked the +12 and -12 volt rails especially where they connect to the big PCB (IMG_0571) on the bottom?
Most likely they will eventually go to the front of the PCB (front of the oscilloscope) try and trace them. You will need to remove the metal shield cover.
Step 1/ They probably branch out and go to many places on the PCB.
When they branch out check for any low value Resistors (probably around 10 to 20 Ohms) they will be in series.
The other side of the resistor (output) will go to some part of the circuit.
Visually check and measure (multimeter ohms range) these resistors in case they are high resistance or open circuit.
If the resistor is ok go back to Step 1/ and continue to trace the main branch of the voltage rail until you find another resistor. Continue and repeat.

I have tested the +/-12V rails right off the PSU circuitry, and both are in good shape.  I will investigate some more with the steps you have listed above, thanks!

[What_NZ]

Channel 1 and Channel 2 are pretty much isolated in the Vertical Pre-Amp section. Unless the same fault happened to both channels - is it possible???

If you are bored waiting and want something to do, you can follow those steps but I think the fault will be elsewhere.

What I'm trying to figure out is why you appear to be getting amplification rather than attenuation when you adjust the VOLTS/DIV knob anti-clockwise and why it is common for both Channels 1&2.

Maybe an open circuit Ground (0V) Visually check all the tracks, especially the grounds (0v) on the PCB in IMG_0571. See if any are broken/blown (open circuit)

sorry I keep adding to this post -

Can you also check the correct operation of the Component Test function Page 16 - 18 of the Instruction  Manual.

Is there a board layout for the PCB shown in IMG_0571? I can't find it. It would be handy if you have it.

[tautech]

Maybe an open circuit Ground (0V)

Possibly, check BNC outer to mains earth for continuity. Haven't looked yet at the schematic, but check if circuit GND is also at the same potential.(GND)

If the probe reference lead has previously been connected incorrectly, it may have fused a track somewhere. 

[ZaUcY]

Maybe an open circuit Ground (0V)

Possibly, check BNC outer to mains earth for continuity. Haven't looked yet at the schematic, but check if circuit GND is also at the same potential.(GND)

All the BNC's outer has continuity to mains.  I checked the GND on the board in IMG_0571 and every GND point I tested on the board had 0 potential with respect to the chassis GND.

Can you also check the correct operation of the Component Test function Page 16 - 18 of the Instruction  Manual.

I tried the component test with a resistor and electrolytic capacitor, but I couldn't get anything to show; except for two EXTREMELY faint and out of focus vertical lines.  I measured the scopes AC signal used in component test with the USB Oscope and got the signal in the image below.  However, I am pretty sure that it's just clipping from exceeding the USB scopes limits.  To check this I generated a 5Vp-p sine wave with 5V offset and -5V offset from the USB scope, and then measured the signal from the USB scope.  The measured sine wave seem to start clipping from -4.9V up to 5.2V, so im pretty sure the signal I measured from the component test is just distorted due to exceeding the USB scopes limits.

Is there a board layout for the PCB shown in IMG_0571? I can't find it. It would be handy if you have it.

The schematic page labeled "(13)" is the layout for the board in IMG_0571.  However, the layout shown in page 13 looks like it is for revision G? and some boards in this scope seem to be revision A.  There are some distinct similarities between the board in my scope and the layout on page 13, but when I compare traces between the two I can find differences.

I have also been comparing component RefDes on the schematics and layout, and it seems the board in IMG_0571 is entirely the vertical pre-amp circuit and nothing else.  The vert mid/final amp circuits are on the main board, schematics page "(14)" and IMG_0568/IMG_0576.

Assuming the comp test does not work, which bypasses the vert pre-amp board, should I bother taking a closer look at the vert pre-amp board (the one in IMG_0571)?  It looks like there is quite a bit of desoldering to fully get that board out and metal cans taken off.

thanks

[tautech]

It is unfortunate you don't have a Service manual that should describe simple test procedures and show test points, voltages and waveforms. But you do have a working USB scope!

Without a full manual, you will have to follow a signal path while powered on.
The CAL output is fine for this.
You can work backwards from CRT Plate drive pre-amp or forward from around Plug 201 Page 2.

It seems from symptoms the signal is going high, that might suggest a divider is open or a coupling cap has shorted. I think I would check this first, powered down of course.
It will help you familiarize yourself with the signal path and PCB layout.

[What_NZ]

The thing that is strange is it looks like a Vertical Pre-Amp problem but it is also the same problem on Channel 2 and the Component test function as well. So unless the same fault happened to both Ch1 and Ch2 and the Component test feature then it is elsewhere.

However (I knew I was going to say this) we should measure some places on Page (1) just to be a the safe side.
I would suggest connecting CAL to Ch1 then test at U101 (correction U102) Pin 9 then connect CAL to Ch2 then test at U202 Pin 9. Those signals may also clip your USB scope so try using it with a scope probe on x10. I would say you should see a signal that looks the same as the CAL output but when you adjust the VOLTS/DIV control you should see it change amplitude. You should be able to trace those same looking signals to the Anodes of (CH1) D304, D306, (CH2) D308 & D310 on page (2)

The above was just really to be on the safe side and I still think we can ignore everything on Page (1) and everything to the left of the COMP. TEST switch S301-A on page (2) as none of that is in circuit when you selected and checked the Component Test function.

Does the Beam Find function work correctly?

Continuing on you can follow the signal path to page (4) P401B pins 1 & 3 and to the Bases of Q403, Q404, Q405 & Q406 but if you want to check any further on with your USB Scope, I would suggest measuring the voltage with a multimeter first to make sure it is a safe voltage AC & DC before connecting the USB Scope.

You could also measure (with the power off) all the low value resistors on page (4)

That's it for now as my brain is hurting - lol

[ZaUcY]

Hey, thanks for the guidance guys!

I haven't had time to follow the signal on the board yet, and I need to wire up a BNC adapter to get the x10 probe connected to the USB scope.

...test at U101 Pin 9...

The op-amps are in 8-pin DIP packages, did you mean pin 6 (the output pin)?

Does the Beam Find function work correctly?

Them beam find does not seem to be working, but I have used an analog scope only once before.  My understanding of beam find is that you press and hold the button down, and you should be able to get a fine dot in the center of the screen that is affected by the horizontal and vertical(channel 1's?) position knobs.  I think I was able to get a vertical line (about a div long) at one point with the beam find, but the lowest I could get it was about 1-2 div down from the top of the screen.  I will have to try this again when I get the scope back together to confirm.

I have posted pictures of the pre-amp board, and visually it looks like it's in great condition.  I proceeded to try some obvious continuity checks (such as making sure GND and +12V wasn't shorted) and it seems the pre-amp schematics and layout really doesn't match my board (mostly based of the RefDes).  So I emailed BK Precision for the schematics of my scope's boards (1:PWB-601P-A4 2:PWB-044V-A1 3:PWB-045H-A2 4:PWB-043D-A3) and I'm waiting to hear back; maybe they wont give me that information .  I guess my next step is to get my probe setup on the USB scope, put the pre-amp board back in the scope (without the solder side metal can) and very painstakingly try to probe the right solder joints seeing how the silk screen isn't duplicated on the solder side.

If you guys want to carefully look at the photos for something wrong, go for it, but I should be posting an update in a about half a week with more details.

Thanks again for the guidance!

Here are the photos:

[tautech]

The Beam finder result IMO is a clue.
That suggests the problem might be after that point, closer to the vertical output stage.

As far as a visual check of the PCB's, I imagine you are as capable as us for spotting any obviously discolored components or bulged E caps.

Download, read and save this: www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Let it be your bible. 

[What_NZ]

Sorry I meant U102 it is the CH1 equivalent of CH2 U202

The Component Test function was the biggest clue and the Beam Finder was the confirmation.

[What_NZ]

I really don't think the problem is on that PCB you would be better off quickly checking this (from my other post)
" You could also measure (with the power off) all the low value resistors on page (4) "

You can post pictures of that PCB if you want to.

[tautech]

BTW if the fault tracing/troubleshooting while powered on is physically difficult (to access test points), you may have to make "extender cables" so boards can be laid on the bench for test and still be connected. This practice is not uncommon for compact designs.

[ZaUcY]

BTW if the fault tracing/troubleshooting while powered on is physically difficult (to access test points), you may have to make "extender cables" so boards can be laid on the bench for test and still be connected. This practice is not uncommon for compact designs.

Good idea, I thought about doing this, but for whatever reason decided not to.  I guess my signals will just accumulate a little noise before they are amplified

thanks

[tautech]

BTW if the fault tracing/troubleshooting while powered on is physically difficult (to access test points), you may have to make "extender cables" so boards can be laid on the bench for test and still be connected. This practice is not uncommon for compact designs.

Good idea, I thought about doing this, but for whatever reason decided not to.  I guess my signals will just accumulate a little noise before they are amplified

thanks

Often in Service manuals, there will be a list of equipment/materials needed for service and extender cables are listed there. Servicing "plug-ins" a prime example.
The links between PCB's are often noncritical regarding "introduced noise"(designed as such), so one must not be ad-versed to using an extender cable if necessary.

[ZaUcY]

So with some wires boged (is that how you spell it Dave?) on the pre-amplifier board for remote runtime, the pre-amplifier board seems to be doing it's job.  The AC/GND/DC coupling switched are good.  I checked the input's to U102/202 and the signal de-amplified as I increased the V/div knob and amplified as I decreased the V/div knob (which I believe is correct).  I also tested the signals coming off 16 wire bundle at the end of the board (which leads to the mid-amp board) and every part of the vertical amp switched seems to work correctly on both channels (5X magnification, all V/div settings{atleast relative to each other}).  So I just soldered everything back together and re-installed the pre-amp board.  I was going to measure on the diodes, D308/310/304/306, but one of my probes broke and it's hard to get a good measurement.  I have ordered a couple more (cheap Hanteks)probes that should hopefully do the job for now.

The main reason why I'm posting before I have tried everything you guys have suggested, is because I managed to get the scope to do something interesting.  I turned on the "CHOP" setting in the scope, which makes channel 1 and 2 sweep simultaneously, and it seemed to show the bottom end of the square wave along with the top end (shown in picture).  This is with only one channel plugged into the CAL signal, but I can get this to show with either channel 1 or channel two.  The only difference is the trigger needs to be set to the correct channel, and the position switches need to be inverted when you switch channels.  I have re-tried to recreate this with "CHOP" off, but I cannot.  Does this give any more clues?

[ZaUcY]

I also want to point out that I can only get both of these parts of the wave to show when V/div knob is set to 50mV/div, no other setting will show both.

[tautech]

So with some wires boged (is that how you spell it Dave?) on the pre-amplifier board for remote runtime, the pre-amplifier board seems to be doing it's job.  The AC/GND/DC coupling switched are good.  I checked the input's to U102/202 and the signal de-amplified as I increased the V/div knob and amplified as I decreased the V/div knob (which I believe is correct).  I also tested the signals coming off 16 wire bundle at the end of the board (which leads to the mid-amp board) and every part of the vertical amp switched seems to work correctly on both channels (5X magnification, all V/div settings{atleast relative to each other}).  So I just soldered everything back together and re-installed the pre-amp board.  I was going to measure on the diodes, D308/310/304/306, but one of my probes broke and it's hard to get a good measurement.  I have ordered a couple more (cheap Hanteks)probes that should hopefully do the job for now.

The main reason why I'm posting before I have tried everything you guys have suggested, is because I managed to get the scope to do something interesting.  I turned on the "CHOP" setting in the scope, which makes channel 1 and 2 sweep simultaneously, and it seemed to show the bottom end of the square wave along with the top end (shown in picture).  This is with only one channel plugged into the CAL signal, but I can get this to show with either channel 1 or channel two.  The only difference is the trigger needs to be set to the correct channel, and the position switches need to be inverted when you switch channels.  I have re-tried to recreate this with "CHOP" off, but I cannot.  Does this give any more clues?

The trigger always needs to be set to the correct channel!
Have you experimented with the Trigger Level?

It seems by your tests that the pre-amp and attenuators are working correctly. 

[What_NZ]

If you want to try a little experiment.

Desolder and lift out of the PCB, one end of diodes D304, D305, D308 & D310. Then recheck the display using Channel 1 with the CAL signal.

Then can you measure the voltage on U301 pins 5 & 6 with the Chop function both Off and then On

[alanb]

This may be a stupid question but do we know that the probe is ok?

[What_NZ]

This may be a stupid question but do we know that the probe is ok?

I couldn't think of any way a passive probe could cause a fault like that and post #25 seems to confirm that now.

[ZaUcY]

This may be a stupid question but do we know that the probe is ok?

I was using the probe on my USB scope and getting a clean signal.  For some reason my other prob recently died   China made crap.  The probes I just ordered are also China made (crap?) but they are atleast branded.

I was getting an open circuit on the bad probes positive lead compared to 228ohms on the good probe's positive lead, in 1X setting.

[ZaUcY]

Hey guys,

Again sorry for I have not completed everyone's recommended tasks yet.  However, I have created another video (Shorter) showing something that I feel needed to be posted right away:

https://www.youtube.com/watch?v=uZ6FOrh_naI&feature=youtu.be

Not really sure what is going on, I have a feeling this is just a coincidence.

If you want to try a little experiment.

Desolder and lift out of the PCB, one end of diodes D304, D305, D308 & D310. Then recheck the display using Channel 1 with the CAL signal.

Then can you measure the voltage on U301 pins 5 & 6 with the Chop function both Off and then On

This is my next step

I really don't think the problem is on that PCB you would be better off quickly checking this (from my other post)
" You could also measure (with the power off) all the low value resistors on page (4) "

You can post pictures of that PCB if you want to.

This is my next next step

Thanks

[What_NZ]

After looking at your video, I'm starting to think there is something fishy with U301, half is used on page (3) and the other half on page (6). Please check U301 as per the previous post #28.

[ZaUcY]

After looking at your video, I'm starting to think there is something fishy with U301, half is used on page (3) and the other half on page (6). Please check U301 as per the previous post #28.

OK, so I have tested what you have suggested.

I de-soldered one end of D304, 306, 308, and 310.  When I tried to display a wave on channel 1, I could not get anything.  The only beam I saw was an extremely faint and out of focus vertical beam; nothing I did would sharpen the beam.  This beam would begin to sweep from left to right as I increased the time/div knob, like how a signal will slowly sweep across the screen as you increases said knob.

With the same diodes still disconnected, I continued to measure U301.

Power:
Gnd looks good, no voltage drop between the chip Gnd and chassis Gnd.  Vcc looks good too, in the schematics the chip's Vcc is connected to 5V in series with a 51ohm resistor.  Measured values are 4.93V at 5V rail and 4.73V at chip's Vcc.  According to datasheet, Vcc can go as low as 4.5V.

U301, CHOP half:
I have attached a sort of truth table for how pin 5 and 6 (Q and Q') were behaving when switching chop and the vertical mode switch.  the on voltages (output high voltages) I was seeing was ranging from about 4.3V to 4.7V, which looking at the datasheet seems to be fine.  However, the datasheet claims that at Vcc = min, the V output high should be 2.4V-3.4V, so the chip is out of spec when vertical mode = dual.  There is one instance when the voltage output can be out of spec, according to the data sheet, when pre' and clr' are both off.  However when my voltage outputs are out of spec I'm measuring pre'=4.76V=clr'.  Something strange.......  Oh, turns out it's a square wave input and output at this setting with the correct output voltage.

All the Off voltage (output low voltages) values were fine too, about 230 mV.

Another thing I just realized, how is Q and Q' on at the same time in a single flip-flop?  Looking at the truth table on the first page shows this possible, I measured pre' and clr' both off, so this is correct.

U301, Trigger half:
I was getting a couple of clks for the trigger's half of the chip, depending a couple siwtches.  I have posted another truth table setup, and snapshots of clks based the switches.

Also is a truth table for how the Q and Q' were behaving.  Everything seemed to be a DC signal and straight forward.

Datasheet:
http://www.ti.com/lit/ds/symlink/sn74ls74a.pdf

Measuring this chip is getting a little cumbersome now, but I think it is working correctly.  If the community is not convinced, I can probably throw it on a breadboard in a more controlled environment.  I also could do a thorough truth table, like in the datasheet, if necessary?

[What_NZ]

Just a quickie, is this a typo (in BOLD)?

<Quote> I de-soldered one end of D304, 306, 308, and 310.

As it should be D305. What I was trying to achieve is isolate Channel one from Channel 2 and also the operation of the CHOP (for the want of a better name) circuit.

I just re-read the whole thread cause I lost my train of thinking on this fault.
Back in Post #15 you confirmed the Component Test Function didn't work, this still is the biggest clue and rules out all circuitry to the left of S301-A on page (3). So don't worry about D304, D305, 308, and 310 or that half of U301. Sorry for that diversion.

It still would be good to see the waveforms (using the CAL signal) at the Anodes of (CH1) D304, D306 and then move probe to (CH2) and do the same on D308 & D310 on page (2)
Those signals should remain similar all the way to the Emitter of Q404 & Q405

[ZaUcY]

Oh, sorry!  I assumed you made a typo with D305, seeing how D304 and D306 correlated the same way as D308 and D310; I will fix that.  Do you recommend trying to re-test U301 after I make the correction?  (U301 seems to be an isolated digital circuit to me.)

So I have gotten around to testing page 4's resistors.  It was a little tricky, because the CRT is laying right over this circuit.  Thankfully the schematic book board layout matched the on in my scope, I was able to probe from the solder side using the schematics.  Some of these values MAY be off due to components in parallel, I haven't looked too closely to that yet.  An example is R433 and R434, I cannot measure those (unless I de-solder them) due to being in parallel with inductors :/  I have highlighted the resistances that seem to be way off, I have also included the percent error in ideal value and measured value.

Hope this will help find something!

[What_NZ]

Sorry I seem to be making a few mistakes swapping between the circuits and the forum, my apologies.

The diodes to lift one end of are D303, 305, 308, and 310.
The reason for doing that was to isolate Channel 1 from Channel 2 and also from the operation of the CHOP (for the want of a better name) circuit formed by U301. Just seeing if you get a correct display would be enough when using Channel 1 and the CAL signal.

I don't think rechecking of U301 is necessary.

Oh did you check R446 on page (4). Also (but maybe nothing) it is interesting that R419 is about 10K different than the same resistor (R422) in the identical circuit below. Could you recheck R419 and then again but this time reverse the multimeter probes while checking it. If it still measures 10K lower could you check Q407 & Q408 for leakage.

It still would be good to see the waveforms (using the CAL signal) at the Anodes of (CH1) D304, D306
Those signals should remain similar all the way to the Emitters of Q404 & Q405

[onlooker]

I am not sure if it is already asked,  did the scope show the flat trace when nothing connected to the input bnc?

If not, a dc level check using a multi-meter along the signal path should already be enough to do an initial diagnosing. At the very end y+ and y- should have a voltage difference following the vertical position knob centered around zero volt difference.

I would also give all the  knobs and buttons some hard turns/toggles to make sure good contacts.

[ZaUcY]

The diodes to lift one end of are D303, 305, 308, and 310.
The reason for doing that was to isolate Channel 1 from Channel 2 and also from the operation of the CHOP (for the want of a better name) circuit formed by U301. Just seeing if you get a correct display would be enough when using Channel 1 and the CAL signal.

This actually isolates Channel 2, the schematics are confusing.  And I now only have the listed diodes disconnected on one end.

Oh did you check R446 on page (4). Also (but maybe nothing) it is interesting that R419 is about 10K different than the same resistor (R422) in the identical circuit below. Could you recheck R419 and then again but this time reverse the multimeter probes while checking it. If it still measures 10K lower could you check Q407 & Q408 for leakage.

I must have missed R446, I measured it and added it to the list.  I have re-measured R419 and I am still getting values that are pretty far off.  If I measure the resistor in one polarity the DMM settles at about 22.5K and in the other polarity the DMM settles at about 25.8K.  The weird thing is the DMM will take about 3 times longer to settle on a value than almost every other resistor.  Here is some information I have on the transistors:

PNP:
Q407 HFE = 5
Q410 HFE = 64

NPN:
Q408 HFE = 20
Q409 HFE = 21

I am not sure how to check for leakage, but it looks like there is something fishy with the PNPs.

It still would be good to see the waveforms (using the CAL signal) at the Anodes of (CH1) D304, D306
Those signals should remain similar all the way to the Emitters of Q404 & Q405

So I have posted the input and output waveforms for the mid-amp in channel 2. 

Negative terminal
Input: Pin14In
Output: D306Anode

Positive terminal
Input: Pin13In
Output: D304Anode
(These screen shots were taken with V/div knob = 5mV/div, most amplified signal)

The signal definitely looks messed up at the diode anodes, like runt pulses maybe.  The other really off thing about the signal at the diode anodes is how the signals behave with V/div knob settings.  The signal output, past the pre-amp board, will de-amplify as I increase the V/div knob and amplify as I decrease the knob; this is correct.  Measuring the signal at the anode of the diode will begin to de-amplify as expected, but will gradually add more DC offset, and at the same V/div setting when the signal disappears off my CRT screen, the signal at the anode becomes a seemingly pure DC voltage at about 2.8V.

So if you are saying I should see a similar signal, and behaving the same as after the pre-amp board, then my problem is probably somewhere in the mid-amp circuit before D304/306.  However, like you have mentioned earlier, it is unlikely both channels get identical problems in isolated circuits. 

I am not sure if it is already asked,  did the scope show the flat trace when nothing connected to the input bnc?

If not, a dc level check using a multi-meter along the signal path should already be enough to do an initial diagnosing. At the very end y+ and y- should have a voltage difference following the vertical position knob centered around zero volt difference.

I can't get a flat GND line on the screen whether or not anything is connected or disconnected.

If you look in the attached screen shots of this post, there is a DC offset in the signal after the pre and mid amplification, but I haven't checked all the way up to the CRT yet.

[tautech]

R419, is this measured out of circuit, if so replace.
Q407 replace

[ZaUcY]

R419, is this measured out of circuit, if so replace.
Q407 replace

It is measured in circuit.  Q407 and Q408 had some burnt flux around the solder, as if it had already been replaced at some point.  So it definitely sounds like Q407 is bad?

[onlooker]

I can't get a flat GND line on the screen whether or not anything is connected or disconnected.

If you do not see the GND line, that should be the 1st priority to work on since it is easier and no need for cal-signal.

Assuming your plots are GND referencing,  then,  for the most part those plots are the wrong things to look at. Other than the 1st part of the pre amp, all other amp stages are differential amps of various types (with emitter or collector current mirror, class B (push-poll emitter follower),...). 

For differential amps, you need to check the voltage difference on the input pair and output pair of each stage. pin 13 and 14 are such a pair. I do not know the reason for the square wave yet. I guess they are due to the diode switches (D303, 305,...) are at working to alter channels for sending the input to the rest amp stags (But, in that case, the plot on D304 should be the one showing the effect??).

The right view of the ch1 input is (pin13) subtracting (pin14) and (D304) subtracting (D306).  If I do this visual math, I do not see things that are visibly wrong.

What you need to have is a DMM hooked to such pair points, then,  varying the V pos dial and  observe whether the voltage on the DMM changes and whether the voltage can be adj to zero.

Since you can trigger properly, the likely place wrong is in the "Vert Final Amp".

More of such pair check points there are
 --collectors of Q402 and Q401,
 --emitters of Q405 and Q404,
 --Y+ and Y- (or collectors of Q407 and Q410). Be careful the 140VDC.
 
The schematics is confusing. It switches around ch1 ch2, chA, chB. You may need to correct my listing accordingly.

Did you check the 140 VDC supply?

 

 

[ZaUcY]

Did you check the 140 VDC supply?

Yes, I have right off the PSU and it is measuring 145V.

I will attempt in checking the differential voltage, with no input signal, on the differential pairs as you mentioned.

[onlooker]

And make sure it is on the single channel mode.

[What_NZ]

Oh that was an easy trap, well spotted. I suppose it became obvious during your testing. An easy mistake but I should have been more careful so CHA on page (2) is Channel 2 and CHB is Channel 1.

[tautech]

I must have missed R446, I measured it and added it to the list.  I have re-measured R419 and I am still getting values that are pretty far off.  If I measure the resistor in one polarity the DMM settles at about 22.5K and in the other polarity the DMM settles at about 25.8K.  The weird thing is the DMM will take about 3 times longer to settle on a value than almost every other resistor.  Here is some information I have on the transistors:

PNP:
Q407 HFE = 5
Q410 HFE = 64

NPN:
Q408 HFE = 20
Q409 HFE = 21

I am not sure how to check for leakage, but it looks like there is something fishy with the PNPs.

Seems that way to me too.
When you get measurements that you suspect as fishy, the only sure thing to do is pull the components and test them on the bench. How hard can that be. Once done they are eliminated from suspicion.
Grab the datasheets and check hFE is to spec.

This will not be the first scope I have seen vertical output stages fail.
There are a few areas in a CRT scope where the componentry is "stressed".

In order of my experience:
1 EHT supply
2 Vertical output
3 LV PSU

Mostly voltage is the killer and the age of the components.
You can with very careful selection substitute with higher spec components.

[What_NZ]

Actually I assumed you must have pulled the Transistors to measure the hFE? Most of those types should have a gain of more than 60 so I'm not sure of your tester or testing method (in/out of circuit?)

Also check that they are what is shown in the Service Manual especially if you suspect someone has been there before you. Toshiba are a big player in the 2SA, 2SB, 2SC, 2SD types so check the brand too.

I'm not sure if this is a good idea but there is a 2SA1360 in the Horizontal circuit page (9) Q658 I can't see any harm if you swapped them. But before you do, what do the others think? If the fault changed axis it least it would be an indication.

[tautech]

First hit on a Google search: http://www.amplimos.it/images/2SA1360.pdf
As remarked on in a previous post, this device is likely to be voltage stressed, especially if any surrounding component gives problems.
Why? It's absolute max voltage rating is 150 V and in is working in a 140 V area.
Take note of: Note 2: hFE classification O: 80 to 160, Y: 120 to 240 from the datasheet.
Identify which version you have, pull it and test.
If replacement is needed, it is likely you will have to do some minor overall gain or balance adjustment, but as far as I can tell they are provided for in earlier stages.
Any component that you suspect pull and test.
It seems you are very close to the problem, let this give you the energy to crack it. 

[ZaUcY]

Alright guys,  Sorry for such a late reply, I had to put my scope away for a couple days and just ended up taking a break on it.

R419, is this measured out of circuit, if so replace.

I have pulled R419 out of the circuit, and its measuring correctly at 32.69k (I have updated the resistor table, and have added one to this post too)


So I have pulled out Q407/Q408/Q409/Q410 and have measured all of the transistors separately.  Q407 was giving me weird results, so I measured all transistors as an NPN and PNP.  It looks like Q407 is bad, but you might also notice that Q408 and Q409 don't agree when they are the same transistor.

I have tried to look for data sheets for these part numbers and could only find on data sheet for C3953 made by fairchaild semi, found here:
http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/KSC3953.pdf

I could not find a data sheet for A1538 (Q407/Q410). Is this the right transistor for where the schematics label A1360?  Actually both part numbers don't fully agree with the schematics.  I am no expert in reading part numbers and understanding the patterns, so I have posted pictures of all the writing that appears on the transistors.

It might be a little blurry to read those part numbers so here is what it all says.

Q407/Q410:
top line: "A1538:"
bottom left corner: "E" (to label emitter)
bottom right corner: "6L"

Q408/Q409:
top line: "C3953:"
bottom left corner: "E"
bottom right corner: "1G"

[ZaUcY]

I also just realized that R419 is connected to the base of Q407.  So because I was getting a bad reading for R419 when still connected to the base of Q407 in the circuit, might also suggest that Q407 is bad.

Still not sure why the hFE of Q409 was 30% higher than Q408, the same transistors.

[tautech]

I also just realized that R419 is connected to the base of Q407.  So because I was getting a bade reading for R419 when still connected to the base of Q407 in the circuit, might also suggest that Q407 is bad.

Still not sure why the hFE of Q409 was 30% higher than Q408, the same transistors.

I would be tempted to replace all 4 transistors in the pre-amp stage.
If your tests are with removed devices, things are up the creek. 

The device #'s have been shortened as is often the case and I usually put "2S" in front of the # for it to make sense.
But do your homework re datasheets to ensure you get the correct devices.

[ZaUcY]

I would be tempted to replace all 4 transistors in the pre-amp stage.
If your tests are with removed devices, things are up the creek. 

Do you mean the final amp stage?  that is where Q407 - Q410 are from.  If that is not what you mean, can clarify please.

Mostly voltage is the killer and the age of the components.
You can with very careful selection substitute with higher spec components.

Could someone possibly suggest transistors with higher ratings for me?  or atleast tell me the parameters I need to match?

[tautech]

Quote from: tautech on Today at 09:25:35 AM
I would be tempted to replace all 4 transistors in the pre-amp stage.
If your tests are with removed devices, things are up the creek. 

Do you mean the final amp stage?  that is where Q407 - Q410 are from.  If that is not what you mean, can clarify please.

Correct, Q407-410 I miss-read the schematic

Quote from: tautech on July 03, 2014, 05:37:19 PM
Mostly voltage is the killer and the age of the components.
You can with very careful selection substitute with higher spec components.

Could someone possibly suggest transistors with higher ratings for me?  or atleast tell me the parameters I need to match?

Gain, Frequency, voltage, pin-out etc
As you have taken a while to work through this, what's a bit longer.
Take your time to hunt out replacements, X match them, Google substitutes etc

[onlooker]

I think a little LTspice exercise can help the diagnoses as to what should not be concentrated on. Yes, for the fun of it, I extracted a simplified version of the upper half of the output stage and did the simulation.

The simulation is done for a DC level sweeping.
 
The results are:

1). Bf = 5 to 200 for Q407 and Q408 has no significant impact on the performance of the output stage, (which has a voltage gain of ~6737). That is, they are non-critical. The much more critical things are the value drifts of the resistors in the feedback loop.

2). The proper Vin range is ~[2,5.5] V, for which Y+ linearly swings from about 130V to 10V. The 0V trace on screen should appear at Y+ ~= Y- ~=70V.

If I were doing the repair, I probably would not take out all the parts without checking these reference operating voltages first.

[ZaUcY]

First off, thanks for the simulation onlooker!

1). Bf = 5 to 200 for Q407 and Q408 has no significant impact on the performance of the output stage...

I am still confused on how you came to this conclusion, it looks like you hard coded the model to have a Bf = 5 for PNP and Bf = 20 for NPN.  Did you change the Bf value in the model and compare results?

The much more critical things are the value drifts of the resistors in the feedback loop.

Are you referring to the resistor values I highlighted yellow in my table, or are you just simply stating the correct resistor values are critical?  I still haven't pulled the resistors from the circuit to confirm they are bad yet.

2). The proper Vin range is ~[2,5.5] V

This is necessarily the proper Vin on my scope?

If I were doing the repair, I probably would not take out all the parts without checking these reference operating voltages first.

Unfortunately I have already pulled out the four transistors, but maybe that was good, because Q407 is more than likely a bad trans.  I have found new 2SA1360 and 2SC3423 trans on ebay that I have purchased to replace the transistors.  I had a look on digikey for upgrades, but I couldn't find a TO-126 package transistor with a high enough transition frequency.

I still plan on performing your DC levels check once I get all the components placed back in.

thanks again onlooker!

[onlooker]

Quote from: onlooker on Yesterday at 12:03:37 PM
   

1). Bf = 5 to 200 for Q407 and Q408 has no significant impact on the performance of the output stage...

I am still confused on how you came to this conclusion, it looks like you hard coded the model to have a Bf = 5 for PNP and Bf = 20 for NPN.  Did you change the Bf value in the model and compare results?

Bf = 5 for Q407 and Bf = 20 for Q408 were using the numbers you provided; Bf=200 was the datasheet number you or some other earlier poster quoted. I did tested the setup using Bf=5 and up for both transistors  (and also varied values of other components). The voltage gain remained about 37.

This is no surprise so long as the open loop gain is much larger than 37; Theoretically, for any Bf>0 (say 0.1 or 0.5) for Q408, the open loop gain can be infinite if the collector CC sink can be assumed ideal. Then, these are just DC analysis that may be helpful for get your 0V trace back if my quick simulation was correct.

Quote from: onlooker on Yesterday at 12:03:37 PM
   

The much more critical things are the value drifts of the resistors in the feedback loop.

Are you referring to the resistor values I highlighted yellow in my table, or are you just simply stating the correct resistor values are critical?  I still haven't pulled the resistors from the circuit to confirm they are bad yet.

The main feedback is through R415 in this part of schematics. Then, many other things can go wrong including the transistors you took out.

But, the 1st thing to do should be to determine if the problem is in this area by simply checking the voltage on Vin and Y+ on the schematics I showed (with auto trig on, no input on BNC and varying V pos,...). To be safe, you may clip the leads of your DMM in place with hands off.
 

Quote from: onlooker on Yesterday at 12:03:37 PM
   

2). The proper Vin range is ~[2,5.5] V

This is necessarily the proper Vin on my scope?

Vin is the test point marked on the schematics in my early post.  [2,5.5] V is the simulation findings for having a linear Y+. It is pretty stable when varying Bf and other things.

In any case, it can be served as a reference range for you, even if your scope has a range that did not fall exactly on it.

Quote from: onlooker on Yesterday at 12:03:37 PM
   

If I were doing the repair, I probably would not take out all the parts without checking these reference operating voltages first.

Unfortunately ...

Then, hope for the best. Be gentle with the pads and traces on "older" PCBs.
 

[ZaUcY]

Thanks for the further elaboration!  I just noticed the Vin on the graph

I will let you guys know what happens when I get the new transistors in solder everything back together, fortunately the solder pads have stayed intact thus far; only the solder mask is giving me trouble, it's fragile.

I will try all the DC measurements once it is all back together.

[ZaUcY]

For future reference, I have contacted BK Precision on the replacement transistors they use for Q407-Q410 and received the following response.

The A1360 have been replaced with NTE 374 and the C3423 with the NTE 373. Hope this helps.

Regards,
-Chris

[ZaUcY]

Hey guys!

Back with awesome news!  I received my Toshiba 2SA1360 and 2SC3423 pairs in the mail, from Germany, purchased off Ebay.  I soldered them into my scope, and soldered all the diodes and resistors back in that had one end de-soldered, and the scope is working now!!!!!

I have posted some pictures of using both channels, using component test on an LED and 10uF electrolytic, measuring a 10MHz 50mV p2p sine wave, and measuring the cal signal.  Everything seems to be working as expected (including everything that I did not photo) except the rising and falling edge of the cal signal looks funky.  I have never seen edges with variable intensities like the one shown in the picture below.  I don't have much experience with analog scopes, so if anyone else with more experience can elaborate, I would really appreciate it!

I haven't tried to measure signals or check calibration of the scope yet, unfortunately I'm writing this post on a train and only had time to solder the parts in this morning and take pictures.  So in about a week, I am going to be focusing on calibrating the scope.  Really excited it's working!!!  thanks for all the help guys!!  I now see that debugging a scope is a very typical and systematic approach, like everything else in EE, but I had no clue how scopes really work so it was daunting for me in the beginning.

Does anyone have tips for going about calibrating my scope?  I should have a reference scope, and function generator when trying to calibrate mine (college lab).  Unfortunately I have no clue on the calibration of these devices, and the reference scope will likely only be 100MHz.

thanks again

P.S.  I bought some extra new NTE transistors as well, in case these transistors die again

[tautech]

  
It is likely the scope is not far from accurate calibration.
Mainly I would check vertical accuracy.
There might be 3 adjustments for gain.
1 for each channel and another for overall gain.
A proper service manual will describe the procedure.

You can use a recently "in cal" scope for comparison and mostly this is quite good enough for hobby work.

Sometimes the Cal signal is adjustable(Amplitude & Freq) and it can be useful to get this exact for a quick reference in the future.

Mark any adjustment pots for a "return to" reference.

[ZaUcY]

Everything seems to be working as expected (including everything that I did not photo) except the rising and falling edge of the cal signal looks funky.  I have never seen edges with variable intensities like the one shown in the picture below.  I don't have much experience with analog scopes, so if anyone else with more experience can elaborate, I would really appreciate it!

I emailed BK Precision about this matter, and here was their response.

This is normal for analog oscilloscopes. The rise and fall is very fast, so it will exhibit those faint string of lines you see. The intensity of the light is based on the speed of the electron beam being traced on the phosphor screen. Adjusting the sweep speed setting will affect what you see. If it is slow, the vertical lines may not even be visible.

Makes sense!  I'm sure you guys already knew this.

[ZaUcY]

I took my scope into an old college lab, and everything seemed pretty calibrated.  I only really knew what about one third of the pots did, but all of those pots seemed to be calibrated pretty good (according to reference scope and function generator).  The only thing I didn't like is that everything agreed in lower frequencies, but when I ran a 10Mhz signal, amplitudes did not agree.  It maybe had something to do with the front end capacitors, but I didn't really have access to them  

Scope works, it seems calibrated, I am a happy engineer