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7834 Missing Trace


Roger Evans
 

That is a very illuminating picture! The two boards that you see edge on in your first picture, one of which is the A6 Logic Board, connect through to the main interface board (the one about 1cm or so to the left, it carries the four plugin connectors) via a whole host of gold plated pins and sockets. If you look at the A6 Logic Board schematic, <4> in the manual, then there are a run of connectors on the left and right hand edges of the schematic with two letter labels like FA, FS, GG,GE .... These carry most of the signals that are routed to the Logic Board from the plugins, in particular these pins carry the Z axis signal (FQ) and the A and B intensities (FV and FU at the bottom of the schematic). The board, probably isn't bent, just very badly misaligned, either through a ham fisted repair or a severe knock. So it is quite likely that the relevant connectors are on the side that is not inserted properly and have moved that crucial millimetre when you removed the plugin!

Reassembling these board pairs can be a pain since you need to align so many pins at once, Tek made it a lot easier by arranging for the sockets to be slightly oversized at entry and tapering down farther in. If you do decide need to separate the boards, make sure nothing is bent (it is not a disaster if they are, just a good bit more difficult) or broken, and reassemble. It would be very worthwhile to have a look at the corresponding place on the left hand side of the chassis to see if there is anything amiss there that could explain why the left hand vertical slot is not working.

Since the scope was working a day or two back you might get away with just trying to realign the A6 board in place and not worry about disassembling too much. You do run the risk of a moderate bump in future causing the board to move again.

Hopefully you will have a very rare and desirable scope working again fairly soon. Then it would be time to worry about the calibrator issue. To dispel any doubt about the calibrator and how the manufacturers of a DMM have interpreted RMS readings I would just measure a DC supply and see how far the trace moves when you move the input coupling selector between GND and DC.

By the way, the traces on storage CRTs are nothing like as bright as the standard CRTs, they should be very sharply focussed and as soon as the trace starts to broaden turn the brightness down.

Good luck with the repair.

Roger


Roger Evans
 

Minor addition - the highest voltages on these boards are the +/- 15V.

Roger


sdferg7@...
 

Ok, I think I'm almost there so hopefully I'll be out of your hair soon, haha. I got U4494 out and cleaned and reseated it. Then, with a liberal amount of wiggling, the logic board popped in place and all the connecting pins appear aligned correctly. It seemed at least one of the pins near the bottom center was not aligned previously. Now when the scope is powered on the and I slowly turn the intensity on one of the traces up, the trace will flicker and pop in and out on the screen until I get to about 1/3 intensity when it suddenly comes in steady and mostly normal.

If there is no signal the trace behaves as it should, but when I attach the calibration signal, the top peaks appear fuzzy and jump up and down about half a division periodically. The left time base also appears about half as bright as the right one. Here is an image: https://imgur.com/a/ARi67OV

Alt mode is working now, but when I first turned it on I would only get a dot from the right time base, but selecting either A or B would work as I described. Any advice?

Shane


sdferg7@...
 

For clarity, the image I took was at the same intensity setting on the knob, but now that I just turned it on after sitting for a while, the issue with the brightness seems to have corrected itself. The traces also no longer exhibit the popping in and out as I described before. They now come on smoothly with the intensity knobs. The problem with the upper peaks seems to come and go a little bit, but is much better now. However, when I try chop mode I get two dots, one for each time base.

Shane


Roger Evans
 

I was going to suggest that the brightness controls would get less noisy with use but you got there first. You can try to get a little cleaning fluid such as IPA into switches and variable controls that are interittent or noisy, but usually they improve a lot with use. Similarly have you tried selecting each of the calibrator voltages in turn, several times, to try and clean the switches. Again the horizontal (A, B, Alt, Chop) and vertical mode buttons and the small trigger selector buttons usually benefit from repeated use.

There may be a variable control on the calibrator which is also noisy, I can't find the calibrator in my manual! If you just move the trace up and down with no vertical signal does that also get noisy?

What about the left vertical slot, is that working?

Congratulations on finding the misaligned board, it was probably only just touching the contacts when you received the scope and removing the plugin just gave it a tiny jolt.

Regards,

Roger


sdferg7@...
 

The trace only gets fuzzy when I have the calibrator signal plugged in, and only the top half. The issue still comes and goes, usually after I change a setting, e.g. the vertical channel. With no signal the only time the trace flickers from moving up and down is when it goes through the trigger.

Unfortunately the scope only came with 1 vertical plugin, in the right slot, and the pull tab is missing. Is there an easy way to get it out without much disassembly? In the manual it seems the only way to get the black follower tracks out is to have their respective plugin removed to access the screws. When I select the empty left vertical plugin slot I get a flat trace slightly tilted clockwise, maybe 1 or 2 degrees.

I also discovered I was mistaken about the 2 dots, they actually appear to be the minimum and maximum of the right horizontal plugin from the calibrator signal I had plugged in. If I select chop with no signal I get 1 brighter dot from the right plugin, still nothing from the left.

I also did more testing on the scope's calibration. Last night after the scope was pretty warm I tried measuring a 1.6V battery at 1V/div which showed on the scope as about 3/4 of a division. This morning I tried the same with the scope cold and it came to almost exactly 1 division. This seems to follow my previous experience with the scope reading lower as it gets warm. The scope also seems to read slightly off on the frequency of the calibrator signal, each period shows as slightly less than 1ms. Of course that could be an issue with the calibrator.

Shane


Roger Evans
 

You should be able to remove the bottom cover from the scope with a few screws. Whether this gives enough access to pull whatever is left of the release tab with a set of pointed pliers I am not sure. You could start a new thread here on 'how do I remove a 7000 plugin with a broken tab?'

What vertical plugin do you have? If it has two channels then I imagine by now you would have checked both. Nearly all vertical plugins have a 'var' setting which gives a continuously variable Volts/div but this should activate the '>' symbol on the readout and make it obvious that the V/div is not calibrated.

You can put a timebase in the vacant left vertical slot and on auto-trigger it should produce a vertical line if you select the empty slot for the horizontal mode. The line should shift up and down with the position control of the timebase and you might see if the top end displayed any noise (not a very convincing test). If you can arrange for both timebases to trigger from the same external input (maybe the cal output but now you are only relying for it for a trigger signal) then you should get some diagonal lines and any noise would be a bit more obvious. This is all dependent on the left vertical slot actually displaying a trace.

The journey is not ended yet!

Roger


Roger Evans
 

Two more thoughts - does the vertical plugin have an 'Invert' button? That would put the noise at the bottom of the screen if the calibrator is the culprit.

Have you tried all the different voltage output from the calibrator? Is the discrepancy the same for each and for any different V/div settings where you can make a meaningful measurement on two adjacent ranges eg 1V/div and 2V/div for the 4V calibrator output?

Roger


Colin Herbert
 

To remove a plug-in with a broken pull-tab:

If you can grasp some of the pull-tab with long-nosed pliers, do so once you have put something to act as a pull-handle onto one of the BNCs (it doesn't matter which, but don't obscure the pull-tab too much and don't use a BNC Tee-piece; they simply pull apart. Something like a Normalizer is perfect). Pull the plug-in forward gently without pulling the whole scope off the bench and then give the pull-tab a pull with the long-nosed pliers while keeping pulling gently at the plug-in. You might hear a click as the retainer disengages and the plug-in start to move out. Once you have got the plug-in out of the scope, don't put it back into that or any other scope until you have sourced a spare pull-tab or disabled the retaining ability of the broken one, otherwise you are back at square one.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of sdferg7@gmail.com
Sent: 08 August 2020 17:08
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7834 Missing Trace

The trace only gets fuzzy when I have the calibrator signal plugged in, and only the top half. The issue still comes and goes, usually after I change a setting, e.g. the vertical channel. With no signal the only time the trace flickers from moving up and down is when it goes through the trigger.

Unfortunately the scope only came with 1 vertical plugin, in the right slot, and the pull tab is missing. Is there an easy way to get it out without much disassembly? In the manual it seems the only way to get the black follower tracks out is to have their respective plugin removed to access the screws. When I select the empty left vertical plugin slot I get a flat trace slightly tilted clockwise, maybe 1 or 2 degrees.

I also discovered I was mistaken about the 2 dots, they actually appear to be the minimum and maximum of the right horizontal plugin from the calibrator signal I had plugged in. If I select chop with no signal I get 1 brighter dot from the right plugin, still nothing from the left.

I also did more testing on the scope's calibration. Last night after the scope was pretty warm I tried measuring a 1.6V battery at 1V/div which showed on the scope as about 3/4 of a division. This morning I tried the same with the scope cold and it came to almost exactly 1 division. This seems to follow my previous experience with the scope reading lower as it gets warm. The scope also seems to read slightly off on the frequency of the calibrator signal, each period shows as slightly less than 1ms. Of course that could be an issue with the calibrator.

Shane


sdferg7@...
 

The vertical plugin I believe is a 7A12. I did check both channels and the variable buttons are pushed in. I could use them to temporarily calibrate the scope if I need to use it I suppose. When I put them in inverse mode the trace is no longer fuzzy, it appears sharp as it should. The calibrator discrepancy does remain consistent for each V/div setting and each calibrator output, at least for any given time.

When I put my right timebase in the vertical slot and selected the blank horizontal, unfortunately I did not get a trace or readout.

I'll see if I can make some headway with the missing tabs, I can't see any of the plastic piece left in the hole, they might have broken at the point where the spring attaches to the plastic.

Shane


Harvey White
 

I remember being able to pull up the plugin towards the top of the scope and then slide it out.  As has been mentioned, you can see and manipulate the remaining latch pieces from the bottom with side/bottom panels removed.

Harvey

On 8/8/2020 8:51 PM, sdferg7@gmail.com wrote:
The vertical plugin I believe is a 7A12. I did check both channels and the variable buttons are pushed in. I could use them to temporarily calibrate the scope if I need to use it I suppose. When I put them in inverse mode the trace is no longer fuzzy, it appears sharp as it should. The calibrator discrepancy does remain consistent for each V/div setting and each calibrator output, at least for any given time.

When I put my right timebase in the vertical slot and selected the blank horizontal, unfortunately I did not get a trace or readout.

I'll see if I can make some headway with the missing tabs, I can't see any of the plastic piece left in the hole, they might have broken at the point where the spring attaches to the plastic.

Shane



sdferg7@...
 

Ok, thanks for the tip Harvey. Still having a bit of trouble with it, doesn't seem to want to budge. I'll keep at it. Also I didn't know there was supposed to be a bottom cover, haha. Apparently my scope is missing that.

Shane


Roger Evans
 

There is information about the 7A12, including manuals on TekWiki at http://w140.com/tekwiki/wiki/7A12. The site has lots of other interesting information if you delve around.

Can you clarify that each channel shows noise on the upper part of the calibrator waveform and on each channel the noise disappears when you select Invert for that channel. The reason for asking this very precisely is that there is very little circuitry which is common to both channels and also affected by the Up/Invert switches. The only suspect I have found so far is the +/-9V supplies generated on board the 7A12 itself (Up/Invert switches between +/-9V to provide current into pin 12 of U225) but measuring these requires removing the side panels so that has to wait until you can get the 7A12 out of the mainframe. In fact the value of the +/-9V supplies is critical in affecting the gain of both channels equally so this could have some impact on the calibration being wrong and also on the warm up drift which is clearly way out of spec. This is just speculation until you can get some access.

The 7A12 is also slightly unusual in having the gain adjustment for each channel on the front panel so it is conceivable that a previous owner simply 'got it wrong' in the same way for each channel.

If we are happy that the noise is being caused inside the 7A12 then there is no need to play around with timebases in the left vertical slot.

Regards,

Roger


sdferg7@...
 

The behavior of the noise is as you describe it, at least in my testing. My 7A12 is actually missing the left cover, so I have access to U225 with it still in the scope. I measured (what I'm pretty sure is) pin 12, I will link an album. The positive setting gives me 0.1346VDC, and the inverse gives me -0.1057VDC.

Here is the album: https://imgur.com/a/6DNMQM3

Shane


sdferg7@...
 

Addition: The measurements I linked were using channel 1. In channel 2 the voltage stays at about 0.13VDC in both positive and inverse settings.

Shane


sdferg7@...
 

Ok, last one hopefully. I went through the supply test points and found that the +9V pin is supplying 8V. This voltage is also on pin 13 of U225. The test point labeled inv is supplying the 0.13V, and the one labeled pos is supplying about -0.045V. The rest seemed to be normal.

Shane


Roger Evans
 

Thanks for doing all those measurements, if my understanding of U225 is correct (the manual doesn't describe its internals) it is something called a 'Gilbert Cell', which functions as a current multiplier. So if one current input is steady and the other is the signal then the steady current controls the gain of the cell for the signal input. You can also change the polarity of the steady current and make the amplifier effectively become an inverting amplifier. Being current driven, its inputs are always close to zero volts and I am not sure how significant are the small differences that you see. If you can find R551 on schematic <5> you can try adjusting it to get the +9V back to its nominal value and see how much the calibration changes. The -9V is derived from the +9V via an inverting op-amp and has its own adjustment, R570. Adjust +9V before testing the -9V.

In order to get the 7A12 working more nearly to its spec you can use the front panel gain adjustments to try and get the two channels somewhere near their nominal value. If you can borrow a second vertical plugin that would help enormously in being confident that the errors are in the 7A12 and not the mainframe.

Any progress in removing the 7A12 from the mainframe?

Regards,

Roger


sdferg7@...
 

I attempted adjusting R551, however I reached its maximum and the 9V pin was at about 8.86V. I tried wiggling the board to make sure it was seated properly. The signal shown on the positive of both channels now shows about double the voltage and the trace is much noisier; the inverse setting seems about the same as before, as I haven't changed R570. For the gain adjustments I would just calibrate the traces to the calibrator signal, correct?

Unfortunately I don't know anyone personally who owns an oscilloscope, let alone a 7000 series. I've been keeping an eye out for plugins on ebay, I thought a 7A24 or 26 would be nice, but they seem quite expensive. I might settle for another 7A12 for now, I'd like to not spend too much until I know I can get her working properly.

I still haven't been able to get the 7A12 out through pulling and wiggling. I think today I'm just going to start unscrewing the bottom portions to see if I can get the latching mechanism out.

Shane


Harvey White
 

On the bottom of the plugin, behind the latch, is a spring.  That spring (in mine) is attached to a plastic piece which is essentially the stem for  the pull tab.  Push that white piece of plastic towards the front of the scope.

Harvey

On 8/10/2020 1:23 PM, sdferg7@gmail.com wrote:
I attempted adjusting R551, however I reached its maximum and the 9V pin was at about 8.86V. I tried wiggling the board to make sure it was seated properly. The signal shown on the positive of both channels now shows about double the voltage and the trace is much noisier; the inverse setting seems about the same as before, as I haven't changed R570. For the gain adjustments I would just calibrate the traces to the calibrator signal, correct?

Unfortunately I don't know anyone personally who owns an oscilloscope, let alone a 7000 series. I've been keeping an eye out for plugins on ebay, I thought a 7A24 or 26 would be nice, but they seem quite expensive. I might settle for another 7A12 for now, I'd like to not spend too much until I know I can get her working properly.

I still haven't been able to get the 7A12 out through pulling and wiggling. I think today I'm just going to start unscrewing the bottom portions to see if I can get the latching mechanism out.

Shane



sdferg7@...
 

Here is the bottom of mine: https://imgur.com/a/B0FAYRI

I'm not sure how to get to the spring or plastic without taking the whole bottom frame apart to remove the black guide rails. I can't access them from the side of the plugin either as far as I can tell.

Shane