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7834 Missing Trace


Leo Potjewijd
 

On Wed, Aug 12, 2020 at 10:17 PM, Roger Evans wrote:

...
I think the C suffix means ceramic case
...
FWIW: The C stands for commercial grade, which has to do with the operating temperatures and possibly bias and offset values.


Roger Evans
 

On Wed, Aug 12, 2020 at 02:14 PM, <sdferg7@gmail.com> wrote:

Looking at your middle picture, it does seem that a good clean around the IC socket would be a good idea. It looks like Q555 may have been replaced and there are signs of corrosion on the surface of R570. If you get the 7A12 working well then the internals of R570 are probably OK. I would certainly pull Q555 and Q575 and give the leads and sockets some IPA.

Best regards,

Roger


sdferg7@...
 

Ok, I will try my best with cleaning the socket. Here are some pictures of the aftermath: https://imgur.com/a/wIVfYYx

The two capacitors near it don't seem to be in the worst health so I'm not sure what caused the gunk. A kind forum member offered to mail me some spare 741s they had, so I should be able to get that sorted out. They also informed me from the pictures that mine are plastic cased. I bit the bullet and ordered a 7A26 that should be in good working condition, but it will take a while to arrive.

Shane


Roger Evans
 

In the UK there are a couple of eBay sellers I know I can trust and I would probably have to pay $5 for two uA741s. I have no idea which sellers in your area are reliable. You can probably get five uA741s for a couple of dollars from China but you will have to wait and once in a while you will get fake or non-working parts. They will refund your money but not your wasted time.

I think the C suffix means ceramic case, they are not normally prone to corrosion and it would be well worth while looking closely at both sides of the PCB to see if the source of the corrosion is somewhere else nearby. Electrolytic capacitors are the most likely culprit and they will need to be changed and the board given a good clean. Check that the IC socket is OK and maybe scratch around the contacts with a sharp pointed tool.

Regards,

Roger


Dave Seiter
 

For TI, the P should be for a plastic DIP.
-Dave

On Wednesday, August 12, 2020, 10:06:19 AM PDT, sdferg7@gmail.com <sdferg7@gmail.com> wrote:

Well, unfortunately when I removed U550 and U570, I broke a leg off U570. Both are heavily corroded so I think I'll just replace both while I'm at it. The 7A12 manual lists both as replaceable by Fairchild UA741C. The closest I can find is a Texas Instruments UA741CP, not sure what difference the P makes. Would this work for my application? Any recommendations on where to purchase? Mouser.com has them for $0.50 a piece but shipping is $8.

Shane


sdferg7@...
 

Well, unfortunately when I removed U550 and U570, I broke a leg off U570. Both are heavily corroded so I think I'll just replace both while I'm at it. The 7A12 manual lists both as replaceable by Fairchild UA741C. The closest I can find is a Texas Instruments UA741CP, not sure what difference the P makes. Would this work for my application? Any recommendations on where to purchase? Mouser.com has them for $0.50 a piece but shipping is $8.

Shane


Roger Evans
 

Congratulaions on getting the plugin removed, it must be a great sense of relief when it comes out without breaking anything. If the actual edge connector was responsible for much of the stiffness then brush some IPA onto the PCB edge of the plugin and on to the sockets attached to the interface board.

While the 7A12 is out, remove, clean and reseat the two 8pin op-amps U550 and U570. If you have a probe lead you can use the 7A12 to invertigate noise on its own power supplies. Use the Invert setting since it gives a clean signal on the calibrator. You need to probe the two test points labelled TP +/-9V on the schematic and also the resistor/inductor combinations LR556, LR557 etc that provide additional decoupling. It is best to set the two DC levels as best you can before doing the noise probing and also make the DC measurements on U550 and U570. If you need to replace U550 / U570 they are very cheap and easily obtainable. It would do no harm to move R551 back and fore a few times before finally setting it, a dirty sliding contact in R551 could be responsible for the noise.

Hope the good progress continues,

Roger


sdferg7@...
 

Actually I just found U550 next to U570 now, there's a lot of gunk covering the silk screen label and I didn't spot it when it was in the scope. This plugin is definitely the worst looking part of the scope in terms of internals.

Shane


sdferg7@...
 

Huzzah, thanks Colin, I got it out. The thing is, I still had a lot of trouble even with your technique, so I tried it on my left timebase which is also missing the latch, and my right timebase without pulling the tab, and both came out very easily. After I had both sides open around it I managed to get a good grip on the plugin and had to use a lot of force to pull it out. It seems the majority of what was holding it up was the actual connector holding the board contact pins. I can see now that the entire pull tab and plastic strip along with the spring are gone in both the 7A12 and my left timebase.

My original plan was to remove the rest of the latch temporarily once I had them out, but it appears that has already been done. What else can I do to prevent them from getting stuck again?

Before I took it out I measured VR550 and the upper leg measured about -8.9V, the lower leg measured 0V. I checked R551 as well again and it is up to about 8.9V now. I was unable to locate U550, is it perhaps under one of the daughter boards?

Shane


Colin Herbert
 

I think I have the answer, having tried this with my 7623A with a 7A12 in the left-most bay:

With the bottom off and the scope on its right side, so you have as much access to the leftmost plug-in compartment as you can get, push an ordinary flat-blade screwdriver (having a blade width of about 6mm so that it has a bit of strength in it and won't bend too easily) under the plug-In below where the latch-knob would be. You don't need to wedge it in particularly tightly, just enough so that you can lever the screwdriver enough to wobble the plug-in up and down in its compartment (that is, left and right since the scope is lying on its side). While you do this wiggling, apply some leverage on the solid parts on the plug-in frame to encourage it to emerge from the front of the compartment. Unless there is something else holding it from moving, the plug-in should eventually pop out of the front of the compartment once its latch release bar has allowed it to do so.

When the plug-in is out, don't put it back in that compartment until you have sorted out the release mechanism so that it will work properly in future. Your next job is to find the person who put the plug-in into that leftmost compartment without being able to remove it easily. Find something big and heavy and hammer that numpty into some crevice where he will find it difficult to get out of, either that or ask for a price reduction!
Good Luck with the wiggling!
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of sdferg7@gmail.com
Sent: 10 August 2020 19:58
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7834 Missing Trace

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


Roger Evans
 

The 7A24 seems attractive because it is 400MHz bandwidth and dual channel. You may find it less useful than you expect because the inputs are 50 Ohm impedance and you will not be able to use the commonly available probes. You can make your own probes for 50 Ohm inputs but you typically get a 500 Ohm or 1kOhm impedance at the tip and they are rather a specialist item. The 7A26 is more generally useful and there are a lot around. eBay prices seem to be forever on the increase, check for completed items to see what they actually fetch.

There is definitely something wrong around the +/- 9V regulating circuitry. There should be -9V on the Zener diode VR550, you could try measuring that and also the voltages on pins 6, 7 and 10 of U550. I am surprised that the Invert position is still showing the same Y (lack of) sensitivity as before since adjusting the +9V regulator should also have changed the -9V supply. Fixing any problems that you identify is going to have to wait until you can get the 7A12 out of the frame.

You can find replacement plastic guide rails should you break the one currently holding the 7A12 in place. Qservice in Rhodes (Greece) is a well respected supplier and has part number 351-0181 advertised on his eBay site. If you were to ask around on the forum you might well find someone who has parted out a 7000 frame and would sell you one very cheaply (I don't have any unfortunately).

Regards,

Roger


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


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@...
 

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


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@...
 

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


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@...
 

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


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@...
 

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