Tek 7603 with noisy traces on the right vertical compartment only

[david77]

Hi there,

haven't been on here for quite some time.

But I've got a 7603 mainframe that has me scratching my head.

The vertical plugin in the left compartment displays nice and sharp traces, however the plugin in the right (Y) compartment displays a nice trace only in the middle of the screen. As you move the trace up or down it becomes quite noisy. It does display your measured waveform, it just imposes noise on it.

It is not the vertical amp, I've checked by switching both of them and also tried it with a third 7A18 that I borrowed.

At this point I fail to see how these symptoms are possible, as the output from both Y-amps goes straight into a switching IC U214. There is no individual discreet circuitry apart from two 50.5R resistors.
I'm resonably shure that U214 is good as I switched it with U324 from the trigger PCB and the fault is still the same. I doubt that both U214 and 324 would have the same fault.

Attached are pictures of the trace first around the middle, where it's sharpest. Then to the top of the screen and on the bottom. The "halo" you see around the trace is real, it's not an artefact of the phosphor or the camera.
The fourth pic show's the settings I used.
Note that I'm triggering off line frequency and get a stable waveform that way. This somehow leads me to think it's PSU related but at the moment I fail to see how exactly. I've recapped the mainframe not so long ago and all voltages are ripple free.
In the last pic you can see the waveforms have roughly 20ms cycle time, wich fits to our 50Hz line frequency.

If there's one Tek specialist out there who can make sense out of this, I'd really appreciate it.

David

[david77]

I've played around with the scope again today and noticed something.

When both vertical plugins are removed the trace for the right plugin still shows the fault, very hard to see but it's definately there.
After discovering that I pulled out the vertical interface board from the back plane. The fault then disappears, the right trace is as flat as the left trace.
I also again switched U214 (155-0022) with one out of one of the 7A18 amps and the fault remains the same.

Somehow it looks like the superimposed waveform I'm seeing is injected between the socket for the right plugin on the back plane and the vertical interface board, also located on the back plane. Very weird. Also: Can that explain why the size of the waveform varies with the vertical position on the screen?

[med6753]

After discovering that I pulled out the vertical interface board from the back plane. The fault then disappears, the right trace is as flat as the left trace.
Somehow it looks like the superimposed waveform I'm seeing is injected between the socket for the right plugin on the back plane and the vertical interface board, also located on the back plane. Very weird. Also: Can that explain why the size of the waveform varies with the vertical position on the screen?

While I'm not familiar with the 7603 back plane assembly I think this is an important clue. Carefully check the back plane for an open ground or bad solder joints. If the signal path from the back plane to the vertical interface board is some sort of transmission line (most likely) lead dress can be very important. Compare it with the left plug in.

It sounds like you are real close to the fault.

[singapol]

I've played around with the scope again today and noticed something.

When both vertical plugins are removed the trace for the right plugin still shows the fault, very hard to see but it's definately there.
After discovering that I pulled out the vertical interface board from the back plane. The fault then disappears, the right trace is as flat as the left trace.
I also again switched U214 (155-0022) with one out of one of the 7A18 amps and the fault remains the same.

Somehow it looks like the superimposed waveform I'm seeing is injected between the socket for the right plugin on the back plane and the vertical interface board, also located on the back plane. Very weird. Also: Can that explain why the size of the waveform varies with the vertical position on the screen?

The vertical position problem or some sort of switching interface may have some similiarity with this link:

https://www.eevblog.com/forum/testgear/tektronix-2465b-vertical-problem/

[Cyberdragon]

Be careful! There are rare transistors in there that are hard to find and expensive! 

Mine started acting weird like this before it totally died while using it. Been in bits for over a year now because I can't get parts.

[richnormand]

These tend to be difficult components to get exact replacements now.
In some cases you might be able to make decent substitutes with modern components depending on the function. i have done a few mounted on a header that could be plugged directly in the original socket.

You might "carefully" try some Freeze Mist on U214 see if the noise is temperature dependent. That would confirm that it is the chip that is failing.
Check and reflash all the solder joints around the socket and associated components too. If you have access to a second scope you might want to go up-stream and follow the noise to the guilty stage.
 
Last time I saw a very similar symptom in a scope (not a Tek) it turned out to be a  "sparkling" tantalum drop decoupling capacitor on the Vcc line next to the chip.

 

[david77]

med6753 was probably right. I ripped the scope to bits, cleaned and resoldered the back plane, cleaned contacts and plugs on the boards.
After putting it back together it now works again. So it was probably just some random grounding issue or a bad soldering joint.
Sometimes all you need is a second opinion.

I didn't want to do it because it's not very nice to disassemble this 7000 mainframe at that level, tons of wires and dozens of not very accessible screws make it a bit of a pain. But now it's done and it works again 

The semiconductors are relatively easy to get, I think. I had to replace one chip in it when I got it and that was easily available from the US. Not cheap, but available.
There's loads of Tek stamped transistors but most of the time you can find a replacement from the open market, sometimes something as simple as a 2N3904 will do. There's a list with replacement parts somewhere on the net.

[tautech]

There's a list with replacement parts somewhere on the net.

http://www.sphere.bc.ca/test/tekequiv.html

[JoeO]

There's a list with replacement parts somewhere on the net.

http://www.sphere.bc.ca/test/tekequiv.html

Here it is:
https://archive.org/details/tektronix_Tektronix_REF_book_text_searchable

[dave_k]

I didn't want to do it because it's not very nice to disassemble this 7000 mainframe at that level, tons of wires and dozens of not very accessible screws make it a bit of a pain. But now it's done and it works again 

Great result, and yes I agree it is not a pleasant job to remove the main interface board from one of those scopes. While it was out, did you happen to replace the 5 electrolytic caps? I had a 7603 where 2 of the original caps went short and was shutting down the +5 volt supply.

[david77]

Yes, I did replace all of them. That massive ground plane was a bit of a bugger, soldering wise  . Needed to heat it up from the top with the iron at full blast while sucking from the bottom side with the desoldering tool to get those pads clean.