Electronics > Repair

Tektronix 7904 Repair

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Hi, I have acquired a Tektronix 7904 for a fairly cheap price. I hope to restore it.

The scope is whole and is missing no parts to my knowledge. The mainframe has an early serial number (B0922#2) The hashtag means the digit is illegible. The red warranty sticker inside says the warranty expires in 1973. Tek usually has 1 year warranty so I think the scope was manufactured in 1972, near the very start when the 7904 scope was rolling out.

The insides are... not very clean. I see mud and A LOT of dust and particles. A spider even decided the scope was a good place to build its home.

Anyways, as for symptoms, the scope goes into "burst" mode, indicating a short somewhere.

All plug-ins have been removed

Resistance on voltage rails from Z-axis, chassis ground referenced show:

+130v ~ 6.63k ohms (bit high)
+50v ~ 314 ohms (low)
+15v ~ 7.4 ohms (very low)
+5v ~ 22.2 ohms (low? Not sure if it's lamp or not)
-15v ~ 84 ohms (low?)
-50v ~ 310 ohms (high?)

According to this: https://w140.com/tekwiki/images/f/f0/Servicing_the_Tek_7904_Power_Supply.pdf journal, the expected resistances are:

+130v ~ 6k ohms
+50v ~ 2k ohms
+15v ~ 90 ohms
+5v LAMP ~ 800 ohms
-15v ~ 100 ohms
-50v ~ 250 ohms

These are resistances with the PSU fully connected

Resistances with the PSU fully disconnected (except HV anode):

+130v ~ 7k ohms (high?)
+50v ~ 2.06k ohms
+15v ~ 7.4 ohms (very low)
+5v ~ 40.7 ohms (lowish)
-15v ~ 192 ohms (high?)
-50v ~ 6.87k ohms (high)

Expected are:

+130v ~ 6.6k ohms
+50v ~ 2k ohms
+15v ~ 90 ohms
+5v ~ 65 ohms
-15v ~ 110 ohms
-50v ~ 2k ohms
+5v Lamp ~ open

With P1896 removed however the blue one in the photos of the Z-axis board, resistances are as follows:

+130v ~ 12.4k ohms
+50v ~ 12.7k ohms
+15v ~ 283k ohms (short cleared)
+5v ~ 1k ohms (short cleared?)
-15v ~ 1.8k ohms
-50v ~ 66.2k ohms

P1896 leads to the plug-in interface board, which is very difficult to access.

This is all I have for now. Unsure how to proceed. I'll try some stuff later.

The manual on Tekwiki has some schematics that are difficult to read. It is also meant for a high serial number scope. The one I have is low, unsure how helpful that manual is.

I do not have any proper formal education on electronics. I only have 4 years of YouTube, books, and self-learning. I have no school for this entire week so I plan this to be a project of mine.

I have experience with these scopes though, repairing a 7613 in the past (though only bad capacitors). I have a few oscilloscopes (Tek 2230, 7613, Hantek DSO2D15), and a 5CT1N curve tracer to check transistors if need be. I think I am quite well equipped, but lack the knowledge to make any proper guesses for problems.

I suspect tants shorted, as they are common. The schematics on Tekwiki, as previously mentioned, don't have crisp scans on some schematics, so I can't trace down P1869 very well.

Stupid me forgot to add photos:

Here ya go

First image is Z-axis

Second is plug-in interface

Removing piggy back boards in main plug-in interface, and disconnecting all sorts of cables and such does not remove the short across the +15v rail. This means the short is 100% on the main plug-in interface board.

I was hoping this would not be the case. The plug-in interface board is the 2nd hardest board to remove after the PSU boards. Wish me luck and I hope I don't zap anything.

I believe I have found the short. C66 is a very large tant that filters the +15 rail. I will go ahead and replace it now.

Will replacing it with a 100uF 25v electrolytic capacitor suffice?

Edit: Why did Tek have to put the decoupling capacitors that come straight from the PSU in the least accessible location...

Update: C66 is indeed shorted. Replacing with 1980s NOS capacitors because that's all I have other than old and used Chinese capacitors from power adapters.

I replaced 3 tants on the A2 board. Now to reinstall the A2 board.

The +15v one had 2 ohms accross it. I replaced the other two while I still had the A2 board out.

I really hate these blue connectors. They always crumble to pieces! How the hell am I supposed to connect THAT back in!?  :rant:

Update: I got them in after a few minutes. I can do these tight tasks more easily than others due to my small hands. If you look at the picture, you'll notice all the wires are naked! [NSFW ALERT]

I fear they will short and cause a whole bucket of problems. Any ideas on how to rectify the issue?


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