Tektronix 7904 Repair

[BlownUpCapacitor]

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.

[BlownUpCapacitor]

Stupid me forgot to add photos:

Here ya go

First image is Z-axis

Second is plug-in interface

[BlownUpCapacitor]

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.

[BlownUpCapacitor]

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.

[BlownUpCapacitor]

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!? 

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?

[colorburst]

They can be replaced with modern Dupont connector housings. The idea and the attached photo are the courtesy of EB4APL from TekScopes.

[BlownUpCapacitor]

So I used colorburst's idea and cannibalized borrowed connector housings from my Tek 7613.

Don't worry, I ordered a shipment of new connector housings. The borrowed ones are just for now to make sure nothing shorts during testing.

Anyway, I have reassembled the whole thing back together again, no obvious shorts found this time from the z-axis voltage points. The PSU is still clicking.

I will investigate further...

Update: Resistances in the mainframe are a little high now... Maybe I forgot to connect something? I don't think so...

What is the likelihood that the grime build-up on the PCBs is causing some sort of short in the PSU?

Also, does anyone know good anode plug removal techniques? Last time I did that I shorted out half of an oscilloscope...

[BlownUpCapacitor]

I see a dead short across C1351 in the PSU. Unsure if the capacitor is shorted or not. Perhaps something else is shorted down the line. Investigating...

I can not desolder C1351 for lack of a powerful soldering iron. I have an old radioshack soldering GUN, but that's really heavy and bulky, and I can't lift heavy objects well.

Update: C1354 is of very very high suspect. Investigating...

Update 2: C1354 was indeed shorted. Like said, I don't have a very strong soldering iron and thus can't desolder things on multilayer boards very easily. So, I destroyed that tant with a hammer and the short cleared!

[BlownUpCapacitor]

I've made progress! The control illumination lights turn on no, well they flash in sync with the burst's of the power supply. So do the neon bulbs in the z-axis.

So we have power!

It is not longer a continuous consistent burst phase now, it is more of an engine trying to start, but struggling as you crank the car. It's trying to work!

Update: I smell something burning...

[BlownUpCapacitor]

I'm not too sure what happened, but without changing anything the thing powers on just fine now!!!!!!!!

The only problem I see now is NO TRACE. I am not sure if the CRT circuit is working. But power issues have been solved I belive. All the voltages are there. Out of spec, but there. Not sure about the anode.

I don't feel any HV on the screen. I need to investigate more...

Any ideas, suggestions, or anything would be of great help.

Update: I was trying to plug in a plug-in while it was on, and accidentally pressed the chassis against the exposed internals of the PSU, shorting out several things.

This didn't break anything though thankfully. It just went into burst mode. BUT THE IMPORTANT THING I NOTICED IS THAT THE SCREEN WOULD GLOW IN SYNC WITH THE BURSTS. This indicates life in the CRT.

[BlownUpCapacitor]

AHAHAHAHAHHAHHAA BAHAHAHHAA IT'S ALIVE IT'S ALIIIIIIIVEEEE I FIXED A $50 SCOPE!!! AAAAAAAAAAAAAA

AND ONLY IN THE SAME DAY AAAAAAAAAAAAAAAAAAAAAAAAAA

Update: It still has some problems such as no trigger for the very right compartment, no calibrator output, and no readout, but it displays a nice sine wave.

Time to reassemble everything but the case and troubleshoot more.

Update 2: All trigger signals are there. It's just one of the horz plug-ins that have no trigger. Read-out system is still funky. No power getting to the calibrator.

[BlownUpCapacitor]

WAHOO THE SCOPE WORKS 100% NOW!

Turns out that the calibrator and readout weren't working because I forgot to plug in a yellow connector on the A2 board...

But hey, it displays a crisp beautiful square wave from the calibrator!

There is still a little bit of wiggling in the readout so AC ripple is still present in the power distribution network. Probably because I didn't use low ESR tants? I'll add some more electrolytes and see where that goes.

The power supply still clicks 3 or more times before powering up still.

The brown spots you see on the scope are not dirt, but rust. I can't remove those.

[alm]

Congratulations on getting it working! That's pretty cheap for a 500 MHz scope . Did you get any plugins with it?

[BlownUpCapacitor]

Congratulations on getting it working! That's pretty cheap for a 500 MHz scope . Did you get any plugins with it?

Yes, I did! 4 of them. One 7B85 (no trigger), one 7B80, one TD-1085/U, and one 7A19 with option 4 installed. Pretty sweet I think.

Originally I was supposed to get it for free if I could get it to power up at the recycle center, but I wasn't able to, so I paid $50 and fixed it in my bedroom.

The next order of business is to fix the 7B85.

[BlownUpCapacitor]

While I was troubleshooting my 7B85, I discovered a really really really neat trick!

The TEK mini coax cables that are used to connect boards together can be tricky to probe, especially the female ports. The probe is too big and won't touch the center conductor.

But modular tek probes such as the P6109 or P6106 have a coax connector design that is the same diameter as the TEK mini ports used on PCBs. So I can just remove the 10x probe part of the probe and stick the probe connector into the port.

This has proven very useful when tracing the trigger signal in my 7B85.

[med6753]

The power supply still clicks 3 or more times before powering up still.

Heed this warning. You still have a short somewhere. Go back and make sure the PSU resistances for each supply are above the minimum amount. If they are OK then disconnect the HV from the CRT and see if the PSU comes up without ticking. If it does come up first try then you have the same issue I did. The CRT developed a leakage path to ground overloading the HV supply. The only fix is replacement of the CRT. I was lucky and I found one on Ebay for a reasonable price and it fixed the issue.   

[BlownUpCapacitor]

The power supply still clicks 3 or more times before powering up still.

Heed this warning. You still have a short somewhere. Go back and make sure the PSU resistances for each supply are above the minimum amount. If they are OK then disconnect the HV from the CRT and see if the PSU comes up without ticking. If it does come up first try then you have the same issue I did. The CRT developed a leakage path to ground overloading the HV supply. The only fix is replacement of the CRT. I was lucky and I found one on Ebay for a reasonable price and it fixed the issue.   

The scope powers up clicking when powering up after a long time of being turned off. But turning it on after it has recently been turned off doesn't trigger any clicking. What could this indicate?

Also, the scope is like I said, likely to have been rained on at the recycle center, and possibly have endured non-ideal weather conditions when it was still in use. I believe this was a US-army piece of equipment judging by the fact it has US-ARMY CALIBRATION stickers on it and the plug-ins. The most recent was in 1991.

The CRT inside looks like it has quite a bit of dirt on it.

[BlownUpCapacitor]

Resistances are as follows from the Z-axis board (all plug-ins removed):

+130v, 6.65k

+50v, 1.884k

+15v, 93

+5v, 58.9

-15v, 186.5

-50v, 3.06k

Update: Powering on without any plug-ins and with the HV anode disconnected, still clicks 2 times before powering up, unlike the usual 5 clicks with full plug-ins, and everything else connected.

I may want to mention that before, when first actually powering up, something was burning. I smelled the distinct smell of an electronic component somewhere in the scope releasing the magic smoke.

I hope that this scope doesn't have a problem with the CRT. It works really really well, and I love it. It's the first time I've ever had anything higher than 200MHz, nevertheless, a 4 bay 7904 mainframe.

Update 2: With everything else connected including the anode, but with no plug-ins, it clicks 6 times before starting up...

[med6753]

The burning smell you noticed was probably another tantalum burning up.

Given the recent results you got it suggests that you have one or more crapacitor with high ESR. I would recommend a complete re-cap of the PSU first, both tantalum and aluminum. Where the parts list calls for tantalum, use them. They typically have very low ESR. And don't be tempted to use cheap Chinese. Use quality parts from like Mouser or Digi-Key.

[BlownUpCapacitor]

Thanks. I'll take this advice.

My plan is to replace all tants with as close values as possible, except to have them rated for double the voltage.

Electrolytic aluminum stays the same.

And of course, I don't don't think ceramic caps need replacing.

The only problem I see with this plan is that my soldering iron is not powerful enough for boards with more than 2 layers. Like the PSU, which has 3 or 4 layer PCBs, I couldn't desolder that shorted tant, and so I smashed it with a hammer and soldered the new capacitor leads to the remaining leads from the old capacitor. I do not wish to do this to every single capacitor. I may have to invest in a new soldering iron. Or just wait 3 minutes for each solder joint to heat up...

[lasseo]

The only problem I see with this plan is that my soldering iron is not powerful enough for boards with more than 2 layers. Like the PSU, which has 3 or 4 layer PCBs, I couldn't desolder that shorted tant, and so I smashed it with a hammer and soldered the new capacitor leads to the remaining leads from the old capacitor. I do not wish to do this to every single capacitor. I may have to invest in a new soldering iron. Or just wait 3 minutes for each solder joint to heat up...

You will likely lift the trace off the board if you try to heat the joint like that with a conventional iron (or two). You'll want something with induction heating / Curie point control. Try and get your hands on something like Metcal MX-500 or MX-5200, or Thermaltronics TMT-9000s, or an Ersa, depending on what's available in your area. Metcal stations seem to readily come up second hand, Thermaltronics not so much. Perhaps you could even borrow one? For old leaded solder, I'd look for "low" temperature tips or again you will risk lifting the traces. The amount of heat these stations are capable of dumping to the joint is something to behold, so there's no need for the higher temperature settings. I recently acquired the TMT-9000s, and it's quite amazing what its 40 Watts can do.

[BlownUpCapacitor]

I would like to update that since the scope has been working near flawlessly. Everything is in spec. I haven't replaced anymore capacitors yet, but I would like to report a crackling noise. At first, I thought this was normal. My 2230, 7613, SC502, 5110 all make a crackling noise when powering up. But over time I noticed the sound of arcing inside the scope.

This is not present in any of my other scopes. The crackling happens near the HV anode line of the CRT. If I touch the line, it crackles a bit, and if I love it with a plastic spudger, it crackles a bit. I believe this to be the HV insulation breaking down over time,  and the performance of the dielectric degrading. I also noticed A LOT of soot on the cable, common with nearly every HV cable.

Not quite sure how to proceed from here. I do not want to replace the CRT because that would be expensive as one might imagine.

Also, the right vertical plug-in bay has some bad connectors. I plug in any plug-in and the amplifier plug-in would have minimal deflection, until a pull the plug-in out a little by pulling the BNC. Then it makes good contact and works perfectly with everything in spec.

Also not too sure how to proceed from here.

Also the electrolytic aluminum caps have seemed to reformed as most of the capacitors I've checked have regained their capacitance and are back to spec.

[factory]

Is any arcing visible in darkness? Cleaning might help a bit. If it's the cable, could some extra sleeving help?

Are replacement connectors available for these?

David

[MathWizard]

Did they add many protection circuits all over the place back then, or was it just the big stuff like the input/outputs, and maybe the main rails ?


IDK what to think of the 1 year warranty back in 1972, at least you had a schematic tho. Even today, IDK if I would take a 1 year warranty or, no-warranty, but I get the schematic for some scope or DMM/etc (but I can't share it).

[BlownUpCapacitor]

Is any arcing visible in darkness?

In a pitch-black room, there is zero arcing, except of course the neon lamps that glow at start up.

Cleaning might help a bit.

I'll try that, but what should I use to clean the HV anode line? It has approx 21kV going through it.

If it's the cable, could some extra sleeving help?

Are replacement connectors available for these?

How would I apply the extra sleeving? Also, someone on the official tek forums tried to do that but it didn't end up working, not matter how much dielectric grease or stuff he put on. Also, replacements would be scare.

[med6753]

I would try 100% IPA then dry completely with a blow dryer. Clean the silicone around the anode cap too.

Some sort of sleeving would also be worth while too. Oversize shrink tubing perhaps that fits over the plug?

[med6753]

This might also work too.

[BlownUpCapacitor]

Clean the silicone around the anode cap too.

I don't think that is possible on this CRT. This is because it looks like the anode cap is glued on using the silicone. I've never taken off the anode cap of Tektronix CRTs. I've taken off the anode plug many times, but never the cap. The silicone seems to be hard.

[med6753]

Clean the silicone around the anode cap too.

I don't think that is possible on this CRT. This is because it looks like the anode cap is glued on using the silicone. I've never taken off the anode cap of Tektronix CRTs. I've taken off the anode plug many times, but never the cap. The silicone seems to be hard.

All Tek CRT's are as you described and it's not removable but the dirt on the silicone can be cleaned off.

[BlownUpCapacitor]

Ah, that clears things up. Thanks.

Before applying some extra sleeving on the HV line, I think it would be best to clean off the soot on it right? Because soot is typically conductive. Would using wet wipes to clean it suffice? Or will the moisture damage the insulator? If so, what should I use to clean it instead?

[factory]

Over this side of the pond we have antistatic foam cleaner, that is what I use for cleaning dirt from most things, I was using Servisol 30 but that has been EOL'd by Kontakt Chemie which is more expensive, I've moved to using Ambersil Amberclens (all these are owned by the same parent company).

Seem to remember dirt causing the HV to not work in one of my 1960s HP scopes and that only uses 12kV in normal operation, cleaning the HV doorknob capacitor & lead cured it.

David

[med6753]

Ah, that clears things up. Thanks.

Before applying some extra sleeving on the HV line, I think it would be best to clean off the soot on it right? Because soot is typically conductive. Would using wet wipes to clean it suffice? Or will the moisture damage the insulator? If so, what should I use to clean it instead?

Well yes, that's what I was implying in my earlier post. Clean the the soot off the HV cable as well as the silicone around the anode cap. But don't use a water based cleaner. Use 100% IPA but check in small area first to make sure the insulation doesn't break down. Then additional sleeving if necessary. 

[BlownUpCapacitor]

As expected, something shorted again. While I was using my 7904 to test the rise time of a PG501, the thing started clicking; slowly at first, then gradually became more rapid until it acted as a complete short. But unexpectedly, after I while of just sitting there in disappointment while the PSU clicked away, the clicking became less frequent until it stopped. Then I saw the magic smoke escape the instrument. The scope still works perfectly fine now that the short has "cleared". I've ruled out any of the plug-ins as the first thing I did when the PSU started clicking was pulling out all the plug-ins.

The issue lies here; I can't find the part that failed upon a visual inspection. The burnt component should be easily discernible due to it likely being burnt to chalk by now. The smell doesn't smell like how I would expect a burnt tantalum to smell, all be it I've never actually smelt a burnt tantalum. Maybe I'll release the magic smoke from the shorted tantalums I got out of it before to compare the smell.

Anyway, the smell smells A LOT like a burnt carbon film resistor. I mean identical smell. I haven't looked at the PSU yet. Maybe it's in there.

Also, I've been stacking a TM504 unit on top of the 7904. Would this perhaps cause excess heat and also be the cause of this failure?

Edit:

Readout, Calibrator, Horz Amp, Vert Amp seem fine from a visual inspection. I don't suspect the failure to be in the plug-in interface boards as the scope still works perfectly fine. That leaves pretty much only the PSU. This gives me a problem. My soldering iron is weak, and can't desolder the stuff on the 7904's PCB unless I put the heat up really high.

Update:
I still don't see anything failed on the PSU. I still haven't checked the HV board and autofocus board of the PSU, nor the plug-in interface. Who knows? Maybe I just have bad eyesight. 

I'm tempted to follow the "If it ain't broke, don't fix it" saying, as the scope works perfectly fine. But I also don't want that failed part to cause problems in the future.

Also following the "thou shall check voltages" saying, all voltages seem to be reasonably OK except for the +15v rail. It is a bit low coming in at +14.79v, measured from the z-axis board to the chassis. This would indicate a shorted tantalum.

Would disconnecting certain boards while the scope is on harm the scope in any way? I want to do this to isolate the problem. My conjecture is to disconnect certain ribbon cables feeding power to certain boards such as horz-amp, vert-amp, plug-in interface, etc... If the voltage comes back up after disconnecting a certain ribbon cable, then I will follow that ribbon cable to the PCB it is feeding power to. From here I will examine the board under very very high scrutiny.

[alm]

Would disconnecting certain boards while the scope is on harm the scope in any way?

Quite possibly. I wouldn't plug in or unplug any connector that's not designed for hot plug (e.g. USB) while the instrument is powered. I'm pretty sure the manual warns against hot plugging plugins. Why would you expect less user serviceable connections to be different?

The problem with connectors not designed for hot plugging is that power and ground pins may not get connected in the right sequence, potentially sending large currents through circuits not designed for it. This is something that may go without problems 90% of time and blow something up the other 10% of time.

[BlownUpCapacitor]

Strangely, after I disassembled the entire scope to find the failed part, the short disappeared. I re-assembled it and the short was gone. The +15v rail was measuring 40.4 ohms, now it measures 90.6 ohms. Very close to the expected 90 ohms. I have no idea what fixed this.

Update: Powered it on, no trace. The display is completely dead. All voltages are there. HV is there, using a 7B53A and 7A26 plug-in for testing, setting bay compartment switches accordingly, no display. Full intensity, no display. I see no signs of CRT life. Beam find has no effect. Investigating...

Update 2: I can confirm Vert amp and Horz amp are working just fine. Plug-ins are getting power, the readout board works fine, calibrator too. I checked these by checking waveforms. I input a signal to the plug-in, the waveform appears on the CRT plates. Readout board is sending pulses to the vert & horz amp.

I haven't checked the Z-axis board... Unsure how to check. I'll RTFM now.

[BlownUpCapacitor]

Z-axis is blanking. I checked that on TP1883. I'm not 100% sure HV is there, but the anode wire clicks with HV static and is attracted to my finger.

Update: What the fuck is wrong with this scope? It works perfectly now?!!!! I didn't even change anything! CRT was dead before, but then after a while, it suddenly came back? I have no clue. I was just probing around when I looked at the 7904's display I saw a TRACE. Maybe bad solder joints? Thermal issue?? Bad capacitor? NO IDEA.

Maybe my scope is haunted. I still do not know what component failed and created smoke. I saw a lot of smoke come out of the instrument. But I can't find what failed. I guess I'll just follow "If it ain't broke, don't fix it" saying unless someone proposes an idea.

[djrm]

I recently repaired some 7804 plugins after the smoke escaped, it was the low value series resistors in the power feeds to the tantalum decoupled supplies to the plugin circuits which had burnt. the burnt resistors became open circuit and where difficult to spot. The rest of the scope continued to function when the resistors were fried. The cure was to replace both resistors and capacitors. hth David.

[BlownUpCapacitor]

The 7904 has two lamps in the A trigger source selector board/switch burnt out.

Replacing the lamps are pretty straight forward, my issue is that I don't know how to remove the trigger selector board. I RTFMed, but it doesn't mention anything for removing it.

Any help would be appreciated

Photo: https://groups.io/g/TekScopes/album?id=293799

Update: Figured it out:  https://youtu.be/1XTmbKJs9PY