Repairing Tektronix 7603 Oscilloscope [Fixed]

[miguelvp]

This scope was giving to me a year ago with the caveat that if the original owner needed it he could borrow it at anytime.

I used it but it did have problems, the position knobs in the amplifier are almost non responsive, and to get any signal to display I had to set the Voltage Divider to 5mV on top of using AC coupling because otherwise the bottom of the wave never goes bellow the center of the screen and it's totally unresponsive if no signal is present.



I have since purchased a Rigol so I had the monster (full rack depth) just sitting around and even if it wasn't terribly dirty I left it as I received it.

Yesterday I decided to restore it, I gave it a good clean, and feed my Rigol test signal (they both share the same earth), at 5mV per division using an x10 probe it barely gets to 0.3 divisions (x50mV) so about 15 mVpp when it was supposed to be 3Vpp, so it's reading 200 times low. But at least it's clean



Feeding it the 4 Vpp calibration signal built into the scope I get 1.7 divisions at 50mV per division (because of the x10 probe) that's 85mVpp so it's reading around 47 times low (not good because the error is greater at the lower voltage).



I have a known working single channel amplifier (not a very clean one, that I got on a recent auction) so I did swap it and the scope is happy. Signal is from the Rigol 3Vpp, x10 probe at 50mV/Div (so really 500mV per division) I get 6 divisions so the output is 3Vpp.



So the problem is the module so I took it out and examined it, and something doesn't look right, the back mounting assembly is broken and I didn't find any of the pieces inside the scope, so probably the plastic piece was broken a while back before I got the scope and probably before the previous owner got it, since it was given to him, and he told me that the scope always worked that way and he thought that's how it was supposed to work.



I took off the shield from one side of the module and noticed that a cable is detached. Maybe someone dropped the module and landed in the back, and the protection armature broke off and got into the board and cut the cable at its junction.



Not bad, since that should be an easy fix, but I took off the shield on the other side to see what was going on in there.



Broken off cap lead 0.01 uF, totally missing cap (same value) and broken off inductor 3.2 uH wrapped around a 10 Ohm resistor as a single component.



I do have plenty of spares, but they are all soldered to the board I'm working with (tek part number 108-084-00). And I don't have broken spare modules that I could take those from.

After looking at the schematic I realized it wasn't just an inductor, it's an inductor wrapped around a 10 ohm resistor.



The cap on one side is totally missing (not found in the enclosure either), the other cap has a broken lead (hard to see in the picture. And finally the LR (or RL) filter is broken with only one lead attached and part of the resistor lost.

So I need to replace C397, C398 and LR397


The caps are Tek part# 283-0002-00
Description: CAP.,FXD,CER DI:0.01UF,+80=20%,500V
So I found what I think is a good match on E14:
http://www.newark.com/vishay-bc-components/d103z25z5vf63l6r/ceramic-capacitor-0-01uf-50v-z5v/dp/80K9840

Capacitance: 10000pF
+80%-20% tolerance
Disc
Radial
Dielectric Z5V
Lead Spacing    6.4mm
Operating Temperature Max 85°C
Operating Temperature Min -30°C
Voltage Rating 50V

@ $0.033 per, I got 100 for $3.30 total.

As for LR397 is Tek part# 108-0184-00
Description: COIL,RF:3.2UF(WOUND ON A 10 OHM,5%,0.5W RES)

I couldn't find a resistor wrapped with an inductor, let alone with a 3.2uF inductor.
So I'm planning to put a 10 Ohm, 5%,  1/4W carbon film resistor since that's what I have in hand, in parallel with this 3.3uH inductor from E14, bought 5 total @ $3 per.

http://www.newark.com/vishay-dale/ihd1eb3r3l/inductor-3-3uh-4a-axial-leaded/dp/53K4757

Would that work? is the 0.1uF going to make a difference, or the 1/4W vs 1/2W for the resistor?

Hmm, element 14 is doing maintenance so the links are not working at the moment.

[miguelvp]

Also it looks like I would have to fix this scratch in the ground plane, even if the +5 restricts the path as well, so maybe I should just leave it as it is.

[tautech]

For the inductor/resistor repair, have you an LCR tester than be set to L only and"roll your own"?

Strange looking scratch, in a form that looks like it's meant to be there?
Plugin PCB right? Does the other look the same?

[miguelvp]

For the inductor/resistor repair, have you an LCR tester than be set to L only and"roll your own"?

Strange looking scratch, in a form that looks like it's meant to be there?
Plugin PCB right? Does the other look the same?

I don't have another module like this one (yet, because tomorrow I might have one).
But I did look at the manual and you are right, it's in there.
Edit: the well known module is a single trace module and different than this dual trace one.

But it seems the resistor next to the inductor did get part of the blow as well, might be connected but unsure what is going on inside it, so I should replace it for good measure. Thanks for pointing that out.


Edit: they look like slightly different revisions, but the "scratch" is in there.


And I don't have an LCR tester (yet, because I got a Diligent Analog Discovery, on its way here, and if I can use it for a decent function generator I could find the value doing some math).

Edit: BTW, those are awesome solder joints (on the discrete components at least).

[tautech]

Looking at your schematic snippet, it's only a 5V rail LR filter, I wouldn't think the value was crucial but with these old Teks there was always a reason they did stuff like this. 

Wack a 10R in and try it.

[miguelvp]

I would have to dig into the 7603 manual to see what pin 1 and 8 do. The have senses and mosfets in the backplane to ensure things are ok. So without analyzing a lot on what is going on between the scope, the amplifier module and the time base module is hard to get the full picture.

Right now I'm just going for the obvious physical faults since it seem the problem is mechanical (as in damage) instead of electrical.

One thing I just noticed, can those ceramic caps actually handle 500V? or could that be a typo?
Otherwise I think I ordered 100 useless caps

[tautech]

Spotted that in your first post and thought it was YOUR typo. 
Saw you had already ordered them and didn't want to break the bad news. 

Na, they'll be fine, they're only across the 5V rail, what could possibly go wrong. 
Murphy is not watching is he?

[sdg]

Also it looks like I would have to fix this scratch in the ground plane, even if the +5 restricts the path as well, so maybe I should just leave it as it is.

 
It's not a scratch, it shows where on the PCB the +5v testpoint is...

[miguelvp]

 , in my defense I had an inductor hiding the tp.

[cncjerry]

Yes, the caps you bought are too low value, and yes, those caps are 500v.  Replace them with the correct cap, keep those you purchased as they are one of the most common bypass caps on earth. 

The 10 ohm with inductor is a filter/trap.  Just about any formed inductor you wire in parallel will work.  If you have some fine magnet wire, just take a large 10 ohm resistor and wrap about as much as you can around it soldering the ends to the resistor and it will be fine.    it is used as a trap to keep noise off the 5v. There are others in there, measure the dc ohms and make sure your wrappings are sufficient to match the ohm readings of others. Don't bother tracking down Tektronix parts.

You can buy those amps on eBay pretty cheap.  Just about all of them have broken knobs.  I mold my own using two part RTV mold products from TAP Plastics.

A large number of problems with that scope are related to poor connections on the back plane.  The second problem I've seen (aside from the knobs) are bad controls on the front, usually corroded.  It is common to have the input blown, sometimes the impedance matching caps short and the users even melt the soldering on  the input. I have two amps that just by jiggling the ac/dc/gnd switch will act the same as yours.

Those scopes are easy to fix if you have skill.  It is the only analog scope I have left, probably because it is so heavy.

[miguelvp]

Thanks!

BTW, I did find a replacement dual trace amp and I'm waiting for it, seems like it's in good condition but we'll see.

I also did find the tek part on a site that sold it as "used" for about the same price as what I paid for the replacement module.

I will still attempt to repair this module since there is not much wrong with it.

Pin A8 is the +5V in from the mainframe. It's only used on the NPN collectors feeding both the +/-trigger signals and +/-channel signals for both channel 1 and channel 2. So the scope must be working just on what little is coming from the NPN bases.



I did use paintshop pro to reduce the schematic from the manual so it fits the forum better. Also the same circuit applies to channel 1.

A1 is the sweep gate ping but that's directly connected to A8.


So I made my own LR Filter, thanks cncjerry for suggesting it.



I did use the 30 gauge and it happens to be red, I'm just not sure if it will handle the max current (typical 0.86A). I ended up with 7 feet and 2 inches to get the 0.80 ohms I measured on the rest of the similar devices. That is, after trimming a couple of times from my computed 7 feet 9 inches.

Maybe the probes and associated leads where part of my 0.80 ohm reading, a cursory glance at UNI-T UT71B manual states that the leads add around 0.1 to 0.2 Ohms and I didn't use the relative measurement, but no need to recompute it because the trimming went ok. I used the relation that 30 gauge copper has a resistance of 103.2 Ohms/1000 ft

For now the connection is mechanical, but i'm going to solder the inductor before I place it in.


On the Caps, what is weird is that when I first did look for the caps, I did a parametric search on mouser three days ago and I only got two hits and they were in the $2-$3 range per, I guess finding it so cheap on E14 I jumped the gun and missed the voltage or misread it, but being on the +5 V rail do they really need to be 500 V rated or would 50 V cut it?

In any event I think I'll go for the ones that I found before:
http://www.findchips.com/search/440LS10-R
or:
http://www.findchips.com/search/440LS10UH-R


Now I'm going to investigate what the broken off wire is all about.

Edit: Oh, and thanks for the TAP Plastics Idea, I do have friends with 3d printers but for something like this, I don't think 3d printing would work because I will think it will start pealing appart fairly soon.

[T3sl4co1l]

The values aren't critical, as long as they're in the general area.  Give or take a factor of two, kind of a thing.

An R || L will indeed do a fine job.  You might also get close with a toroid, if it's a #26 (yellow-white) core. (Joke being, they're practically resistors with a little inductance; as inductors go, they're pretty lossy.)

At higher frequencies, a ferrite bead would be used (being similarly lossy in the, usually, 20-500MHz range); but mind that they saturate with little DC current, so aren't always suitable for power rails.  Probably, something in the 600 ohm (at 100MHz) range would come closest, but may not have enough losses at the moderate frequency suggested by the values here.

Tim

[miguelvp]

Thanks, but I had to try to get it right since the others where spot on .80 Ohms, so using the same leads this is what I get.



But since the original is way thinner that means it can carry less current, so with the thicker wire I should be fine in regards of amps.



One thing is that the inductance most likely changes with a thicker gauge.

Once I get the parts, I'll solder them to the amp board and I'll check it's all good

Edit: forgot to embed the images.

[cncjerry]

Yes,  on the 5v line those caps are fine.

The first time I blew or broke one of those traps I looked all over before just making my own as you did.

As to the molding,  a little black dye stirred in is all you need to get close to the Tek color.   Mine have come out near perfect. I use a drill bit with tightened set screws to support the original knob in the casting mold compound.   I then tighten the insert on the same drill bit before pouring the harder mix in and it will stand up in the hole from the original drill bit.   You obviously don't tighten the set screws when doing the cast. If you are careful you can align the set screw holes in the mold with the insert.

I've made a bunch for all kinds of equipment.   I like restoring the old gear.

[miguelvp]

The broken-off cable is the +5V going to the rest of the scope, so I did fix that.

Also, I did purchase some modules of ebay and they got here today:



The one on the left is a 7A11 but had a missing pull tab.
I have a single channel 7A15A that is a mess, it's impossible to clean up the front, has a broken knob and cal shaft and even if it's a 100MHz module it's just a single channel, so I designated the module to be a donor module, it's first donation was the tab that went to the 7A11, I will have to use some whiteout and paint a 1 over the 5A.

The donor has the needed caps and LR, and I was impatient even if the caps should be here tomorrow (well today since it's 3am now).

But, because I was curious if the homemade LR filter would work, I went ahead and use it with two spec caps from the 7A15A, which now it doesn't have a +15V rail. Also the 7A15A was kind enough to donate the rear panel.



Here it is with the rear panel installed:



Tested channel 1 and channel 2 with the Rigol's 1KHz 3Vpp test signal and it's working fine. Position knobs work, AC/DC, well everything works! 





Then did a test with a 26MHz OCXO with 5Vpp DC coupled, you can see the donor LR resting on the probe BNC, since I'm about to take out the homemade one and place that one instead, so that the module has all OEM parts.



I took it all apart again and placed the final LR filter, I did measure the resistor that got a bit banged and it's reading what is supposed to be, so I left it alone.



Cleaned up the little flux that was left behind, closed it up and tested it with it's internal calibration test signal 4 Vpp at 1KHz.



So it's fixed, only thing left is the display knob which is the same as the one in the 7A15A (checked the part numbers), but that one is broken, but it has most of it. I do have a 2nd 7A15A with a good knob, so I might make a cast using that.
Edit: The knob on the 7A15A is the same as the Volt/Div ones on the 7A18, not the display mode one. But since I'm getting a couple dual trace amplifiers I could make the mold with one of those.

Now to calibrate it all, including the rotation because it's a bit off.

PS: the donor card now has the homemade LR filter but no caps yet.
Also I have two dual trace amplifiers coming in next week.

[miguelvp]

Funny thing,

The 7A15A doesn't have a filter on the +5V rail at all. I wonder if I could have gotten by just by bridging the LR filter.



Compared to the 7A18:


Oh, well, it's done anyways.

[cncjerry]

Yes, it would have worked without the filter, maybe a little hf noise in the trace, or an intermittent logic error, but you learned something along the way.  Also, someone paid to be smart at Tektronix put that filter in there so you did what I would have done and that is repair it spec. 

Someday when you least expect it you would be looking at the trace and see some noise, then fight for two days on some circuit until you figure it is the scope. I'm a purist when it comes to repair.

Glad it is working for you.  I like mine but I have five DSO scopes as well. 

[tecman]

The filter is a small inductor for noise on the +5 v feed.  The value is not particularly critical.  The resistor is mostly a "carrier" for the coil.  A jumper with some ferrite beads would likely work just as well.

paul