My Tektronix 310 "Basket Case" Project

[Smoky]

I found a Tektronix 310 oscilloscope at a garage sale a few weeks ago. It's in really bad shape and was going to be thrown out.

Even though I'm a beginner in the electronics hobby, I know we're going to bring this thing back to life!

I'm starting with a good cleaning since it's full of cobwebs and over 60 years worth of dirt.

The serial number stamped on the faceplate is 1499, and from what I can see, most of the tubes are dated 1956.

Let the fun begin

[xrunner]

Cool deal Smokey, please keep the thread going so we can follow! 

[Smoky]

Will do Xrunner!

I didn't power-on the 310 because I could see bad tubes through the side panels. The tops of some of the tubes are white and two were cracked.



While I was slowly in the process of removing the heavy layer of "mold" from all of the wires, and when I made it to the HV board area, I noticed the same "white" area on a "5642" rectifier tube!

[Smoky]

While I'm moving along the interior of the 310, I'm checking resistors as I go.

Several of the resistors just on the HV board are out of tolerance. Most have silver bands (10%).

Now that little black resistor, that's a 1.5 Meg Ohm 1% type, it still measures 1.495 Meg Ohms on my DMM after all of these years!



What would be the best choice of replacement resistor for the bad Allen Bradley's that I find?

[Smoky]

On Wednesday night, I started to snip all of the wire loom wrap to release the tightly-gripped wires.



This allowed me to clean all of the crap easily from the sheathes and bring out all of the colors. I used a small bucket of hot water with dish-washing soap and a little bit of Clorox bleach along with small brushes. Now I can easily trace wires around the chassis.

I removed the piano hinge from the back of the case which made access even better.

I have 1/2 of the scope cleaned very well and you can see in the last picture where that transition is.

[Smoky]

I removed the cathode ray tube to gain more access too. It's a Dumont 3WP2.

The heater pins are 1 and 12 and I do get continuity.

[DaJMasta]

What would be the best choice of replacement resistor for the bad Allen Bradley's that I find?

They're just reasonable power carbon composite resistors, so virtually any comparable resistor will do the job.  I'd probably go for thick film or metal oxide 5%s, maybe avoiding wire wound in case there are parasitic inductance issues... maybe look for the ones that best fit the look you want.

Worth mentioning that it seems like a lot of the brown half watt or 1W Allen Bradley mil spec resistors drift up over time, so a huge number of even the old stock ones won't be their original value, so I'd probably avoid replacing them with direct replacements.

[Smoky]

Thank you DaJMasta.

I pulled a couple of resistors to test from around the scope. Here's a 2 watt Allen Bradley 33K testing high.

[Smoky]

I was just sent the 310 schematics from 1955 through 1957

Right-click the picture to enlarge them even more. I uploaded the schematics to Imgur.com to save space and to guarantee that they'll be around for a while. These photos are huge!

Block Diagram



CRT



Heater & LV Power



Calibrator



Preamplifier

[bd139]

Thanks for writing this up so far. Good to see someone taking care of such a nice instrument 

As for the carbon comp resistors, they always go high. Haven't found any vendor that isn't vulnerable to that...



I usually replace carbon comp with the blue TE connectivity metal oxide resistors.

[Smoky]

...and the rest of the schematics 

Again, just right-click the photo for an even larger view.

Horizontal Amplifier



Vertical Amplifier



Sweep Generator



Sweep Trigger



Sweep Timing Switch



What is so nice is that the schematics also identify the wire color in the Low Voltage and CRT circuits!

[duak]

If you replace the resistors with metal film types, make sure their voltage rating is not exceeded.  eg., a 1M0 1/2 W RN65 would dissipate 1/2 W with 707 V across it, but it has a maximum voltage rating of 350 V.  This is most likely with the parts in the EHT and around the CRT.

Looks like it's cleaning up well.  Interesting project - hope there's no show stoppers.  I don't think there's anything too biodegradable (ie. self-destructing) like in some modern equipment.

[Smoky]

Thank you Duak!

 I found this chart that converts Allen Bradley resistors to Vishay resistors.

 It has the voltage information in each column so it's easy to see if that rating is equal or greater.

 I believe that the Allen Bradley EB (1/2w), GB (1w), and HB (2w) series resistors are in the Tektronix 310.



...and here are some new tubes I picked up this morning at $1.50 each:



And the new 5642 rectifiers tubes under $2 each too.



My plan is also to bypass the Selenium rectifiers with silicon rectifiers. I have these 1000VDC Silicon bridge rectifiers from a previous project and I plan to use them. I'll probably fabricate an aluminum heat shield/protective cap/cover for them too. Maybe even mount some of the old rectifier parts to keep the look original. Here is a picture of the Selenium rectifiers in the 310 and the new rectifiers:





...and these new tubes have a radiation symbol on the tube and box!

[Smoky]

This weekend I'm going to spend a little time to put together a parts order for Digi-Key and Mouser.

For example, here is C722, a 3Kv .0068uf film capacitor from the HV circuit board in the CRT circuit, being tested on the Sencore Z Meter 



...and a sample of some of the plastic tubular .01uf caps (C612, C613, C639, and C671). All from the LV power supply circuit and reading 5 times higher!



...and here's what I'd call The Big Kahuna, C315. The 0.1uf cap on the UHF input jack of the preamplifier. 5.43uf!



**Parts are starting to come in. The only resistors I had a hard time finding new on Digi-Key, Mouser, or Newark are R721 on the HV board, R722 on the Intensity pot, and R742 on the Focus pot. These are 2 Watt 5.6 Meg Ohms and a 3.9 Meg Ohm. I'd like to thank Kb4dmf on Ebay. He sent me NOS Ohmite and AB resistors better than 1% tolerance.





The original P6017 probe is in really bad shape along with a severed cord. It connects to the oscilloscope through an Amphenol UHF input jack. I recently bought a new P6006 probe from a seller in Chapel Hill, NC for $10 but it has a BNC connector on the end of it. The original UHF input jack mounts to the back of the faceplate of the oscilloscope on two threaded studs. I searched Ebay and I found two new BNC input jacks with the correct mounting flange! It's a Tektronix part number but I'm told they're made by Bendix (131-0279-00)   







I also read that these 5726 tubes are the same as a 6AL5, so I bought two.

[Smoky]

I was able to accumulate enough parts to start rebuilding the 310. I started last night with the CRT circuit and its printed circuit board.

All of the resistors are new Vishay PR01/02/03 and the Meg Ohm are MRS25, all of which meet or exceed the voltage ratings of the old Allen Bradley resistors. The only original resistor that tested excellent was R740 (1%, 1.5 Meg Ohm), which I left intact.

The ceramic capacitors are C0G or NP0 and the film caps are metallized PP.  The original .0068uf caps were not available in a 3Kv version so I went to a PP .0082uf Panasonic, which I'm told by the Tek Museum folks, is just fine.





The reason why I soldered some of the leads of components over the grommets and onto the traces is because the brass grommets are known to lose contact since pressure is all that keeps them in place. I flowed solder to bridge connections that were close. I'm now moving on to the remaining portion of the CRT circuit within the chassis.



And what turned out to be a lot of fun was shopping for vacuum tubes

They came from all over the place. All of the sellers were able to test the tubes and match their internal triodes and health. It was cool to find Tektronix "selected pairs" like in the 6BQ7A and 6AU6 types. Other tubes like the 6AN8 were a little harder to find and more expensive. Who knows how all of this comes out in the wash, but I just wanted to have a variety of tubes to experiment with.

[george.b]

...and these new tubes have a radiation symbol on the tube and box!

https://www.orau.org/ptp/collection/consumer%20products/electrontubes.htm

[SeanB]

When replacing the selenium rectifiers please add a series resistor to the one lead, so that the capacitors it is driving do not get too much inrush current and also have a lower ripple current. Around 33R 2W works for most small ones in valve equipment, though larger higher current ones need a lower resistor value.

[Smoky]

Thank you Sean!

Here is some good news. Soon after the 310 was on the market, the 310A followed. It is the same oscilloscope but it was built with Silicon rectifiers instead of the Selenium rectifiers.

I was able to print the schematics to the 310A's rectifier circuits and it shows the added resistors and values in the legend. They list resistor R601 as a 10 Ohm 1 Watt. Not only that, the capacitance changed for C601. It went from one pair of 40uf caps in parallel in the 310 to three pairs of 15uf capacitors in parallel in the 310A.

Here is a shot of the "310A's" Silicon rectifier circuit surrounding C601:

[Smoky]

Here is how I rebuilt C601 today.

I first gutted the 2 x 40uf can.

I purchased six Nichicon 18uf 450v capacitors for the new assembly.

In the 310A schematics, C601 is listed as -20%/+100%, so I imagine, going from ~90uf to 100uf is just fine.













Ok, I'm now on the Selenium rectifier/power transformer side of the chassis. I'm ready to tackle the removal of the Selenium rectifiers and following the color-coded wires and reading the schematics. Again, it is so nice to have the schematics noting the colors of the wires. It's making tracing the circuit so easy with all of the jumpers everywhere. I found a broken solder joint right on the back of the power socket, it happens to connect with the thermo-cutout switch of the scope!



...I also carefully tagged all of the wires and then removed the old rectifiers

[Smoky]

Next up, I went to work on fabricating a mount for the new Silicon bridge rectifiers.

I trimmed a length of aluminum angle and carefully milled a "rabbet" on one edge. When the heat sink compound is applied to the backs of the rectifiers, this ledge will keep the assembly from spinning when they're bolted down.





Here is the rectifier mount mocked-up in the chassis:



I'm also going to make a small cover too that will screw to this mount and hide the wiring connections. I'll probably drill random holes in it too to add to the heat-sink-ness

The hole just above the mount that isn't being reused will be enlarged and a rubber grommet will be installed so that wires can run through.

[james_s]

Nicely done, that all looks really clean, especially the capacitor.

Nice to see an unloved instrument like this coming back together. I probably would have parted it out and scrapped the carcass, I like the outcome you are achieving much better.

[Smoky]

Thank you James!

Just moments ago I removed the next large can capacitor, C630 (150uf at 250v). This can has its case grounded to the chassis.

It wouldn't test on the Sencore capacitor tester so I connected it  to a Peak ESR+ meter:



I have a brand new Nichicon UCY 180uf at 350V to stuff in it    ...it measures 160uf on the Sencore, a little more on the Peak:



So I cut C630 open and installed the new Nichicon capacitor. This cap was easy, the Nichicon leads reached both the (+) and (-) terminals outside of the can through small drilled holes. I then found a plastic sleeve and glued it inside of the cover:



I next put glue around the perimeter of the base and closed up the can. All that is visible is a fine hairline along the base:





C660, the 40uf at 450v capacitor, got the same treatment too:

[Smoky]

Hey Guys, I'm comparing the early 310 schematics w/Selenium rectifiers to the later 310A which uses Silicon rectifiers.





If you notice in the legend of the later 310A schematic that capacitor C661 (.02uf/1400v) was added just after the transformer and before the Silicon rectifier.



I read that Silicon diodes can create RF/EMI interference by the way they "switch" in circuit. I'm guessing that C661 is being used as a low-pass filter.

The .022uf CDE film capacitor I bought for this purpose is rated at 2KV DC/500VAC.



I did notice that one of the ceramic terminal tie strips is cracked, but it seems to be fine and stable since there are so many wire "ties" in that area. None the less, I put a "want ad" out for a used one.

*Update* Thanks to T3sl4co1l for sending me a mint-condition 9-notch ceramic strip





Also, I found two original Allen Bradley 2.2 MOhm 1/2 Watt resistors installed in place of, what should have been, a 2.7 MOhm (R506) and a 4.7 MOhm (R519) in the "Calibrator Multivibrator." I bet they didn't have them at the time, so they went with what they had in stock? Well, I installed the correct values as read from the schematics (these resistance values are listed the same in both the 310 and 310A schematics).



The new resistors are starting to go in now too!





All of the original black 1% precision resistors are still spot-on or maybe reaching just under 2%, so they were tested and left alone.



I like jumping forward and checking components around the scope that I haven't touched yet. Get a load of this one, it's C321, a 275uf capacitor on the Preamplifier PC board 

[Smoky]

I'm digging into the heart of the beast 

I removed the Vertical Preamplifier board today to get to the hard-to-reach parts. All of the old resistors and capacitors were replaced with new. The old parts are so out of whack!







The only part yet to be replaced is the Selenium 100mA rectifier plate (SR321). I compared the Early 310 schematics with the newer 310A schematics. The Selenium rectifier plate was replaced by a Germanium diode (T12G, AKA 1N270) in the newer version 310A scope.





I found two NOS Tektronix T12G diodes (part# 152-0008-00) on Ebay and here they are



Now, within the 310 and 310A, there are only two resistors that were required to be a "matched" pair. They come off of the preamplifier board (R320 and R401). I put yellow dots on them.



So I decided to pull and measure them. They are marked as 1%, 1 MOhm resistors. I tested each one on my Fluke 45 multimeter. Each resistor was given one minute to settle down. One of the resistors measured 1.0144 MOhm and the other 1.0015 MOhm.

I purchased four of these Dale .1% CMF60 resistors and put them through the same test. Two of these resistors settled at 1.0006 MOhm! They're going in



Here are the new Silicon rectifiers along with isolated turrets to mount the new 10 Ohm 2 Watt resistors. New PTFE 20 AWG wire is being used to connect the rectifiers to the rebuilt can capacitors. In the background, I bought PTFE 22 AWG striped wire to replace the old wires in the transformer area. I also used the spacers from the old Selenium rectifiers to lift the new rectifier mount off of the chassis.



With the hard-to-reach soldering done beforehand, I installed the rectifiers. I drilled out two of the four old rectifier mounting bolt holes and installed rubber grommets to pass the wires through. Next is to wire the transformer back up.



Most of the low-voltage section is now back in place with new wires attached. I wanted to repeat what I said about the wires that attach to the back of the power plug socket. I found one with a broken solder joint. It's because the plug is made out of heavy brass bar stock. Tektronix used very little solder. I cleaned the brass on the bench and pre-tinned the brass with ~740 degrees at the solder iron tip. The new joints will be rock-solid for another 50 years



After the large resistors were re-wired, the rebuilt capacitors were installed. The new Low-pass film capacitor (C661) was also installed just above the topmost rectifier too.



I'm over to the other half of the oscilloscope now, and working around C250 was no joke. Two soldering irons would've been needed to lift it from the ceramic strip without breaking it, so instead, I carefully removed the old Allen Bradley resistors from below and installed the new resistors the same way.



When I got to V150, a 6AL5 tube, I noticed pin 7 wasn't soldered to a lead tying both pins 2 & 7 (the plates of the tube) to capacitor C115. Maybe it had an intermittent problem? I don't know, but it's soldered-in now. C115 is a 5% 180 pf Mica cap, so I measured it. It came out to 200pf, a little high, so I purchased a new 1% Mica. One of the NE-2 neons nearby had rotted a leg off, so I replaced both neons with new ones. David Hess suggests using neons that have a little bit of radioactivity so that they have a consistent strike voltage whether in the darkness or the light in these circuits, so I did.







Here's the before and the after of this "quite-busy" Sweep Generator section of the scope

         

...and here's the rebuilt Vertical Amplifier portion. All pairs of resistors on the schematics are matched (under .2%). I really like these new Vishay resistors. Their smaller size fits these tight spaces much better than the AB's.



I rebuilt my Sencore Z-Meter last year. This is the most accurate capacitor it has tested so far! A Tektronix can (.01uf/.1uf/1.0uf)!







Now on to the cleaning, rebuilding, and lubricating of the switches. Can you see the split in that large tube capacitor



...and now the Time Base "trigger" switch is rebuilt and installed into the sea of new red resistors. I flushed and cleaned all switches with Deoxit F5 too. The contacts shine and the movements are smoooooth. The Time/Division switch is next with only just a few caps and resistors left to test/replace. I see the light

[Smoky]

Things are going well with the oscilloscope rebuild. I just finished the install of the replacement input jack. I went with a new BNC jack versus the original UHF one. I even use dark red paint from Testors to dab over the reinstalled nuts. The Preamplifier board goes in next







I also went back to the Preamplifier board to install the Germanium diode that is replacing the original Selenium rectifier plate. I bent the leads so that it would be stable once soldered to the copper traces. The anode points to the positive side of capacitor C321, as shown in the schematics.