Tektronix 512 Restore/Repair

This is a story set in an evening where I was in high spirits (literally), and that led me to another Tek scope. However, this is quite special as it is the 2nd scope model that Tektronix commercially produced – they started with 511, 511A and then produced 512 which is our scope for today.

Overview/internals video.

And here is the detailed restore/repair video.

 

However, if you’re the kind of person who likes to read, this post is for you! Here, I also cover points that I forgot to include in the video and document the bugs in the video. In the language of software, it would be called “release notes”. Yes, I am documenting the defects I knew but never acknowledged while releasing the software – just to get it out the door. (Sounds like someone you know?)

To start with, I would highly suggest you check the Tekwiki page for 512  @

http://w140.com/tekwiki/wiki/512

This is where I started my journey –

Right after reaching home, I opened it up to explore the beauty inside.

Note: The brown stuff that you see in the picture is dead electrons which fell off the switches/connectors. They accumulated inside the instrument over the years. Since there is no fan, these electron corpses kept on decomposing under high voltage and over the past 65+ years, turned into electron fossils.

The snake and its family living inside the scope was relocated to my truck and is now suing me for trespassing their home.

After a couple of hundreds of hours of cleaning, troubleshooting,  restoring, drinking and repairing – here it is.

No, I didn’t just wash the face, I cleaned the intestine too.

Note: The description and schematics are for my SN# range which is the later model. If you have an earlier SN, your schematic will be slightly different. Refer to the service manual or call 1-800-SPOONFEED.

WARNING :

THESE INSTRUMENTS CONTAIN DEADLY VOLTAGES INSIDE THEM, DO NOT ATTEMPT TO REPAIR OR RESTORE UNLESS YOU ARE QUALIFIED.

WHAT IF YOU DON’T CARE?

SIMPLE, IT CAN  KILL YOU.

WHAT IF YOU STILL DON’T CARE?

PLEASE DON’T CRUSH A NICE VINTAGE SCOPE WITH YOUR DEAD BODY, LET SOMEONE ELSE RESTORE IT.

(Jokes apart, these things are dangerous. Please do take care. It is not the +1500 V which can kill you, rather it is the +450 V or +250 V DC)

Assuming that you have cleaned it or you’re trying to restore it with the vintage pickle inside, you may wonder what’s next? Let’s talk about the easy parts first.

1. Capacitors – Yes, the bad crowd inside every electronic instrument.

The bigger and fatter ones, called electrolytic are damn nasty. Sometimes, you can rehabilitate them using an intervention and counseling mechanism called “reforming”.  Check my 575 blog post for details (I’m too lazy to link it here). Unfortunately, the electrolytic gang in this scope was too old, bald and bad. All of them were too stubborn and leaking. So I had to do the inevitable – replace it with modern Japanese Nichicon bouncers.

The second type, the larger population of paper bumblebees capacitors are nasty, just shoot them at sight and replace with modern “film” chicks.

Don’t forget to open the HV module and recap. (Pictures in Tekwiki)

Now, as in any instrument, there are a few hundred caps you could live without.  For 512, here it is – C303. It has a weird rating, don’t waste money on it. It is used for coupling external modulation signals to the cathode for intensity modulation. This is to watch 4K movies in this scope. Since I am a nice guy, I replaced it, but you need not.

2. Tubes – Yes, test them but do not squeeze their neck to test Gm or other extraterrestrial parameters like inter-molecular capacitance. Just confirm that there is no short and the filament is good. This scope is a 6AU6 farm.  I found a few bad tubes with open heater and shorted elements. I replaced all of them and buried the bad ones according to their faith.

3. Clean contacts, switches, sockets, and your neighbor’s kitchen – You can use any divine solution as long as it doesn’t kill the switch or your neighbor’s wife.

Now, that was the easy part which was done in a few days. How do we proceed to the next step?

This scope does not use the time delay relay because there is no solid-state rectifier. Do you wonder why? Check here – http://w140.com/tekwiki/wiki/Delay_relay

So the first step is to take out all the rectifier tubes so that the system will send only the heater voltage to the tubes. The tubes to be removed are highlighted in the red-blocks below.

Powering up with a variac should bring all the tube filaments to life, except the ones below,  because they are fired from -150V bias rail. Why? To avoid noise from 6.3 V AC filament. Of course, these are the tubes in the vertical amplifier.

If you were smoking the “right stuff” while recapping and cleaning, your scope will not smoke when you power-up. Let it sit and warm-up for  30 minutes. You can continue smoking.

Now go ahead and insert the rectifier tubes back in place and power it up with the plate voltages. If all goes well, your scope should shine and draw all your thoughts on the screen in 3D. However, I was not so lucky.

I am listing all the issues I have faced. My horoscope claims that this time is not good for me. However, depending on your place of birth and zodiac sign, you may face different issues. I hope you realize by looking at my issues that “Things could be worse for you 😛 “. So be happy.

Problem #1 – Power Supply – 250V and 450V volt rails will not adjust/regulate.

Yes, it is displaying absolutely irresponsible behavior. If I adjust -150 volt using the pot, these rails don’t move. They are stuck at 40% over the rated value. I tried talking to their parents, they don’t care either.  -150 V is decent, perfect and adjusts to precise value.

This is a very simple power supply compared to the 555 or other models. Check my 555 power supply explanation video in case you want to understand how these things are designed. Be warned, I may put you to sleep.

This is a stacked design, -150V reference which has totally different output voltages as later models.

The 5651 provides the reference voltage, which is used to generate stable  -150V. This is then used as the reference for +250 and +450 rails. However, the sampling of -150V is done via a precision resistor divider. To ensure accuracy. R217, R218, R208, and R209 are of 1% tolerance.

For me, all of these resistors were bad. Considering the season, I suspect that all the three resistors were following the CDC advisory for COVID-19 and decided to stay open a.k.a. practiced social distancing.

So I had to replace all three of them to get the power supply back in shape.

Problem #2 -No Vertical deflection – Beam is off the screen.

Yes, it is too shy to show up. It was a simple issue, the frequency compensation coil L6 was open. A quick test with a multimeter in resistance mode traced the culprit.

Here is the mug shot.

Are you wondering how to identify such issues?  In any analog circuit – tube or semiconductors – check bias voltages first, for all the elements:  plate, grid, screen, base, collector, emitter, destroyer, etc. The stage with incorrect bias voltage is where the problem lies in most cases.  Here, the plate voltage for V10 was not correct.

Since my spirits were too high that day,  my hand was stable and my brain was in slow motion I was able to trace the broken strand of wire in the coil and solder it together.

Problem #3 –Touch screen with vertical position control.

Vertical position control was very touchy, the trace disappears if I touch it inappropriately.  Not a serious issue, it is a well-known problem of a dirty pot. It was a bit extreme here as the pot was filled with uranium nitroxide  – UNO₂ 

(Just kidding, I have no idea how water got into this unless the pot was having a bladder control problem)

I had to clean it. However, the question is – why did the trace disappear if the pot is bad?

And here is the explanation. This pot controls the cathode bias for the 3rd stage of V-Amp. So if the bias supply goes away due to the dirty pot, your trace can disappear, especially if you are on an internal trigger. So watch out for this.

If you are wondering why this weird dude is bringing in triggering,  poking and rocket launching into this simple issue of a dirty pot, check R71 and R72. They are the trigger pick-off divider, sitting right at the edge of V-Amp. So if your V-Amp is dead, no int-trigger and no trace. Since you questioned me, I will not highlight those resistors in the diagram, find it yourself.

Problem #4 – Weak intensity (Failed to Fix)

Yes, you need to wait till night to see the trace. It’s a bit of an exaggeration but the trace is definitely weak. I confirmed all CRT biasing voltages and the blanking signal and they were all okay. Since it is the CRT and I don’t want to spend another $100 on a replacement CRT, I decided to try the poor man’s method – CRT brightener.

What is it? Nothing but an auto-transformer, which converts your 6.3 filament voltage to say  ~10V so that the cathode gets a bit more !hot!  – to compensate for her poor skin due to aging (coating) – producing more beam current a.k.a. intensity.

Why is the elevation to -1500V? I guess I mentioned this before, the heater and cathode need to be at a closer potential. Otherwise, it will be a spark-plug. So to avoid arcing every tube specifies a maximum heater-to-cathode voltage. For 5ABP7, it is 125V. In order to keep it low, elevate the heater to cathode potential, or close to it. This is why most filament transformers have a special winding for CRT filament. That winding is with higher isolation, as it is expecting an “elevated” potential with respect to ground.

Remember, there are two variants of these transformers – parallel and series. The series variant is used for TVs with series filaments, and the parallel variant is used for TVs with 6.3V parallel filaments. The brightener for series application is a step-down auto-transformer, whereas for the parallel it is step-up. Why? You need to present more resistance in the series filament string to get a higher voltage drop across and then pass it to the CRT. It is a bit out of the way, but just a warning if in case you follow my path.

It is worth noting that this is not a permanent solution, rather it helps extend the CRT’s life by a few hundred hours of usage.

For me, this attempt failed to yield any results in brightness. I decided to call off further attempts to rejuvenate the CRT and live with the existing brightness. It is not terrible, but not great either! I wonder why the brightener failed to make any change for this CRT. Either the CRT is too weak, or there is something else to do with its construction.

Note #1 – Blanking

The blanking in this scope is very special.  It is a CW signal, or modulated signal to allow coupling from low voltage blanking side to the high voltage CRT grid and allow a blanking time of several seconds. These are things explained in Tekwiki, so I’m not going to duplicate the information here. In summary, it is a CW AM receiver on the CRT grid side. No, no BFO if you are ham and asking for it.

The 6AL5 de-modulator or rectifier is elevated to 1500V. Be careful.  Also, the coupling transformer is a very interesting construction. Engineering creativity at its best, I should say.

If you like to see the CW blanking signal which goes to the de-modulator for blanking, here it is. This is rectified (demodulated) and the -ve signal pulse is applied to the CRT grid.

Note #2 – Regulated plate and screen supply

V11 and V12 in the vertical amplifier are voltage regulators. They are used to supply a low-level plate and screen voltage to vertical pre-amp, CF and 3rd stage. The designers wanted a low impedance low voltage source. Instead of doing a cheap resistor divider which will have temporary compensation issues as well as high output impedance, they used a dedicated triode from a 12AU7 to supply plate/screen to each section of the V-Amp.

So that was it. The scope came back to life. Unfortunately (or fortunately, for me) it turns out to be another heavy iron box, which I have no place to keep, neither can I forget it and move on. I will try carrying it with me for the next blind date.

Video Bug#512-1

In the overview video, I have explained all controls, except one – The CAL output switch. This switch selects the CAL signal to be either used internally or coupled to the UHF connector for external use.

Also in the troubleshooting video, this is the switch which caused no input into the vertical section. All I had to do was to clean the contacts with contact cleaner.

 

 

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Disclaimer: I have no commercial affiliation with any person, object, institution, alien, tube or entity mentioned here. This is purely for education and entertainment and the sole reason for this entire circus is that the medicines are failing again in my brain. If you try to steal parts of the content or any tubes from here, your car won’t start the next day.

P69 WARNING: The restoration activity above creates chemicals known in the State of California to cause arguments and fistfights with your partner,  and can harm your reproductive exercise.

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