Tektronix 2215A probe adjustment adjust

[Redcat]

Again a little fix on one of my oscilloscopes I want to share with you  .

It bothered me a little bit that the probe adjustment signal on my Tek 2215A was out of calibration.
It was only 940 Hz and 45% duty cycle instead of 1 kHz and 50% (0.5Vpp  is ok) so I read the manual if it could be calibrated.


(Edit: sorry, I forgot to post the Tek circuit  )

Of course it could not  , but it was mentioned, that R987 and C987 were responsible for the output frequency - R987 should be 120K but was a little higher.
I looked at the circuit diagram and found that it is a simple oscillator made with an OpAmp.
After looking around, I found an interesting website with some diagrams showing, how this circuit works and I decided to modify this circuit.

http://www.onegentleman.biz/Hardware%20Design/Op-Amp%20Oscillators/2013-Op-Amp%20Oscillators.php
(really interesting website if you are looking for OpAmp oscillators  )

I soldered R987 out (ok only the top leg  ) and first soldered a combination of a 33K resistor and a 100K pot in. With this I could now adjust the frequency. But it did not fix the problem with the duty cycle.
Looked again on this oscillator website and found a circuit that matched my needs. I had only to add another pot and two diodes.
With these diodes, the cap C987 can charge and discharge on two different ways (adjustable by the additional pot) and so the duty cycle can be set.

I have painted you a little circuit diagram in fritzing and made some photos (sorry I could only make a few on the fly with my phone).

Hope you like it and perhaps it helps you if you have an 2215 and the same problem (have to fix my 475 too - hmmm maybe there is a similar solution?).

Of course I take no responsibility  and perhaps some more experienced engineer can look at it, but for me it works fine as it is now (look at the meter readings and the last photo) .
(Rise time is around 9uS, is this ok so or have I totally deformed/slowed the signal?)







Nice greetings, Tom 

[valentinc]

      Hi Tom,


           Nice work in modifying that circuit... You could also done it with a little microcontroller (like the PIC12F675 or ATTiny13), if you wanted to use less components... You only needed the microcontroller and maybe a bypass capacitor...

           That signal I think was initially thought for probe compensation only, in which case it only needed to have sharp edges and no under or overshoot... I don't remember what is written in the Tektronix manual (I have done the adjustments for a 2235 some time ago), but I think that the verical and horizontal calibration were meant to be done with an external function or pulse generator... Correct me if I'm wrong...

           If you do more oscilloscope repairs or tweaks please share them

[Redcat]

Hi Valentin  ,
This way it was the easiest way for me to fix the probe compensation signal - but I would be very interested in how it could be done with a little ATTiny (have used some ATMega8 in some projects before with bascom many years ago, but there is not so much I could remember  .But I have still my selfmade parallel programmer cable ).

And yes, the signal is only for probe compensation (have a little signal generator circuit and my DMM for calibration), but it looks much nicer if it's accurate adjusted  . But you'r right.


I took the train today to visit my mother and stopped for some hours in a nearby electronics shop in the next bigger city to buy some more parts, because my next target is the probe compensation from my 475  . Already planned this. The signal is generated a little different, but it's way more out than the 2215 (900 Hz or so) and I want to adjust it too - so there will be more  .

Two weeks ago I had to renew the "triggered" lamp of the 475 (there is now a green LED in - this was not so easy to do  ), but unfortunately I didn't take photos. There is still the lamp of the "uncal" timebase not working (all the other do now), but I will not fix this at the moment, because it's not so easy to take apart.

Thank you again for your interest and nice greetings 

[valentinc]

     It would be very simple with the ATTiny... You just need to configure the PWM module: set the clock prescaler to select the frequency, choose between TIMER0 and TIMER1 (8 or 10 bits), although I'm not sure if ATTiny has 2 timers... and set the OCR register accordingly to obtain a 50% duty cycle... And that's it... And of course you could select the crystal frequency accordingly to obtain the frequency you want (to have some period that would be a integer multiple of the timebase setting)...

      I like the Tektronix 2000 series scopes... Personally have 2 of them: a 2235 AN/USM (military version) and a 2465A (which I scored from ebay for $150), which would not start... I repaired the power supply... 1 electrolytic capacitor, 1 or 2 resistor and 2 mains filter capacitors and the scope was working just fine... And it has continued to work... And no other components were changed since then...

[Greg65]

Very good,
I also have a 2215A that is in need of some attention as it's blowing the main fuse at the moment. I will be looking at this calibration modification when I get up and running again.
Hi Valentic, could you also possibly draw up and post a schem of how I could install the mod with the micro-controller as you described that it would be very easy to do. Either way would be ok for me though, I don't mind.

Thank you Redcat of you for sharing this.   

[Redcat]

Ooooh - now I understand  .
You would use the ATTiny instead of the scopes oscillator circuit (soldering it to the output wire).
Ok  . PWM didn't came to my mind. Yes this would be easy.
Of course I would do this only if the circuit was completely not working anymore, as it is a little modern and not using the original circuit (I would like to maintain it as original as I can).
But I have to say nice idea   .

I like the 2000 series too  . It has even a function my 475 doesn't have (trace separation for A and B sweep), and a nice sharp trace.
First I wanted a 2225 but I could get the 2215A for a good price and I dont't regret it.
The caps are always suspect No 1 when the scopes are not working and sometimes some transistors, and have even seen a broken resistor (but you couldn't see that it was broken in half) shown in a scope repair report.

Have another story for you:
Two weeks ago when I repaired the light on the 475, I had another interesting problem.
On one channel all Voltages with 2 didn't work.
After taking the input attenuator box apart I found out, that one resistor had the completely wrong value (I could compare it to the other channel).
But it was not broken. And after pulling it out and measuring and setting it back into, the channel worked again and the other didn't  .
Problem is: many components on the 475 are socketed (which is really nice - normally - I miss this a little bit on the 2215). And so the resistor.
It simply had bad contact  . ... again something learned 
Now all works fine again.

Your 2465A is really a nice scope ( and the 2235 too  - looks like the 2215 - but as far as I know the Tek scopes from 100MHz and above use many/all integrated circuits - this was the reason I bought one with lower bandwidth).

I would not really need my scopes for work, but I ever wanted one when I was a kid  . And now I have three of them.
I wouldn't want to miss them. I hope I have more useful things to do for them in the future.
And repairing and maintainig it's function is part of the fun for me  .

Nice greetings, Tom 

[Redcat]

Hi Greg, thank you  .

You could also solder the mod to a tiny pcb and make it a little bit nicer than I did  .

I also would like to help you with the fuse, but I'm not at home this week (but could download and look at the manual at least if you need help).

But as I said just a minute before - caps are always a good suspect, also resistors and rectifier diodes.
If you replace caps, take the good ones from a known brand  with the high temperature rating and the right voltage rating.
Be careful, when you touch the caps . I always have a little fear with these.

The power supply is under some metal shielding at the back. I have not opened it so far, but looked through the slots.

I'm really interested how this is going on. Perhaps you can post if you find something. Good luck for this  .


Nice greetings, Tom   

[Greg65]

Thanks Redcat for offering me some help on this as I will be looking for some help and asking many questions here at the forum to get up and running as I am a newbie around here. 
I have taken the scope apart already and have had a little look around inside, plus I do have original printed copies of the service and user manuals for the 2215 which should be very helpful.


 

[valentinc]

      Tom: Indeed, the 2400 series especially uses a lot of hybrid ICs (each channel frontend attenuator and preamplifier, Y amplifier, X amplifier, trigger, they are all implemented in a hybrid IC...), but from what I have read so far, I drew the conclusion that they can last a lifetime... The only problem is to be properly cooled (i.e. the fan working correctly and the case mounted)... There had been problems with the horizontal deflection circuit, there are discussions about that on this forum also... I have preventively put a small heatsink besides the small metal tab that the IC already has... but from what I have seen it doesn't become too hot (maybe a little above 40 degrees C without case, running for a couple of minutes, with the case and air flow from the fan it would be even cooler...)

      The 2000 series also have custom ICs in them, Tek branded, but from what I remember there are nothing special (there was an array of matched transistors in the front end, for example), but I wouldn't worry too much... I mean, if the scope is in working order when you purchase it, it's improbable that you will have problems with that ICs...

       Looking forward to see your 475 (I wanted to buy one in the past, but I just don't have enough space in my lab)

[Redcat]

Hi  valentin  , yes the airflow is very important. I have heard too, that these scopes schould not run too long without the cover.
Similar to a modern computer.
But sounds like you are experienced EE/user so I have no doubt that your scopes will last forever  .

An old scope like the 475 is nice to have (your scopes are way better and you wouldn't need one), you could also go for a 465/B (wanted this but similar to the 2215 I got another) or 485 if you have more space one time  . But more for collecting scopes reasons .
But I'm really surprised how good (at least some of them) work even after 30 or 40 years.
On mine the crt is a little bit damaged but I have decided now not to change it. It's still usable.

The trace is not as good as from a more modern scope like the 2000/2400.

And the pcbs are a mess - they look like drawn by hand and are full of old analog components.

End of the week I will be back at my home so then I will play with the 475 fix  .
By the way, my favorite locomotive (BR119) was build in your country  , just saw from where you are.
Have a nice sunday 


@Hi Greg, could you make a new thread with your power supply repair and take some photos?
As I said I'm really interested how you fix it .

Have you checked if the right fuse was in? Sometimes there are only little things.... .
I downloaded the manual this morning. Would be interesting if the problem is before or after the power switch.
And when it's after, on which of the different voltage lines.
Be very careful with the high voltages (maybe some caps are still charged).
Sad that it doesn't work at all, so you could go backwards but now I would suggest to look from the beginning.

Good luck and nice greetings, Tom 

[Redcat]

Hey Greg, just found some document. Might be useful for troubleshooting your power supply:
http://www.hakanh.com/dl/docs/troubleshooting_tips_on_2200_ps.pdf
At least you have some part numbers.

  Tom