My oscilloscope needs surgery...

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Externet
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My oscilloscope needs surgery...

Post by Externet »

Hi all.
A well known flaw in Tektronix 2465. The horizontal final amplifier chip U800 gave up, as expected. :x

The chip is made of unobtanium, Tek part number 155-0241-02, available only by canibalizing another of similar series, or at usual armtwisting fees plus much luck to find one chip that will not equally and shortly crapout :cry:

http://www.qservice.tv/vpasp/shopexd.asp?id=1131

Its 2 differential outputs connect directly to the CRT 2 horizontal deflection plates. The minus output died. The image is thrown stuck off screen as only one deflection plate has signal. I believe the rest of the chip functions are alive.

I would dare to build a simple inverting buffer to replicate the still working plus output as a resource to re-create the dead minus mirror opposite signal pin.
It would have to be a ~100V rail handling capable, very linear, to ??MHz

---> Any suggestions which Op-amp to select ? Other circuitry considerations you may think of ? Would this work ?

Pin9 good + out------------------------------(+) Horiz CRT plate /|/|/|/|/| sawtooth waveform
Pin9 good + out-----------inverter------------(-) Horiz CRT plate \|\|\|\|\| sawtooth waveform
Pin4 bad - out-------------------X cut and delete pin connection

Or, perhaps any of you have that Tek chip in your bins/deceased oscilloscope for sale?
Miguel
- Abolish the deciBel ! -
Robert Reed
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Re: My oscilloscope needs surgery...

Post by Robert Reed »

Oh yea - that infamous Tek chip. The horror stories I heard almost kept me from buying my 2465. If I recall there is more to it than just an H Plate driver - several inputs go to it and selects from among them. Still pretty straight forward and simple circuitry within it but not easy to duplicate. Before I bought mine, I did a little research on that part. Apparently the early versions of the 2465 (<30000 s/n) had reliable chips. Then the fab house was sold and the new Manu cut corners. After a short while they decided it was too limited production to be profitable and discontinued it completely.I found no cheap way out of this and maybe buying a dog on E-Bay would be the cheapest way to go (as long as it still had horizontal sweep). I did come across a web site where a couple engineers were in the process of constructing a replacement device for it at a reasonable price. I will try and relocate that site and see what their progress is.
Robert Reed
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Re: My oscilloscope needs surgery...

Post by Robert Reed »

Well I spent an hour web searching and then threw in the towel looking for the site I mentioned. Either they moved to some remote location or abandoned the project completely.
My Tek manual shows a sawtooth excursion of +28 to +46 volts on each of the differential outputs and of course 180 degrees out of phase. I have no idea of what the supply voltage is to that chip as this is one of the worst Tektronix manuals I have ever seen. No flow and data scattered all over the place. If all else fails and you feel confident about the reliability of the rest of the chip, it seems a single stage transistor -unity gain & inverting- biased at the proper level might work.you would need adequate collector breakdown voltage and moderately high Ft, but it doesn't look like those requirements would be difficult to meet. Maybe 100 V/200Mhz?The 5 nsec/div sweep rate would correlate to a 20 MHz sawtooth,add the required harmonics (tenth?) and it would seem like a run of the mill transistor by todays standards. Who knows, if successful, you could market it and make small fortune :grin:
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Externet
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Re: My oscilloscope needs surgery...

Post by Externet »

Thanks, Robert. :grin:
Yes. As you say.
The supplies to that chip are on schematic "diamond" 11 if I remember well, feeding +87V; +15V; -5V and something like +4.3V
The simple transistor approach seems safer as the Op-amps found run max. to 70V; even when not all the swing would be needed. About 20Vpp could suffice. But they are not much current capable.
The linearity of the circuit is something I do not know how to tailor. A mosfet could be selectable too. The current is unknown to the electrostatic deflection plate, but there is a 150 ohm limiting resistor; which tells cannot go beyond 300mA, well within TO220 devices.

Now I have no older oscilloscope schematics to see how is it done with discrete components. :sad:

Well, I will play with some circuits and look for suggestions:

---> The intention is to linearly mirror-invert a 25Vp-p sawtooth waveform riding on top of +25VDC from +25V to +50V.

Edited - added: Perhaps an APEX PA19, if I can find it qty 1.

Miguel
- Abolish the deciBel ! -
Robert Reed
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Re: My oscilloscope needs surgery...

Post by Robert Reed »

Probably could not find an Op-Amp fast enough anyhow. The 150 ohm resistor may be for oscillation suppression. The Horizontal plates form a capacitor of some small value so there may some amount of charging current involved. If you want to download and review some shems in and about that area go here - http://bama.edebris.com/manuals/tek/
If you want to talk to other Tek users go here - http://www.tek.com/forum/viewforum.php?f=3
If I run across anything I will get back. Good luck!
Javen
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Re: My oscilloscope needs surgery...

Post by Javen »

Yes, I also heard about that Tek oscilloscope flaw. It's horrible.

Javen
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Externet
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Re: My oscilloscope needs surgery...

Post by Externet »

Done, works.
Workaround the IC U800, Tektronix unobtanium part 155-0241-02

Now to fit my canteen style contraption circuit within the guts, with smaller power resistors and heat sink. It does not run hot at all, and there is room for it.

Much better than a dead 'unfixable' oscilloscope. :grin: :grin: :grin:
Miguel
-sorry , poor quality pictures-

http://i588.photobucket.com/albums/ss32 ... closer.jpg

http://i588.photobucket.com/albums/ss32 ... ek2465.jpg
- Abolish the deciBel ! -
Robert Reed
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Re: My oscilloscope needs surgery...

Post by Robert Reed »

That is remarkable job Externet. Now I have to ask- is the sweep linear up to the maximum sweep rate? Did you add the chips complete function or just the mirrored sawtooth output ( looks like probably just output). Could you post a schematic diagram for the rest of us worrywart 2465 owners?
boroko
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Re: My oscilloscope needs surgery...

Post by boroko »

Hi,
Same problem with a Tek 2445. Trying to find a solution. I found someone that was working on a new circuit, but so far, nothing that I can test, so nothing that I can recommend.

Externet, are you willing to share how you solved the problem?

Thanks
Bo
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Re: My oscilloscope needs surgery...

Post by Dean Huster »

That is the final question, Miguel: how is the linearity at the maximum sweep rate? Were there any horizontal center issues during calibration with the substituted circuit? Does the 2465 have sweep magnification? If so, how does your new circuit handle it?
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.
johansen
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Re: My oscilloscope needs surgery...

Post by johansen »

hi, new guy here.

i bought a 2465 before i knew about this problem, and the chip promptly failed 3 months later. (this was 6 months ago)
so i've got quite an incentive to fix it.

good news is u800 works, for about 20 seconds when cold, and i've also got a peltier chip arriving in the mail sometime, which should give me ample time to take detailed measurements of the functioning chip (or indefinitely, if i get lucky and it works forever at -10C or something like that.)
i've also got a tek 475 on the way, but that might be 2 weeks away due to the holidays.

I'm aware of the complexity of this chip, but if just mirroring two sawtooth waves is enough, i would suggest using one of those three channel crt video display drivers. i've only had my hands on two of them (15 inch dell crts have them)
they a 10-30:1 voltage gain but less than 20ns rise time and about 30Mhz bandwidth. i've used them to drive pulse transformers and they don't seem to mind, specifically the stv9552.
my email is johansense at gmail dot com if you want to contact me.
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Re: My oscilloscope needs surgery...

Post by Robert Reed »

Johansen
Very interesting. Please let us know the final outcome of your project.
I am another concerned owner of a 2465, although so far so good.
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Re: My oscilloscope needs surgery...

Post by johansen »

If you just want to get the scope working, traditional sawtooth is all that is needed.
photos here: www.johansense.com/other/2465/

i have yet to look at pin one, which i believe is the sawtooth input on my 475 while the 2465 is running.
when you look at it with the 2465 all you see is the triangle, i'm sure there's more to it than what these photos show.

what i can tell you is that the digital display readouts are not locked to the main sweep, they run at a constant 80 hz or so.
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