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Test Equipment Anonymous (TEA) group therapy thread

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m k:

--- Quote from: Vince on March 20, 2023, 10:36:35 pm ---This a Tek 7504 scope.... from my big TE haul from a few months ago.
It's a parts unit in extremely poor shape...

--- End quote ---

So you lost a perfectly good sink wash demo possibility?

TERRA Operative:

--- Quote from: Vince on March 21, 2023, 05:03:50 pm ---
--- Quote from: TERRA Operative on March 21, 2023, 03:26:53 pm ---Annddd, now I have a Tek THS720 (with carry bag and charger) on the way... It has the usual burnt polarizer on the LCD, but it seems to work according to the pictures in the listing.

I'll upgrade it to a THS720P and update the firmware if needed. Might even try and see if the TDS210 to TDS220 upgrade hack (Tell the scope it is the higher model then run the appropriate adjustment routine before rebooting it) will work to upgrade this THS720 to a THS730. ([EDIT] Just looked at the schematics, might be a case of swapping resistors and maybe one or two other parts to make the upgrade)
Probably end up doing the LED backlight mod, and building a new battery pack while I have it in pieces.

Will be handy for working on the car and for the youtubes, easier to lay on the bench under the camera than the TDS220..


Should provide a few hours of fun giving it a bit of love on the healing bench.

--- End quote ---

Oh so you are expanding your old Tek scope expertise to this line of prodcuts as well ?!

GREAT !.... because I really like them and would get one one day.
Very low on my priority list money wise, so it's only a remote idea.
But just in case, get whatever model is the top of the range, give it all the mods it can take, polish it up, and stack it on that TDS 784D that's marked "Vince".
It should not increase the price of shipping all that much I would think...


--- End quote ---

Ha, I'll keep an eye out for another one. :D




Tonight's work was replacing the typically snapped off 'Left Right' toggle switch on the 177 test fixture for my Tek 577 curve tracer.
Got a switch from Mouser (they cost way too much for what they are, but such is life) and pulled the toggle lever out and replaced the broken one in the switch in the fixture.
You can't buy the whole switch in the test fixture configuration, but as the switch is part of a series using common parts to build up the needed configuration, there are many common parts, so swapping the lever was just a simple matter of knocking out a pin to extract the lever, then performing open heart surgery with a little rocket science to reinstall in in the test fixture.

While I was there, I replaced the carbon composite resistors with equivalent metal film (gotta order a 22Meg 1/2W still) and it all went rather painlessly.
Now to find some replacement knobs for the front.

Next I'll have to lug the 577 onto the bench and figure it out. I know there's at least one shorted tant in it so far...

But before that, I'm waiting on parts to build my calibration and retro gaming PC, and I have a TM5006A mainframe to finish repairing, and a toaster to fix, and a shitty 30MHz scope to get the triggering working, and, and, and.....

ch_scr:

--- Quote from: Vince on March 21, 2023, 09:45:48 pm ---
--- Quote from: factory on March 21, 2023, 09:23:32 pm ---AD571 seems to be analog to digital
--- End quote ---

Geez you are right, made a mistake in my spreadsheet indeed !  :palm:
ADC it is, not DAC....
OK so I have zero DAC in stock then, problem solved...



--- Quote from: factory on March 21, 2023, 09:23:32 pm ---There is no rush to choose parts from Mouser, I'm not in any hurry to place my order.

--- End quote ---

Ah cool, thanks.
In this case I will take a few days to hurry up and start working on the project. A quick first pass, back of the envelope design so I can evaluate how complex it would be to implement a "normal" DAC.
If not too much trouble then might go straight for that and call it a day, but if it proves troublesome then I think I would just use cheap R-2R jobbies to just "get the job done"...
This would allow me to concentrate on the rest of the design, and once I have a working "proof of concept" board, I can always modify the design with a "proper" DAC later on, if really I deem the performance of the R-2R unacceptable after real world testing.

--- End quote ---
Hi Vince, I've looked and IMHO you "might as well buy a few of these" . Yeah, thick film, 2% but at the price they are well worth it to "have a play", at least!
Don't forget what comes "before and after" - the logic buffer to drive the bits (it's output transistors in particular) influence dac performance - make sure to go CMOS, TTL won't cut it. Also, you probably want to buffer the DAC output - take care that you would need either an opamp with "rail-to-rail in and output" or a negative supply to make it "go to zero". Good luck!

Vince:

--- Quote from: m k on March 22, 2023, 10:57:11 am ---
--- Quote from: Vince on March 20, 2023, 10:36:35 pm ---This a Tek 7504 scope.... from my big TE haul from a few months ago.
It's a parts unit in extremely poor shape...

--- End quote ---

So you lost a perfectly good sink wash demo possibility?

--- End quote ---

I don't know what that is  :-//

Vince:

--- Quote from: ch_scr on March 22, 2023, 05:45:32 pm ---Hi Vince, I've looked and IMHO you "might as well buy a few of these" . Yeah, thick film, 2% but at the price they are well worth it to "have a play", at least!
Don't forget what comes "before and after" - the logic buffer to drive the bits (it's output transistors in particular) influence dac performance - make sure to go CMOS, TTL won't cut it. Also, you probably want to buffer the DAC output - take care that you would need either an opamp with "rail-to-rail in and output" or a negative supply to make it "go to zero". Good luck!

--- End quote ---

Yes I will probably do that.
I checked all the ladders on mouser.... they don't have that many all in all, and you quickly realize that they really only have one brand and one series... Bourns series 4000, same datasheet for all, all 8 bits or nothing. The choice is more about value and package type.

So if even Mouser has only 8 bits ones, no need to waste my time trying to find 9 bit jobbies, they either don't exist or can't be bought by a hobbyist.  So I will have to make one myself using an 8 bit one and adding a couple 0.1% resistors to extend it and make up the ninth bit. I checked the price of such resistors on Mouser (so I can order everything from the same place...), and they are freaking expensive in leaded / through-hole type, but way, way cheaper in SMD packages. In 0805 (easy to solder) 1/8W (same power rating as the ladders), they are 35 cents a piece from Bourns, and only 17 cents, less than half price, in 10 quantity.

https://www.mouser.fr/ProductDetail/Bourns/CRT0805-BY-1002ELF?qs=h2IHEVivlqCjNQtH%252BVLkLw%3D%3D

As for buffers, hopefully I don't need any. Won't be using TTL for the reason you said.

Basically, what I want to do is have binary counter chip, 74HCxxx, drive the DAC.
So I thought, if I use a 10K or 100K ladder, it should not load the 74HC outputs enough to cause trouble.
At any rate, even if it did, every output would behave the same, so it should not affect the linearity of the DAC should it. Might affect the output voltage, I would get a bit less than 5V full scale, but I could not cat less in my application.
All I want is a DAC, any DAC... just convert the counter output to analog and that's it, no fuss. I am not designing a metrology grade instrument to say the least.... I guess anything would do.

Buffering the output of the DAC, might no even be necessary, it will drive 1Mohm loads.
And again even if the voltage dropped, I don't care, I can adjust full scale deviation afterward, no big deal.

I will post somewhere on the forum to gather suggestions about DACs that can be used without a CPU, maybe there are some out there that exist, or maybe not. The experts will tell me...

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