TDS680B to TDS684 - Page 1
Forum section: electronics / repair

author: S57UUU
author_rank: Regular Contributor
timestamp: June 29, 2025, 07:09:03 pm
content: Friday, I have bought a TDS680B at the Friedrichshafen HAM Radio fleamarket. A heavyweight, but I like the fact, that I can connect a probe, without having to hold the scope with the other hand!Somewhere I read that you can upgrade the type by soldering some resistors, but can't find that again. The tek forum linked above is offline.I would like to transform mine into a 684, to be able to display all four channels simultaneously. Can this be done without installing additional sampling hardware (like by reducing the max sample rate)?Marko Cebokli

author: madao
author_rank: Frequent Contributor
timestamp: June 29, 2025, 07:41:35 pm
content: Are you a guy which buy him for 200 bucks ? (Front plate said TDS680B, inside is a TDS620B )I have "repaired" him, Errormessage is away after running of SPC (it take long ~ 10 minute, )Yeah, resistor moving on acqusistion board change it to other faster /better modell. But i must looking after good pictures of TDS640B Acqusistion board. Now, i can't tell, which resistor should be inserted /removed.Here is option hacking for TDS640 & TDS540 without suffix. TDS6xx B Familiy is much newer units.

author: S57UUU
author_rank: Regular Contributor
timestamp: June 30, 2025, 05:34:43 am
content: Yes, that's me. Nice to meet you here! Now at home, I have done more tests, and it seems to work OK, displays a 1GHz signal adequately (haven't checked the amplitude or sample rate, though). It is just that a four channel scope that can only display two, feels wrong to me :-) On the boot screen it says 680B, so somebody has already hacked it, if there really is a 620 inside? (How do you determine that?)

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 30, 2025, 06:29:48 am
content: I have some info at home about what resistors to change which I'll post later tonight. You'll have to change resistors on the bottom acquisition board to change its personality, and IIRC if you are changing the number of channels, you'll have to swap some resistors on the top processor board too.If you are wondering about enabling options, it is easy using GPIB:https://www.eevblog.com/forum/testgear/enabling-options-in-tektronix-tds500-tds600-and-tds700-series-scopes

author: madao
author_rank: Frequent Contributor
timestamp: June 30, 2025, 06:42:46 am
content: Oh, TDS680B is 2 Ch 1 Ghz, not 4 Ch 1 Ghz (as like TDS684B).. My mistake with Tektronix numbering schema :-) (normally x80 and x84 numbering schema = 4 Ch 1 Ghz )I have here a TDS684C (use same Board as TDS680B/C) this is quad channel.-> 2ch is limited by software ->You must convert CPU board to color display for getting of quad channel digitzing.Yeah, my repair on stand is successful :-D . Proably has Seller power off, while TDS680B SPC is running.

author: S57UUU
author_rank: Regular Contributor
timestamp: June 30, 2025, 09:41:27 am
content: @T.O.Thanks, I have read your post about the "PITBULL" method, and I definitely want to enable the extra math functions. Not sure that the 680 hardware is capable of externded memory operation, though. I have an old Pentium 1 with an HP ISA GPIB card, windows 3.1 and IBASIC, should do the job.@madaoOh, and I thought it was just a resistor... When looking at four chans at the same time, color is certainly a desired feature. But on the other hand, I like the brilliant contrasty and sharp CRT, viewable from any angle, on the 680, more than the mushy (of age?) color "DPO" LCD on my 3052.Since they already put the four 5Gs/s A/Ds (expensive!) in the 620, I would expect also the (cheap) color video interface driving the BW CRT.But what can we do - this will obviously be more work that I thought - trawling ebay for suitable vintage VRAMs etc...BTW, I measured the -3dB bandwidth at 1060MHz and -6dB at 1260. "intensified dots" also shows it samples at the full 5G/s.

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 30, 2025, 09:47:56 am
content: Quote from: S57UUU on June 30, 2025, 09:41:27 amYou can use any GPIB interface you like. As long as you can send the commands, the scope doesn't care. I use a USB to GPIB adapter in Windows 10 and it works fine.

author: S57UUU
author_rank: Regular Contributor
timestamp: July 03, 2025, 01:42:11 pm
content: Still thinking about 680->684 conversion. Found this video about 620->680:now I know where the resistors are, just not what combination to put in.But there is the color display problem. The color scopes use a funny sequential RGB display wtih a LCD "color wheel".Also, the frame rate is 180 Hz, to acomodate the sequential RGB. I've used a photodiode to measure the refresh rate on the 680, and it is 60Hz. I doubt it can run at 180. When set to 684, the SW will write parts of the image into the non-existent part of the VRAM, so they won't be displayed.Possible solution 1:In this video (about the 744):at 0:48 I see a "monochrome" option in some menu. Would that mean that I can just set the resistors, then set this option and all will be OK?I am not sure. This option might just switch off the LCD, but still write to the color VRAM. Also not sure about the frame rate.Possible solution 2:Add the required VRAM. This reportedly works with an external VGA LCD. But I would prefer to keep the original CRT, mostly to avoid the mechanical fitting work. There is no Y (luma) signal to feed a B&W tube, but I could probably just use the green channel, and set suitable palettes. But the 180Hz problem remains. Maybe the external VGA runs at 60Hz, and could be used for the CRT?Any suggestion welcome!

author: madao
author_rank: Frequent Contributor
timestamp: July 06, 2025, 03:28:48 pm
content: QuoteYou can read my posting.There is no way to avoid of replacing of CRT with NuColor CRT (most expensive option, but brilliant display) or with TFT with conversion with notice of my posting (added 4 VRAM, remove one resistor ).