TDS 754D: swapping AS7C256 SRAMs by AS7C1024 to enable 1M option - Page 1
Forum section: electronics / repair

author: tverbeure
author_rank: Regular Contributor
timestamp: May 16, 2025, 05:48:51 am
content: I just bought a cheap TDS 754D with 32 AS7C256 SRAMs.If my understanding is correct, these RAMs are too small to support the 1M option. For that, you need AS7C1024 RAMs.When I added the RAMs of my old TDS 420A, the firmware checked the RAM size by writing and then reading back. I see that the TDS 754D acquisition board has 4 unsoldered pins per SRAM for the AS7C1024. I'm hoping upgrading will be equally painless?The Digikey price for AS7C1024 is not horrible, $3.1 a piece for a total of ~$100), so I'm wondering if anyone has tried replacing those 32 SRAMs?On the tekwiki page, I see that the acquisition board has IDT71024S12TY SRAMs mounted. Those are only $2.48 a piece... if you don't take into account those !@#$!@# tariffs. (https://www.flywing-tech.com/p/9671577)

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 16, 2025, 10:50:07 am
content: I was under the impression that 1M was able to be enabled on all C and D TDS700 series scopes, and 2M on all later serial number scopes (RAM became cheaper so they just stuck the larger chips in it seems).Have you tried just enabling option 1M (and also 2M) to see if it appears on the boot screen?If the scope doesn't support it, it just won't be activated from my experience.(Just remember to turn the option off if it doesn't work to avoid errors...)But to answer your question, I do believe that simply replacing the RAM chips with larger capacity chips will allow for the 1M or 2M options to be enabled.I would be inclined to install the RAM needed for option 2M if you are going to all that trouble.

author: tverbeure
author_rank: Regular Contributor
timestamp: May 16, 2025, 01:52:28 pm
content: Doing the upgrade will be my weekend project.Seeing the AS7C256 on mine while others have RAM that is 4x larger, I just assumed it wasn’t possible.There is also the Assembly Parts Numbers section at the bottom of the tekwiki TDS754 page, which lists different parts for the regular, the 1M and the 2M option.But you’re making me doubt again!32 x 32KB chips per channel is 1024k samples. Sufficient for 500k samples per channel for 784D/option 1M even if you’re doing double buffering. So the AS7C1024 version might be for the option 2M version.Maybe the different part numbers for the regular and the 1M option are just different bits set in the EEPROMs that are on the A10 PCBs.I’ll report back. :-)

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 16, 2025, 02:44:27 pm
content: The option bits are set in the NVRAM on the processor board, the EEPROM's on the Acquisition board only hold the calibration data.

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 16, 2025, 03:58:42 pm
content: Quote from: tverbeure on May 16, 2025, 05:48:51 amActually if you go down the road to replace the SRAM on your acquisition board, then we're talking total of 64 chips: 32 on side and 32 on the other side since you have 4 channels..Normally you should obtain 1M option with your actual SRAM via GPIB setup, otherwise for 2M tektronix used on these D serie IDT71024 chip.

author: tverbeure
author_rank: Regular Contributor
timestamp: May 16, 2025, 04:34:15 pm
content: Quote from: Tantratron on May 16, 2025, 03:58:42 pmOof, I had no idea! I've been relying on the pictures on tekwiki and some other places and none of them have the backside of the board. I plan to take the whole thing apart to evaluate whether or not to recap, so I might as well do a little photo session and have it uploaded to tekwiki for others.Speaking about recapping: have you done that for all your TDS scopes?

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 16, 2025, 04:53:31 pm
content: Since 2019 I've been dealing with more than ten TDSxxx, I need to make precise count including models but if you focus on C serie and D serie, no need to recap, they are free from capacitor plague.The problem of recap is necessary with older TDSxxx models (no suffix, A serie and B serie) but my advice is rather to focus on C or D, they are super good and clean.As for the main PSU, on C and D serie they redesigned the PSU a bit so again, works super good. However if you have old style PSU which can be found on C or D serie, you need to change some RIFA and one cap otherwise problems.Lastly about the CRT unit, I tend to remove it and rather use LCD kit: you save weight and save 75W from the grid

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 16, 2025, 05:17:56 pm
content: I was going to write a reply, but Tantratron basically summed it up perfectly.

author: tverbeure
author_rank: Regular Contributor
timestamp: May 16, 2025, 08:25:42 pm
content: Using the PITBULL GPIB method, I was able to successfully enable the 1M option, with the scope still in 754D mode. The 2M option didn't do anything, as expected. 500k points already makes the scope super slow to operate, so not sure if I'll ever try the 2M option by replacing the SRAMs...