After replacing Tek2440 Ram U664 and U350 by FRAM no scope display anymore - Page 1
Forum section: electronics / repair

author: Alectronic
author_rank: Newbie
timestamp: October 10, 2021, 04:12:24 pm
content: I hoped that I still could self diag and externally calibrate my oscilloscope like they tell it is easy and only about 2 pages in the service manual.Sadly after replacing the troubled old 32K SRAMs by 28 pin FRAMs FM1808 my scope no longer reacts on any of the buttons and no longer gives any display READOUT text messages anymore.And copying both SRAMs failed so not what I hoped or expected. Is there any way to restore and calibrate my 2440 scope without using the external Gpib bus? Or does anyone made a copy of a working SRAM chip that I can use to at least use my scope again? I already removed jumper J156 but it made no useful difference.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2021, 12:10:37 am
content: Losing the battery backed up SRAM data will not stop a 2440 series DSO from operating. I hope you installed sockets instead of the FRAMs directly.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 03:10:13 am
content: I have the same model, same problem. The scope has been bricked. I replaced both NVRAMs and now, nonsense display, no response to controls.

author: james_s
author_rank: Super Contributor
timestamp: October 11, 2021, 03:26:39 am
content: Have you tried putting the original SRAMs back in place? I've tried using FRAM to replace NVRAM in several different devices and it hasn't worked in any of them. The issue comes down to a quirk in the way the address is latched or something like that, IIRC the enable line has to go inactive between each write. If you had access to the source code it would be easy to modify an existing device to work with FRAM but without that whether it works is a matter of luck in how it was designed.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 06:43:07 am
content: I put one NVRAM back in but the other got broken. No change in performance.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2021, 04:47:04 pm
content: The Dallas nvSRAMs use the standard JEDEC 32Kx8 SRAM pinout and timing so standard parallel SRAMs can replace them. The original 2440 did exactly this and the SRAMs were replaced with the Dallas parts and the parts for the battery backup circuits were left off of the board.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 05:11:10 pm
content: So what should I do to get the scope working again?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2021, 06:17:24 pm
content: Quote from: bob91343 on October 11, 2021, 05:11:10 pmI would start by installing a pair of 28 pin 0.6" DIP 32Kx8 JEDEC standard SRAMs to remove the memory being the problem:https://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Memory-ICs/SRAM/_/N-98xofZ1yzvvqx?P=1yyxb3oZ1z0vhtuZ1yzmm18Z1z0z63x&Ns=Pricing|0I replaced the Dallas nvSRAMs in my 2440 with AutoStore nvSRAMs from Cypress but the DIP versions are no longer available.

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 07:32:30 pm
content: David, which specific parts? Is it a drop in, or is more required after that?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2021, 10:01:41 pm
content: Quote from: bob91343 on October 11, 2021, 07:32:30 pmI looked through that list and any of them should work, but I would use a slower part. The Autostor SRAMs that I used are 45 nanoseconds. My original Dallas nvSRAMs are 120 nanoseconds. Maybe it does not matter.Nothing needs to be changed. Of course all data will be lost when power is removed. In theory you could install the missing parts for the battery backup which was my plan if the Autostor SRAMs had not worked.Has anybody reported FRAM being compatible with the 2440 series DSO? I think the only reports I have heard were for the 2465 series of analog oscilloscopes. As mentioned earlier, FRAM requires toggling of -CS (?) to trigger precharging.

author: bob91343
author_rank: Super Contributor
timestamp: October 12, 2021, 05:20:34 pm
content: What are the part numbers and sources?

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2021, 05:33:37 am
content: The Alliance Memory AS6C62256 in the above list is an exact replacement for the original SRAM:https://www.mouser.com/ProductDetail/Renesas-IDT/71256SA25TPG?qs=sGAEpiMZZMtm4RFapVkzrLgp2JtZKHKucFTmbTbWRn0%3DThe Renasas/IDT parts and the other Alliance part are much faster but lack low power standby. They should work, but draw too much current for battery backup.

author: bob91343
author_rank: Super Contributor
timestamp: October 13, 2021, 04:43:46 pm
content: David, I can buy those Renesas chip via the link you provided, for $3.50 each. If I do that, do I just pop them in and the scope should work? Do I have to add a battery backup? What about calibration?Please guide me here, as I am over my head.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2021, 05:30:47 pm
content: Quote from: bob91343 on October 13, 2021, 04:43:46 pmJust drop them in and it should work. Without the battery backup circuits, the data will be lost when power is removed. I am only suggesting this as the least expensive way to verify that the 2440 is working properly without the FRAM. While it is theoretically possible to install the battery backup circuits to use with the original CMOS SRAM because the printed circuit board was not changed between versions, I do not know of anybody having done that and it would be a serious effort.The 2440 will run without external calibration, but of course the accuracy specifications will not be met and it will warn you about this.If you have a preferred supplier other than Mouser, then I can check them for suitable parts.They are no longer produced, but the AutoStor SRAMs that I installed in my 2440 were SIMTEC/Cypress STK16C88-WF45. These use EEPROM storage to backup the SRAM when power is removed.https://www.mouser.com/ProductDetail/Cypress-Semiconductor/STK16C88-WF45?qs=iZGZjqWbGbSQbSEl2oBSxA%3D%3D

author: bob91343
author_rank: Super Contributor
timestamp: October 14, 2021, 04:53:15 am
content: Mouser wants $3.50 each, making it $7, plus eight dollars shipping. That hurts.

author: james_s
author_rank: Super Contributor
timestamp: October 14, 2021, 07:34:39 pm
content: So only $15 total to diagnose your oscilloscope? Can you even go out to lunch for that? Good test equipment doesn't come cheap, I don't like paying more for the postage than the cost of the items but even so it sounds like a bargain to me.

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2021, 02:20:57 am
content: 32Kx8 JEDEC SRAMs are available from multiple sources if you want to look for a less expensive source than new parts from Mouser, included used.

author: bob91343
author_rank: Super Contributor
timestamp: October 15, 2021, 05:11:44 pm
content: David, do you have any specific sources?James, I don't even know if this will get the scope working; I hate throwing money at a job without some assurance it will do it.

author: james_s
author_rank: Super Contributor
timestamp: October 15, 2021, 09:51:18 pm
content: You could look on ebay, or post in the wanted section on the forum. 32k x 8 SRAM with a JEDEC pinout is a very common part.There is never any guarantee with anything, but with no risk there is no reward. I've paid a lot more than $15 on occasion in an attempt to repair something that ultimately didn't work, and other times I've been successful, that's life.

author: bob91343
author_rank: Super Contributor
timestamp: October 16, 2021, 05:42:00 pm
content: Will this work?https://www.ebay.com/itm/392649066989?hash=item5b6bb549ed:g:XioAAOSw9J9eKAWg

author: james_s
author_rank: Super Contributor
timestamp: October 16, 2021, 05:54:29 pm
content: If it's a genuine part it should work. It's a crapshoot buying cheap ICs from China though, if you get them and the scope doesn't work is the scope broken or are the ICs fake? If you have a EPROM programmer some of them can test SRAM.

author: David Hess
author_rank: Super Contributor
timestamp: October 16, 2021, 11:52:22 pm
content: The original part Tektronix used according to the 2440 manual is the Sony CXK58257P-12L, and according to the datasheets, the Hitachi HM62256 is practically identical.I did a search but the only discussions I could find about FRAM and the 2440 series are mine from 8 years ago. There are reports of FRAM working in the 2465 series but it is only speculation based on that that FRAM will work in the 2440 series. (1)I used Autostor SRAM when I did my upgrade because of uncertainty about whether FRAM would work in a 2440. Autostor SRAM might have failed to backup properly if the supply voltage dropped too quickly on shutdown but testing never revealed a problem.(1) A web search finds this much more recent discussion saying that FRAMs in the 2440 *do* work, and maybe some commentary about your problem:https://www.antiqueradios.com/forums/viewtopic.php?f=8&t=364933&start=40

author: Trader
author_rank: Frequent Contributor
timestamp: October 17, 2021, 02:32:35 am
content: This video at 38 min, Marco Reps replaced the "Dallas SRAM" with a "Ferroelectric Memory".https://youtu.be/upTgM_S5rAQ?t=2284That memory can replace the Genuine Dallas DS1225AD SRAM for Tektronix 2467B?Someone can explain what memory did he use to? Thanks

author: Alectronic
author_rank: Newbie
timestamp: October 18, 2021, 08:21:50 am
content: Yesterday I now also fully calibrated my Tektronix 2440 scope externally!And because my Tektronix 2440 scope repair now also was FRAM safe and fully calibrated I posted it after my last 2440 scope article on Jestine Yong's blog.https://jestineyong.com/tektronix-2440-oscilloscope-repaired-part-4/I first upgraded my oscilloscope by replacing both old battery backupped 32k SRAMs U664 and U350 by FerroMagnetic FM1808 DIP 28pin chips. Because without the calibration data in these chips the scope won't work, no working screen, no nothing, so I also used the uploaded firmware bin files from this great guy - see following link - to keep my 2440 alive after replacing both SRAMs. And pre-programmed my FM1808 with them so my scope would still work to be able to calibrate it again afterwards! Those bin files (from an almost identical 2430 oscilloscope) can be found here:https://www.diyaudio.com/forums/equipment-and-tools/312894-tektronix-2430a-lost-cal-data-repair.html. And if you have a dead 2440 most likely these SRAMs lost there original firmware and need to be restored first! PS: I used my even almost as old SYSTRON 100A Pulse Generator to finish the last external CTE calibration check. And I now have a fully Internally (Selftest) and externally calibrated Tektronix 2440 oscilloscope. Also the above given link provides the easy Calibration steps you can follow.I also read some guy had problems with calibrating the attenuator at 0.2V, 2V , 20V on Channels 1 and 2. I had the same problem but noticed when checking the voltages with my DMM that only a voltage of about 1.999 V passed. And 2.0 was too much which failed passing.And in Part7 of this Youtube video you can see how to setup the signal for the final CTE calibration step:

author: bob91343
author_rank: Super Contributor
timestamp: October 18, 2021, 04:24:10 pm
content: Okay I ordered the Hitachi parts. Now must wait for the high speed shipment so typical of these things.Wish me luck.

author: Alectronic
author_rank: Newbie
timestamp: October 18, 2021, 07:42:31 pm
content: Good luck with installing both new 32k U664 and U350 memory chips Bob!Those are the only ones that I measured that both were connected to the Backup Lithium 3V Keeper Battery in my 2440 scope.And they are the ones we need to restore to get the Tektronix back alive! So whoever tells us that the scope is still calibratable when the battery is fully drained and the 32k Srams (so not the Dallas NVRAM ones) completely lost their memory and so their calibration/configuration data is most likely ill advised.If the calibration still causes any problem, know that for the last CTE calibration the 100 KHz frequency is probably not critical. But I did use a good square wave pulse from my very old from the 70's Systron Donner 100A Pulse Giver to successfully finish that final calibration check. After that I also tested my scope's bandwidth with my homemade improved Jim Williams Fast Rise Avalanche Pulse giver. And it was as it is to be expected of a 300 Mhz fast oscilloscope.

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2021, 09:38:07 pm
content: The fast rise calibration is for matching the delay between channels. I used the upgraded sync output from my 2 MHz function generator which was like 4 nanoseconds and plenty fast. The oscilloscope measures the delay between channels 1 and 2 and then uses that value to shift one channel to match the other.

author: bob91343
author_rank: Super Contributor
timestamp: October 18, 2021, 11:40:23 pm
content: Thanks Alectronic! It will be a couple of months until I receive the parts, most likely.I measured my other, working, 2440 and its bandwidth runs around 280 MHz, close enough I guess. I measured it by looking at the output of my HP 8756B and increasing its frequency until the display dropped 3 dB from what it was at low frequency. The usual monotonic droop in response runs -3 dB at cutoff, and -1 dB at half cutoff. So to be accurate, I think I used 1 MHz which should be low enough. How could I raise the bandwidth to 300? Is there a hardware adjustment?

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2021, 03:55:01 am
content: Quote from: bob91343 on October 18, 2021, 11:40:23 pmMore likely the real bandwidth of your the 2440 was higher than 300 MHz, but the output of your HP 8756B (?) fell enough by 300 MHz to give a false measurement. This could be confirmed using a sampling oscilloscope or sampling RF voltmeter.Even the effects of the patch cable between the signal source and oscilloscope needs to be accounted for when making this type of measurement.

author: bob91343
author_rank: Super Contributor
timestamp: October 19, 2021, 04:21:45 am
content: I verified the signal input with my other fast scope, the 500 MHz HP 54542A and it comes in at about 600 MHz. Same cable.

author: Alectronic
author_rank: Newbie
timestamp: October 19, 2021, 09:08:10 am
content: About measuring the scope's bandwidth after complete calibration I measured a pulse rise time set about 500pS/Div on 2 Div which gives about 1 nS on my 350MHz Tektronix 2465A oscilloscope with my upgraded Jim Williams Pulse Generator. Which was around 1GHz rise time and when calculated in the formula this results in a bandwidth of 350MHz on my 2465A and about 300MHz on the 2440.How to make one with todays 2N2369A fast avalanche HF transistors that are protected against avalanching at lower Collector emitter voltages which makes them harder to use as an Avalanche Pulse giver can be read in this link:https://jestineyong.com/making-a-working-jim-williams-based-pulse-generator/

author: bob91343
author_rank: Super Contributor
timestamp: October 21, 2021, 03:53:36 am
content: Here is the nonsense display I get on my 2440.

author: m3vuv
author_rank: Super Contributor
timestamp: October 21, 2021, 06:54:00 am
content: Quote from: james_s on October 14, 2021, 07:34:39 pmwhat a tight ass!!

author: BradC
author_rank: Super Contributor
timestamp: October 21, 2021, 07:06:54 am
content: Quote from: bob91343 on October 21, 2021, 03:53:36 amYowza. Checking the schematics and theory of operation, U664 is the likely culprit. Looks like the system uP isn't running properly.

author: bob91343
author_rank: Super Contributor
timestamp: October 21, 2021, 09:37:49 pm
content: Thanks Brad! I will start to pursue that avenue. What part is U664? Working on these multilayer boards is worse than a pain.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2021, 12:12:03 am
content: It's easy if you have the right tools. Get a vacuum desoldering station, even one of the Chinese knockoffs will do. Trying to do a job like this without the proper tools is a recipe for a ruined instrument. Also when you replace anything that has a high failure rate, such as static RAM, install a good quality IC socket if at all possible.

author: BradC
author_rank: Super Contributor
timestamp: October 22, 2021, 12:25:39 am
content: Quote from: bob91343 on October 21, 2021, 09:37:49 pmOne of the RAMs you replaced. U664 is the main processor memory along with the calibration data.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2021, 12:31:10 am
content: A thought occurred to me, if that RAM was replaced without proper desoldering equipment, it's possible the board has been damaged, such as tearing out the through-hole plating. If that has happened then that would explain why the scope is not working, and it could be very difficult to repair.

author: bob91343
author_rank: Super Contributor
timestamp: October 22, 2021, 04:29:52 am
content: That is entirely possible. When I get the new ICs I will check for board damage and decide whether repair is required or advisable.