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TDS 644A Blank Screen and Solid 8 on LED

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azupanchick
Posts: 2
Joined: August 1st, 2020, 10:18 am
Country: United States

TDS 644A Blank Screen and Solid 8 on LED

Post by azupanchick » August 1st, 2020, 10:39 am

Hello,

I recently purchased a TDS644A and getting into trouble shooting the issues with the scope. The scope does not display anything and the entire front panel lights stay on constant. I can crank up the display brightness and don't think the issue is with the display. The LED shows a solid "8". All capacitors were replaced and proper voltages are present. I put sockets in for the DS1650Y and DS1286 and replaced them. I re-programmed a DS1250Y with a 644A file from the tek site that I found. Still getting the solid "8" on the LED. Not sure where to look next. By the way, re-programmed the DS1250Y using a BP Systems 1200 programmer. Seemed to program correctly without errors, but wondering if I found the correct .BIN file. Let me know your thoughts on this problem. Thanks!

strick
Posts: 199
Joined: March 22nd, 2016, 11:23 am
Country: United States

Re: TDS 644A Blank Screen and Solid 8 on LED

Post by strick » August 3rd, 2020, 8:49 am

azupanchick,
I've attached one of my known good DS1650 images (same as DS1250) for a TDS644A that I recently calibrated. you can give it a try. if that doesn't work, then you will probably never get it going without making a serial debug port. the 644A generally had the option 13 board, so you can modify it to be a serial port and get the errors while it's still running the bootrom.
let us know what happens...

:) Strick
Attachments
nvrac_8.zip
(227.86 KiB) Downloaded 1 time

strick
Posts: 199
Joined: March 22nd, 2016, 11:23 am
Country: United States

Re: TDS 644A Blank Screen and Solid 8 on LED

Post by strick » August 6th, 2020, 12:47 pm

Also, if the DS1250 is bad, the scope will show a flashing 8 due to the scope not being able to write to it. I just tested a bunch that I got from my seller in China and 3 out of 5 failed to keep a good write and one of them did the flashing 8 trick. When I put that chip in my programmer, it immediately failed whereas the others would take a write, but would fail verification.

Strick

azupanchick
Posts: 2
Joined: August 1st, 2020, 10:18 am
Country: United States

Re: TDS 644A Blank Screen and Solid 8 on LED

Post by azupanchick » August 7th, 2020, 5:47 am

Hi Strick,

Thanks for the info! I used the data and re-programmed the DS1250Y, but it didn't like it. It would not let me access the error log menu. The data you sent over had the 1M feature included, and the original one I had was 13, 1F, and 2F. I think math functions were included on the 644A. I did have some progress with the scope, however. Started looking at clocks and functions on the processor board and saw that the U1175 Reset chip had a 33uF capacitor missing. Replaced it, but did not start. Started poking around and saw capacitor C1221 was discolored a bit. When I hit it with a soldering iron, it fell apart! When I replaced it, the scope fired up! That was good. Still getting attenuator and acquisition faults on powerup. Relays were clicking, but think I have an attenuator problem. When I tried to use the 50 ohm channel setting, it locked up the waveform on the screen. And the fourth channel was giving me a funky waveform with a 10kHz sine wave input. It was letting me change voltage scales on all four channels. Someone in the past replaced all the caps, but some on the processor board were missing or installed backwards from the circuit board polarities. Started investigating the caps on the acquisition board too. Where would I look next for these issues? Appreciate all the help I can get! I also have an HP82357B GPIB interface and associated software loaded up. I can identify the scope, but won't let me run tekfwtool or any of the Zenith programs for TDS scopes. Not sure if I need to change any settings for the HP stuff to work with the Tek scope?

strick
Posts: 199
Joined: March 22nd, 2016, 11:23 am
Country: United States

Re: TDS 644A Blank Screen and Solid 8 on LED

Post by strick » August 7th, 2020, 7:17 am

azupanchick,
I was going to post a blub on the site a while back about the 600A series scopes (both 644A and 684). If the ds1650 chip is corrupted in any way, you will get strange stuff. I redid a 644a and it was working perfectly until I put in the new ds1650 from china. It would boot, but suddenly would put strange pulses (looked like static) on the screen. I suspected the power supply but it was pretty clean. I went ahead and changed the power supply and it did the same thing.
then I started thinking about the ds1650 chip and when I programmed it in the scope (using tektool, not tekfwtool), the first 1000 bytes or so were correct (first 3 are always different due to the clock), but had tons of fails after that. The actual process is to write to the chip with "tektool -w nvram.bin -b 0x4000000 -l 0x80000" and then to do a read after cycling the scope off using "r" instead of "w" and "rdnvram.bin" vice the nvram I sent you. then, do a file compare with the following: fc /b nvram.bin rdnvram.bin only the first three words should be different.

Tek puts some boot up stuff early on in the NVram address structure for both the 544A and 644A series. if it reads the right stuff, the scope will boot with good retention of calibration and options, but may dump some of your later error stuff. It really doesn't need the whole 80000 bytes.

However, the 600A series takes a lot longer to do a SPC, and it includes a bunch of frequency testing between channels that I don't see in the 500A series. If this part of the chip is not taking a good read, then the scope will boot normally except for a "failed calibration" error. Then when you run SPC, it seems to work, but still has the issues. Only with a fully functional ds1650 (or ds1250) will the scope behave.

I actually figured this out when a 584A that I had calibrated a month earlier (and getting ready to market) suddenly started showing the "failed calibration" error when I put in a new ds1650 chip ( I already had put in a new ds1486). This cpu board is like the 744A and later series and has a clock chip with 2000 bytes of memory (vice 100 in the 644A clock chip) and normally stores all the bootup and SPC stuff in that chip. In fact, I've seen 744A DS1650 chips that had either nothing or garbage and the scope worked fine. BUT, on the 684A, the calibration is apparently written to the DS1650, not the DS1486. Probably has to do with the length of the file.

Once I replaced the DS1650 with a known good chip and reran SPC, the scope works perfectly, as it did before. BTW, SPC in the 684A takes 15 minutes to run, so you know it's testing and comparing a lot more than the nominal 744A or later.

good luck!
Strick

strick
Posts: 199
Joined: March 22nd, 2016, 11:23 am
Country: United States

Re: TDS 644A Blank Screen and Solid 8 on LED

Post by strick » August 7th, 2020, 11:10 am

Sorry,
didn't reply to your last part of the text.
Attenuators for the A series are rarely at fault (in my experience of 30+ A scopes). Certainly not true for B and later.

the tektool that I use works fine under a Win98 dos window. It will not work under windows alone or in a straight Dos session, like the calibration software. If you have a gpib adapter, I can help you to get a win98 partition set up and allow you to run tektool. Other folks have run with different systems, I believe, but mine works and I haven't experimented with many other system. If you want, I can run a test on a second XP computer that I have that I use to run my older eprom burner and stuff like that. Almost nothing with Win10 will allow you to communicate.

I have found that having the caps make your system cleaner, but don't stop the system from running properly. I now clean off all the surface mount caps and do my troubleshooting with no caps at all. then, when it's fixed, I put on new Panasonic low-ESR radial type caps. When I did it the other way around, the caps would just get in the way of any circuit fixes I had to do.

If you want to get win98 working and don't have the proper tools to partition the disk, send me a PM..

Strick

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