TDS744a Fails to complete boot

To whom may be able to help,

I have a TDS744a, the power supply had gone out and after repairing the unit booted once(see photo with probe compensation signal) but then never fully booted again. When the dip switches are set to run the full diagnostic, it comes up with processor, acq/proc interface, acquisition, and front panel failed. Please see photos attached.
Thought it might be the Nvram chip but replaced and the same thing. no change in the boot up of the scope. Went through the boards and verified voltages, and all look good. 

If someone might have suggestions on what to check next i am all ears. I have been through all the older forms and haven't found any that have had or what they would have done to identify the problem like this. 

One other thing i have noticed and i find odd is that, the clock never increases, this is what made me suspect the NVram chip and watch dog chip, but once replaced it made no difference. It is as if it is waiting on a signal that never comes back and may be the same signal that tells the clock to increase in time....All other scopes even when not working correctly the clock seems to increase, and this one, stays right at zero, but I may be way off here too. As I find out more, I'll add it to this post.

thanks for all you help......

Hi Michael,

I wonder if it's a power rail that has gone bad?
The fact you get partial failures across such different systems suggests to me that it's power rail that has dropped out or is badly out of tolerance, so that's what I'd check first.

NVRAM failures usually show up as the scope working fine with power on, but losing data once you power it off (as the NVRAM becomes volatile), so I don't believe that's your problem.

Afonso,

Thank you for your insight, I had not considered this as it had gotten so far in the boot process, I will have to set up a scope to monitor the supply lines the 5,-5 and 15 and -15 volt lines to see if one is dropping out during the boot process, 

I am also going to try and connect to the debug or console port to hopeful read what it is doing during the boot up process. 

would you, or anyone really, have a step-by-step procedure to do this? I can figure it out just would be a big time saver. Once i get the output recorded i'll post it here to see if anyone here can decipher it. :-) 

thanks again for your feedback.

mike

Mike,
The 744A should have a DS1486 and a DS1650 (or DS1250) NVram on the CPU board.  There is code on the DS1486 that has to be there to boot properly.  Did you copy the old NVram image to it?  I have tons of images from such scopes and can provide one or more if you don't have it.

If by chance you have an older CPU board that has the same watchdog timer as the older TDS500 series, then that code will have to reside on the DS1650 as there is these are not ramified timekeepers.

Also, you did not say if the LED on the CPU board increments from 0 to F and then starts to flash alternating segments (like top/bottom on one side).

Strick

Strick 

Thank you for the information on the chips, I had thought these DS chips were gone, so I bought new/ replacements from kellyjo_13. And kellyjo has been great to work with as they too have images of these chips for the specific machine. Kellyjo burnt me the chips and sent them to me, however the chips didn't solve the problem. 
Tektronix TDS784D 700x 600x exact functional replacement for DS1486 and DS1250Y | eBay
The LED display inside on the processor board runs through a set of numbers. then if looking at the front of the machine, the left side alternates top to bottom and keep this pattern until power down. The one thing I do see is that the last digit to flash is ".L" is this significant? 
I did the sw1001 DIP switch options, and it didn't give any error when, I selected these settings. Please see the attached jpg of the settings from the TDS520B manual. I could find anything like this for TDS744a.

Please let me know your thoughts as I am at loss right now on the correct way to proceed forward. Going to try and see if I can capture the boot data as it is booting. I have found how to hook it up, but this is bits and pieces I have complied from multiple sources. Think I have it correct but I'll find out maybe tonight. would you have any suggestions here? Anyway, thank you for your support.

Thanks
Mike

 

Mike,
I would leave the dip switches in the normal position.  I read Kellyjo's ebay ad and it would appear that he knows what he is doing. The image on the DS1486 may or may not be correct for the TDS744A, unless he still has one to test it on.  The ad indicates he tests the chips on a TDS784D, but the images for the DS1486 are very different.

So, I'm not sure the DS1486 is the problem or not.  Could you try putting the old DS1486 back in with all dip switches in the normal position and post a picture of the screen results after boot.  If it fails to show a screen, then could have a NVram problem or some other CPU board issue. 

Also, make sure that the connector board on the right side is in the proper position. It should have the part where the board extends past the plug in connector above the CPU board, not below the ACQ board.

With dip switches in normal, getting to the point where the LED is alternating and not showing numbers or letters generally indicates that all boot up checks are complete and the routine passed to the Flash memory. 

Finally, do you have a GPIB capable computer to talk to the scope?

Strick

Strick

Strick
 Thank you again for your thoughts, however all connections to the boards are correct. And I have tried putting in the old ds1486 chip and the same result. Please see the attached photo. it gets to the finial boot screen and then locks up, and by locks up I mean the front panel is not response, all led are lit, but cannot do anything. I have attached a photo of the boot screen and of the diagnostic screen with switch 3 set to run the diagnostics. The one thing i can help but wonder on is why the clock isn't increasing in time in the lower right-hand corner. It always stays at 00:00:00. I have seen other machines with the wrong date, but the clock is still running. 

Anyway, i am look at the debug port as well as the gpib port. I am in the process of getting the gpib setup. trying to track down an old pc Pentium 3 or older, i have all the software's required dos, windows etc., still not sure if I have the right Tek tool but figure I would figure that out once I got the hardware. 
I seen on a forum somewhere can't remember where right now that the debug port could be used to acquire the boot data and was going to give that a shot as i have the stuff to do this.

Also, you mentioned that it passes to the flash memory.  you would happen to know the chip numbers? i could at minimum check voltages at those chips...just a thought...

anyway, thanks again for your thought and advice.

Thanks
Mike

Mike,
It could be that your problem may be with the power supply. You may be getting the 5 volts DC to run the CPU board, but you may not be getting all the voltages to the ACQ board.  The correct voltages are listed in the TDS520 component service manual under the power supply.  I once had a power supply I fixed that would power up the CPU board, but failed miserably once the ACQ board was attached.  What failed on the power supply that you had to fix?

BTW, when you say the dip switches are on power up with full diagnostic, do you mean that the dip switches are all off (closed)?  The manual indicates to set dip 3 open - this avoids the full power on self test.  That's fine to avoid the long self-test that TEK fixed with later C and D scopes.

Still, freezing after booting with the LED working normally is typical of a bad NVram image on loadup.  The flash is checksumed in the initial boot sequence and won't even load if the checksum is incorrect.  I sincerely doubt it's the flash.  However, I can assure you that an incorrect image on DS1486 will cause all kinds of strange things to happen...
Strick

Strick 

I believe that there may be something to the voltage theory. I do not know much about the acquisition board circuitry, but I did stumble upon one area on the board that I cannot seem to understand. (this was also before I had any set of schematics) Now this may make some cringe, but I was going over the board using a diode check across the ceramic caps the ground plane with my multimeter, and found that the ceramic caps on the  (+ and -)5 volt line appear to be shorted to ground, again this was a very basic check, flipped to ohms to verify if it was a dead short it is not about 34 ohms I believe...can't remember right now however not dead short, since I had nothing to lose, I applied + 5 volts to the circuit, and it was drawing 2.5 amps maxing out the dc power supply I have, which seems high but again I am not sure what is driven on this circuit. (also nothing even got a hint of being warm...... Also, this+ 5-volt line does not seem to go to the processor board. The processor board has two 40 pin connectors as the acquisition board has a 50 pin and a 40 pin, same as the power supply, the last 10 pins or so have this+ 5-volt rail I am speaking of. 

Ceramic capacitor C1407 was cracked and was open, 1307,1207,1107 of the other three channels appear to be fine, in the sense that they are not cracked visually. Now all of the capacitors check with about 34ohms to the ground plane of the board, on the other side it is showing a dead short meaning that this was the ground/common / whatever you want to call it lol.. So, I have pulled the attenuator board and set aside, check again no change, in readings so do not believe the attenuators of the four channels are (NOT) the problem;-)

Also, as I was writing this I looked up the drawing for this area in the TDS520 manual and remember the cap that was broke mentioned above was on the -5 side... both the negative and positive rail were showing this the strange reading. I will tonight after work revisit this and get better confirmed results to talk over, is it perhaps when initializing this line is dropping out after the boot is completed???? Is the board drawing to many amps and shutting down the power supply or the other way around, the power supply cannot handle the current draw form the acquisition board and shutting down??? I will try to confirm this tonight hopefully.

With what I mentioned here sparks a thought or hey do not do that dumdum...;-) let me know, as I will check this before I start...LOL. Thanks again as we may have a smoking gun here or at least something to track down,  For Sure.

Thanks
Mike

 

Mike,

If you don't have the TEDS520  (not the 520B...) component service manual, then I strongly recommend you get it.  It will give very good instructions on troubleshooting - especially the power supply.  Although the power supply in the manual is the 300 watt supply, most of the circuits are the same as the 400 watt supply you probably have.

the only real changes are the addition of 10 more pins on the connector to the ACQ board to allow more power for a +5 volt line.  Between this manual and the TDS520B manual (downloaded from TEKwiki or KO4BB's web site), you can figure out all the circuitry for the various power supplies that were used.  The TDS520 manual can be ordered online from Artek Manuals and is delivered electronically in quick order.

Strick

Strick

I found a copy of the TDS520 not the b...I will go over this and report back on what i found..thanks again foryour help

Thanks
Mike