Tektronix TDS 694C not booting

[cf4044]

Hi I'm new here, well sort of.  I've been following Dave's channel for years and even reading this forum but never registered until today. Maybe because I've found lots of answers in the forum without needing to ask the questions but now I do have one which maybe someone can help with.

I've just purchased a TDS 694C which is not working. It was initially listed as 'does not power on', so assuming it would be a relatively simple power supply issue I went ahead and ordered it. Before sending the seller told me (and showed video) that it was now powering up and passing self test but after sometime the screen goes dark and all the front panel LEDs are lit up and it does not respond to any controls.  I probably should have cancelled as outright not powering up would probably have been an easier issue to fix. I also have a TDS784C which works fine and found help in this forum with regards to backing up the NVRAM and how to replace them when it becomes necessary.


Well now I have it. I plugged it in and turned on the switch at the back. I tried turning it on by the on/off button at the front but nothing happened. I left it there, still switched on at the back, and after some twenty minutes I heard signs of life as it started the fan and lit up all the LEDs on the panel. Screen still dark and no output from the CGA connector.
I followed the procedure in the service manual but there was no path that described this behavior. I then went ahead and checked the supply voltages as instructed and they were all correct.

I have tried following the flow chart in the service manual but the behavior does not quite fit with the flow chart.
I checked the power supply voltages against the table in the manual and they are all correct.
While it rarely powers up at all from the front panel I can reliably turn it on/off every time by shorting two pins on the power connector (as described in the manual) or pressing the small push button between the two power connectors. The front panel button rarely works though.

When it powers up the diagnostic display starts at dot-8 then proceeds rapidly 2 3 4 5 6 7.  I had to film slo-mo to actually see the count up. It then pauses for some time on 7, dot-8, dot-6, and then alternates between 6 and 8 forever.
I tried disconnecting stuff one by one, hard disk, floppy, front panel and then even acquisition board until there was only the power supply, the fan and the main processor board remaining and I always get the same result, which seems to indicate it may be the main board.

I've been through the forum regarding issues with this scope but none seem to match what I have.

Any ideas?

[m k]

For those a usual thing is a bad capacitor, but since you can use other methods it is not.

Your front panel switch must be connected somehow.
Can you verify where it goes or should go?
Maybe front panel connection is the key.

[Jwalling]

Every time I've seen the LED alternate between 6 and 8, it was a defective DS1486 NVRAM. Working ones are tough to find, lots of counterfeit devices on Ebay...

[TERRA Operative]

Yeah, sounds like dead batteries in your NVRAM. There is a guy on ebay who is selling redesigned ones that work great, I have them in two of my scopes. Definitely recommended.
https://www.ebay.com/itm/302913384290

You'll need a good firmware and NVRAM image if he doesn't have that model though (Unfortunately I haven't had a TDS694C come across my desk yet so I don't have anything).
If you do manage to find firmware etc, send it to me and I'll upload it here:
https://w140.com/tekwiki/wiki/TDS694C


As for the power supply, check for capacitor C17 (shown in blue) and replace it, it will save you a blown PSU needing repairs.
(There are a couple of PSU revisions, if yours doesn't have the noted capacitor, then you don't need to swap anything).

[m k]

Every time I've seen the LED alternate between 6 and 8, it was a defective DS1486 NVRAM.

Is front panel switch thingy included?

[cf4044]

Thanks, I eventually sorted out the matter of it not turning on by the front panel switch - it was just very dirty carbon contacts on the rubber button.  Unfortunately although the power button now works all it does is power up into the same 'dead' state with all the LEDs on and no display.

I have since done a lot of work on it but with out much to show. I'll be adding an update as a reply to my first post.

[cf4044]

That would be great, if so it should be quite easy to fix, relatively speaking.  I hadn't thought the NVRAM could prevent it from booting, I was expecting more like some error in the startup self test.

[cf4044]

If it is the NVRAM it would be great as it is relatively easy to replace.  I think it should boot even with a blank NVRAM and 'only' be missing the calibration data.

Thanks for the advice on the power supply. I did actually check the capacitor marked in blue, mostly because there was a stain on the PCB which I though was a leak from it. However the capacitor tested fine so I put it back in place as it is probably a better quality one than the ones I have to replace it.

Eventually I found that the power supply is actually working fine, or at least so I believe.  More on this in an update I'm posting.

[cf4044]

Update on the progress, or lack thereof :-(

First of all thanks for the replies and apologies for my late reaction.

So first I found that the scope has at some time in its life suffered a big impact though I'm not sure if it has anything to do with the fault or possibly something that happened after it was put aside because it didn't work. What I had originally thought small cosmetic damage on the frame left of the screen turned out to be much bigger inside with the magnesium frame being broken around the floppy drive and the frame visibly skewed, even making it hard to remove the cover. I did not manage to find any electronics actually broken though, such as cracked PCBs, components ripped from the board or anything of the sort.

I checked the power supply as per service manual and all voltages were correct and free from significant ripple or other issues, both on load and unloaded.

I tried all the dip switches that are meant to skip certain self tests. The only difference they make is that with some combination I get a different response from the diagnostic LED but I haven't managed to find out the meanings. I hate that the service manual doesn't even give a clue of what the diagnostic codes mean.

I also tried disconnecting things, trying many different combinations of thing s connected or not, just in case one of them was causing the bootup to stall. In the end I even removed such that there were only the motherboard and the power supply and still I got the exact same result on the diagnostic LED.

I then disassembled the scope completely down and cleaned everything as it was absolutely filthy, it seems mostly from storage as not much of the filth appears to be what the fan would have drawn in. I also carefully inspected all PCBs and connectors for physical damage, leaky capacitors and such. The only damage I found was a totally busted trigger chip but that is something I was half expecting even when I ordered it. In any case although this definitely means faulty triggering it shouldn't stop the scope from booting.

I then put it back together, perhaps naively hoping that by some magic it would work but as I should have reasonably expected it was exactly the same as before, except that the front power button worked and the instrument looked almost like brand new :-)

The next thing I tired was quite risky. I swapped the motherboards between my perfectly working TDS 784C and the dead TDS 694C. Now the display on the TDS 694 lit up and showed bootup in progress went through the self tests.  I got an obvious failure on the acquisition board since it is a totally different one from what it expected. But other than that everything worked and it even displayed waveforms loaded from the hard disk, all the buttons and menus worked, so in short it could do everything except the most important thing.  But at least that sort of pointed with certainty at the motherboard itself being at fault.  In the meantime the motherboard of the TDS 694 in the 784 behaved exactly the same as when it was at home in the TDS 694 (all LEDs on, no display, no reaction to controls) further confirming the mother board faulty.

I found a board with the exact same part number on eBay and ordered it. It is from a different scope but since it has the same part number I guess it differs only in the firmware loaded into its flash. 'Only' perhaps not being the best word. In any case once it arrives I'll put it in and see what happens and how to proceed.

Thanks to Jwalling for pointing out that it is probably the NVRAM.  I will order the replacements, two sets actually as the ones in my working TDS 784C are probably close to their end of life.

One thing that I have also yet to do is hook up the debugging serial to see if it reveals anything useful.  I still have to figure out the connections though and make the adapter.

[nctnico]

I'd check the fuses and power rails on the acquisition board. That trigger chip looks like it could take a whole lot of other circuitry with it when it went up in smoke.

The dipswitches on the processor board can be used to put the scope in a diagnostic mode that uses the 7 segment display. There is a component service manual for some of the TDS500 series; these scopes are all very similar.

[TERRA Operative]

The cal data is stored in an EEPROM on the acquisition board, so it should be pretty safe.

If you need chassis parts, I have a few spare bits, pending postage costs from Japan... Might be able to get in under the limit for 'small packet' if we are lucky to get cheapest rates.

I think replacing NVRAM with new is a good next step, as long as you can get a good firmware and NVRAM image from another scope you should be golden. The batteries in the chips are weellll past their use-by date....

There are many shared parts, so your test on swapping boards is valid.
I have loaded different firmware into a board and was sucessful in changing it to suit the new scope it was going into. If the part numbers match, it's good to go.

[cf4044]

....
I think replacing NVRAM with new is a good next step, as long as you can get a good firmware and NVRAM image from another scope you should be golden. The batteries in the chips are weellll past their use-by date....
......

I just ordered the NVRAMs as you suggested from kellyjo_13 on eBay. Actually I ordered two sets, one for the TDS694C and another for my (still working) TDS 784C as the batteries on that will definitely run down some time soon.
The NVRAMs will be pre-programmed with the appropriate data for each model, so if nothing else is wrong I'm hoping it's just a matter of putting them in and switching on.

It looks like they will take a while to arrive though, the estimated delivery being 27th February :-(

In the meantime I will prepare by desoldering the NVRAMs of the 694C, which I am not expecting to be particularly easy.

....
If you do manage to find firmware etc, send it to me and I'll upload it here:
https://w140.com/tekwiki/wiki/TDS694C
......

Sure, if I manage to get it to work I'll be glad to send you the firmware, but I'll probably need your help on how to download the firmware from the scope. 

[cf4044]

IT'S ALIVE !!

It was actually the NVRAMs -  thanks for the advice in that regard.

That was probably the last thing I would have thought of.  I would have thought that it would go through the boot, come up with some message that it is loading with default values or something like that and then proceed to work, possibly with limited capability and performance. It's kind of weird that it's designed to instead just stop with a blank screen not letting you know what is going on.

I just received the replacement chips/modules yesterday, installed them and it worked just right away, even passing all the self tests and successfully doing the signal path compensation.

Unfortunately I confirmed the triggers are not working though, not entirely unexpected given that one of the trigger chips is literally burnt through. There seems to be some limited triggering ability - I could get it to trigger through the Ext trig and the Logic triggers but very unreliably and only work at relatively low frequencies.

In any case in an effort to prevent the triggering system from deteriorating further I jury rigged a heatsink for the three remaining chips. Even so the heatsink still gets very hot so I can only imagine how hot the chips were getting with no heatsink at all.

I had considered swapping around one of the chips as it seems that fixes the trigger problem, probably at the cost of the currently working (sort of) logic trigger.  That would be fine but at this point I'd rather not risk making it worse by a bodged repair as I'm not very skilled at SMD repairs.

I have attached some photos of the heatsink addition.  The main challenge was how to secure the heasink in place.  I didn't want to glue it as it would make it next to impossible to remove the chips intact if I eventually decide to swap them around. So the only way I could figure out to secure it was to use a longish brass bar to span the nearest two screws that mount the PCB to the chassis.

[darin_i]

I like your heatsink mounting method. Sort of reminds me of the spring clip heat spreader retainers in GPUs, etc.
Only way beefier. Did you work from a drawing of some kind you can share, or just wing it with show tools?