Electronics > Repair
Tektronix TDS 694C not booting
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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:

--- Quote from: Jwalling on January 09, 2024, 09:45:10 am ---Every time I've seen the LED alternate between 6 and 8, it was a defective DS1486 NVRAM.

--- End quote ---

Is front panel switch thingy included?
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