Adding Video Trigger to TDS744A

[andy]

Does anyone know if it is possible to transfer the video trigger board (option 05) from a TDS544A into a TDS744A? The board is a perfect fit, and the 744A recognizes option 05, and passes all self tests, but video triggering isn't working properly. It will only trigger over a very narrow range of vertical offset when in video mode, and at that point half the trace is off the top of the screen. There doesn't seem to be any way to adjust the trigger level when in video mode.

The video trigger board still works fine in the TDS544A it came from, so I suspect it's either incompatible with the 744A, or it requires calibration to work. Unfortunately, I can't find much information about option 05 such as part numbers for the boards, and what calibration may be necessary.

[cball]

I am not an expert on these, but the part number for the TV trigger board for the TDS544A lists compatibility with the TDS510A, TDS520A, TDS524A, TDS540A, TDS544A, TDS620A, TDS640A and TDS644A.

The board for the TDS744A has a different pat number, and is listed as being used in TDS520B, TDS540B, TDS620B, TDS644B, TDS680B, TDS684A05, TDS684B, TDS724A, TDS744, TDS744A, TDS754A, TDS782A, TDS784A.

I suspect they are both very similar, but have some differences.

[andy]

That would explain the problem then. I wonder what the difference is? Can you tell me the part number of the board I need for the 744A?

[maxxxpower]

It seems to me it should work.

[andy]

It seems to me it should work.

I doubt there can be much difference between the boards since it almost works (maybe it's just a revision, rather than a completely different board). I'm guessing maybe a slight difference in output level to the main trigger circuit.

I'm still not completely ruling out a calibration problem. It's possible that there's an adjustment for the TV trigger level that isn't set unless the option is installed?

Does anyone know where I can get a copy of the calibration software? Based on the service manual, it looks like you can make individual adjustments without doing a complete calibration. Hopefully I will at least be able to see if there's a TV trigger adjustment. I need the TDS600B and TDS700A adjustment software according to the service manual. I have already made a backup of the NVRAM, so it can't hurt to try.

[cap]

I always thought these were freely exchangable, but I never really looked at function beyond option presence and the new menus being present. They do use a front riser card too; I think they did anyhow. Spacing of the risor cards was a bit different; D1 Buss, ribbons etc.. So, the D1 bus at the very least was different, but it would take someone at tek to say if that affected pinout. It could be that the on board firmware is different too, some unsupported commands. The TDS744A was a significant design revision from the 500A's, the power supply and especially the acquisition board. cap

[andy]

I may just stick with my 544A if I can't get the TV trigger working, and I've found a new problem. SPC is failing intermittently (more often than not), particularly when warm. There are occasional SPC errors going back to 1998, but who knows how often they ran it. The exact error is "Cal meas clipped at max dig o" (the rest is cut off). It still passes all the self tests, and there are no obvious performance issues on any of the channels.

[andy]

After doing some more digging, it seems that intermittent SPC errors are fairly common on the 744A (common enough that one person advised to run it repeatedly for an hour to see if it ever fails before buying a used scope). Can anyone tell me exactly what "Cal meas clipped at max dig o" means? I assume it means the compensation has reached its limit without succeeding. Are there any common causes of SPC problems in this series. Is it possible to find out which channel and which range failed?

Cooling U1250 on the acquisition board appears to improve the chances of a successful SPC, but it might just be compensating for something else. Freezing it does have a small effect on the DC offset of the trace, but it's only a fraction of a division, at 1mV/div (this doesn't seem excessive). Even when it's failing SPC, there is no significant DC offset on any channel, and it seems stable during warm up. The gain on all ranges for all channels is accurate.

The acquisition board doesn't have any electrolytic caps on it like the 544A, but there are some on the processor board. None are leaking, but their ESR is on the high side (5-8 ohms, in circuit for the 33uF caps).

My plan is to re-solder U1250, and replace the caps on the processor board. I would still appreciate it if anyone can share any experience with this problem, and I'm still looking for the cal software.

[cap]

No. Unless you can observe a pin moving with probe pressure under magnification, leave U1250 alone. Do change out the caps on the mother board (your option 05 board, and everyone you can find in your 544A if you have not). Failing SPC is bad for resale but so long as it is still passing, after those times, all is good; for hobby anyhow. Very rarely, but sometimes, calibration can help but if it is failing often probably not; and it may not even calibrate at this point and then your cal menu might not show pass anymore.

cap

[andy]

I won't disturb U1250 other than checking it under magnification for broken connections.

The 544A was a mess of leaking caps, which I replaced as soon as I got it a few years ago. Fortunately, there was no permanent damage.

At least the 744As caps aren't leaking, but as I said, their ESR is higher than I like to see.

I'm planning to sell one and keep the other. Ideally, I would like to keep the 744A with option 05, and no SPC problems.