Adding Video Trigger to TDS744A

[hkp7]

tech8, I have a question for you. I replaced ALL of the caps with 33uF 35V, before I did all of the work I was getting a clean power on self test but failing SPC. Now I get s failure on the aquisition on the power up self test and of course, it still fails SPC. What would you suggest checking? Is it critical that the 10uF 35V caps have a 10uF value as oppoed to what I replaced them with (33uF)? The aquisition board didn't have any leakage or corrosion at all before I changed out the caps. It had a sticker on it from 1996 with a technician's initials. I'm wondering if the caps were replaced in '96. I checked all of my work and everything appears to be good so I'm a little perplexed that it now shows a fail aquisition on start up. BTW, I also replaced the 10uF on the processer board with 33uF and that tests clean. As an aside, the worst caps I replaced were those on the control board in the front. They were leaking and starting to corrode.

[tech8]

If the board has a blue or yellow sticker with a job# and technicians initals, then yes. It most probably had all the caps replaced in 1996. The rework started in late 1993 in manufacturing (like Nov.?) and not long after in customer service. Since I don't have the schematics memorized and it's been years since I've looked at them, I can't say for sure if all the caps are on supply lines or not. If they're all on power, then I wouldn't expect your replacing the 10uf caps with 33uf would cause a problem. I do recall that 33uf 10v caps are on the +/- 5V supplies, while the 10uf 35v are used on the +/-15v supplies. What is the diagnostic failure you are getting (check the error log)? Are you sure you didn't get one in backwards or bridge to a nearby via? If you removed the acquisition board, make sure you didn't bend any of the pins on the coax cables from the attenuator board when you plugged them back into the acquisition board.

[hkp7]

I get on startup:

pass--processor
pass--cal initialization (see error log)
pass--Aquisition/Processor Interface
FAIL++Aquisition
pass--Attn/Acq Interface
pass--Front Panel
pass--Floppy
pass--RS323/Centronics
pass--TV Trigger

I get on the error log:
Error diagnostic test failure
OPTVEdgeAnd Level + Edge SWT)-Sig 1 No NTRIG2 Output (Maybe because I had the TV trigger board out when I powered up?)
Error diagnostic test failure
CvrReg Diag, **EXP Data=0x100 Actual=0x2100 for bit 0
Error diagnostic test failure
Error 2260 calibration failed

[tech8]

The TV option failure may or may not be from when you had the board out. I don't recall if early on that option was software enabled or not. Later it was enabled by the presence of the board, so no board would mean "no TV option installed" to the processor. The older TV option boards do have the same type of electrolytic caps and leakage did occur. The CVR circuit failure could possibly be due to incorrect cap value(s). There are 4 10uf caps in that area. Although, I don't recall there being any eletrolytics in the CVR circuit itself. The CVR circuit is around U900 and RP911. The resistor pack is the voltage divider for the cal reference. You should closely inspect that area, especially around those 4 caps. You said originally this unit failed SPC. Could be that you did nothing wrong and the heating/cleaning just caused something to get bad enough to finally fail diagnostics. That circuit is used during SPC. There's a DG444 in the circuit and the cal voltage output for SPC is from U903 (I think it's a TL072). We used to replace that opamp a lot.

[hkp7]

Thanks tech8,
I will check around the places you mentioned. I don't think the board was at the point of iminent failure because, as I mentioned, it appeared to have been serviced in '96 and all of the caps had nice shiny solder joints. I will also put the TV board back in and see if the error related to it disappears. I checked for aly inadvertant solder bridges and didn't find any. All of the caps are installed with the correct polarity. I guess if I can't find anything else I could always get the 10uF and place them on the board where it originally had them. I'm kind of scratching my head on this one.

[hkp7]

tech8,

I just put the TV board back in and powered up. The error related to the TV board disappeared but I still see the "exp data 0x100 actual=0x200 for bit 01". Checked voltages at the caps and got mostly +/- 5V and +/-15V with some lower voltages on some.

[tech8]

What were the lower voltage readings?

[hkp7]

The lower readings were between 1.8V amd 3.8V. Should they be getting 5V or 15V at all of them?

[tech8]

Well, there's a 2.5v reference, but 1.8 and 3.8 aren't ringing any bells. I'd suspect an open somewhere. It's not that hard to fracture the connection between the run and the pad. Without a microscope you might not be able to see it. Could be a cracked ferrite bead or resistor somewhere. Had you removed the acquisition board? Could be something got damaged or knocked off the bottom of the board in the cap changing process.

[hkp7]

I took it out very carefully. I will check the table to see if anything fell off while I was doing the work. I guess it will be time to take out some high powered magnification and go through the board again to see if I can find anything open. BTW, when I was checking the caps for continuity of a leg to ground, I found a few that will give continuity to ground on both legs, that doesn't make sense to me but I don't have any schematics to go by to try to make sense of it.