1. Home
  2. Electronics Repair
  3. Tektronix TDS-744A SPC fails

Tektronix TDS-744A SPC fails

I'm working on a TDS744A digital oscilloscope that fails SPC (signal path compensation) most of the time. It usually passes when it's cold, but after warm up, it almost always fails with the error "Cal meas clipped at max dig o". The self test always passes.

I haven't been able to find any obvious performance, or accuracy problems on any channel, or range, and it seems to be stable during warm up. Are there any known common causes of this error on the TDS744, or other similar model? It seems to be a common complaint about this model, but no one has mentioned a fix.

So far, I've checked it over for obvious problems. Cooling U1250 on the acquisition board seems to increase the chances of a successful SPC, but it's hard to be sure. It might be a coincidence, or just compensating for another problem. It doesn't have the leaking electrolytic problem of earlier models.

I'm also looking for the TDS600B and TDS700A adjustment software (both are needed for calibration according to the service manual).

Reply to
Andy Cuffe
Loading thread data ...

Andy Cuffe schrieb:

May I dare to direct you to the Tek Scopes Yahoo group? You'll find a lot of Tek specialists there.

HTH

Reinhard

Reply to
Reinhard Zwirner

That's worth a try. I'm waiting for them to approve me now.

As a side question, has anyone here ever removed the LCD color shutter from the CRT? I have two similar scopes (a 544 and 744). The 544 has a mint CRT, but there are some bad spots on the shutter. The 744 has a lot of burn in, but a perfect shutter. I would like to swap the good shutter onto the good CRT, but only if I can expect the results to be good.

Reply to
Andy Cuffe

I wouldn't trust my exact measurements, but there was consistently more resistance in two of the relays, which is obviously not good in a high precision circuit.

I wasn't bothering with those for now, but it's good to know they don't cause many problems. I suspect the blue ones are just for switching between 50 ohm and 1M termination. They looked like they would be tricky to replace due to their short pins, and they are a lot more expensive.

It sounds like a mess, but at least you don't have to cut all of the RTV off.

I know replacing the CRT with an LCD would be more sensible, but I like the look of those CRT + LCD shutter displays. If anyone reading this has an old TDS5XX, 6XX, or 7XX color CRT that they want to get rid of, please contact me. I'm interested in anything that has a decent looking color shutter and/or CRT. I'll even take a known bad one for practice if the price is right. I'm located in the US.

Reply to
Andy Cuffe

Good news, replacing the relays appears to have cured the SPC problem. Half of the old relays had an ohm or more of resistance, so it's no surprise it was having problems.

Replacing the relays was fairly easy, but you'll need a good soldering station with a long tip. The attenuators are ceramic hybrids, so I used silver bearing solder just to be on the safe side.

I'm still looking for a CRT or two, if anyone has one left over from an LCD upgrade, or parted out scope.

I will post back if I find the cal software.

One other question I have is about the TV trigger board (option 05). My TDS544A has this, and I would like to move it to the TDS744A. I already built new power ribbon cables and added the second connector to the bus board. The TV trigger option shows up in the 744 and passes the self tests, but the trigger level appears to be off. It will only trigger when the trace is adjusted so that it's cut off at the top of the screen. I suspect the board is either incompatible, or that there is a calibration for the TV trigger level that needs to be done.

Does anyone know if any calibration is required after adding TV trigger? Did Tektronix even allow you to add option 05?

Reply to
Andy Cuffe

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.