Tektronix H2462 acquisition module TDS 5xx, TDS 6xx, TDS 7xx

[EEVuser]

Hello,

I have a H2462 Acquistion module, that has a sporadic problem. Sometimes the 50 Ohm serial resistors seems to be broken:
https://w140.com/tekwiki/images/f/f9/H2462_schematic.png
Is it even possible, that this is a sporadic problem?

Does anybody know, where this resistor is located? Behind the shield?
https://w140.com/tekwiki/images/9/93/Tek_h2462h.jpg

And which kind of resistor is required to replace it. Are there special types, special HF capabilities?

[TERRA Operative]

Often it's faulty relays, you can test them using a good multimeter and a power supply to activate the relays.
The contact resistance should be in the tens of milliohms (I like to use a 4-wire measurement). I think the relay for the 50ohm resistor is the smaller one (usually blue?) at the front of the module. I wouldn't be surprised if the resistor is printed on the ceramic directly rather than an actual SMD part.

Replacing the relays is doable, if a little fiddly. I have a video on the subject on youtube.

As for the resistor itself, I'll have to take a look at a spare hybrid module to be sure. On a related note, I have spare tested good hybrid modules if you end up needing replacements I can send you for a reasonable price.


Oh, don't forget to replace the thermal paste when you reinstall the hybrid module, you don't want to overheat the chip!

[EEVuser]

Thank you for the fast answer.
I have attached an image of the problem (channel 2).

It is observable in all vertical scales. That let me conclude, that it is not one of the relays. Is this conclusion reasonable? However, beeing a sporadic problem, a relay is not a bad place to look for, since it is mechanical.

Since a small AC signal is getting through, that's the only path, branching before the 50 ohm resistor. That's why my guess was the resistor. If you have a spare part available, could you confirm, that the resistor is printed or a SMD part?

I have already switched the attenuator module and the error switched accordingly.
Another interesting observation is: Two channels are affected. So far, always both channels are faulty at the same time. Either both are running or not. That's strange, since switching the modules will clearly indicate that the problem is on the attenuator module. But having it always on both channels simultaneously indicates a common cause. But right know, I would give the independent cause on the modules the priority.

[TERRA Operative]

Ok, so my memory was a little rusty. The smaller blue relay is actually for AC/DC coupling.

With the hybrid oriented 'portrait' looking at the relay side with the small relay at the bottom, it seems the top left relay is for the 50 ohm switching, and it appears that the 50 ohm resistor is on the other side of the hybrid.
If you flip it over so the part number is oriented correctly (so 'landscape') and you look in the little window in the metal shield, I think the blue square to the top of the hole (above the lower blue square connected to a small black square) is the 50 ohm resistor (I measured 55 ohms on the 6 hybrids I checked, you'll see two vias there that make it easy to gently measure with some sharp probes).

Good luck repairing it if it is faulty..... It will be a fiddly repair to install a resistor there, and you won't get the original performance from it..

I'd test that relay (or all while you are at it) after checking the resistor and see if they are worn, it is a common problem, especially with channels 1 and 2 as they are most used.
If they are ok, the fault might be in the silicon, in which case the hybrid is scrap.

[EEVuser]

Thank you for the hint, where the 50 ohms are placed. I measured them and have exactly the same values like you. So that's not the problem.

The error is sporadic and in case the signal is getting through, sometimes only with significantly wrong levels (20-40% less). This could indicate the relay with an increased contact resistance. But I don't have a failed self calibration test. According to the schematic, the self calibration lines start behind the 50 ohm switching relay?

What I find confusing is, that the schematic: https://w140.com/tekwiki/images/f/f9/H2462_schematic.png does not show all the relays. Specially the black one in the 50 ohm switching path.

I don't see a chance, to measure the contact resistance. The contacts are too hidden and even, if I reach them, it is not sure, if the measurement really gives the right direction. So I problably have to replace the black relay without a 100% evidence.

The form factor of the black TN2 relay is not available anymore from a big distributor. Only option is, to take an ebay offer with new samples from china or original, but used ones.
What is your recommendation?

The yellow relays might not be the problem, since it is independant of the attenuation level and it should throw a self cal failed then.
Anyway, I have to remove them, to remove and solder the new black one.
Would you replace the yellow relays at the same time prophylactically or keep the old ones?
 

[Andy Watson]

I had a similar problem with intermittant attenuator operation. The fault was traced to poor solder joints between the relays and board (ceramic IIRC).

[TERRA Operative]

The mention of a bad solder joint is a good point, I have heard of that being an issue on occasion.
That's an easy-ish thing to rectify with some flux and a soldering iron.

I would have a go at pulling the relays off the board (carefully!), testing contact resistance, then replacing if needed.
The act of resoldering them will deal with any bad solder joints.

[EEVuser]

Actually it is not so easy to resolder the black relay. At least two other yellow relays have to be removed for this. Doing it without destructing them is also very difficult, since at least 3 solder points have to be hit at the same time and it can't be reached with a desoldering tip/blade.
I also think, that the heat only reactivates the contact plates in the inner of the relay a little bit, but does not solve the problem in general.

Also desoldering the black relay without destruction is even more difficult. The SMD pads are again too hidden for the desoldering tip. At least mine does not fit in the small gap below the relay body. I don't want to hot air the pcb, since any damage to the pcb is even worse.

So conlusion is: either carefully destruct and replace all the relays or nothing.