Help desperately needed on TDS744A scope repairs

[denimdragon]

Hi everyone. I hope you are enjoying you're Sunday.

A week ago, I purchased 9 Tektronix TDS scopes from a local business that closed their doors (for a nice price). Two of the scopes are working well based on me running the self-test and SPC. I also measured and checked some waveforms using my BK Precision 4011 5MHz FG. One is a TDS784A, and the other is a TDS644B. I have placed those to the side until I have learned what further tests I can do in order to make sure they are working properly.

I am currently working on a TDS744A with intermittent SPC failures, but here is what I have left to work on:

2x TDS744A - One scope would not boot when I first received it. I took the case off of it and started poking around. With further examination, I noticed that the J2 ribbon cable that connects to the A11 Processor/Display board was unplug. I plugged it up, and booted the scope again. This time it came on, but only lite up all the front panel LEDS. A few seconds later, I smelled a weird smell which had somewhat of a grapefruit odor to it (that's my best description. It was not a familiar electronics problematic smell, but please keep in mind I am new to scope repair, but not electronics repair in general). Maybe it was from bad caps.

The other 744A just has SPC issues (intermittent Pass and Fail when SPC is run). It passes start up self test. I have decided to use the non booting 744A as a parts mule. I am currently checking the relays* from  the hybrid Attenuators on the non booter to see if they are okay and I can switch them out to the working scope with SPC issues.

1x TDS724A - Consistently fails SPC test - Passes Start up self test.

1x TDS694C - I started a thread about this one. The trigger chips seems to be the issue but I'm not
sure how to test if they are bad or not. Any suggestions would be greatly appreciated. I have yet to look at the chips under the microscope for a more detailed visual inspection, but I have eyeballed them and didn't see anything (Thanks to @Jwalling for his input on the other thread about this unit). I do know for certain that there is an overheating issue because it has shut down once. After I put an external fan on it, it stayed on (very hot that day). Since then, it hasn't shut off again after initial boot, however it still radiates a ton of heat by the trigger ASICs under the casing.

1x TDS684C - Powers on, All LEDs light, but doesn't boot. It just sits with the LED on (I already checked the write-protect switch position)
 
1x TDS544A - Powers on, All LEDs light, but doesn't boot. Caps need replacing. I've examined the board under my microscope.

1x TDS7154B - Came with no hard drive. I haven't a clue as to how I should address this, but I have a few HDDs from laptops but I'm not sure how this would work. I am very partial to this scope and this is the only one I plan on keeping for myself. I will work on it once I am done with the others.

Pardon the long winded post. I just want to make sure I have given enough information so you guys can understand what I'm dealing with. Right now, I want to know the best way to test the hybrid attenuators and the relays. I have been applying voltage across pins 1 and 6 on the DS1E-M-DC12V until I hear them click (right at around 6.3-6.6V .02a) and slowly go to 12V and the current increases. I then take a test light and check pin 12 to see if my test light lights up. I also checked for resistance across pins 1 and 6 (inductor) and they are within spec (360? +- 10%). I have yet to test the TN2E. So far, for the one I have check, 3 of the 4 are bad. My question here is, am I checking them correctly? Does anyone have a better test method other than the DMM and power supply method I am using? I would be most grateful for guidance.

Lastly, I would like to know what boards are interchangeable. I've been told that the 744A are which is what I am working on now and attempting to do (thanks @eKretz), but I wanted to test the A11 board from the donor to make sure all was well before I go through the trouble of reassembling only to find out that I have the same issues with the transplant board.

I know I have a tall task ahead of my but I'm up to it and I really want to get good with repairing scopes. Any advice or suggestions would be greatly appreciated. Thanks everyone.

[eKretz]

Check the contact resistance to test the attenuator relays.  On mine the bad ones were awful.

[denimdragon]

Check the contact resistance to test the attenuator relays.  On mine the bad ones were awful.

Pardon my ignorance, but how?    I'm assuming that's the resistance across the metal contacts on the relay legs?

[eKretz]

Connect an ohmmeter (ideally milliohmmeter) across the NC and NO contacts while the relay coil is not energized and energized,  respectively.  You'll need to either use some minigrabbers or remove the relays first to reach the legs. The resistance should be in the sub-ohm range (IIRC, they are spec'd at 0.02 ohm). Anything higher and you have contact problems.

[eKretz]

BTW these relays basically control the signal path through the attenuators, so they switch in and out to change the path of the signal through the x1, x10 etc. signal paths. Hence,  if the contact resistance is high,  the signal path's parasitic resistance can't be compensated for when SPC is run because it's outside the range of adjustment.

[denimdragon]

Connect an ohmmeter (ideally milliohmmeter) across the NC and NO contacts while the relay coil is not energized and energized,  respectively.  You'll need to either use some minigrabbers or remove the relays first to reach the legs. The resistance should be in the sub-ohm range (IIRC, they are spec'd at 0.02 ohm). Anything higher and you have contact problems.

Oh ok. I will check that now. I have been using minigrabbers because those little legs are hard to get to.
 

BTW these relays basically control the signal path through the attenuators, so they switch in and out to change the path of the signal through the x1, x10 etc. signal paths. Hence,  if the contact resistance is high,  the signal path's parasitic resistance can't be compensated for when SPC is run because it's outside the range of adjustment.

Thanks for this info. This helps me understand the circuitry better. I'll do some further testing and post tomorrow. Also another quick question, I was looking on Mouser for these relays and they are pretty pricey (well to me anyways, but I am a rookie after all so I'm sure it's relative). Seeing the amount of scopes I have that could be suffering from this same condition, I may have to buy bulk to get them cheaper. Any suggestions on purchasing relays and the replacement caps for this and the other units? Thanks in advance.

You've been a big help! I've also realized from discussing this with you, that I have to buy more test equipment to test the test equipment   

[tautech]

There's a small selection of Tek relays at the foot of this page on Sphere's site:
http://www.sphere.bc.ca/test/tek-transformers.html#catalog

[eKretz]

Mouser or DigiKey are the only two places I'd buy the relays - can't trust that you'd be getting new ones or even legit ones on eBay and doesn't look like Sphere has them. The relays were something like $5 apiece when I bought mine IIRC. If you have good Google-Fu you might be able to find a list of  the capacitors you need online from guys who have done the same recap previously.

Needing more test equipment to test our test equipment - the bane of my existence! It does tend to pile up,  heh.

[alm]

Qservice is another place where you might find obsolete Tek parts. Just search for the part number.

[denimdragon]

Qservice is another place where you might find obsolete Tek parts. Just search for the part number.

These look to be the ones I need but I'm still learning the nomenclature on these components. The "-H166" is added but the specs look the same. What's the difference or is there one?

There's a small selection of Tek relays at the foot of this page on Sphere's site:
http://www.sphere.bc.ca/test/tek-transformers.html#catalog

Unfortunately they didn't have any but I made sure to bookmark it. Here is the one I am working on now and the actual relay pictured.

[eKretz]

For that price I would go with new ones personally.  Those are take-offs according to the link.

[denimdragon]

For that price I would go with new ones personally.  Those are take-offs according to the link.

Oh ok. Well I was debating on getting these from mouser http://www.mouser.com/_/?Keyword=DS1E-M-DC12V 10 @ 4.60 isn't that bad and I might as well get use to it. I just want to insure I'm getting the right thing. Then I have to deal with the NVRam next. All these scopes need them changed but I need a chip programmer and a bit of reading so I know what I'm doing. One things for sure, I'll be a little more than a rookie when I'm done with this. I have 2 months left before it's back to school so I'm putting in as many hours as I can to learn the ends and outs. While those TDSXXXs are on hold for parts, I've been working on the 7154B to keep me busy and learning.

Thanks again for all the help guys!

[james_s]

Definitely buy new relays, that will fix the SPC failures. It's a real PITA to replace them so you only want to do it once. I forget where I ordered mine from but they were a bit cheaper than digikey or mouser, I can search for the invoice if you can't find a source.

You have to be really careful replacing the relays, the ceramic substrates are extremely fragile and they soak up heat from the iron like crazy. There is also very little clearance, I had to fabricate a custom long tip for my iron out of solid copper wire and then snipe the old relays out with precision cutters. Do it one attenuator board at a time and mark the boards before you start so you don't get them mixed up and need a full calibration.