Tektronix TDS784D: Successful Attenuator Repair - Page 1
Forum section: electronics / testgear

author: kc7gr-15
author_rank: Contributor
timestamp: October 15, 2017, 01:43:18 am
content: Good day,Having successfully resurrected my TDS784D O-Scope from its 'bricked' state, my next task was to address the Signal Path Compensation errors I was getting, consistently but on different channels, every time I tried running SPC.As has been pointed out by others, the usual failure point is a set of SPDT relays, four per ceramic attenuator hybrid assembly, made (now) by Panasonic, their part number DS1E-M-DC12V. These were, last I checked, readily available from various distributors. Jameco.com had the best deal as of Oct. 7th. There's one additional relay, much smaller than the cluster of four, up near the front of the hybrid. My current experience is that this one does NOT need to be replaced with the others. I suspect it's the relay which selects the input impedance, or perhaps the one which changes between AC/DC input.The ceramic hybrids are fairly fragile, so you need to be very careful during relay replacement and post-replacement reassembly. The best way I found to remove the old relays was to snip three of their five legs with flush cutters, then gently rock the relay on its remaining two legs a few times and it'll pop right off the board. Next, use a pair of needle-nose pliers to rock the remaining lead-stubs the same way to remove them.Replacing the relays themselves requires a small, fine-point soldering system (I use a Metcal MX5000 series with the UF handpiece, and a UFTC-7CHL206 tip) and tin-lead solder with 2% silver content (just like Tek used in so many of their early tube scopes). Don't skimp on these requirements! It's way too easy to damage the ceramic substrates, and replacing them can be very expensive (judging from what I see on Ebay, at least). The solder I use is Kester '44' series, alloy and part number details: SN62PB36AG02, #66/44, .031 diameter, QQS571F. It'll run about $70 or so for a one-pound roll.Whne you disassemble the attenuator section, you'll see a couple of black plastic inserts well-populated with gold contact wires. Try very hard NOT to touch the contacts with your fingers. Wear gloves, if you have to. It'll avoid intermittents later down the road.Once you've got the new relays attached, do a thorough cleaning-off of ALL flux residue with 99% Isopropyl alcohol (don't use anything less than 99%) and soft cotton swabs. Once you get the hybrids done, and back in the attenuator casting, thoroughly clean AND DRY every single one of those little gold contact plates with the same 99% IPA and cotton (or, ideally, foam) swabs. Don't leave any residue of anything! It can and will interfere with the contact surfaces.Overall, it took me about four and a half hours to do everything right. Be patient, and take your time (especially if it's your first time doing this, as it was for me). Yes, I was completely successful.If you have any doubts about your own abilities or repair equipment, don't chance it! Seek help from someone who knows how to deal with these 'scopes. You'll save yourself endless grief.Happy tweaking.

author: snoopy
author_rank: Frequent Contributor
timestamp: October 15, 2017, 11:29:39 pm
content: When I did mine I too cut the relays off and found that there was a certain sequence of soldering the relays that made it easy to solder them without contacting the relay body with the iron. I used normal led solder because I did not have any silver solder at the time and there doesn't seem to be any repercussions so far.cheers