CaptDon's TDS420 Oscilloscope Repair (w/Update) - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: June 09, 2020, 05:28:25 am
content: Many thanks to CaptDon for sending me his disabled TDS420 oscilloscope for the cost of shipping!Not only that, he stuffed the TDS420 User's Manual, Programmer Manual, and the Reference Guide in the box tooNow CaptDon's scope suffers from a bad power supply, but the fortunate thing is, I have a TDS420 parts machine with a bad Acquisition PC board but a newly restored power supply.I'm hoping to make a working scope by combining parts.The serial numbers on the scopes are B021343 and B021008 and both have the "13" option stamped on the tag.Both Acquisition PC boards are stamped 671-1679-10 too!Let the fun begin

author: Smoky
author_rank: Frequent Contributor
timestamp: June 09, 2020, 05:47:31 am
content: I did plan ahead for this moment. I stripped my chassis and cleaned the CRT assembly last Fall.I started the assembly process by re-capping and installing the backplane board:The power supply was cleaned, re-capped, new TVS installed, all solder joints re-flowed, and bench tested:The make-or-break though is CaptDon's Acquisition board. I re-capped it tonight. CaptDon said that the scope passed SPC before the power supply went south. What I found was every original electrolytic capacitor was wet but there was minimal damage. I did find and repair one SMD ceramic capacitor near a leaking electrolytic capacitor with no continuity off of its soldering pad. We'll cross our fingers! I could not find any other lost connections:I like using leaded electrolytic capacitors since they are easy to install and because I have a bunch of them!The battery on the DSP board measured over 3 volts, so it was left alone. The CPU board battery was hovering around 2.6 volts, so it was replaced:The Display board is part number 671-1784-02:I know by now to go straight to inspecting and cleaning boards soon after pulling the scope from the shipping box. This is the membrane behind the front panel board. The leaking electrolyte makes its way through the holes from the scale knobs and gets trapped in the sandwich. It attacks the carbon and copper traces from behind. If your scale knob skips a step, this is where to look:Even though CaptDon's Front Panel board had all of its original capacitors leaking, the board itself is in excellent condition. I'll be saving it for another project since I already have another FP board re-capped and ready to go:I also cleaned and re-capped the RS-232 board:

author: CaptDon
author_rank: Super Contributor
timestamp: June 09, 2020, 02:57:00 pm
content: I recapped the 644A CPU using all radial lead caps.I recapped the 644A Aq board with SMD ceramiccaps. The original 644A Aq board had a bad memorycontrol mux chip which was unobtainium indeed soI bought a 644A Aq board from Ebay and cleaned andrecapped it instead. The 644A now works 100%. My644B did not have capacitor issues at all. There is aworld of difference between the 644A and 644B Aqboards and the 644A does a 1,2,5,10 sequence in gainor time settings and the 644B is different, somethinglike 1,2,4,10. Personally I prefer the 1,2,5,10 as it ismore like the analog scopes we grew up with and I amused to that order of magnitude steps. I am only sadthat our Cal lab threw out our TDS540A saying theycould not get it to pass SPC. Idiots didn't return it tome or send it for repairs, just threw in the metal scrap.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 09, 2020, 09:49:15 pm
content: Yep, I know what you mean. On a previous TDS420 repair project, the vertical scale knob would skip all of the "2's"It turned out to be the leaking electrolyte of the capacitors on the front panel board leached through the holes to the back side and in between the rubber membrane. It severed the connection between the carbon pad at the start of the copper trace. A fine wire and some solder fixed the issue:Btw, I'm almost ready to fire up the newly-rebuilt TDS420. A mix of parts were used from both scopes. I'll post some pictures of any good or bad errors that pop up

author: Smoky
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:37:49 pm
content: I love it!And she's running right at 1.25amps at 120VAC:The Signal Path Compensation test is coming next. I'm going to let the scope warm up for a half-hour or so then shut it down for a fresh re-start.I have it out of its case for now. The case will go on for the SPC test:So what really matters is, will it pass SPC?...it sure doesBut here's the icing on the cake. CaptDon sent me his TDS420 and it was full of General Electric stickers on it. I thought it would be too much work to get them off ...not!And not only that, I noticed the case was coated differently than I had ever seen on a Tektronix oscilloscope. It has a very heavy texture with the correct color. All of the stickers came off with ease using Goo-B-Gone without leaving a hint of a mark or shade. So I took the handle off and put the case in the tub and scrubbed it good. Holy-crap, it's like new! It's Rhino-bedliner tough and in perfect shape.Thank you CaptDon!

author: CaptDon
author_rank: Super Contributor
timestamp: June 10, 2020, 02:34:32 am
content: Very nice!! That makes two that were headed to the dumpsternow repaired, the TDS420 and the TDS644A that Norway Labssaid could be repaired for $1195 plus shipping both ways, whichI repaired for a total investment of $170. And there is also theTDS644B that I actually pulled from the dumpster sitting in threeinches of water and repaired for about $10. I am thinking of sellingthe TDS644A and donating any profit to the lab to buy hand tools.Funny thing about industry, they often pay to have scrap hauledaway (all those dangerous CRT's and PCB's and stuff, don't getme started) but an industrious person could live like a king fixing,cleaning and re-purposing these industrial waste streams. We havebooks in the cabinet for some outrageous Tektronix, HP and Wavetekgear, and sadly the gear itself is gone!!! I bet we have 4 AM503Current Probe Amplifiers each in a TM501 Power Housing but noprobes to be found. All of that stuff is currently on a shelf markedscrap also!!! I did repair and calibrate one of the units and I havethe ferrite jaw 20A probe as well as the 70A gun stored with italthough I am the only one who ever uses it. Glad there are stillsome of us old school hams and techies who love bringing thisold gear back to life and actually using it!!

author: oventech
author_rank: Contributor
timestamp: June 22, 2020, 01:17:03 am
content: Smoky great work on bringing the scopes back. Capt Don, i would be pissed too if a scope sent for cal was just tossed. I'd try to get the cost of the same scope out of them. Unless somebody higher up ok'ed the toss. Like the old Teks myself. Picked up 2 Tek 745's for 40.