(Attempted) repair of a DC505A - Page 1
Forum section: electronics / repair

author: Ero-Shan
author_rank: Frequent Contributor
timestamp: October 30, 2017, 12:56:55 pm
content: One of the first pieces of equipment of the recently gotten stuff that I tried to play with was the Tektronix DC505A Universal Counter/Timer. The display and range LEDs lighted as expected, and the gate LED blinked happily.But I wasn't able to make it count anything, on neither input channel. That looked like a problem I just might be able to fix. First of all, I needed an extension cable. Luckily, EEVblog hinted at JAMMA. While not perfect, they're cheap and need only little modifications to make them fit.This weekend (which is extremely long in Germany) I started to look after it. And immediately faced the first problem: that bugger was not designed for ease of assembly, let alone disassembly! To remove the front plate, you need to remove the BNC sockets, but this proved almost impossible to do as long as the front plate is attached.After a embarrassingly long time I managed to take the whole assembly apart, without damaging anything. Phew! (The upper right edge of the plastic subpanel fell off when I got it. I tucked it away so I wouldn't lose it - which is the most effective way to never find it again.)While being able to get at all the test points, I was constantly afraid of shorting anything with the lower aluminum frame - Tektronix used it as a heat sink for two pass transistors. Maybe I should have made some extension cables for them. I measured all the supply voltages, and all of them looked good. Then I wanted to input a signal and see where it went. The manual does have a parts location plan, but this is hard to read - this beast is cramped. Some more turning and flexing, and disaster stroke. The flex connectors between the boards just disintegrated - all 3 of them at once.Thus ended my first day of the DC505A.Incidentally, I was reading about desoldering stations in this blog about 2 weeks ago. Having cannibalized electronics for 40 years (I probably did more desoldering than soldering), I finally learned that those machines made the task quite a bit easier. I immediately order one and lo and behold, I never could have dreamed how easy desoldering could be! And just at the right time! Without that machine removal of the smeary leads would have ended in a total mess. And trying to free the holes of solder would have ended in destroying the PCB. So it was a real easy and neat job.I've now made new connectors from ribbon cable, twisted 2 wires together to get the right spacing, and soldered (and glued - never allow stranded wires to bend at solder's end) to headers, so they could be easily inserted.That's where I stand now. If anyone has tips regarding this plug-in I'd be very grateful.And I still have to learn to make more photographs (now that I have a working camera, again).

author: texaspyro
author_rank: Super Contributor
timestamp: October 30, 2017, 05:15:51 pm
content: I've worked on a couple of these several years ago. I didn't have to disassemble the unit. Old flex cables can be a horror... as you found out, the conductors like to come unglued from the backing. Another common failure for the DC50x counters is failed LED display segments. Some use individual 7-segment digits, others use multi-digit modules. I used to have a bunch of the (Fairchild) single digit displays but have used them all.I got disgusted about the price, quality, and availability of TM500 extenders so I built my own. Here is a picture of my TM500 extender cable kit. It uses two 40 pin ribbon cables to connect the two boards. I still have some kits available. They cost $20 each. Shipping is $6 for any quantity in the US, $16 international. I also have some assembled extender cables for the TM5000 GPIB connector... same price.The closeup of the PCB is not quite the same as the final version which has a power LED and some other tweaks. The high current lines use two wires in the ribbon cables.

author: Ero-Shan
author_rank: Frequent Contributor
timestamp: November 02, 2017, 09:16:53 am
content: Your cables are very neat. I especially like the descriptions on the PCBs - no need to constantly look up what's where. You can also exchange your ribbon cables for longer ones if need be (I would like mine to be a tad longer; the TM50x sometimes gets in the way, as well as the extender itself). The JAMMA extenders are no match to yours, but do their job after the simple modifications. And they were cheap (10 € apiece, can be had even cheaper).I also found the diode CR264 to have failed open. Now I have a signal at the input of DCU0, but still no counting. (I also did an embarrassingly bad job with my PCB connectors that cost me a whole day to discover.Serves me well!)On to the next round.

author: texaspyro
author_rank: Super Contributor
timestamp: November 02, 2017, 04:29:56 pm
content: Quote from: Ero-Shan on November 02, 2017, 09:16:53 amYes, I have tested mine with 3 meter long cables. The cables I include with the kits are old style IDE disk drive cables. The board layout accommodates the missing keying pin on a lot of IDE cables. My cables are usable, but could be longer. I originally got some samples from a seller and they were a few inches longer than the ones they shipped. The PCB board house left off the rear silkscreen. The manual includes some overlays you can cut out and attach to the back of the board.The Jamma style cables were the only affordable ones out there when I did the project. A major problem with those is having to solder 56 wires. Plus the wires like to break off. Plus the edge tab is does not have slots for the TM500 keying tabs.

author: Ero-Shan
author_rank: Frequent Contributor
timestamp: November 02, 2017, 08:00:13 pm
content: Quote from: texaspyro on November 02, 2017, 04:29:56 pmMy JAMMA extenders came with about 40 wires fitted (and hot-glued, shouldn't break too easily). The slot was easily added.I'd prefer yours, but for the time being I can cope with my cheapos.

author: Ero-Shan
author_rank: Frequent Contributor
timestamp: November 02, 2017, 09:00:58 pm
content: Although this thread did not attract a huge crowd, it has to be concluded one way or another. Maybe someone looks for a DC505A story 2 years from now. And stories that start but have no end are just frustrating. So here it is, the final chapter.I was wrong (nothing new here). The short between wires 1 and 2 of the PCB connector cables I replaced wasnotin my replacement cable, as I had assumed (it seemed so obvious). Making a new cable and replacing my first one didn't change anything: DCU0didn't count. The Set inputs of the FFs were at the exactly same voltage (3.8 Volts - between High and Low, obviously seen as High) as before. Out again went the cable, and on when my watchmaker's loupe. Now I saw it, a very fine glittery thing between the two pins. I removed it, and before soldering in the cable again, I made sure my multimeter measured a high impedance. I blame this on sloppy soldering/desoldering on my side, although it would explain the channel B failure from the start.The only certain fault then was an open CR264 which made it impossible to set the trigger level for channel A such that it actually triggered.I hardly couldn't believe it when I saw that thing actually showing a frequency measurement!But there was another problem: The clutch on the display time potentiometer looked like it wouldn't last much longer. And it didn't. When I cautiously unscrewed the thing, it fell apart. With no replacement in sight, I decided to make an emergency repair (I'm not the kind that allows only original parts even in non-visible places). A piece of wire through the small slit in the potentiometer axle seemed enough for the needed torque. Not exactly beautiful, but works all right.Another two hours were spent for assembling the beast. All in all, this was quite a harrowing experience for me.What did I learn, though?1. Make more photos! Document absolutely everything (would have helped tremendously with assembly).2. Don't print just the schematics (especially for Tektronix!). Having to look up everything on a computer is very awkward.3. Make some more photos.4. Be meticulous. No, beextremelymeticulous.5. Remember what you learned ages ago. Faults can be found, even in analog circuits.6. Don't assume, even if everything seems oh so clear.Alwaysmeasure.7. Don't be too afraid of rather hot ECL circuits and inaccessible spaces.8. And finally: Make even more photos!I can assure you that I'm relieved at last. And somewhat elated.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 02, 2017, 09:58:52 pm
content: Congrats! Just as I was going to call dibs on the wreck!Those extenders from texaspyro sure look nice. Except that I just aquired the original Tek cable, but if I encounter a problem on a TM5000-type module, I am not covered.Maybe texaspyro could elaborate on how he wants to handle his sales?

author: texaspyro
author_rank: Super Contributor
timestamp: November 03, 2017, 12:01:55 am
content: Quote from: Neomys Sapiens on November 02, 2017, 09:58:52 pmJust send a PM and I'll send you my Paypal address.Before I did my extenders, I already had a couple decent ones. I needed to work on a module with three connectors and was shocked at what they were selling for and also did not like the non-replaceable cables. A friend had experience with the Jamma ones and warned me off of them, so I did my own. I figured (apparently correctly) that others might find them useful and, taking a big leap of faith, bought enough parts for 150 sets... I'm down to around 30. A lot of people really like the TM50xx stuff. It's well documented and easy to keep running (very few unobtainium parts).I also did a run of extender boards for the HP-5370 counters... in case you have one of these fine old venerable beasties. The 5370 is around the only way to get 20 ps res for time intervals without spending $5k+I'm currently working on a time interval calibrator for the HP-531xx counters (replaces the unobtainium HP-59992-J06 / high dollar pulse generator) and an interface/breakout board set for the Symmetricom X72 rubidium oscillator.

author: frantisek
author_rank: Regular Contributor
timestamp: January 18, 2018, 10:10:23 am
content: The TM500 extender from texasypro is absolutely excellent! I can recommend it to all.