Calling all Tek TDS694C experts, I need help! - Page 1
Forum section: electronics / repair

author: denimdragon
author_rank: Regular Contributor
timestamp: June 18, 2017, 11:09:42 pm
content: Hi guys,I purchased 7 used TDS5XX, 6XX, 7XX oscilloscopes from a business auction, and want to keep the TDS694C. I am having some problems with it and I want to repair it first before moving on to the others.It passed the start up test, and it has passed the SPC test 3 times and with no fails so far (I test each 10 times). I hadn't tested it initially. Now to the problem.Friday when I turned it on, I walked in the house (it's in the garage with 80-90F temps) and was gone for about 15 minutes. When I came back out the scope had turned off. I booted it up again and read the error log. It gave an ERROR: 240 hardware error a/d overtemperature. I set it to the side and continued testing the other units. (See pictures below)Today I decided I would focus on it again since I know the functionality of the others. I started it up, but I was in the garage this time. It shut down after about 5 minutes. I turned it on again, and this time I set up a CPU 12V fan to blow directly in the area behind the front panel (The cabinet is off). It stayed on around 25 minutes this time and then turned off again. That was about 30 minutes again.From the reading I have done for the last couple of days, I think it's the trigger chips. There was also a tag that says that the unit wouldn't trigger on channels one and two. Am I correct in assume the trigger chips are the culprit? What else could it be? The other scopes didn't have this problem and they all came from the same place.I would like to keep this as one of my personal scopes because of the high BW. Any suggestions about this scope would be greatly appreciated. I'm new at repairing scopes, but it's something I am committed to learning. Thanks in advice, and happy father's day to all the dad'sP.S. What probes can I use with this? I only have a P2220 and I know that won't cut it. I have a P6139A arriving tomorrow and I have read it will work with the other TDS units but I'm not sure about this.

author: KE5FX
author_rank: Super Contributor
timestamp: June 19, 2017, 12:34:26 am
content: The trigger chips can be shuffled around to restore operation (seeJwalling's thread) but they are far from the hottest chips on the acquisition board. The ADCs are probably the hottest, as suggested by the error message. Was the cover off of the scope when you left it running for 10+ minutes in an 80-90F garage? If so, there's a good chance that did further damage.You may be able to trigger on the delayed timebase, even if the main timebase doesn't work.Probe-wise, this scope doesn't have the traditional 1M inputs. You'll need active probes from the P624x series or a "buffered passive" probe (P6339A). Could also use high-frequency passive probes with low-impedance tips (e.g., P6156A would probably work).

author: denimdragon
author_rank: Regular Contributor
timestamp: June 19, 2017, 02:38:16 am
content: Quote from: KE5FX on June 19, 2017, 12:34:26 am@KE5FX Thanks for the info. The first time I didn't have the cover off, the second time I did but it had additional fan blowing on it. I have the case back on it and, I have been running test (onboard) on it. So far, it's been running well over and hour and hasn't shut down again. I saw a mod to put heatsinks on the chips and I plan on doing that after I can see what's going on with the triggering, and of course I can't do that until I get one of the recommended probes you mentioned. I just missed a P6339A earlier today by 5 minutes but I wasn't sure if it was the right oneI'm going to review the thread link you sent me and get more information. I was actually watching Louis Rossman do the repair live, and had no idea I would be in possession of one 2 months later. It's a pretty straightforward process but how can I tell which is good or bad?Board schematics for this model seem non-existent, but I know there's got to be one out there. The TDS520B has a component service manual on the Tek site, so I'm hoping there's one for this one.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 19, 2017, 03:14:33 am
content: There's nothing out there for any of the other later TDS scopes. From what I have read, the TDS520B component level service manual was only produced at the behest of a very large customer (likely U.S. military) and was never intended for public consumption but got out somehow. Any other TDS manuals that may possibly have been used internally at Tek have never been publicly released. Many people have spent a lot of time looking - unsuccessfully. The TDS520B can give some guidance for the latter scopes as well though, many of the boards are the same or similar.

author: KE5FX
author_rank: Super Contributor
timestamp: June 19, 2017, 03:17:41 am
content: Quote from: denimdragon on June 19, 2017, 02:38:16 amThe P6339As are fairly common, but note that they will only give you 500 MHz BW, same as a P6139A. To really get the most out of a 50-ohm scope like this one, you need active probes. Prices on eBay vary wildly for P6245s, anywhere from $100 to a kilobuck or more.Also keep an eye out for the P6248 differential probe and the higher-performance P6249 single-ended model. I've found it useful to keep two P6245s and a P6248 handy for concurrent operation.That said, if this is going to be your only scope, the P6339A is the only way to look at signals of more than a few volts. Overall the TDS 694C is not the most economical choice for general-purpose poking around. It willworkbut the short record length and the need for expensive/delicate probes can be inconvenient.QuoteAs far as I know it's always the same two that fail for a given combination of defective channels. Hopefully someone will correct me if I'm wrong there. If that's the case, and if you have the same problem jwalling did, then simply duplicating Louis's repair job may yield a working scope assuming no other major failures exist. Keep that acquisition board cool!QuoteNo one has uncovered a schematic for the 694C yet, unfortunately. If they have, they're being awfully quiet about it. The 520B schematic is better than nothing, but there are obviously a lot of differences.One thing I find curious about the 520B schematics is the references it makes to a "garbage trigger" channel and/or chip. Perhaps that nomenclature is a clue to the seemingly-redundant nature of the trigger chips in the 694C. Again, if anyone knows what that really means, they're not saying.

author: Jwalling
author_rank: Unknown
timestamp: June 19, 2017, 09:39:01 am
content: Quote from: denimdragon on June 18, 2017, 11:09:42 pmA 12V CPU fan is not enough airflow! When I work on these scopes, I use an 18" diameter fan of the type that you would use for cooling a room, like this one:http://www.homedepot.com/p/Lasko-Cyclone-18-in-Adjustable-Pedestal-Fan-1823/202563946I have it blowing in from the right side of the scope. The right side of the scope I keep lifted about 3 inches, which helps to cool the power supply as well.As far as which trigger chip is bad, look for discoloration on the pins and package that may indicate which one is bad, if you're lucky.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 19, 2017, 11:31:35 am
content: These scopes rely heavily on the enclosure to guide air flow into the right places. Running them with the lid off for an extended period is not recommended; the air simply doesn't flow across the boards correctly.

author: Jwalling
author_rank: Unknown
timestamp: June 19, 2017, 12:44:58 pm
content: Quote from: eKretz on June 19, 2017, 03:14:33 amThere is a guy by the name of Hakan who occasionally posts on the Yahoo Tekscopes group that claims to have the entire TDS500, 600, and 700 component level service data with schematics on microfilm or microfiche - forget which. Dave from Artekmedia has tried to get him to lend them to him to make proper pdfs of them, but he refuses as he's worried something might happen to them. A great pity...

author: denimdragon
author_rank: Regular Contributor
timestamp: June 19, 2017, 02:54:20 pm
content: Quote from: Jwalling on June 19, 2017, 09:39:01 am@AndyC_772 @Jwalling Yeah I found out about the case/airflow situation a few minutes after the unit shut down initially. And I will also take a look at the chips again under my microscope. To be honest, I'm not entirely sure that the heat issues aren't due to Friday being a very hot day, and that could have caused the problem mainly. Yesterday was fairly cool, for Houston, Texas anyways. I turned the scope back on last night and left it on overnight and it's currently on now. It hasn't turned of at all. During the time, I have ran 20 SPC manual test an it has passed every one. I won't be able to check anymore results until I receive a probe that will work with the scope. This being the case, I'm going to have to set it aside and look at some of the others I have to work with that have smaller issues I should be able to repair (784A, 744A, and a 724A. I also have 684C, 544A, and another 744A, but they have power on issues and they will be my training scopes).Now, a bit of advice from you guys who have worked on the TDS5XX, 6XX, 7XX series of scopes and any other high BW scopes, if you don't mind, but first let me explain my situation a bit better so that you will understand where I am and what I'm going to do.I'm currently a fresh EE student (new career switch at 43, just started from scratch fall 2016. I graduated from school the first time in 1996, so I decided to go through the whole process again), and I'm footing the bill for school. I plan to repair, calibrate, and restore/refurbish oscilloscopes as a side business to pay for school, plus I just love test equipment. I purchased these scopes (and 2 others which should arrive today, Tek 7154B, 644B) as a start. With this in mind, here are my questions to you:1) What standard set of tools/test equipment do I need to perform what I want to attempt? I have a function generator currently that I use for waveforms, but I'm currently looking for a signal generator, and a high current probe for my Fluke 87V which should be here this week.2) What scope should I use as a daily driver for this type of work and what type of scope should be my specialty scope (which is why I was thinking about keeping the 694C. I actually have a Tek TCP0030 High current probe coming as well this week. I was planning on using them in tandem if that would work, but now I'm not sure about keeping the 694C after hearing about the trigger chips and overheating issues)?3) In your opinion, is the 694C worth the trouble of fiddling with, or should I just sale it as is and move on? The scope seems very solid other than the issues I have spoke about and it has the 1M option for the extended memory. I have to make money so I can't afford to waste time on equipment that would be better suited sold as is. I do, however, want to become very good with scope repair and I know the challenging jobs are the ones where I grow the most, but there is a cut-off point.As usual, I really appreciate the guidance