Racal-Dana 1992 - Page 1
Forum section: electronics / testgear

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 02:21:15 am
content: I thought I'd start a new thread for my questions rather than resurrecting the older threads here.I picked up an RD 1992 from eBay (not working). I replaced all the buttons and it powers on and broadly seems to work fine. I can't run through all of the tests in the manual since I don't have a signal generator, but I was able to run through several of them. There are a couple of things that I wanted to run past people here before I go fiddling.Channel A just displays random numbers when nothing is connected. Channel C shows all zeroes. lowimpedance herehttps://www.eevblog.com/forum/testgear/racal-dana-1992/msg341984/#msg341984indicates that this is due to worn capacitors. I just want to verify that what I'm seeing could be caused by that? Also, when I connect a simple signal (from my cheap eBay DDS signal generator), the randomness persists unless I use the x1 filter, in which case the display shows a reading which looks broadly correct.I also tried test 61 from the manual. This should output a saw tooth waveform from the trigger level outputs on the back of the unit. I connected my scope to each of the outputs (left the ground clip dangling, but even clipping it to the case or a screw had no effect). I didn't see the sawtooth shown in the manual - just a bouncing horizontal line. Trouble is, the manual doesn't give any indication of what to do if you don't see the results they provide! Could this also be due to bad capacitors?Lastly, how can I check the accuracy of the internal oscillator? I was lucky enough to get a unit with the 04E option, which appears to be an undocumented OCXO option. If I plug the external output on the back into my scope, I get 10 MHz, but I was expecting a square wave - what I see is a squarish wave with a lot of ringing on it.Thanks in advance for ideasAnthony

author: VK5RC
author_rank: Unknown
timestamp: August 26, 2014, 02:35:01 am
content: Re the 10Mhz output, was the scope in 50ohm input impedance mode? Some devices use a 5Mhz oxco and double it, several bits of my gear do this and the ref output is not a pretty sign or square wave.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 02:42:12 am
content: Quote from: VK5RC on August 26, 2014, 02:35:01 amI'll check the scope tomorrow. This OCXO is a doubled 5Mhz unit.

author: VK5RC
author_rank: Unknown
timestamp: August 26, 2014, 02:46:22 am
content: Sorry I can't spell SINE wave, d'oh. I think there are some harmonics in there which can be useful at times.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 26, 2014, 02:57:24 am
content: You can use your scope to check the ripple on the +/- 11.2v rails, if present or excessive on the regulators inputs then the 680uF need replacing.Does selecting the 50 input termination stop the display fluctuation too ?.The x1 filter is for low frequency stuff and is generally a good idea when measuring below 100kHz. Have you got a higher frequency source (the DDS ?, maybe quite noisy on the output also jitter). I recall some one else needing to replace some smaller bypass electro's distributed around the PCB to reduce random readings with no input.If you do have a higher frequency does channel C work?.The reference outputs are generally not very pretty!!.I can also try test 61 to see what I get.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 26, 2014, 03:21:56 am
content: Okay test 61 gives the correct waveform (remember its around 2 Hz).Also with no input I get all 0 on the display with no A or B trigger LED's flashing (Freq A selected). Flashing triggers with no input could be a hint of possible supply issues.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 09:41:51 am
content: Quote from: lowimpedance on August 26, 2014, 02:57:24 amThe 50 ohm termination doesn't stop the fluctuation. My DDS source is <100kHz, so the x1 filter usage makes sense. I was surprised that the source does seem to be quite accurate and stable. The DDS can also produce 2, 4 and 8MHz and that displays fine on ChA, without the x1. On ChC, the results are incorrect (eg 2MHz displays as 13MHz). I assume that is because ChC is for higher frequencies?Quote from: lowimpedance on August 26, 2014, 03:21:56 amThanks for verifying the test. I'll try that again and make sure I don't have something odd on the scope, but I didn't see anything except a line bouncing up and down. With no input on ChA, the trigger LED does flash, so sounds like I need to poke around the supply. I'll let you know what I find.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 26, 2014, 09:59:20 am
content: Quote from: aroby on August 26, 2014, 09:41:51 amCorrect. The C channel is only specified down to 40 MHz.QuoteMake sure the scope is set for DC input.QuoteI wouldn't be too concerned about channel A reading randomly when there's no input. I have a 1991 that does this. It depends how sensitively the hysteresis is set on the channel comparator. On the other hand - it could be ripple on its power supply.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 05:46:21 pm
content: I just ran some quick tests. C52 and C59 measure 600uF and 602uF respectively (with an LCR), which is within the 20% tolerance described in the manual. Measuring the voltage across gnd and output on the two regulators, IC3 and IC4, shows 11.979v and -12.19v respectively. On the scope there's no discernible ripple (I just connected the scope probe and gnd across the regulator pins).I've since found a troubleshooting flowchart, so I'll investigate that and see what I find.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 05:59:48 pm
content: Quote from: Andy Watson on August 26, 2014, 09:59:20 amWould be nice to see if I can find a cause though! I'm doing this for fun and learning, so it's not critical.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 10:00:19 pm
content: I am beginning to suspect the two large caps, C49 and C50. Both have large bulges.My LCR meter only shows 6pF for C50 (the one on the right). It should be 4,700uF. I'm not sure how to test C49 - it has 4 leads. Any advice?I guess these are vintage components. C49 can be found herehttp://www.ebay.com/itm/380100849540.I've looked on mouser and digikey and can't see any 4 lead caps like it. What should I search for? The 4,700uF seems easy to source and replace. They are both 16V.Anthony

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2014, 10:57:34 pm
content: The extra leads on the big capacitor are part of the can and provide extra stability for mounting. They are also one of the electrical connections but since they are all connected together, the printed circuit board may use them as jumpers as well.A snap in or radial leaded replacement capacitor can usually be made to fit.My 1992 does not trigger and show random readings unless I touch the center conductor of the BNC. If I do so and set x10 attenuation, then I see the power line frequency.

author: aroby
author_rank: Regular Contributor
timestamp: August 26, 2014, 11:35:01 pm
content: Thanks for that. I can't get much more than pF out of the larger cap, so I'm going to replace them.Anthony

author: lowimpedance
author_rank: Super Contributor
timestamp: August 26, 2014, 11:50:46 pm
content: The bulge is only the plastic disk on top, not the can of the cap.Is your LCR meter capable of reading 10000uF ? , maybe not reading correctly (6pF !!) Can you verify the LCR meter on a known good electrolytic of a similar value. If it is indeed 6pf then 'bobs your uncle'. I would also suggest you replace them all at the same time as they are not getting any better with their advanced age.Okay I have just tried my other two 1991's that have been in the cupboard for quite some time (ie not used). And when first powered one unit failed to start properly at the first few power cycles but then powered up correctly from then on BUT it also has random triggering with no obvious repetitive nature. The second unit started up first time and also had a few odd triggers with no input, but then the longer it was on the more stable it got with no odd triggers .The unit that I use has no issues, just sits with no random triggers at power up.So my guess would be to suspect all the electrolytics as being passed their prime. One other thing to suggest is to re seat the socketed IC's first before any mass re cap.EDIT: Just re seated IC39 (main chip) on the unit that had trouble starting and still had random triggers, and now it appears to be stable with not random triggering !. Could be just coincidence though.EDIT2 : Still might be something with the crusty old sockets, however after being stable for a while a few more random triggers appeared.My money is on both caps and the old IC sockets.Footnote : After being off for a while and just turned back on its stable from start up !, will have to see if triggers creep back in as IC39 heats up.

author: aroby
author_rank: Regular Contributor
timestamp: August 27, 2014, 01:00:27 am
content: The readings weren't correct. I have a DE-5000 LCR meter which I was using on Auto. When I set the frequency on it to 100Hz, I got 4.76mF and 11.35mF from the caps, so they seem fine.I'll check out the ICs.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 27, 2014, 07:03:59 am
content: After some time powered up there was only the odd trigger or two noted.I gently tapped the top of IC39 and was able to induce triggering with random readings with each tap, (suspect the socket)Do be careful when getting IC39 out of its socket as the TEC hybrid is close by, you do not want to crack that assembly !!.

author: aroby
author_rank: Regular Contributor
timestamp: August 27, 2014, 12:55:20 pm
content: ok, I'll check that out.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2014, 01:35:21 pm
content: Mistaking 6pF for 4760uF is not exactly a ringing enforcement for the DE-5000 LCR meter's reliability. I have run across large can style capacitors which failed and read that low but it was because internal corrosion cut one of their leads.

author: aroby
author_rank: Regular Contributor
timestamp: August 27, 2014, 01:58:39 pm
content: Quote from: David Hess on August 27, 2014, 01:35:21 pmIndeed, but I need to read the manual more closely. I just had it set on Auto, and also manually set it to Cs (serial capacitance) mode, ,and got these low readings. I couldn't see how the circuit would still work with next to no capacitance, so I then played tried switching the frequency and at 100Hz got more reasonable readings. I assume this has something to do with the physical size of the capacitors? To be honest, I've never really used an LCR and bought it recently to play around with. It's always worked fine on smaller components.Anthony

author: PedroDaGr8
author_rank: Super Contributor
timestamp: August 27, 2014, 02:14:40 pm
content: UQuote from: David Hess on August 27, 2014, 01:35:21 pmI have never had any issues on any capacitors of any size. That being said it's really easy to get strange values if you don't know what you are doing. Since things like test frequency, serial/parallel, open/short calibration all can affect the readings. I think most LCRs can be finicky due to their precision nature.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 28, 2014, 10:26:30 am
content: Following on from my previous post in regard of the suspect socket for IC39 (40pin). I have replaced the socket with a new turned pin high quality one, the chip certainly requires more careful force to re seat in the new socket. There are a few holes that are plated through to the top side ground plane with no thermal releif which make it a bit of a pain but with some care and good desoldering tools its reasonably easy.The result was no random triggers and fluctuating readings with no input, and no amount of tapping would cause triggers either so if any one is getting random readings I would suggest to look at the socket.

author: aroby
author_rank: Regular Contributor
timestamp: August 28, 2014, 09:26:25 pm
content: Quote from: lowimpedance on August 28, 2014, 10:26:30 amGood to know, glad you had some success.

author: aroby
author_rank: Regular Contributor
timestamp: August 30, 2014, 12:06:03 am
content: Looking some more at this, I ran the tests to check the relays on ChA. You connect the external output to ChA, enter Check mode and use code 77 to run the test. It failed with Er Code 51. I had to take the shield plate off from the underside of the main board to test the relays.One of the resistors (I think it was R169) was cracked, so I replaced that (thank goodness for the board layout in the manual!) and ran the test again and got Er Code 52. That is for RelayC, which is the 1Mohm/50ohm switch. Sure enough that relay doesn't work. Not sure how I can replace it since it seems to be unobtanium. It's a reed relay with 5 pins - 3 on one end, 2 on the other. It's the top G57D-X424 in the picture.I still have the issue of the random display with nothing connected and no trigger waveform on the external trigger test points. So my hunch is that there is something wrong in the trigger circuitry. I don't want to fiddle with the trim pots that are used for setting up the device, since I don't have the signal generator needed to do that, as described in the manual, so I may be a bit stuck right now.