Solartron 7150 check and a small repair - Page 1
Forum section: electronics / repair

author: h.bashar
author_rank: Contributor
timestamp: August 18, 2016, 09:09:55 pm
content: Hi, earlier this week I scored a crusty old Solartron 7150 multi-meter, this is my first 6.5 digit multi-meter so naturally I was very excited about it, when it got delivered the meter case has a missing handle and part of the case is broken, quickly plugged it in and it booted successfully. As many of you know the LCD display back-light in these meters had four yellow LEDs that are dead after many years of service, so those got replaced with bright blue ones.This was my initial diagnosis of the problem which I will keep below just for future reference but it waswrongand I will explainfurther down:I had a problem with testing the meter because I didn't have any accurate voltage source, so I hooked up a 9v battery to the front test leads and while I think it measured it to 9v, the LS digits where not stable and had a negative trend, I tried the resistor range with 0.1% 10K Vishay resistor ( I think it was 25ppm) and the same problem occurred, so I opened the meter and checked the voltages on the power supply regulators and they all checked out as per the service manual, the only problem I encountered was when I tried to measure the output voltage across C311 (the +15V rail) which is a 1uF tantalum capacitor, while I was placing the positive lead of the meter to the positive marking on the capacitor and the negative lead to capacitor negative I measured -15V, so I was confused so I re-checked the test meter leads and they were hooked the right way so something fishy was going on, I consulted the service manual for diagrams and also the PCB marking under this capacitor and it all pointed out that this capacitor was soldered the wrong way around!Imagine in all these years someone was using this meter not knowing about this fault, I am surprised it passed any calibration, even the 15v regulator IC was running hot, considering it’s a one amp regulator supplying opamps. So I disconnected one capacitor leg and checked its value with my DE-5000 LCR meter and lucky enough it was still in working order. I disordered the other capacitor leg and put it back the right way around this time and tested the meter again and this problem has disappeared. I don’t think this meter has been repaired before but it’s surprising that such an error passed unnoticed all these years.There is still some work to be done for the meter as the mains input filter needs replacing, many dry joints on the PCBs and a calibration. I hope this post will be helpful for someone.There was nothing wrong with C311 except that the marking on the capacitor was printed in reverse showing the wrong polarity, which I didn't catch and as a result I put it back the wrong way around based on the reversed silkscreen print, now I am pretty certain that I fried it and it will need replacement which will be done in due time, I will be changing this cap and check the rest as well for miss- labelling and report my findings.For all people who helped me with this thank you so muchUpdates:++de-soldered C311 and tested with LCR meter and its still functionalsee picture WP_20160820_001 (Custom)++ went over with LCR on the remaining polarized CAPS on the board two were toast C305, C306 (100uF) on the 25V rail (pictures WP_20160820_015 (Custom) and WP_20160820_016 (Custom))C302(220uF) is fishy (WP_20160820_017 (Custom))++Rest of CAPS were fine but at this point I lost all faithso all have been de-soldered and pads cleaned waiting for new replacements.New Update:CAPS were all replaced with Vishay ASM series and new mains entry module was installed, everything looks in working order nowBest wishesHasanain

author: Prehistoricman
author_rank: Regular Contributor
timestamp: August 18, 2016, 10:36:33 pm
content: Did the cap appear to be soldered in that polarity from factory, or did it look as if somebody tampered with it?

author: h.bashar
author_rank: Contributor
timestamp: August 18, 2016, 10:43:09 pm
content: As far as I can tell yes, evident by the dried solder joints, what surprised me further that it didn't leak, open or short in all those years, trust me I didn't expect to find that thing in such a high quality multimeter.

author: h.bashar
author_rank: Contributor
timestamp: August 18, 2016, 10:59:22 pm
content: The 100uF tantalum are similar to this from RS:http://uk.rs-online.com/web/p/tantalum-capacitors/5381714/i am not sure about their behaviour when the polarity is reversed other what they taught us in school (dooms day and all), the data sheet for this one says it can take 0.35V reverse voltage at 125degC so this one was taking 15V reverse for God know how many years of operation, and it still checked Okay by the LCR meter! wonderful!

author: Prehistoricman
author_rank: Regular Contributor
timestamp: August 18, 2016, 11:19:21 pm
content: It's a miracle! Would this unit be hand-soldered? Maybe it's human error.I noticed something interesting. I put up a post today about my switch-mode bench supply and uploaded pictures. Those pictures were taken from a Windows Phone, and cropped with a tool that adds (Custom) after the name. I look at your file names and... wow!

author: h.bashar
author_rank: Contributor
timestamp: August 18, 2016, 11:33:10 pm
content: I know right, some of the parts must be hand soldered, so probably a human error. yes its an old Lumia920 WP and yes it was a free tool from the net, how is it going with power supply repair? opto-couplers tend to fail trust me I know because I repaired some equipment before that had an issue with TL113, not a SMPS but it failed because of age. Now put that in a closed power control loop and the magic white smoke will escape.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: August 19, 2016, 12:10:17 am
content: Opto-couplers? But they're just a light and receiver... wouldn't have ever thought to check them. Now I love magic smoke... but only in shit I don't want!I'm realising I have a fairly large lack of tools as far as debugging/repairing goes. I don't have a scope or freq. generator, only a clamp meter.

author: h.bashar
author_rank: Contributor
timestamp: August 19, 2016, 12:43:12 am
content: You are right, but the problem with any LED it fails after some time, if you look for example LED data-sheet it will specify life time in hours, now I wont say I know much about SMPS failures because I tend to steer away from non-isolated mains in general, of-course it will be much easier if you had an oscilloscope to see the change in PWM frequency as you load the power supply. If you are going to keep playing with this thing (which can easily kill you) I suggest an isolation transformer and heavy precautions specially around the mains parts of the supply and make sure the voltage on the input filter CAP is bled by the bleeding resistor each time you approach that part. As for a frequency generator it all depends on what sort off frequency you are after, an old 555 timers can work wonders so try that if you want. Finally I will stay away from using inductive loads to test a SMPS, unless they are designed for it, try to approach the problem with a resistive load first and see if you can approach the stated current of the device without problems.

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 01:29:19 am
content: Tantalum caps that are installed backwards are like time bombs with an unknown fuse length. I've seen them explode almost instantly, and I know of situations where it took months, setting fire to circuit boards that had been in the field for quite awhile. In the latter case, a company I worked for had a memory board (ISA bus 4MB DRAM memory card) that had the silk screen polarity marked incorrectly for one of them. We had calls for many months after the ship date to the support team about systems that caught fire.Nasty things, I don't know why they're still in use...Jay

author: h.bashar
author_rank: Contributor
timestamp: August 19, 2016, 01:38:19 am
content: Hi Jay, I am just surprised it took about 30 years until I discovered it, and it still working!

author: Prehistoricman
author_rank: Regular Contributor
timestamp: August 19, 2016, 01:41:16 am
content: @h.basharSo far, I've only operated the supply with its earthed and insulated case on. Both the mains caps have their own bleeding resistors, thankfully. If not, I'd install some.I've just ordered a really cheap kit for a component tester and frequency generator.The inductor was the first component on my desk that I'd used many many times before as a load, as a spark generator, and a brick that absorbs heat. I've got some resistance wire that I'll use in the future though.@JwallingDo tantalums have a certain niche? I can't see them being so dangerous and used without reason.

author: h.bashar
author_rank: Contributor
timestamp: August 19, 2016, 01:59:11 am
content: Very interesting read on Tantalums inhttps://en.wikipedia.org/wiki/Tantalum_capacitor, basically high capacitance per unit volume so physically smaller than aluminium caps. Also there is a section about reverse voltage that reads:"Tantalum electrolytic are polarized and generally require anode electrode voltage to be positive relative to the cathode voltage.With a reverse voltage applied, a reverse leakage current flows in very small areas of microcracks or other defects across the dielectric layer to the anode of the electrolytic capacitor. Although the current may only be a few microamps, it represents a very high localized current density which can cause a tiny hot-spot. This can cause some conversion of amorphous tantalum pentoxide to the more conductive crystalline form. When a high current is available, this effect can avalanche and the capacitor may become a total short.Nevertheless, tantalum electrolytic capacitors can withstand for short instants a reverse voltage for a limited number of cycles. The most common guidelines for tantalum reverse voltage are:10% of rated voltage to a maximum of 1 V at 25 °C,3% of rated voltage to a maximum of 0.5 V at 85 °C,1% of rated voltage to a maximum of 0.1 V at 125 °C.These guidelines apply for short excursion and should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently"So what the hell is wrong with my capacitor still working after all this abuse! Go figure!

author: picburner
author_rank: Frequent Contributor
timestamp: August 19, 2016, 06:20:45 am
content: This thing intrigued me so I opened my 7150 (mine is newer than yours, Solartron changed designator's names, and I could verify that C161 (your C311)is properly placed and the tensions across it is right.From the photos you've done seems that the manufacturer has printed the label on your capacitor C311 incorrectly rather.In fact, C308, C309 and C310 have the right label (+sign) while C311 not.At this point I think you've put C311 now in the wrong way....

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 09:37:43 am
content: Quote from: Prehistoricman on August 19, 2016, 01:41:16 amThere may have been good reasons in the past, but with all the advances in cap technology (high value ceramics and solid polymer types for example) since then, I don't see a good reason to use them in modern equipment.Maybe someone else would know better?Jay

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 09:44:44 am
content: Quote from: h.bashar on August 19, 2016, 01:38:19 amMe too. Must be a world record. Given that the particular rail is supplied a low current three terminal linear regulator, maybe it got used to being abused?I wonder if it would blow up if correctly connected to a high current source?Would make an interesting experiment.Jay

author: h.bashar
author_rank: Contributor
timestamp: August 19, 2016, 11:17:30 am
content: @picburner, the silkscreen is the correct way and the current placement of the capacitor as well, how do I know? using continuity test the CAP+ is now connected to output of the +15V regulator (IC301) and the CAP- is connected to ROME (0V GND) and to + of CAPS C308 and (C309 which you can see in the picture on the top layer photo of my board) and finally it measures the right polarity voltage across C311 when testing with the multimeter so I am pretty sure it was the wrong way around. In fact the orientation of capacitors in both of our pictures are the same

author: kutte
author_rank: Contributor
timestamp: August 19, 2016, 05:48:15 pm
content: Quote from: picburner on August 19, 2016, 06:20:45 amjust opened my 7150 to see it looks like picburner's device. So I agree with him: your c311 has been mislabeled and is is now put in the wrong way.Kutte

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 06:08:56 pm
content: Quote from: picburner on August 19, 2016, 06:20:45 amGood catch!In any case, I would discard that cap if it's been powered up while in backwards.Jay

author: Kleinstein
author_rank: Super Contributor
timestamp: August 19, 2016, 06:31:53 pm
content: If in doubt with the polarity one could do a test with a non polarized capacitor (e.g. MKS should be similar size) and than measure the actual voltage at the capacitor. There might be a voltage higher than the normal positve supply, e.g. if the cap is part of a charge pump.

author: h.bashar
author_rank: Contributor
timestamp: August 19, 2016, 08:33:34 pm
content: @Jay, Kutte, my mod picture look exactly the same as picburner picture, the capacitor was the wrong way around, nothing wrong with the silk screen, it looks the same on both photos!, it might be that you are pointing to the one with red arrow, that one is how I found it and the last picture is what you should be looking at.

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 01:01:57 am
content: Quote from: h.bashar on August 19, 2016, 08:33:34 pmWell, all I know is that the the picture titled WP_20160818_016 (Custom).jpg the C311 cap itself is marked incorrectly - picburner is right.Jay

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 02:10:28 am
content: Dear Jay, the picture you just referenced is how the capacitor was originally found, i.e in the wrong position, the correct orientation is in the last picture of the first post. The silk screen in both cases is the same my picture and picburner picture unless I am missing something.

author: Gertjan
author_rank: Frequent Contributor
timestamp: August 20, 2016, 07:14:48 am
content: Hi h.bashar,I think you misread the posts of Jwalling and kutte.....They are not referring to the print on the silk-screen, butthe print on the capacitor itself.Just compare your C301 with the same capacitors next to it. Look at the shape of the housing, and where the plus mark is printed....regards, Gertjan.

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 09:54:57 am
content: Quote from: Gertjan on August 20, 2016, 07:14:48 amCorrect!Jay

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 11:07:38 am
content: I can't believe I didn't notice this, you guys are absolutely correct, I am sorry for dragging you in a wild goose chase, I will be editing the first post to reflect this finding and thanks again.

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 11:43:22 am
content: Quote from: h.bashar on August 20, 2016, 11:07:38 amDid you apply power to it with it in backwards? If so, replace it to be safe.Jay

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 11:58:31 am
content: Unfortunately I did, definitely it will get a proper replacement thanks Jay.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: August 20, 2016, 01:53:22 pm
content: So... how come the problem appeared fixed when you placed the cap in the wrong polarity?

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 02:55:16 pm
content: Quote from: Prehistoricman on August 20, 2016, 01:53:22 pmPossibly just opening it up and messing around inside for a bit. Could indicate a bad connector/solder joint...Jay

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 09:50:04 pm
content: All polarized CAPS have now been removed waiting new replacements, first post updated to reflect new findings.

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 09:53:46 pm
content: Quote from: Prehistoricman on August 20, 2016, 01:53:22 pmIn addition to what Jay said I have no idea but further checking revealed more broken CAPS and that magical CAP is still functional

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 10:01:28 pm
content: Quote from: h.bashar on August 20, 2016, 09:53:46 pmBroken/bad or mis-labeled polarity?

author: h.bashar
author_rank: Contributor
timestamp: August 20, 2016, 10:58:50 pm
content: Quote from: Jwalling on August 20, 2016, 10:01:28 pmso far just one mislabelled on the capacitor body itself, two broken and one near bad. just a word of caution: some of these CAPS have some sort of enamel cover around their leads so care is needed to where LCR probes placed specially when they are still on board.

author: guido
author_rank: Regular Contributor
timestamp: August 21, 2016, 08:41:55 pm
content: The smaller silver caps look to me like they are wet tantalum caps. If so they should beverygood caps.So i'm wondering if the faulty caps you found were regular caps or tants.

author: cncjerry
author_rank: Unknown
timestamp: August 21, 2016, 09:03:28 pm
content: a lot of people needlessly wholesale replace caps in old equipment. I have yet to find one of those small silver electrolytic caps to be bad and I have about 40 pieces of old gear, maybe more.Can you plot the negative trace and post it up here? Are there relays in the circuit? A poor relay contact could cause your problem as well.

author: h.bashar
author_rank: Contributor
timestamp: August 21, 2016, 09:08:46 pm
content: Quote from: guido on August 21, 2016, 08:41:55 pmThe faulty ones are regular caps. the smaller silver caps are fine (STC TAA hermetically sealed solid tantalum cap).

author: h.bashar
author_rank: Contributor
timestamp: August 21, 2016, 09:16:10 pm
content: Quote from: cncjerry on August 21, 2016, 09:03:28 pmI agree, not to mention expensive! compared to what was paid for the old unit itself, however with units like a bench multimeter switched on for long times its probably best to replace the electrolytic caps at least. as for the negative trace, until the caps get replaced I cant check it again, it could be as you mentioned a bad relay contact (highly probable considering the age of the device).

author: kutte
author_rank: Contributor
timestamp: August 22, 2016, 05:30:06 pm
content: Quote from: h.bashar on August 21, 2016, 09:16:10 pm...and do not forget to replace the mains-power filter. They tend to explode with nasty consequences.Kutte

author: h.bashar
author_rank: Contributor
timestamp: August 22, 2016, 07:05:23 pm
content: Ordered the filter from RS today

author: h.bashar
author_rank: Contributor
timestamp: August 25, 2016, 11:10:04 pm
content: Did some resistance checks using 110, 1K, 10K, 100K all 0.1%, 1M and 10M 1%, mostly everything is within tolerance(except for the 10M resistor), will get REF102 to test DC voltage range.