Sanity check 7812 voltage regulator for Minolta DImage Elite 5400 II scanner - Page 1
Forum section: electronics / repair

author: robca
author_rank: Frequent Contributor
timestamp: June 10, 2019, 12:26:46 am
content: Hello,I'm repairing a Minolta DImage Elite 5400 II scanner. I'm not new to electronics repairs, but I'm always super-nervous when working on expensive equipment (the scanner sells for $500 on eBay currently, even if old). There's no schematics for this scanner, and the "service manual" you can find online is a joke (just how to replace improved parts, plus a calibration procedure requiring tools that are not available anymore)The scanner in in good conditions, but doesn't properly initialize. Halfway thru the self test, after a few whirrs, it stops flashing the front LED 6 times, pause, and repeat. Alas, no error codes I could findI started measuring all the various voltages. It uses a 24V brick, which works well. I found the 3.3V rail, as well as 1.7V. There also seems to be a 12V rail, connected to the CCD assembly (a separate PCB). It uses a d2pak 7812 regulator (picture attached). Input is 22.5V (there are 2 beefy 240 Ohm resistors in parallel dropping the voltage to, I assume, lower the input voltage). I measured only 2.2V on the output, so I lifted the outpin pin, and sure enough 2.2V on the disconnected pin as well, so it seems to be a failed voltage regulator, not a problem with the load (granted, the load might also have problems that caused the 7812 to fail in the first place, but still worth replacing). Ordinarily i would just replace it and try if it solves the problemBut it seems strange to me to have 12V going to a CCD assembly, given all the other voltages already present, and I don't want to risk ruining that PCB with a high voltage (and that CCD assembly is powered even without the output from the 7812 connected, so there are other voltages there... it's very hard to take measurements on the CCD PCB assembly, but there are 3 TP test points there and I can see a signal on those, so that board is powered even without 12V)Is it possible that an SMD chip marked 7812 and looking exactly like a d2pak 7812 voltage regulator could be something else? I mean, I know of no circuit using 2.2V, especially considering that there already is a 1.7V and 3.3V supply, and there are no members of the 78xx family with 2.2V outputI guess I'd like just a confirmation that the chip is indeed a 7812, and I can simply solder an identical replacement

author: amyk
author_rank: Super Contributor
timestamp: June 10, 2019, 02:17:55 am
content: The capacitor on one side is a 22uF 50V, and the other side 100uF 16V. You can check if there are any other parts with obvious voltage ratings on what you think is the 12V rail, and if they're all above 12V with some margin, then it's a pretty safe bet that it's 12V.12V for a CCD is not unusually high, and they often require a set of various non-standard voltages --- Google some CCD datasheets (even better if you can find the exact one your scanner has) and take a look.

author: robca
author_rank: Frequent Contributor
timestamp: June 10, 2019, 02:50:04 am
content: Thanks for chiming in... The only other capacitor on the CCD board is 25V, so that would point to 12V as well. As for the CCD, it's soldered on the opposite side of the PCB, and the only way to look at it would be to take apart the optical assembly, which means completely throwing off the optical path and requiring extensive re-calibration. Given that the scanner was discontinued at least 10 years ago, there is nobody with the right tools anymore. Once the optical path is out of alignment, the scanner would become pretty much unusable (and I learned a long time ago that I should never, ever touch anything with precise optics)In any case, it's likely to be a custom or semi-custom chip, given it's a linear CDD with 5400 dpi and 3 colors, and I doubt I'd find the datasheet of a chip that old and specializedAny other guess/input is welcome

author: PKTKS
author_rank: Super Contributor
timestamp: June 10, 2019, 01:36:53 pm
content: I would be very skeptical with this tooIf I am not mistaken the silk screen label is "Q" not "U" or "IC"or whatever...My hopes with SMT markings are long gone and would not assumethat as a standard 7812 with that "Q" label....2 cents of caution herePaul

author: robca
author_rank: Frequent Contributor
timestamp: June 10, 2019, 03:38:23 pm
content: Comments like these is why I asked the question here in the first place, thanks!But I'm more and more sure it's a voltage regulator. While it's true that all the big ICs are marked Uxx, there are at least 2 other voltage regulators (a 1117-33 and an equivalent to a 1117-25) marked with Q12 and Q13. For both, the pinout is consistent with a 1117 (Vin, Vout, GND, including the tab being Vout) and the voltages are all correct: Vin=5V, Vout 3.3 and 2.5 respectively. So it looks like voltage regulators are marke Qxx on the PCB, not Uxx as one would expectAs for the Q3 in question, I don't know of any other component that would make sense in a circuit connected to GND and 23V, if not a voltage regulator... any guess what else could it be?I also did some more searching on the type of linear CCDs used in photocopiers and scanners, and the majority seem to use 12V or even higher (up to 15V), so my concerns about the likelihood to have 12V going to the CCD PCB are diminished.Any other input before I try replacing that component with a 7812 voltage regulator?

author: ptricks
author_rank: Frequent Contributor
timestamp: June 10, 2019, 03:48:51 pm
content: I would draw out a schematic and see what it looks like on paper and see if that fits what a 7812 would look like in the circuit.It definitely look like a voltage regulator, it even has the decoupling capacitors at c139 and c104.Whenever I replace something like this in a circuit I leave the output pin disconnected and measure the current on the output and limit that current with a resistor till I am sure something else didn't cause the regulator to fail (often saving me from repairing a pcb). Could also use a power supply and limit the current.

author: robca
author_rank: Frequent Contributor
timestamp: June 10, 2019, 04:49:06 pm
content: Unfortunately it's a 4 layers PCB, with the top and bottom layers mostly power planes, so following the circuit is pretty much impossible outside of that nearby area where it's obvious as you say that there are both ceramic and electrolytic capacitors, usually decoupling for a power supply (C139 is in parallel with C156 and C104 with C114). And as I said, I'm positive that one pin is Vin at 23V, one is ground, and the other goes to the CCD PCB thru a connector. And as far as I can tell, that "supposed 12V line" goes nowhere else on the main boardVery good point on the current limiting resistor, definitely something I'll do. Thanks!

author: amyk
author_rank: Super Contributor
timestamp: June 11, 2019, 01:41:50 am
content: Agree with not disturbing the optical alignment unnecessarily, but just in case (different make/model, but probably not too different):http://www.shtengel.com/gleb/Nikon_8000_9000_CDD_alignment.htmQuote from: robca on June 10, 2019, 02:50:04 amThey're not as special as you think --- and companies like Toshiba and Sony make tons of them, for both regular flatbed scanners (using a reduction lens) and film scanners (without the reduction); here's an example of a 3x7450 pixel, roughly 5400dpi sensor (using 5V and 10V supplies):https://www.eureca.de/datasheets/01.xx.xxxx/01.04.xxxx/01.04.0122/TCD2716DG_090629_E.PDFThey're also not particularly expensive, as a search online for the above part number will show.One thing to be aware of if you're going to try testing with an external current-limited supply is power sequencing requirements; the CCD I linked above doesn't appear to have any, but if it's one with more complex active circuitry, you may need to be careful to prevent latchup (that is, assuming the regulator failure didn't already cause/was caused by other damage.)That said, more pictures of the assemblies would be helpful --- maybe someone can recognise certain components and give you more info.

author: robca
author_rank: Frequent Contributor
timestamp: June 11, 2019, 07:35:02 pm
content: Quote from: amyk on June 11, 2019, 01:41:50 amYes, I know that in some cases optical alignment is not too bad, but I also had experiences where a highly collimated device didn't work properly afterwards. This is a film scanner, and scans with 5400 dpi and the CCD has 3 degrees of freedom, plus there are a mirror and focusing lense, each with its own screws. And from what I gather from the scarce documentation, there are parameters to insert in firmware (with tools that do not exist anymore) for the final calibration after the optical path is aligned... Like you say, that would be last resortQuote from: amyk on June 11, 2019, 01:41:50 amYes, you are right, thanks. After I wrote that i did some more looking online, and I did find quite a few from Sony and Toshiba, but none yet with the right characteristics for mine. This has 24 pins (which is very common), in a DIP-like arrangement of 12 per side, but with a gap in the middle (i.e. 6 pins, big gap, 6 more pins on each side). I did find some with that pin layout, but none with the same arrangement (I'm checking the GND and 12V pins to see if they at least are in the right places). Still, if there's a CCD problem, the scanner will be resold for parts, since it would go beyond my ability to fix it (once again, optical path, but with a new sensor, there's no hope without the tools). Still, I'm satisfied that having 12V makes sense, since most of them seem to either be 12V, or 10V to 15VQuote from: amyk on June 11, 2019, 01:41:50 amAgreed on the power sequencing risk. Assuming that the dead 12V supply has not caused problems, I would not want to apply 12V out of sequence, so I'm waiting for the 7812 to get here and use a current limiting resistor just in caseMy main concern at this point is that I measure 88 Ohms between ground and 12V, with the 7812 disconnected. The CCD board is populated on both sides, and I can't see all components, nor follow the traces. My hope is that the 47 uF electrolytic capacitor is defective and partially shorted (yes, very uncommon failure mode, not that likey), or that 88 Ohms is for some reason expected... much more likely, though, I will replace the 7812 and discover that still it doesn't work, due to the failure of some other component on the CCD PCB. I'm enclosing a picture, in case someone has more ideas on what to check (I disconnected the main board from the CCD PCB, and I verified that the 88 OHm between 12V and GND is on the CCD PCB)The current plan is to see what happens with a working 7812 and a current limiter and if it still doesn't work, I'll remove the main board that is blocking access to the CCD PCB and probe more. The reason I'm not removing the main board yet is that there are 2 flexible PCB cables and there's a non-negligible risk of cracking those (they are 15 years old), so I'm willing to risk connecting 12V to 88 Ohm for a short time and see what happens. I know it's not very smart

author: bob91343
author_rank: Super Contributor
timestamp: June 11, 2019, 08:06:41 pm
content: I have been repairing stuff like this for decades. I wouldn't hesitate to replace that 7812 with a voltage regulator. I would cut the leads so I wouldn't need to solder to the board directly. I have lots of 7812s salvaged from various discarded gear like copiers and monitors.If you want to be safer, put an ammeter in series with its output. But 7812s are supposed to be overload protected so even that is overkill.

author: robca
author_rank: Frequent Contributor
timestamp: June 11, 2019, 08:33:06 pm
content: Quote from: bob91343 on June 11, 2019, 08:06:41 pmI'm really not that worried about the 7812... and in any case I bought 2, since I didn't want for another shipping in case of problemsI'm more concerned about sending 12V down a trace that measures 88 Ohm (actually 88 when measuring with the black lead of the tester on ground and red on +12V, 92 when the leads are used the other way around). itt looks like there's a semiconductor between 12V and GND that is not working properly. I only now realized the resistance changes with the polarity, so that would rule out the capacitor theoryAnd, yes, I was planning to connect the new 7812 with wires just to see what happens

author: amyk
author_rank: Super Contributor
timestamp: June 12, 2019, 03:10:16 am
content: How good of a match is the pinout to this?https://www.eureca.de/datasheets/01.xx.xxxx/01.01.xxxx/01.01.0031/ILX550K.pdfSame "gapped" 24-pin DIP package, although twice the resolution (but this scanner could just be averaging two pixels), single 12V supply. If it's not that one, I would still guess Sony because I've only seen their sensors with that package.Quote from: robca on June 11, 2019, 07:35:02 pmThe way I see it, fixing broken wires is easy; burnt semiconductors, not so much...88 ohms static resistance across the 12V rail is far too low if there's only the CCD and the ADC (the AD9826); I'd check whatever other components are on that board, and try to reverse-engineer the pinout on the connector.

author: robca
author_rank: Frequent Contributor
timestamp: June 12, 2019, 06:20:11 pm
content: Yes, Sony seems the most likely, but my CCD has different power connection (GND and 12V) compared to the ILX550. Really appreciate the pointer, though. I looked at many Sony sensors, but not that one yetIn any case: after measuring everything again, and double checking that no other semiconductor is connected to the 12V rail (thanks for the suggestion), I decided that the safest course of action was to try powering the 12V rail with a proper power supply, limiting current first to 50mA. After all, the fact that the static resistance was different depending on the polarity suggested a possible ESD diodes on the power rail, and not a real short. My other reasoning was that if the CCD needed proper power up sequencing, in any case didn't get it with the 2.5V output from the faulty 7812. And since I confirmed that the CCD only had a 12V supply, no other voltages, I decided the the risk of powering up with a 7812 capable of 1A output was much higher than using a bench power supply and risking worst case a CCD not powered up the right sequence. At best it would work, at worst would confirm a short or the need for proper power up sequencing, both outcomes vastly superior to feeding 1A on a possibly shorted PCBI'm delighted to announce that upon increasing the current to ~80mA, the scanner is working properly. It's recognized by the scanning program and scans with what looks like good quality (I didn't want to do a proper scanning at high resolution, since that can take in excess of 6 minutes per frame, and didn't want to have the scanner work semi-dismantled to avoid mechanical issues due to removed bracket and optical problems with dust and similar). At 50mA, the power supply was limiting current, but at 80mA it shows that the CCD only uses 60mA, well within the specs of most CCD chips I sawI want to really thank everyone who chimed in, especially amyk for providing continuous feedback and helping me think thru the steps (sometimes just thinking about what to write as an answer gave me a clearer understanding of the problem, beyond all the great suggestions)The 7812 will arrive today, so after a final soldering and cleanup it's time to start attaching the 3,000+ slides from the past... at 6 minutes each