Linear Power Supply Problem - HELP!!!! - Page 1
Forum section: electronics / repair

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 04:44:28 am
content: SO - I hope this isn't uncouth to do but I thought it would be good to start a new thread - though i started a previous one herehttps://www.eevblog.com/forum/repair/bjt-replacement-pulling-my-hair-out/under different auspices ... and starting with different questions - so really the problem is now completely different. I hope that's cool with you folks - i just thought that continuing under the 'older' thread would be confusing since there were a few false assumptions i'd made etc etc ... so this would be a clean slate but I am in need of help now more than ever ... so i am at your mercy folks ...SO - i have this 1974 era Yamaha Synthesizer (like a big organ) - you know the soundtrack to blade runner? that was the big brother to this one. ANYWAY - it was working just FINE until one day i turned it on and popped a fuse (F1 in the diagram). So after removing the PS from the keyboard assembly I did lots of reading etc while i waited for the parts to arrive (I figured i would do a complete rebuild - mostly) - ok - well i ended up just changing all the electrolytics on board primarily - and i changed one 2SA490 that i inadvertently pulled a leg from. SO ANYWAY - now - having HOPED that the problem was one of the capacitors that had gone dopey after so many years ... i tried (without success) to start it up again ...symptom 1: there is an open load across the hot and neutral of my power cable.symptom 2: when i apply power with no load the dim bulb doesn't even light - and I measure the voltages/resistance thusly (see diagram linked) at these points.symptom 3: when i apply power when it's CONNECTED to the boards downstream IN the keyboard ... the dim bulb becomes a very very BRIGHT one and stays that way (so i try not to keep it on too long!)here is the circuit diagram and relevant (I think?) measurements:so - what do you think??I thought it was a blown primary for a bit but it doesnt' seem to measure like it ... secondaries SEEM to measure fine (as indicated). I'm just scratching my head here - I can't seem to get any voltage into the reservoir caps ... could it be these confounded SCRs?? But even so - I should think I'd be getting a much higher reading pre-SCR (?) or is that the result of having no load across the outputs?? Should i try to attach a dummy load?? (it's a bit tricky doing that however because of the sense lines imo).or could it be some kind of monkey business with a bad X-CAP?? (at left on diagram) what would cause such symptoms?Thank you for reading this ...Jon T.

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 07:04:46 am
content: quick update - looks like those aren't SCRs but just dual diodes - so no 'control issues' i guess ... AND ... i traced through from the hot to neutral pins of the power cord and indeed it's ONLY the X cap which is blocking the continuity .. hmmm ... shouldn't that be normal or shouldn't the X cap normally be ACROSS the power input?

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2018, 10:04:47 am
content: You seem rather confused.Go back to first principles, disconnect the transformer secondaries and measure the output voltages.

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 10:29:41 am
content: well that's why i'm trying to look at this simply now ... and just look at 'evidence' only - so to me it says there's a problem with the primary somehow ... you probably looked at the old post - in which case i might not blame you for thinking so - but that was a whole other topic .. about replacing bipolar transistors .. and it morphed into something else ... so i spent some time looking at the power supply problem I started with and set it up as cogently as i could.at any rate ... i may well do that though it will require severing all the cables going down to the board in order to disconnect it - so it's like four hours of work which (understandably) i'd LIKE to avoid ... anyway

author: Armadillo
author_rank: Super Contributor
timestamp: September 26, 2018, 11:02:23 am
content: Your primary wiring looks open circuit.The primary connection is not clear and incomplete in the diagram. And your dimmer wiring is unknown. You should use a incandescent light bulb 40W type instead if you meant to connect it in series for testing purpose.Why don't you try alternative primary [if it is not used], by connecting ac input power onto BE and RE instead? [if it was designed as primary].

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2018, 11:59:41 am
content: Quote from: jaunty on September 26, 2018, 10:29:41 amNope, I was only referring to this post.I don't think I read the other one (I may have).You need to verify the transformer is working before you go any further because your voltage readings suggest it's got a problem.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 26, 2018, 12:00:52 pm
content: I have drawn red circles where normally you should isolate the regulator sections, does your (15 and 15s) and -15 and -15s) pins are connected together ?? The S lines are normally senses line ...The green voltages values on the x-former secondary side are definitely wrongHave you tested the main capacitors for shorts leakage, esr, and bulged tops, outside the circuit before ordering / changing them ?Normally you isolate the sections, remove the mains capacitors for tests, test the rectifier bridge forward and backwards and you test the continuity of the transformer primary sides and secondary sides, you may have an blown thermal fuse in it.The x-former secondary ohms values seems good, i do hope you take measurements in AC before the rectifier bridge and DC after the rectifier bridge ??To me your x-former main ac voltage should be between the gray and brown colored wires, you see an capacitor between theses pins, i think the green 117.7v value between yellow and brown is not okay, it may simply an induced voltage in the wiring ... you have voltages taps to select the main ac voltages, you check continuity between all the primary wirings.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 26, 2018, 01:45:58 pm
content: When you say: “when i apply power when it's CONNECTED to the boards downstream IN the keyboard ... the dim bulb becomes a very very BRIGHT one and stays that way (so i try not to keep it on too long!)”. That indicates the transformer can’t be open. However, with a good transformer all the readings you put on the schematic don’t make any sense so there appears to be something wrong with your testing procedure.The A.C. voltage from BE to BE should be the same as from YE to YE and perhaps about 10 to 12 VAC. The D.C. voltage from D5 to common should be the same as from D7 to common and perhaps 12 to 15 VDC. Double check your meter switch settings.With no load connected are the measured outputs +/-15 and +/-8.5 as you have listed? I assume this bulb you speak of is in series with the A.C. power going to the unit. What wattage size is the bulb and how many watts does the unit draw? If the label on the unit says it draws 80 watts and you have a 40 watt incandescent bulb, the bulb may light with the supply connected to the load but the unit will not receive much or any power. Does the bulb also light with no load connected to the supply?The primary wiring diagram to the transformer is incomplete and doesn’t make any sense as shown but if that wiring hasn’t been modified and the bulb lights with the load connected, I assume that wiring is o.k.. I would not recommend disconnecting any of the transformer leads except as a last resort, it could lead to more problems. You should recheck all your meter readings. If the output voltage readings are correct with no load, the problem is probably in the load and not the supply. The fuse that blew did its job by protecting the rest of the circuitry.

author: kc7gr-15
author_rank: Contributor
timestamp: September 26, 2018, 10:06:32 pm
content: FWIW -- I would be suspicious of the bridge diode modules, D1/D2 and D3/D4. I've fixed other power supplies where the bridge module itself developed one or more shorted diodes, internally. I've also seen them fail open (though that usually comes with visible or odiferous signs -- translation: smoke and stench).In any case: Unlike many switchers, linear supplies don't need a load to produce normal output. Power the supply without any load, then poke around with a multimeter to see if the regulators are working.Happy tweaking.

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 10:13:26 pm
content: Quote from: kc7gr-15 on September 26, 2018, 10:06:32 pm

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 10:16:59 pm
content: perhaps THIS is the missing part and explains the weird behaviour - why it only lights up the bulb when connected -the primary side is open circuit unless the power supply is connected to the rest of the circuitry (though this is kind of dangerous for me - with a ps in unknown condition and 'unobatinium' parts downstream ...)

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 10:19:10 pm
content: although i measure 6.1 ohms across the primary winding i suppose i ought to test the transformer in isolation from the power supply ... it's just that it's such a bugger to disconnect it from the rest of the circuit ... so not an easy feat ...note: I will answer individual question in detail shortly - thanks

author: Fryguy
author_rank: Regular Contributor
timestamp: September 26, 2018, 10:28:32 pm
content: Quote from: kc7gr-15 on September 26, 2018, 10:06:32 pmDon't forget to connect the sense lines to their respective outputs ! Load or no load . And yes - happy tweaking

author: ArthurDent
author_rank: Super Contributor
timestamp: September 26, 2018, 11:22:12 pm
content: This is what the primary side actually looks like without the confusion caused by the connectors. With the switch closed there should be resistance between the two active prongs on the A.C. cord. If there is no resistance then the last paragraph may be the answerI'd still like to know some of the answers to points I raised in post #7. I still think disconnecting transformer leads is a bad idea unless you know there is an actual problem with the transformer.There is one possibility that might explain what you're seeing IF the bulb you reference is a neon indicator lamp and not a test incandescent bulb in series with the A.C. line, which is what I originally thought you meant. If the GY lead is open at the transformer there could be leakage through the R-C circuit across the switch and the neon might glow dimly with the switch open. Some transformers have a thermal fuse in the transformer to protect from overheating and if this transformer has one, it may have opened, disconnecting the GY wire. When the switch is closed that neon would glow brightly IF the GY wire is open at the transformer. There appears to be an unconnected 110VAC winding on the transformer and if you really know what you're doing you could disconnect the primary leads now used on the transformer and connect a fused cord to those unused leads to test power the power supply with no load. These leads may have been there to wire in series with the other winding to operate the unit from 230VAC, I don't know. Don't try this unless you're pretty sure that the gray wire is open and you know what you're doing. I'm not responsible for burns or smoke damage. I would only try this for testing with no load connected to see if the rest of the power supply works.

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 11:34:35 pm
content: Quote from: ArthurDent on September 26, 2018, 01:45:58 pmYes sorry Arthur i was in the middle of some work and i'm just getting around to 'processing' some of this - i'm grateful for the answers (and questions) so far ...no - with no load connected (and what looks like a disconnected primary vis a vis the second diagram posted (so maybe that's my problem - it's a very weird setup compared to most such keyboards i've worked on) i only get millivolts ... maybe just spurious readings bleeding back through the circuitry??? my dim bulb tester bulb is a 60 watt bulb ...so i guess my confusion is due to the fact (it looks??) that it cannot be tested outside of the keyboard - not without jumpering some of the connections anyway ... i was under the impression ONLY the sense lines needed jumpering but apparently it's not so ... (duh!)PS - Zaphod told me to remove the transformer for testing ... i shouldn't trust him?

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 11:42:16 pm
content: Quote from: coromonadalix on September 26, 2018, 12:00:52 pmyes - i have the pos. and neg. 15 volt sense lines connected (shorted to their respective feeds). just before this I went through and did a 'full refurb' and put in new nichicon 105 degree capacitors (3300 uf instead of 2200 - all others same value) and new 4558s on the board ...i think my problem with the primary side might be 'fixed' (see previous post to this) - due to a total oversight (second diagram) ... can you point me to a procedure to check the bridge rectifiers? i have only tested the diodes in circuit - nothing shorted that i can tell and seem to be acting as one would expect while in circuit and unpowered ... i will also check the situation with the yellow primary etc ... could be an error somewhere.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 27, 2018, 12:14:56 am
content: Sorry, I just thought of something else and edited my post #13. I also was enjoying an intergalactic gargle blaster so I might be a little messed up.

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 12:15:59 am
content: THANK YOU SO MUCH ARTHUR for helping me see the problem in my 'measurement' .. stupid though it is not to have seen it ... hopefully i will get to the repair solution much more quickly now .. can't be understated the problems myopia can cause

author: james_s
author_rank: Super Contributor
timestamp: September 27, 2018, 12:41:10 am
content: If I understand your post right, it sounds to me like the problem may not be in the power supply at all, but in the keyboard.Also I see you mention the problem occurred after you did some other work on it. Did the problem occur immediately or did it work for a period of time and then fail? If the problem happened immediately then the first thing I'd suspect is a mistake somewhere. Something plugged in the wrong place or backward, a solder bridge or the wrong part installed somewhere.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 27, 2018, 12:49:13 am
content: With both + and - supplies it might be easy to put one of the replacement caps in backwards. Not that I've ever done that.

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 03:04:45 am
content: Quote from: ArthurDent on September 27, 2018, 12:49:13 amwhy would you think any caps were put in backwards ... no they were put in the same way ... i did that with a tant before but luckily i caught it !! helps to double check thingsi'm terrified of putting caps in backwards

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 03:09:40 am
content: Quote from: james_s on September 27, 2018, 12:41:10 amwell in a way ... since i didn't realize part of the primary circuit was in the keyboard itself - so i had to jumper some things to get the primary to work ... silly me.as for the 'work' - no the work started AFTER the fuse blowing started ... but now that i have the primary side figured out i'm lighting up the dim bulb indicator with the PS alone ... so there's definitely something going on there - MAYBE a diode pair ... i need to investigate further ... that's also the problem i guess - with doing refurb work before investigating a problem - it's possible to create new problems! i hope to god i haven't done that but i do think i've been very cautious

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 03:12:49 am
content: SO - now that i have the primary wiring issue solved i have NEW RESULTS:dim bulb LIT (finally) (means at least the transformer might be good)YELLOW PAIR: 3.1V AC BLUE PAIR: 2.9V AC RED PAIR: 4.5V AC ORANGE PAIR: 8.6V ACcorresponding capacitors read: 3.1VDC 4.4V DC 7.8VDC and the last cap i couldn't read since it was inccessible and I didn't want to short anything with the meter probes in this case .. but after i disconnected the power i probed around the corresponding diodes and found what MAY be a short (reading close to zero under diode testing mode on my meter - I'll pull the offending diodes and check them properly ...does anyone know where i can get a recplacement for a 10DC-4 and 10DC-4R diode bridge pair?? looks like this:https://www.jauce.com/auction/p573041848

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 05:45:23 am
content: SO - i just connected grey/brown primaries directly to mains and get the following:yes i did thanks - - new results: YELLOW: 2.2 BLUE : 2.9 RED: 4.5 ORANGE: 2.2 (all readings VAC) -that's across the secondary windings - relative to ground each is 27-30 VACcapactors (VDC) : 3.1, 4.5, 7.9 ...this is with secondaries connected ...

author: jaunty
author_rank: Regular Contributor
timestamp: September 27, 2018, 05:46:07 am
content: could it be a bad (partly shorted) transformer??

author: Fryguy
author_rank: Regular Contributor
timestamp: September 27, 2018, 11:50:19 am
content: This smells like a dead primary - check if there is any current on the outputs . At these low voltages try to connect a small load to the secondarys - e.g. a flashlight bulb or something similar and look if the secondary outputs drop dead or the lamp lights up . Your multimeter alone has a too high input impedance to show that .

author: ArthurDent
author_rank: Super Contributor
timestamp: September 27, 2018, 01:49:16 pm
content: I had previously posted: “The A.C. voltage from BE to BE should be the same as from YE to YE and perhaps about 10 to 12 VAC. The D.C. voltage from D5 to common should be the same as from D7 to common and perhaps 12 to 15 VDC.” (actually YE-YE might be a little lower)So let’s analyze one regulator and see if the voltages you get make sense. Looking at the +/-15VDC supplies, the A.C. voltage from RE to RE should be the same as from OR to OR and perhaps about 17 to 20 VAC. The D.C. voltage from D1 to common should be the same as from D3 to common and perhaps 20 to 25 VDC.If you make a block diagram of what just the +15VDC supply would have to have for voltages to work, what you’re getting is not right. To get +15VDC out the A.C. secondary voltage would typically be a few volts higher and 17-20VAC is a possibility. When this is rectified you will have the diode forward voltage drops for 2 diodes but the capacitor will try to charge to the peak value of the rectified A.C. so you might have 20-25VDC, depending on load. The regulator works like a variable resistor so the input has to be a few volts higher than the output to compensate for drops across the pass transistor, the current sense resistor, and for low input line voltage. To get a constant +15VDC out of the regulator you would need a few volts higher on the input for the regulator to work properly.When you say: “new results: YELLOW: 2.2 BLUE : 2.9 RED: 4.5 ORANGE: 2.2 (all readings VAC) -that's across the secondary windings - relative to ground each is 27-30 VAC” the secondary voltages are way low as I explained, above. The 27-30VAC you measured to ground is just capacitance coupling and not really there. Put a 1K resistor between any wire to ground and that measured voltage will drop to near zero.If there are shorted turns in the transformer it would be really overheating and/or blow a fuse. If you are now powering the transformer through the other primary (RE-BE), measure the A.C. voltage across the original primary wires (GY-BR) and you should measure about 120VAC as well.

author: Armadillo
author_rank: Super Contributor
timestamp: September 27, 2018, 04:29:40 pm
content: Refer to your description of symptom 1 and now you said you have the primary resolved? Can you measure the ohms from thehot and neutralto confirm. Should be around 6.1 ohms, otherwise clean all contacts, fuse contacts, fuse etc...Quote from: jaunty on September 26, 2018, 04:44:28 am

author: jaunty
author_rank: Regular Contributor
timestamp: September 28, 2018, 09:14:27 am
content: HEY thanks for the help guys - i wasn't online yesterday so didn't check ... but i made HUGE headway tonight ... i was chasing a red herring - i thought there was maybe an intermittent short in the primary too - but part of that was the result of not understanding that the primary circuit routed OUT of the power supply and i had to jumper something -but then the transformer was very much (through mental inertia) still under suspicion - so i ripped the whole thing apart again (part of the inherent evil of this design due to the 'wrapped' wire and soldered terminals which are a BITCH to undo .... measured all the secondaries LIVE with dim bulb (no bulb lighting) and THEN i decided to pull ALL the diodes just because ... 'what else could it be???' for the second time - and I actually found ONE SHORTED DIODE (GAAAAHHHH) - i guess i never found it before somehow because of it's awkward location ... difficult to probe even ... so yeah .. OF COURSE it HAD TO BE that one ... (grrr) - anyway - I'm happy at least i found something now - after checking the transformer - all secondary voltages looked great - very much within spec ... so i feel confident the transformer will live on for some time now ... THANKS for the help and i'll update if i can get this fixed up and running ...now i just need to figure out how to build a replacement IR 10DC-4 diode pair ... seems impossible to find online NOS ...

author: jaunty
author_rank: Regular Contributor
timestamp: September 28, 2018, 09:19:22 am
content: Quote from: ArthurDent on September 27, 2018, 01:49:16 pmthanks arthur dent for your characteristically thoughtful and kind input! but yes i was getting 117V across the BRN/GY pair as expected ... after disconnecting the secondaries I ended up with about 21.5 for both orange and red windings - then about 14 then 10 for the remaining two ... so it sounds about right to me ...and a big monkey off my back ... thanks so much for that ... sometimes you just have to take everything apart ...

author: ArthurDent
author_rank: Super Contributor
timestamp: September 28, 2018, 01:24:47 pm
content: Without a photo it is hard to tell but looking at the parts listed on the schematic it appears the rectifiers are three lead full wave with just 2 diodes. The 10CR-4 has a + out and the diodes are reversed in the 10CR-4R to give a – out. Using the two types together form a more common four lead full wave bridge and that is what I’d use for replacement. Here is just one listing for hundreds on Ebay that should work.https://www.ebay.com/itm/5Pc-FULL-BRIDGE-RECTIFIER-2W06-2A-600V-Full-Wave-Rectifier-FREE-SHIPPING-US-SHIP/163263426481

author: jaunty
author_rank: Regular Contributor
timestamp: September 29, 2018, 05:46:40 am
content: Quote from: ArthurDent on September 28, 2018, 01:24:47 pmyes - they are 'dual diode' packages with three legs as you say ... they look just like this though i can't find the voltage/current capacity ratings for this specific model ... and I DO have 1N4004s on hand ... i fabbed one up and I will try it with a dim bulb i guess and see what happens - though the 'main' fuse is only a 500mA ... so I'm guessing that the 1.0A rating of the 1N4004 might be enough .. (?)https://www.radiomuseum.org/tubes/tube_10dc-13l.html

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2018, 01:32:27 pm
content: 1N4004 diodes would work to make a replacement bridge but I would check any of the 10DC-4 diodes you remove to see if they are the problem. It could be the diodes are o.k. and it is something they are feeding that is the problem.Old electrolytics are a common failure point in older pieces of equipment and I would suspect them before the diodes. It is pretty common to just replace all old electrolytics in old equipment before powering it up as a safety measure and I would replace all the 2200, 1000, and 220 uF caps. It still could be some other component but if the caps aren't bad now they could fail at any time plus they are probably dried out so the value is far less than the stated value.

author: PKTKS
author_rank: Super Contributor
timestamp: September 29, 2018, 07:30:52 pm
content: 2 cents of opinion....- It looks a rather simple PSU.- By today standards I would ditch that PSU aside...- and quickly put together a "3 pin IC" discrete like thing equivalent using 78xx/79xx or clonesJust in case I would stress the transformer or just ditch it altogether with a new oneCHEAP. SIMPLE. Low and common voltages and simple topology.May use an island board even single sided with no further problems...and bye bye old PSU

author: jaunty
author_rank: Regular Contributor
timestamp: October 03, 2018, 03:00:07 pm
content: YES - i did precisely that Arthur - the 1N4004s are in and have been doing well for the last three days or so ... i'm wondering if maybe i shouldn't replace ALL the old diodes now

author: jaunty
author_rank: Regular Contributor
timestamp: October 03, 2018, 03:02:14 pm
content: not so easy to do considering the sense lines and all ... it's an old school 'safety feature' i guess ... but i'm not confident enough with my skills yet maybe

author: HpMad
author_rank: Newbie
timestamp: October 03, 2018, 11:27:23 pm
content: First: you have to verify the primary of the transformer you have to get high ohm reading for primary for the secondary low ohmsa) in this case you have to unsolder the wire of the main transformer just only the first wire for either bridge and with the multimeter in ohms range take the reading.b) is everything is fine verify the voltage from secondary you have to read around 18vac to 20vac, could be more up to 25vac because the caps on the secondary are all 35vdc, but could be less depending of the transformer wiring.Second: Verify the bridge rectifier if you don't know how to do it, read this,1: Put the negative test lead of your multimeter in the positive sign of the bridge rectifier of course you have to isolate both bridge rectifier from the board. with the positive test lead of your multimeter test the pins of the bridge (if the bridge is inline) the second pin is one diode voltage drop so se third and the fourth is two diodes voltage drop if you check the bridge with test leads reverser i should not read any voltage.Third: You said that you replace the caps but check the zener diodes and transistors in case you solve the rest.

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2018, 12:34:55 am
content: No need to redesign the supply, there's nothing wrong with the original design, it just needs to be repaired. You'll get to the bottom of it, just follow along the suggestions in this thread and post your findings. If you get stuck, set it aside for a few days then come back to it.