Question about Dual supply power supplies and shorting - Page 1
Forum section: electronics / repair

author: shockpoint
author_rank: Contributor
timestamp: September 17, 2024, 02:02:53 pm
content: Hi all,I have a few general questions hoping someone can shed light on them.I am repairing an old digital piano. Initially the problem was thought to be on the amplifier board that takes +12 and -12V to power the op amps. However, thorough circuit tracing did not show any glaring abnormalities.So my search led me to the digital pianos mainboard, where the +12V and -12V feeds to a network of 5 LC833 op ampsThe reason I suspect the mainboard is because I found that by disconnecting everything else, reconnecting just the amplifier board and the mainboard's signal/voltage cable caused the same fuses to blow (which is the original condition it came in). I depinned the JST connector and disconnected the +12V, -12V and GND pins and retested, and the new fuses I put in stayed intact. So that is highly suggestive that the fault is in the mainboard.Unfortunately I do not have any schematics for the mainboard as Kawai is rather protective of its proprietary devices I guessSo I have a few questions that are more conceptual/general in nature1. Between the -12V and +12v rails on the main board, I have measured 128.2 Ohms with a DMM. This resistance is not seen on the amplifier board which has 5 other op amps, when the mainboard is disconnected from the amplifier. On the amplifier board, the 12v and -12v rails have a resistance in the 3.8k+ region. Does the 128.2Ohms sound suspicious to those who are experienced in op amp fault finding?2. the 128.2 ohm also tests as a voltage drop of 0.041V in diode mode when I test either between the -12V and +12V pins, or on the opamp -Vcc and +Vcc pins; I have not seen such a small voltage drop before across op amp power rails and in my own personal experience this is edging towards a short3. What happens when you connect +12V to -12V? Logically I think it shorts just like if you connected +12V to GND but just wanted to confirm.4. Is it possible that the 128.2ohms demonstrates the 'wrong side' of the short on the shorted device? by that i mean if the short is on the other side of the SMD I am trying to find, then if I measure on the incoming voltage side or the outgoing voltage side my short may either be 0.001ohms (a true short) or 128.2ohms (not technically a short but only because im measuring in the wrong place).My next step is to begin voltage injection to try and find the shorted SMD because the op amps are in parallel. The other option would be to use a milliohmmeter.

author: Harry_22
author_rank: Frequent Contributor
timestamp: September 21, 2024, 08:40:38 pm
content: Hi!Try the following: in diode mode check the drop voltage between the +12V buses and the GND, then between the GND and -12V. Compare the values.A short circuit on the power bus is very easy to find if you have a thermal imager.To do this set the laboratory source to, for example, 1V with a current limit of 1A (the values ​​can be adjusted based on further results). Connect it to the buses in correct polarity. Next look for the heat release point.All other methods including searching with a milliohmmeter are more labor-intensive.

author: shockpoint
author_rank: Contributor
timestamp: September 22, 2024, 02:26:29 pm
content: Quote from: Harry_22 on September 21, 2024, 08:40:38 pmHi, thanks for your replyThe diode mode between +12V and GND, and -12V and GND compare similarly and they are: 0.565 and 0.537 respectively. Should these values be closer together?With voltage injection, is it voltage set too high that will kill a given IC or is it the current?If I keep the voltage at 1V, with this board having inputs of +12 and -12V, and increase to 2A-3A (in case 1A doesn't get the chips warm enough to show up on the thermal imager), is there a risk of damage by current to the ICs?I have checked though and it doesn't seem that the 12 nor the -12 go to any of the ICs directly. +12V goes to 78LM05 (voltage reg) before it goes to the ICs and there is no short on the voltage regulator

author: Harry_22
author_rank: Frequent Contributor
timestamp: September 22, 2024, 08:36:47 pm
content: Hi!Seems you have short only between +12V and -12V rails, or between the opamp +Vcc and -Vcc pins.In this case connect plus of laboratory PS to +Vcc and minus to -Vcc for test.One volt limit usually sufficient to detect the short line. You will see that your PS will go into the current limitation mode with around zero volt at output.This is absolutely safe for your device under test.But please start from lowest current limit, for instance 1A to avoid restoring the short circuit.Thermal imager is very sensitive. Below you can see the termal image of leaked capacitor 4mA at 12V.

author: shockpoint
author_rank: Contributor
timestamp: September 23, 2024, 03:02:26 am
content: Quote from: Harry_22 on September 22, 2024, 08:36:47 pmThanks again for your insight.I will test it as you have suggested and post the results.A general question I have about +12V and -12V rails. I noticed you mentioned plus of the Lab PSU to the +12V and minus to the -12V.That makes sense as I am trying to see which component 'completes' my short circuit.But would there be any value in purchasing a power supply that produces both positive and negative voltages that have isolated grounds from each other? such as the HP 3631A?

author: Harry_22
author_rank: Frequent Contributor
timestamp: September 23, 2024, 06:23:34 pm
content: Hi!There is no point in buying anything to solve this problem.Also try measuring in diode mode the paths from the output of each amplifier to the power buses. The task is to find difference.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 23, 2024, 07:07:51 pm
content: With a short between the +12 V and -12 V the weaker side could see a reversed voltage.Ideally the circuit and / or supply PCB should have diodes that limit the reverse voltage to ground to something like 0.7 V. Most circuits can survive this.If a bit paranoid one may add such diodes (e.g. 1N400x, or 1 A shottky), at least for the tests.With a larger ciruit (many parallel OP-amps) the test current from the diode mode can be a bit on the low side. The 200 ohm range resistance measurement is usually similar (e.g. 1 mA range).Testing OP-amp pins to the supplies is at least worth a try, as quite some semiconductors fail short.

author: shockpoint
author_rank: Contributor
timestamp: September 25, 2024, 12:48:54 pm
content: Quote from: Harry_22 on September 23, 2024, 06:23:34 pmHi harry, thanks for the response!Yes I noticed one of the op amps had an input (non inverting) that was reading 0.700V in diode mode with the red lead on -Vcc, and every other op amp was reading 0.560 on the very same input. I have my suspicions on that particular op ampHowever, I can't seem to get it to show up on the voltage injectionWhat I did:- connected -12V to black lead of PSU- connected +12V to red lead of PSU- limited voltage to 1.00V and current to 1ANothing lit up, and I checked the current draw of the supply - it was 0.007AThe effective resistance of the shorted component read 128ohms with the DMMAt 1V the numbers check out with Ohms lawMy inclination is to increase the amperage output to get this effective resistance to burn off more wattage as heat, but is this the right way to proceed?Quote from: Kleinstein on September 23, 2024, 07:07:51 pmThanks for the response Kleinstein. What I understand from your comment is that the diode mode alone may not produce an accurate reading because of the (6) op amps connected in parallel?Also what do you mean by the weaker side could see a reversed voltage?Thank you!

author: shockpoint
author_rank: Contributor
timestamp: September 25, 2024, 02:15:13 pm
content: Update- with the voltage at 1.5V and the current at 3A, the op amp that started lighting up on my thermal imager was in fact the one with the wonky 0.700V reading on the DMM in diode modeI think I have found the culpritAlthough the temperature wasn't really that much higher than the surroundings, it consistently lit up on many test runsThere was an offset in the overlay between the actual board and the infrared image so that threw me off, thought it was a resistor which tested completely fine78LM05 runs off +12V and GND to produce 5V for the circuit- connecting -12V and GND to PSU black and red respectively demonstrates a voltage on the 78LM05 input (which technically shouldn't be there?)- I assume the voltage from the -12V rail is leaking over to the +12V via that short on the faulty opamp.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 25, 2024, 02:31:46 pm
content: Some "leakage" from the negative side to the positive side is not a big surprise. It is kind of expected to a low level, e.g. for power a few OP-amps, of a resistive divider.Depending on how important and complicated the ciruit is, it may be worth doing at least a crude reverse engeniering.Reverse diodes at the supplies (input and / or output) would not be for regulation, but for protection, e.g. when one half drops out or just during turn on.The weaker half would be the one with the lower current limit, intentional or just from the tolerances. If both outputs are current limited and the +12 V are shorted to the -12 V one of the regulator will go into current limit and the other voltage could cause the wrong polarity for that half. Diodes could limit at least the voltage in the wrong direction and this way limiting / preventing additional damage from reverse voltage.

author: shockpoint
author_rank: Contributor
timestamp: September 26, 2024, 04:17:24 am
content: Quote from: Kleinstein on September 25, 2024, 02:31:46 pmKleinsteinYes the digital piano's modular power board has those elements that you have mentionedthe current limiting is 400milliamps - I suppose that's why the fuses (M205 slow blows) blew on both the -12 and +12 railsthere are reverse diodes at the outputs of the voltage converters tied to GND for each of the regulators for -12 and +12I have disconnected that module and am assessing the op amps where the short seems to beThere was only one op amp that was lighting up on the thermal cameraWhat doesn't quite make sense at this stage is 24V across the effective resistance of the short (128ohms) would only actually produce a current of 0.1875amps - considering my fuses were rated for 400milliamps I am not sure why they would blowmaybe I am understanding this wrongI confirmed the draw the op amp - with injection of power at 1V and limiting at 1A, the resistance of 128ohm was causing a current draw of 0.007A

author: Kleinstein
author_rank: Super Contributor
timestamp: September 26, 2024, 07:01:35 am
content: The current drawn from a semiconductor circuit may well be nonlinear. So one can not extrapolate from the 1 V measurement to 12 V. The 128 ohms could be closer to a diode and than maybe 40 ohms of resistance.There is also a chance a chance the the "short" that blew the fuses was temporary, like a latch up event that can clear once the power is removed.

author: shockpoint
author_rank: Contributor
timestamp: October 14, 2024, 01:05:42 am
content: Quote from: Kleinstein on September 26, 2024, 07:01:35 amThank you - this makes sense.An update - I removed the faulty op amp and the 125 ohm reading is gone. I think the op amp was letting current through between Vcc and -VccWaiting for a new LM833 op amp now and I will report back.I took some time to learn how to use a hot air gun without blowing away everything else on the board