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Cannot get this circuit to power up, plus some components I cannot identify

looxuser

Dec 28, 2015
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The attached schematic represents an audi device i'm trying to get to power up. R82 was burnt, low value resistor, Q19 seesm good on the component tester, cannot identify sot23-5 component its marked 504 or 50B. Any troubshooting tips, PCB marked 6v for VCC yet 8 volts measured. No12v on TP2???stack exchange3.JPG
 

kellys_eye

Jun 25, 2010
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How did you derive the schematic? There are numerous errors in it so determining the cause of the problem isn't possible unless the schematic is 'correct'.
 

looxuser

Dec 28, 2015
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How did you derive the schematic? There are numerous errors in it so determining the cause of the problem isn't possible unless the schematic is 'correct'.
Yes I reverse engineered it to a fashion, please point out errors to help me refine?
 

Delta Prime

Jul 29, 2020
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Any troubshooting tips, PCB marked 6v for VCC yet 8 volts measured. No12v on TP2???
Please provide a link to the original schematic.
No 12 volts on test point number 2. I have to take your word for it.There's no TP2 designation on the schematic.
Why did you alter(reverse engineered to a fashion) the schematic?What would you like the schematic to do.
You say it is from an Audi device. You mean like the vehicle Audi?
 

kellys_eye

Jun 25, 2010
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Yes I reverse engineered it to a fashion, please point out errors to help me refine?
I can't do that - I'd have to develop my own schematic (using your own hardware) to find ALL the potential issues with yours. I'm not prepared to do it on a one-after-the-other basis. We'd be here all week!
 

looxuser

Dec 28, 2015
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Please provide a link to the original schematic.
No 12 volts on test point number 2. I have to take your word for it.There's no TP2 designation on the schematic.
Why did you alter(reverse engineered to a fashion) the schematic?What would you like the schematic to do.
You say it is from an Audi device. You mean like the vehicle Audi?
Unfortunately there is no original schematic, i had to reverse engineer and draw this myself with KiCAD, I measured voltages around the board on no 12v but that looks to rely on a Vcc of 6v as marked on the pcb, re TP2 that was a typo its just the 12v TP as is the audi typo should have read Audio. The pcb is from a Japanese novelty item called "little jammer"
 

kellys_eye

Jun 25, 2010
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If you know what regulators they are then you can figure out the voltage that should be coming from them (obviously) then you can trace the fault BACK if the voltage is missing. The input voltage must be higher than the output voltage.....

IC13 is obviously wrong as you show 8.8V on a GND pin........
 

Delta Prime

Jul 29, 2020
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I'm not prepared to do it on a one-after-the-other basis. We'd be here all week!
Outstanding:... The direct approach...
On top of that... Thorough:p
Come on now you started it... I mean that in the best possible way!
Between you and me let's just keep this Undercovers.
 
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kellys_eye

Jun 25, 2010
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Outstanding:... The direct approach...
On top of that... Thorough:p
Come on now you started it... I mean that in the best possible way!
Between you and me let's just keep this Undercovers.
It's too easy to get too involved. We all do what we can, as we can, but there has to be a limit. I can't blame/fault the OP for the way they approached this issue as people come here for help and advice without knowing 'how to ask' for it - maybe that should be a 'pinned' page that all new members should be required to read?

Stuff like adding links to datasheets, listing measurements they've already taken, advising of procedures already done etc - for most new posts we inevitably end up requesting more information - very basic information - that takes up valuable time. Many forum members will no doubt have other jobs and offer their time as-and-when (like me) but we can't be here all the time....
 

looxuser

Dec 28, 2015
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If you know what regulators they are then you can figure out the voltage that should be coming from them (obviously) then you can trace the fault BACK if the voltage is missing. The input voltage must be higher than the output voltage.....

IC13 is obviously wrong as you show 8.8V on a GND pin........
Yes thats where i'm struggling a little on IC13. The component is SOT23-5 and has "504" printed on it and i've seen a photo of another board with "50B" on it, yet I cannot seem to find an ideal fit datasheet. In most datasheets I find pins 1 and 5 could not be linked together as they are in this case. I guess unless someone out there knows what it is it would be worthwhile discussing what its there to do and looking to bypass for the purpose of proving its faulty. Please endulge me here; So what I know for further testing yesterday is that R82 is boiling hot on the thermal imager, so alot of cuurent must be passing through it, so my guess is IC13 is short. I know that in order for Q19 to pass cuurent through to the collector the base must be low and therefore IC13 does that. Lets assume IC13 is powered on pin 2 (yes its not GND) is monitoring pin 3 voltage and is pulling pin 4 low. I assume then to do this its switching pin 4 to GND on Pins 1 and 5 and that there should be some internal resistance which there isn't which is why R82 is red hot. Would anyone concur? So i'm thinking of either circumventing it to prove its faulty or injecting the 6v for Vcc from the bench power supply with 504 removed. Any pointers gratefully received?
 
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looxuser

Dec 28, 2015
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It's too easy to get too involved. We all do what we can, as we can, but there has to be a limit. I can't blame/fault the OP for the way they approached this issue as people come here for help and advice without knowing 'how to ask' for it - maybe that should be a 'pinned' page that all new members should be required to read?

Stuff like adding links to datasheets, listing measurements they've already taken, advising of procedures already done etc - for most new posts we inevitably end up requesting more information - very basic information - that takes up valuable time. Many forum members will no doubt have other jobs and offer their time as-and-when (like me) but we can't be here all the time....
There's no expectation from me on peoples time, I'm merely reaching out, to a potential body of minds that can help me yes get to a solution of a fix but importantly too share their knowledge so I learn more that i can apply to other fixes in the future. If a small number of people each give me just me 5% of this goal, i'm much more enriched than when I started from 0% no matter how slight their input and views right or wrong. Fair points made and here are my responses.

1. This item does not power up it has a mometary switch to the front panel and an led indicator.
2. I've measured the 9v input and about 8.8v after the Schottky diode.
3. R82 was burnt so i replaced it.
4. Resoldered most connections in the power area.
5. Thermal imaged the board, nothing alarming.
6. Q19 removed from pcb and tested with component analyser, says ok.
7. Produced schematic on KICAD
8. Modelled component behaviour in EveryCIrcuit app, (if i bypass IC13 to GND and put a larger resistor at R82 that will give me Vcc won't it at least proving IC13 is bad?)

There's no circuitry to get Vcc at 6v though, surely its not a misprint on the PCB?
 

kellys_eye

Jun 25, 2010
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This is one occasion where a picture of the board may be of advantage.....
 

Delta Prime

Jul 29, 2020
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approached this issue as people come here for help and advice without knowing 'how to ask' for it - maybe that should be a 'pinned' page that all new members should be required to read?

Stuff like adding links to datasheets, listing measurements they've already taken, advising of procedures already done etc
I will read this right now . Is there a link for that document?
I meant no offense. It is just banter.
 

looxuser

Dec 28, 2015
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Delta Prime

Jul 29, 2020
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A game of light and shadows perhaps but that transistor looks like all three pins are shorted against the heat sink.
 
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