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Electronics => Repair => Topic started by: donkey on September 20, 2018, 07:27:53 am

Title: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 20, 2018, 07:27:53 am
Have a nice day to all of you.

I have a very sophisticated 665W power supply unit with an output failure. The board have 90 degree angled 10 to 18 pin soldered pcb modules all over the unit which make it impossible to probe without desoldering the modules. I have found an in-circuit shorted capacitor and diode readings that i have not desoldered and tested out of the circuit yet.

Before desoldering the components to test them out of the circuit, i would like to ask you if there is any possibilities to have an in-circuit shorted capacitor and diode readings by design? Let's say may it be possible that the DMM is reading short because of a low resistive parrallel connection to these components for example a resistor, transformer and such..

Is it always the case that having an in-circuit shorted reading capacitors and/or semiconducters, means that there is a short circuit in somewhere else or is it possible to find such in-circuit shorted reading components in a 100% perfectly functioning devices?

Thank you!
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: capt bullshot on September 20, 2018, 08:30:58 am
There's always a chance that a 100% OK component measures defective in-circuit.
You'll need the schematics to judge the in-circuit measurement. If you need to be sure, you have to de-solder the component.

Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: PA0PBZ on September 20, 2018, 08:49:28 am
Just saying…

(https://i.stack.imgur.com/1OLBo.jpg)
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 20, 2018, 11:02:41 am
Thank you all for your replays! I have desoldered the components and they where fine. It will be very difficult to diagnose the unit without a shematics. Pitty that the firm TECTROL is not exists anymore to ask for schematics or the possible failure components.
Thank you!
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: PA0PBZ on September 20, 2018, 11:40:27 am
Quote from: donkey on September 20, 2018, 11:02:41 am
Pitty that the firm TECTROL is not exists anymore to ask for schematics or the possible failure components.

I'm not giving you much hope to get schematics but here is where you should ask: https://www.cui.com/tectrol-power (https://www.cui.com/tectrol-power)

Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 20, 2018, 12:46:06 pm
Thank you very much for the link. I have sent an technichal requirement for a schematics.  :-+
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 20, 2018, 01:20:46 pm
What i have assumed so far from probbing that: The optocoupler FOD617C ( http://www.mouser.com/ds/2/149/Fairchild%20Semiconductor_foffod617a-185209.pdf (http://www.mouser.com/ds/2/149/Fairchild%20Semiconductor_foffod617a-185209.pdf) ) has to short it's collector pin to the ground but it isn't. Why i assumed this is that the collector pin of the optocoupler has a continuity to a resistor which is in series with the cathode pin of the output voltage failure check led ( which is not lighting ) and the anode pin of the output voltage failure check led is connected to the MC33161 ( https://www.onsemi.com/pub/Collateral/MC34161-D.PDF (https://www.onsemi.com/pub/Collateral/MC34161-D.PDF) ) Universal Voltage Monitors output pins. The anode and the cathode of the output voltage failure check light emitting diode's pins are both sitting at 13.5V's which creates a potential difference of 0V's :phew: .

Is it a common practice to use an optocoupler to short out it's collector pin to ground or is my assumption totally wrong?

Thank you!
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: Samogon on September 20, 2018, 04:20:20 pm
Bent soldered modules can be removed and you can solder extending wires. Sure it is lot of work, but sometimes only an option.
Here is my successful repair.
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 20, 2018, 04:47:45 pm
Tomorrow, i will try to send you my jigsaw PSU pictures  :-/O  :-+
Thank you for sharing!
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 21, 2018, 04:36:05 am
This is the beast, I am dealing with :palm:
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: Samogon on September 22, 2018, 06:39:53 am
Yea it is PITA to work on.
Can you describe what symptoms are?
And in power supply shorted filter cap is no good. But it could be other active components shorted in parallel.
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: Rasz on September 22, 2018, 03:22:29 pm
is that 48v? looks like telecommunications/server gear
whats the failure mode? does it signal error/overload? not power at all?
how did it die?
Title: Re: Short circuit capacitor readings in a circuit, do always means there is a short?
Post by: donkey on September 24, 2018, 11:51:36 am
The CUI replied my e-mail that the product is not belong to them and they did not share their schematics with public.
 
It is a telecomunications power unit Rasz. I do not have any idea how much it should output  :-//
 
There are two indication purpose light emitting diodes at the back of the unit. The input stage indication led is lighthing up but the output stage indication led is not lighting.

Thank you!