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transformer discharge

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sclarke523

New Member
I am new to stereo repair but would like to tackle a recap and was curious about if it is necessary to discharge voltage from the transformer before working on the stereo. I have watched videos and read articles about the process but nothing is mentioned of the transformer. I am not that experienced with electronics but would like to try to do the job myself. Is stored voltage in the transformer even though the unit is unplugged or am I worried about nothing?
Thanks
1980 JVC RS-11
 
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No, transformers do not store power when disconnected. It's strictly the capacitors in the power supply circuit that you need to be careful with. But you might want to read up a little on electronic theory before you start attempting any repairs.
 
You can either wait for supply caps to discharge, or discharge them yourself. To do that, you could use a 1K or so resistor and place it across the cap and wait for the cap to discharge, using a voltmeter to check it. NEVER discharge a cap by shorting it out with a screwdriver or other conductor, as that could ruin the capacitor and possibly other components connected to it. Of course, if voltages are high, as those found in tube amplifiers, then use extreme caution to avoid getting a severe electric shock.
As was mentioned in an earlier post, transformers store no charge once the magnetic field in them collapses, which is very quick.
 
egellings said:
You can either wait for supply caps to discharge, or discharge them yourself. To do that, you could use a 1K or so resistor and place it across the cap and wait for the cap to discharge, using a voltmeter to check it. NEVER discharge a cap by shorting it out with a screwdriver or other conductor, as that could ruin the capacitor and possibly other components connected to it. Of course, if voltages are high, as those found in tube amplifiers, then use extreme caution to avoid getting a severe electric shock.
As was mentioned in an earlier post, transformers store no charge once the magnetic field in them collapses, which is very quick.
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Thanks. it seems pretty straightforward based on the youtube videos I have watched. I just didnt see anyone do anything other than unplug the unit and check the voltage of the two large capacitors. in my case the two large capacitors are 35v, 6800mf. I understand that the capacitors store voltage but no one seemed to be concerned about it in the videos. I always understood that transformers can store voltage on electronic equipment but perhaps I am wrong, just wanted to make sure, which is why I posted. I will likely tackle it next week. there are less than 50 capacitors. I have experience soldering but I have never really tackled stereos
 
egellings said:
NEVER discharge a cap by shorting it out with a screwdriver or other conductor, as that could ruin the capacitor and possibly other components connected to it.
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Of course a possible take away from this is, just supposing you get your trusty screwdriver out to short out a capacitor that you know holds a charge, and you 'think' you've powered off the unit, but you haven't - how dangerous is that?

Compare this to the much safer resistor method, with a voltmeter check to confirm.

Maybe the above scenario is unlikely or alarmist, but I was always taught not to get into bad habits where high/lethal voltages are involved.
 
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ic-racer said:
Not sure how anyone is 'working' on anything without knowing the schematic; i.e. if it has a bleeder resistor.
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Bleeders can take a long time to discharge a large cap, since low valued, high discharge rate ones would be dissipating heat in the device when it is in operation.
 
Simply look at the voltage rating of the capacitor(s) in question. In post #6 you say it is a 35 volt 6800uF capacitor. The most it will allow is 35 volts. It is probably operating on 20 or 25 volts when powered up. 35 volts is its max rating and is certainly not going to hurt you as long as you are not wet or barefoot on a cement floor with it powered up. :rolleyes:

Solid state equipment operates on low voltage and high current. Tube electronics operates on high voltage and low current. :thumbsup:
 
@sclarke523 Don't let comments discourage you from venturing into electronics repair. I won't name any specifically but you may benefit from one of many available online electronic courses.

To the idea of recapping that receiver I would ask yourself if there is a specific issue you are trying to address. Primary power supply capacitors are common targets but as for the rest I would question. The service manual for that model is available on HiFiengine. It appears a challenge to work on based on phsical disassembly required including stringing of the tuning cable. Might want to pick an easier candidate to get your experience.

Press on and good luck.
 
safebet said:
@sclarke523 Don't let comments discourage you from venturing into electronics repair. I won't name any specifically but you may benefit from one of many available online electronic courses.

To the idea of recapping that receiver I would ask yourself if there is a specific issue you are trying to address. Primary power supply capacitors are common targets but as for the rest I would question. The service manual for that model is available on HiFiengine. It appears a challenge to work on based on phsical disassembly required including stringing of the tuning cable. Might want to pick an easier candidate to get your experience.

Press on and good luck.
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Thank you. I have been on many forums and there are always idiots that pick apart every word in order to get under your skin. I will look up the manual. I think you are right though I will likely replace the power capacitors and leave the rest
 
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