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Thread: Repairing a cursed Polaroid 8x10 processor

  1. #11

    Join Date
    Nov 2017
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    393

    Re: Repairing a cursed Polaroid 8x10 processor

    Quote Originally Posted by Matthijs View Post
    So does that mean the varistor may have been destroyed (even though it still looked fine) before the fuse blew when the previous owner put 220V on it? When you say they have high voltage when they go into protection mode, you mean they still hold a charge and I should be careful not to short out the pins as that will make it discharge?
    Nothing to be afraid of, a varistor has no capacitance to speak of so it won't have a voltage over it after a time. It is a bit more complicated.

    A varistor is a resistor that has a resistance that depends on the voltage over it. When it is below its rated voltage (150Vac for a "241") it has a high resistance. So it conducts almost no current. But when the voltage rises above its rated voltage its resistance becomes lower. Very much lower for high voltage. So it starts to conduct a high current (100's of amps) so that the fuse is forced to blow. That's how it protects.

    Now when its conducting current there is still a voltage over them, they are not a real a short circuit. So a "241" can have about 300V over them when conducting 100's of amps. So that can have an influence on whatever circuits are behind them. It also means they have to absorb a lot of power. If they are overloaded for too long, their internal structure changes and they become low resistance. That's why your varistor smokes, it has become defective.

    Another thing is that a varistor is typically used to protect against lightning. These are high voltages that last a couple of microseconds and are not very frequent. When you subject them to a wrong mains voltage, they are active 50 or 60 times a second until the fuse blows. And that is a long time in electronic times! That is why I find a varistor that has had to cope with mains voltage suspect. One of them is already giving smoke, so it IS defective, the other one will not be far behind. As these are cheap items it is best to change both of them. Get "241" types of similar size.

    You could check the transformer by disconnecting it from the circuit. Both input and output should measure a low ohmage. Transformers are very robust items but some do have internal fuses. Otherwise, disconnect the output and measure it when powered.

    Another thing I would check is the circuit board next to those varistors. It looks brown so it has been overheated. Look at the copper traces and make sure they are not "lifted" (still well glued to the glass fiber board) of cracked.
    Expert in non-working solutions.

  2. #12

    Join Date
    Feb 2013
    Posts
    9

    Re: Repairing a cursed Polaroid 8x10 processor

    Quote Originally Posted by Havoc View Post
    Nothing to be afraid of, a varistor has no capacitance to speak of so it won't have a voltage over it after a time. It is a bit more complicated.

    A varistor is a resistor that has a resistance that depends on the voltage over it. When it is below its rated voltage (150Vac for a "241") it has a high resistance. So it conducts almost no current. But when the voltage rises above its rated voltage its resistance becomes lower. Very much lower for high voltage. So it starts to conduct a high current (100's of amps) so that the fuse is forced to blow. That's how it protects.

    Now when its conducting current there is still a voltage over them, they are not a real a short circuit. So a "241" can have about 300V over them when conducting 100's of amps. So that can have an influence on whatever circuits are behind them. It also means they have to absorb a lot of power. If they are overloaded for too long, their internal structure changes and they become low resistance. That's why your varistor smokes, it has become defective.

    Another thing is that a varistor is typically used to protect against lightning. These are high voltages that last a couple of microseconds and are not very frequent. When you subject them to a wrong mains voltage, they are active 50 or 60 times a second until the fuse blows. And that is a long time in electronic times! That is why I find a varistor that has had to cope with mains voltage suspect. One of them is already giving smoke, so it IS defective, the other one will not be far behind. As these are cheap items it is best to change both of them. Get "241" types of similar size.

    You could check the transformer by disconnecting it from the circuit. Both input and output should measure a low ohmage. Transformers are very robust items but some do have internal fuses. Otherwise, disconnect the output and measure it when powered.

    Another thing I would check is the circuit board next to those varistors. It looks brown so it has been overheated. Look at the copper traces and make sure they are not "lifted" (still well glued to the glass fiber board) of cracked.
    That makes total sense. So when the voltage is too high, the resistance of the varistor drops and the amperage over the circuit increases enough to blow the fuse. Never knew!

    I’ll try to source some of those 241 varistors and install them. Would any other varistor of the same size rated 150V also work? Or are there other specifications I need to take into consideration as well? The traces on the bottom of the board look okay, so don’t think anything has been damaged there. Again, many thanks for your insights.

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