Electronics Novice with maybe stupid question - bridge rectifier

Discussion in 'General Electronics Chat' started by ColoradoRobert, Jan 24, 2016.

  1. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    I have a worth-nothing guitar amplifier from the 70's (non-tube) and thought I would try my hand at repairing it. One of the first things I noticed when I opened it up was there is a bridge recitfier that looks like it exploded and has white corrosion all around it. So I am thinking that this is one part that I need to replace. Question is after I measure the DC voltage (3 red wires) coming out of the transformer they are less than 50v.... does the replacement rectifier voltage matter (remember novice). Reason being I find no direct replacement for this 70's part but I have found a really affordable cheap one form China on ebay but it is 1000v. Thanks in advance.
     
  2. thumb2

    Member

    Oct 4, 2015
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    You should replace it with a bridge rectifier which has current rating greater or equal the current consumption of your whole amplifier circuit.
     
    Last edited: Jan 25, 2016
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It doesn't matter as long as it is over 300 v or so. What also matters is the current (amperage). I am guessing that your amp outputs less than 100 watts so a few amps is good enough. I am guessing this eBay bridge will be 10s of amps.

    You can also run to a local electronics store (radio shack if there is still one in your neighborhood) and get one for 2 or 3 dollars today. In reality, bridge rectifiers are so cheap that postage or packaging is more expensive. If you lived in my neighborhood, I would give you one.
     
    Last edited: Jan 24, 2016
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    If you're measuring 50 volts at the transformer secondary, the peak voltage will be 70 volts. You'll need a minimum of 2x peak input voltage rating. So, 150 - 200 volt peak inverse voltage rating is required.
     
  5. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Thank you for the replies and comments. Back to the beginning Stupid Q 2 = This white corrosion where the rectifier is mounted on the metal chassis.... is there any chance that what I am seeing is old hardened dielectric paste. It is not glue it looks like corrosion but might it be paste possible dried for 40+ years. But why would they put that underneath it.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    Hard to tell. Overheated components often leave a white residue.
     
  7. paulktreg

    Distinguished Member

    Jun 2, 2008
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    What is under the rectifier? It may be thermal paste?
     
  8. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    The underneath of the rectifier would have been metal to metal (bottom of rectifier to chassis). Thinking out loud. I touched the white stuff with a swab of isopropyl alcohol and it is dissolving it. I think it is corrosion but that thermal paste comment concerns me. I guess my next step will be = learn how to test the rectifier. I have not unsoldered it yet and my wife might put a stop to it.
     
  9. paulktreg

    Distinguished Member

    Jun 2, 2008
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    If it's metal to metal then it's probably thermal paste to help dissipate the heat to the metal chassis. Isopropyl alcohol is often used to clean it off.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    What you have found may only be a symptom, not the cause. What caused the rectifier to blow up? Bad reservoir caps are popular. The big capacitors close to the rectifier.
     
  11. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Thank you hp1729 = They are probably 40 years old and I have been watching youtubes on the 2 nearest and largest caps (60vdc surge 75vdc 7500mfd) and have tested them with some of the ideas in those youtubes and I believe both are defective. Backing up to the paste info also = Uh-oh to the thermal paste (because I removed it all) and further uh-oh regarding testing the rectifier = my old (non digital meter) looks like it does not have a diode setting on it. Can you check diodes any other ways.... or does your meter have to have the diode setting on it.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Trust you mean AC voltage since transformers don't output DC. :confused:
     
  13. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Thank you crutschow = you have caught a huge mistake in my thinking. So if the transformer does not make the conversion to DC then all it basically does is lower the voltage that comes out of my wall outlet. Right or wrong? If so, I would bet it's voltage must be lower than the surge rating on the cap (75v surge). So it sounds like the first thing I need to do is unsolder the rectifier and see what the AC voltage is coming out of that transformer because if it is too high (over the surge amount) maybe that will put a stop to this project right away. Because I think those are expensive and I haven't got much more money to waste than the 20 dollars for the 2 new big caps.
     
  14. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    In case anyone wants to see a picture of her, here she is. BTW 'thank you' to all. * I told you I was a novice. I am trying to save and reuse this old amp (for one thing).... because Bud Ross the founder of Kustom amps, who when his Company lost favor in the market early 70's.... then for a short time became Road out of Santa Ana, CA. So it is sort of nostalgic even though it is completely transistor. Secondly, almost any 10 inch speaker amp is going to cost way more than what I hope to get this fired-up for (still hopeful). BTW, I do not even know what the output on this amp is (watts) because there is dead end all over the internet about this name. I have checked price guides also and they don't mention them.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    Some manufacturers fix components down with some weird glue that deteriorates and becomes both conductive and corrosive - I've seen it eat through component leads very often.

    In the monitor repair trade it was called "brown glue syndrome" - but its quite possible it was originally white when freshly applied.

    Sometimes a glob of it on a cluster of components causes plenty of mischief before its got corrosive enough to etch the leads away.
     
  16. hp1729

    Well-Known Member

    Nov 23, 2015
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    Diode check scales are best, but as long as they don't show a notable resistance at all this is good. Diodes fail by shorting out before they burn open and "let the gas out". Electrolytic capacitors like that 7,500 uF are the most common failures. They have a short life expectancy by design.
     
  17. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Now that I am thinking better about AC/DC and which is which.... when I measure the two wires coming in from the transformer, it looks like about 75+ volts AC. There is another red wire to the transformer but that came off the pos side of the 2nd cap. Unless I buy a new meter (that has a diode mode), I can't really check the rectifier well. * Just by reading ohms across the rectifier they read about the same except for the green one which would be going out to the neg on the 2nd cap. So I might just buy the 1 dollar 1000v rectifier along with two caps and try all that first. From China it will probably take two weeks or more. I am in no hurry though. On the caps do they need to stay the same (75vdc 7500mfd) or could I buy 100v 10000uF without causing and conflict ?
     
  18. hp1729

    Well-Known Member

    Nov 23, 2015
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    If the caps physically fit that should be okay. Watch polarity. They have a positive and negative side. Some manufacturers mark one side some mark the other.
     
  19. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    A photo of the overall electronics assembly and closeups of the power supply area will help immensely. The size of the transformer relative to the other components tells us the approximate power level / current capability of the transformer, the starting point for any power supply stuff. Main capacitor(s) value and voltage rating are additional clues.

    ak
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    Regular silicon diodes in a bridge rectifier should be rejected if they show *ANY* reverse leakage on test instruments in the typical workshop - you need special laboratory instruments to see the miniscule reverse leakage that would pass factory QA.

    Shottky-barrier diodes usually show a small amount of leakage as normal, but you'll probably only find them in a SMPSU.
     
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