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54 posts

Master Geek


#192363 8-Mar-2016 13:13
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Hi there.

 

 

 

I am repairing an amplifier and have sourced replacements for most parts except for one of the resistors. The colour code does not conclude with any matches when using online tools. The resistor has likely been damaged and does not read with a multimeter.

 

 

 

From what I can see, it has four visible bands but has a space for an extra one/two. Red, Black, Grey (could also be faded black), Gold. The closest match I can find is a 20ohm E24 resistor. Would somebody mind looking at the picture and confirm that this is what is likely.

 

 

 

https://cdn.geekzone.co.nz/imagessubs/7b191f03454416a98b71d52e3ef32379.jpg


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485 posts

Ultimate Geek

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  #1508088 8-Mar-2016 13:23
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Looks like red-black-black with tolerance gold, so 2 0 0   5%, so yes, I would say, yes to 20Ω, if memory serves.

 

https://en.wikipedia.org/wiki/Electronic_color_code#Resistor_color-coding

 

Does it get hot?

 

 

 

 


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Vocus

  #1508089 8-Mar-2016 13:29
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Agree with roobarb, 20 ohms.  What size is it?  Looks like a 1w or 2w resistor in the picture but hard to say without any scale reference.


 
 
 
 


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  #1508096 8-Mar-2016 13:37
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There will usually be an identical one in the other channel of an amplifier that you can compare with.





Richard rich.ms

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  #1508112 8-Mar-2016 14:03
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Where do you think it is connected in circuit?

 

That would help identify the likely value.

 

 





Gordy


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  #1508117 8-Mar-2016 14:15
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Sorry to perhaps add confusion BUT :

 

 

with the ends being Red and BRown it is a 1% or 2% tolerance and would have 4 bands for value.

 

 

I think I can see a 4th band ?? grey? white? silver?

 

 

Most likely Red-Black-Black-Silver = 2 0 0 divide 100 -> 2 ohm 1%

 

 

or Brown-White-Black-Black = 1 8 0 0 -> 180 2%

 

 

???

 

 


406 posts

Ultimate Geek


  #1508157 8-Mar-2016 15:08
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There are heaps of free iPhone app's that do resistor colours - there are only certain colour combo's too so that might help narrow it down.

 

If you use an iPhone, this is the one I have installed:

 

http://download.cnet.com/Resistance-Calculator-v3/3000-2094_4-75087829.html

 

(oddly though, I just click the link from my iPhone, opened in the appstore and it said the item is not available in NZL - weird.  May work for you perhaps its coz I already have it installed)  

 

Says 20 ohms, assuming its not a 5 band with colour rubbed off! (unlikely)

 

So a multimeter reads nothing at all?





 

 


406 posts

Ultimate Geek


#1508162 8-Mar-2016 15:16
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...looking for the 10th time, I'm still picking Red, Black, Black, Brown (or possibly Gold) - obviously just a variation on tolerance, still 20 Ohms. Whats worse that can happen, it'll blow and you'll just have to source the parts all over again laughing





 

 


 
 
 
 


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  #1508164 8-Mar-2016 15:21
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If you just change the resistor then it probably will cook up again in anycase, amplifiers are usually a cascade of failures leading to almost every transistor and large resistor on the output stage needing to be replaced.





Richard rich.ms

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  #1508172 8-Mar-2016 15:24
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chimera:

 

...looking for the 10th time, I'm still picking Red, Black, Black, Brown (or possibly Gold) - obviously just a variation on tolerance, still 20 Ohms. Whats worse that can happen, it'll blow and you'll just have to source the parts all over again laughing

 

 

OP said gold, I'm picking they're not going to misread brown for gold in person...

 

But richms advice is good, don't just swap the resistor and cross fingers, it probably blew for a reason, you will need to check everything


406 posts

Ultimate Geek


  #1508273 8-Mar-2016 16:13
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OP said gold, I'm picking they're not going to misread brown for gold in person..

If that were the case then he wouldn't have posted here in the first place...




 

 


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  #1508322 8-Mar-2016 17:18
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Looks like it got hot, judging by the discolouration.

 

 

 

Is it currently open circuit ?

 

 

 

Could be a 1 watter, and vertically mounted to aid heat dissipation - could be the resistor on the Zobel network ( output stage to minimise impeadance mismatch)

 

 

 

As richms mentioned, should be another to match against.





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54 posts

Master Geek


  #1508355 8-Mar-2016 18:25
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Definitely won't just replace it. The amp was running with a horrible combination of speakers and was insanely overdriven. Have replaced a couple of cap's as well so hopefully will do the trick after some serious improvements to the speaker configuration.

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  #1508376 8-Mar-2016 18:44
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Test all the transistors too. There was some test with the output transistors removed that you can do to check that they will not both be turned on as soon as the amp gets power.

Or just measure the voltage of the transformer and order a class d amp board off aliexpress that can use that transformer and bin all the old analog stuff.




Richard rich.ms



54 posts

Master Geek


  #1508735 9-Mar-2016 11:06
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The resistor is currently open circuit and a multimeter is no help, cheers for the advice, I agree it is most likely 20ohm.

 

So it turns out that finding a 20ohm 1W resistor in the right is pretty hard. The resistor in the picture measures 15.8mm in length and 4.5mm in diameter suggesting 1W. If I were to obtain a 20ohm 5W would that have bad any effect on the circuit? or am I best to join two 10ohm 1W in series?

 

My electronic knowledge is somewhat limited when it comes to component wattage.

 

Cheers for the advice.


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  #1508739 9-Mar-2016 11:11
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livefornow851:

 

The resistor is currently open circuit and a multimeter is no help, cheers for the advice, I agree it is most likely 20ohm.

 

So it turns out that finding a 20ohm 1W resistor in the right is pretty hard. The resistor in the picture measures 15.8mm in length and 4.5mm in diameter suggesting 1W. If I were to obtain a 20ohm 5W would that have bad any effect on the circuit? or am I best to join two 10ohm 1W in series?

 

My electronic knowledge is somewhat limited when it comes to component wattage.

 

Cheers for the advice.

 

 

If you can fit a 2W or a 5W that would be fine (5W are usually pretty big tho).  So would 2 x 10 ohm 1/2W or higher in series because each one will dissipate half the power. (edit: corrected)


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