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Author Topic: .51 ohms? Really?  (Read 3400 times)
KE4JOY
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Posts: 1384




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« on: July 20, 2013, 12:05:56 PM »

Yup I really borked it this time.  Roll Eyes

A voltage divider on the output of an audio amp has two .51 ohm 2 watt resistors  Huh

Does not seem like much more than a piece of wire to me but... where in the world am I going to find such a thing??
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KE3WD
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« Reply #1 on: July 20, 2013, 01:09:44 PM »

I've never seen that on the output of any audio amp and I've seen thousands of audio amps. 

More than likely those are the Emitter Resistors for a complementary pair audio output amp that are fried. 

These are available labeled as such from outfits like MCM, Mouser, etc.  MCM is a good source IMO. 

BUT -- if the emitter resistors are open, you likely need to replace both output transistors as well, as they are likely shorted. 

Shorting the output of a TransformerLess Output solid state power amp should never be done, as you've found out, Ohm's law again, the amp sees that short as a load only the devices in the amp can't pass enough current for such a low impedance and something has to give.  It is the device which fails. 


73
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KE4JOY
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« Reply #2 on: July 20, 2013, 01:24:22 PM »

You are absolutely right they 'were' emitter resistors. They are not open but test out at 1 ohm each and are swelled and cracked. Yup the transistor pair are both shorted from what I could tell using a diode checker.

I am having a hard time finding an replacement.  Tongue
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AA4PB
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« Reply #3 on: July 20, 2013, 01:32:25 PM »

Here's one that should work that I quickly found from a Bing search:

http://www.amazon.com/0-51-Resistor-Wire-Wound-Tolerance/dp/B0066AYNB0
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KE3WD
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« Reply #4 on: July 20, 2013, 02:47:43 PM »

I gave a link to the exact replacement in one of your other threads. 

twenty seven cents each. 
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WB6BYU
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« Reply #5 on: July 20, 2013, 06:13:09 PM »

Or get some 1 ohm 2 watt resistors and put two of them in parallel for 0.5 ohms.
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WD4HXG
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Posts: 189




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« Reply #6 on: July 21, 2013, 02:03:09 PM »

If you have bias diodes in the base-emitter of the output transistors
replace them. If used and they fail they cause the finals to run hot.
They also shift the bas point of the finals and cause crossover distortion.
If the stages of the amp are DC Coupled check the preceding stage.
You likely took out the driver stage if DC Coupled.
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KE3WD
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« Reply #7 on: July 21, 2013, 03:25:09 PM »

Diode is probably okay. 

He should check Q509, though. 
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KE4JOY
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Posts: 1384




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« Reply #8 on: July 21, 2013, 03:48:40 PM »

Driver Q509 checked, passed.
Found a few resistors out of tolerance besides the bias resistors esp R557 which weighed in at 300 ohms.
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KB1LKR
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Posts: 1898




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« Reply #9 on: July 21, 2013, 07:03:47 PM »

See Mouser or Digikey:
e.g. 0.51 ohm 2% 2W axial lead @ $0.24 ea

http://www.mouser.com/ProductDetail/KOA-Speer/MOSX2CVTPAR51G/?qs=sGAEpiMZZMtlubZbdhIBIC1UYE%252bULtAwMwsz3jMYMkA%3d

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PBPP
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Posts: 50




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« Reply #10 on: July 22, 2013, 07:58:45 AM »

Ratshack has a 0.47 Ohm 5W 10% Wirewound Resistor:
http://www.radioshack.com/product/index.jsp?productId=2062289&filterName=Type&filterValue=Power+resistors

~ Mitch ~
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