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eHam Forums => HomeBrew => Topic started by: KE4JOY on July 20, 2013, 12:05:56 PM



Title: .51 ohms? Really?
Post by: KE4JOY on July 20, 2013, 12:05:56 PM
Yup I really borked it this time.  ::)

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

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??


Title: RE: .51 ohms? Really?
Post by: KE3WD 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


Title: RE: .51 ohms? Really?
Post by: KE4JOY 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.  :P


Title: RE: .51 ohms? Really?
Post by: AA4PB 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


Title: RE: .51 ohms? Really?
Post by: KE3WD 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. 


Title: RE: .51 ohms? Really?
Post by: WB6BYU 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.


Title: RE: .51 ohms? Really?
Post by: WD4HXG 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.


Title: RE: .51 ohms? Really?
Post by: KE3WD on July 21, 2013, 03:25:09 PM
Diode is probably okay. 

He should check Q509, though. 


Title: RE: .51 ohms? Really?
Post by: KE4JOY 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.


Title: RE: .51 ohms? Really?
Post by: KB1LKR 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



Title: RE: .51 ohms? Really?
Post by: PBPP 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 ~