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Craig
Craig, Electrical Technician
Category: Electrical
Satisfied Customers: 4529
Experience:  33 years experience electrical troubleshooting industrial equipment and control systems.
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i have built a rf amp for 10 meter using eight 2sc2879 transistors

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i have built a rf amp for 10 meter using eight 2sc2879 transistors and am having problems with the resistor on one of the output combinders heating up and burning out. i have tested the transistors as you explained how to and all tested good. i have a schematic diagram but do not know how to send it to you too look at. any help would be wounderfull.

Craig :

The resistor that is heating up suggests high current or possibly under rated wattage spec for the resistor. Is it the same as the other output resistors on the the other circuits?

Craig :

You can upload here doing this:

Craig :

-






see the top right there is a red and white file icon, click that






then you will see this







Craig says:



click browse and go into your folder and click the photo






then you have another box...






then this box comes back and you click INSERT







That will drop it in like I'm doing here







I press enter and it's on the post...




 

Customer:

sorry i did not get back to you sooner. life consumed me. here is the schematic for the rf amp and the resistor i am having problems with is # XXXXX 22 -100 ohm 5 watt if you can see any reason this resistor gets hot and burns up please let me know. i am willing to pay bonus

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Craig :

I don't see a R-22 anywhere. With the circuit on and functional, read the voltage drop across the R-13 through R-20 resistor. Compare it with the other identical circuit resistors. Is it R-20? They all look like 100 ohm resistors is it R-20 that is burning. What is the voltage drop across it compared to the other 100 ohm resistors.

Craig :

Now I see it sorry...

Craig :

Compare the voltage drop across R-21 with what R-22 has, what is the voltage across each?

Craig :

I think it could have something to do with the polarity of the coils on the lower circuit with R-22 and the current is way high from this. I bet the voltage drop on R-22 is way high from this.

Craig :

Get the polarity of the coils associated with R-22 to match the coils for R-21 circuit and it's good to go.

Customer:

i am unable to key the amp long enough to get a voltage drop reading before the risistor blows. the combinders are wraped identical. what kind of polarity would there be on the coils? and witch coils?

Craig :

You have a short there some where for it to pull that kind of current to blow that resistor.

Craig :

Un hook the #16 jumper from FC 12 and also un hook C34 cap. ompare some

Craig :

Compare some of the component values with the identical circuit next to it. For that resistor to blow, there is a short or ground there on the output may at FC12. Check around with your ohm meter and look for something that is way low ohms. Compare it with the other side.

Customer:

i will see if i can find a short

Customer:

i found the problem it was a bad transistor i am suprised because all eight are brand new and a maching set of eight. thank you for youre help

Craig :

That's what I suspected, a short circuit and high current. Glad to help. Craig

Customer:

i found the problem it was a transistor. i am suprised because all eight are a maching set and brand new. thank you for your help

Customer:

sorry for repeat

Craig :

That's what I suspected, a short circuit and high current. Glad to help. Craig

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