mrf454 transistor installation.

[lost bandit]

Need some information on how to power up a set of 454s after being install in a 200 watt penetrater.I have a variac adjustable power supply.Transistors has the pot above them for voltage adjustment but havent a clue what voltage to set these on.Any information on how to do this would be greatly appreciated,.

 

[Crusher]

Mrf454's like 12-15vdc. Not sure what you mean by variac on top of transistors?

 

[vkrules]

Variac for dc solid state????? Are you referring to the bias control by any chance?

 

[9C1Driver]

Variac for dc solid state????? Are you referring to the bias control by any chance?

I think you are correct, older Messenger amplifiers had just such an adjustment as I recall. I don't see the need for a variac in such an example, set the power supply for 13.8 volts and turn it on.

 

[lost bandit]

I think you are correct, older Messenger amplifiers had just such an adjustment as I recall. I don't see the need for a variac in such an example, set the power supply for 13.8 volts and turn it on.

Sorry i did not mention this in a better way.this is a older amp 120 volts.the radio tech said he would charge a 180.00 dollars to installed these.This price was for the two 454s and also said these would have to be power up slowly to a certain amount of voltage or it would fry them.i have installed both new transistors myself but just want to make sure of the voltage i need before i power it up.

 

[vkrules]

No a variac on the 120 v AC is not needed or desired.
. Keep the drive low to start with not the AC!!.
Your amp has an inbuilt regulated DC power supply (or it should have, I have seen some POS FM amps that are only rectified and the volts swing with load) the transistors run off about 13.8 volts dc. Varying the ac in wont achieve much, it will work or it wont, that's all. Good idea to always try an amp for the first time with low Rf input power before switching to full and never exceed manufactures rated power( usually a little less than rated)
What you should be asking yourself is why did the transistors blow in the first place and is that problem fixed ?i.e did you overdrive it, bad swr. or is the inbuilt power supply faulty .If you don't know or can't fix it .Don't turn it on.

 

[Tallman]

Do you have a schematic of the amp? It could be the bias adjustment for the Quiescent current. If you get that set right all should be fine. The following is a link to the spec sheet.
http://www.advancedsemiconductor.com/pdf/mrf454.pdf

 

[Onelasttime]

You can not just drop in any 12V transistor with a matching case type. When change out from one type of transistor to another assuming BCE and such are all in the same place you have to set the bias specifically for that transistor type. A MRF454 is not going to be the same as a Toshiba 2SC2879 ,SD1446or an MRF455 or MRF422 etc.... They can all be made to work. To work well when going from one transistor to another type wraps on the transformers, input and output tuning and biasing need to be checked. This assumes you put a tiny bit of heat past on each one, tightened them down properly and did a good with the iron. Unlike changing a fuse swapping out one make and model of transistor for another is not exactly plug and play! Doubtful that powering it up is going to be the issue. The issue will be what happen when you key it for any length of time. With out at least checking all of those things you are really rolling the dice of chance and it is not in your favor.