2970N2 Service Manual

[dxhound]

I am working on a 2970n2 for a friend of mine and I have some questions. I have the service manual for 2950/70dx, and most of the procedure is the same. My question is specified more toward the TX alignment of the IRF520 driver and dual finals. I looked up the Specs of the transistor but I would like to know what RCI recommends for these.

The radio gets VERY hot and only produces 150-175 watts. The voltage regulator get VERY hot as well. The funny thing is, while tuning the PLL, I had the mirror board remved and while transmitting for only 10-15 seconds at a time, the 8 transistor amp got pretty hot even though no RF was applied to it.

Any help appreciated.

 

[Shockwave]

If it gets hot with no RF applied the bias is too high on the amp. Sorry to be the bearer of bad news but these MOSFET amps can be quite troublesome. Anytime you need to use 8 transistors in a single stage to make a couple hundred watts, guaranteed sure thing is you're using the wrong transistors.

 

[dxhound]

If it gets hot with no RF applied the bias is too high on the amp. Sorry to be the bearer of bad news but these MOSFET amps can be quite troublesome. Anytime you need to use 8 transistors in a single stage to make a couple hundred watts, guaranteed sure thing is you're using the wrong transistors.

I agree!! The fact that these transistors were not even meant to be used in an RF application should have been a clue to Ranger not to use them. Unfortunately, The bottom line usually wins in radio design.

Luckily I found the Bias info I needed for the driver, finals and PA

 

[dxhound]

Well, I had the privilege of working with the 2970N2 all night. I tried different voltages and combinations for the Bias levels and got it where the radio will not get too hot or draw too much current. After all that, I have to say.....................I dont like it. The amplifier section is very prone to oscillations , and without any real way to adjust any harmonic filtering, the spectral purity of this radio is a joke at any level above 175 watts.

RCI needs to find something better than this!

 

[JASTECH]

Dxhound, If I may inquire, what harmonics would need to be adjusted in such short pathways of copper? Also if so, how would you adjust them?

 

[AudioShockwav]

http://www.worldwidedx.com/export-radios/107050-rci2970n2-bias-help.html


73
Jeff

 

[Shockwave]

Dare I say these MOSFET 11 meter amps are complete garbage? If it's not bias problems, it's oscillation problems. If it's not oscillation problems, it's bias problems. If it's neither of those, it's probably sounding distorted, running hot or not making what you would expect from 8 transistors in one stage.

The problem is the transistors themselves. Some claim to be specifically designed for RF amplification rather than high speed switching applications. I don't really care what the specs claim if the device is in the wrong package for the application and power rating.

There is nothing you can do inside the package to reduce the inductance those three wire transistors have and that's the problem. The TO-220 or even the TO-3P packages are a waste of time for anything over 40 watts at 30 MHz. Especially if the transistor has high gain or beta like these do.

If the MOSFET doesn't have four, wide gold plated tabs with dual source connections, it has no business in an amp making over 100 watts at 30 MHz. These problems will persist until Ranger learns it is necessary to use real RF power transistors if you want any reliability. We are not on 80 meters were power supply transistors work well.