Question about the VR's on the Ranger amp boards being used in the 2995 base stations

[loosecannon]

Hello all,

Does anyone have any experience manipulating the trimmer resistors on these amp boards?

here is a link to one: https://stores.goldeneagleradios.com/rci-x9-amp-board-with-hardware-feedback-no-warranty/

there is a VR near the input/output section, and another on the opposite end near the mosfets.

What are these for?

It would seem to me that the one near the "finals" is a bias adjustment for all four mosfets.

to me this seems like it would be adjusted for 400mA while checking that the gates are kept under 4 volts.

the other one i have no idea about.

thank you in advance for any insider knowledge/insight anyone can offer on these.

here's a pic. its not great but if you look closely you can see the trimmers.



LC

 

[nomadradio]

The one on the corner next to the input transformer sets the gate bias voltage. Pretty sure the one at the opposite side between the coax sockets is for the relative-power meter calibration.

We set bias by taking the power lead to the amp board loose from the power supply and inserting a meter to read the board's current draw while keyed in sideband mode with zero mike gain. The trimpot is turned for minimum current. This reading becomes the baseline, where the MOSFETs are drawing no idle current at all. The trimpot is advanced until you see an ABRUPT rise in current. We set it for 80 mA above the baseline reading. Too high and you risk overheating and poofed MOSFETs. Too low and sideband transmit is distorted.

So far this has worked for us.

73

 

[loosecannon]

thanks nomad!

i guess i was confused about how much idle current to use for a four mosfet setup.

You stopped at 80mA which tells me it tried to take off on you above that level.

Can you remember what the gate voltage ended up being at this level?

also thanks for the tip on the other VR!

i can see why people have commented that it doesn't seem to do anything lol.
LC

 

[nomadradio]

I only pay attention to the gate voltage Vth shown on our tester. That's how we match pairs and fours. Just what voltage the trimpot sets to is of no interest to me. I only care about the resulting drain current.

73