Rebuilding the Silicon Vally 700 Watt Amp

04-15-2021

Transistors for this amplifier are readily available for around $100 each. Although there are much improved pallet amplifiers out there, this is still a very reliable amplifier. It isn’t unusual to see these running at full power today after 20 plus continuous 24/7 years in service. They are used in Harris and Continental transmitters with some minor variations. A few different power levels are made, but the one I rebuilt is a 700 Watt unit for a Harris transmitter.

The first thing I did, was inspect the PCB carefully to make sure there was no damage due to excessive heat. If the output hybrid is burned badly or any other areas of the board are burned the module may be unpractical to repair. However, the module may still function without some of its circuitry but with less protection against overloads. If there is no protection for reverse power for example, the module could be destroyed in a Harris transmitter when the Final is mis-tuned. After inspection, I removed the 2 power transistors and cleaned the board with denatured alcohol.

There are a couple of capacitors that should be replaced with hi temp maybe even metal electrolytics. The 20+ year old capacitors in this module actually checked good. They are 47 uF 100V. I replaced the transistors with Macom transistors.

One thing that always really bugged me about these amplifiers is that the bias is set the same for both transistors even though I have never seen 2 that have the same threshold voltage for gate bias. Spec sheet says it can be anywhere from 1 to 5 volts and it does vary typically from 2-3.5 volts. When you have this variance, one of the transistors will dissipate significantly more power. Especially at lower than 500 Watts or so. As the power increases, there is more saturation and the power tends to equalize. I installed a variable voltage divider to equalize the Drain currents for the transistors. This will extend the life of the module. The circuit acts as a balance control for the main bias source so all the protections are still active. It simply skews the offset one way or the other to compensate for the difference in threshold voltage. To adjust it, I placed a .1 Ohm 5 Watt resister in series with the drain supply of each transistor and adjusted the control until the current was the same at about 300 Watts output.

Copyright 2021 Singletone Software and Charles Singleton