LED Diagnosis/Troubleshooting

Board 1 LED Diagnosis/Troubleshooting || Board 2 LED Diagnosis/Troubleshooting

Using the LEDs to Diagnosis Problems

Mosfet Amplifiers || Checking the MOSFETs
Reverse Polarity Damage

History

In order not to use a negative voltage in a MOSFET AGC controlled amplifier, the operating potential of the MOSFET has to be raised 1.5 volts. In a simple 455kHz IF receiver in the ARRL Handbook, the author used an LED as a 1.5 volt zener so that the AGC voltage could run from 0 to 6 volts (instead of -1.5 to 4 volts).

I tried this circuit and found out that not only did the LED light, it's intensity varied with the drain/source current. It also proved to be very useful as an indicator of proper circuit function. When the receiver is finished, a dead amplifier can be quickly located and diagnosed.

Because of the easy diagnosis, the LED's were left in all the amplifiers, regardless of whether they were AGC controlled or not. Gate 2 bias voltage was raised to 6 volts using two 100k resistors.

Over the course of experimentation, the following diagnosis procedure evolved:

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