Troubleshooting a dead pre-amp

[dustyz]

I have a few pre-amps that have "died" over the years. Both still show voltage from the injectors, but neither show any boost in signal.

Is there a basic troubleshooting guide for the electronics in these things? Something that tends to go bad with electric spikes or lightning? I see no signs of burning or failure on the boards... mosfets seem fine.

Links or ideas? Maybe just a place to start? Thanks!

[rabbit73]

Originally Posted by dustyz

I have a few pre-amps that have "died" over the years. Both still show voltage from the injectors, but neither show any boost in signal.

Is there a basic troubleshooting guide for the electronics in these things?

There is no basic troubleshooting guide for the electronics in preamps.
Even if you found a bad part, could you remove and replace a tiny surface mount component?

There are external tests you can make like measuring the current to the preamp, measuring the voltage at the preamp when it is connected, and measuring its gain. If it fails those tests, replace the preamp.

[dustyz]

Gotcha. I've replaced plenty of parts on PC/videocard motherboards, so I could probably do ok with what I see on these. What could it hurt to try, ya know? I'd need to dust off the extra hands/magnifier, probably!

I was just curious if you typically started with the power, or the signal side... or if spikes caused X to go bad before Y. I'll see what I can dig up, and test what I can. Thanks.

[rabbit73]

First power, then signal.

The spikes from nearby EMP or static discharge would take out the first LNA IC.

In the Winegard LNA200, it would most likely be Q2 even though it has 2 transient suppressors.

[dustyz]

Sorry for the late reply, but that is very helpful. Thanks for sharing. I will delve into my broken amps and see what I can find out.

I appreciate it.

[Tower Guy]

The LNA-200 appears to amplify UHF and VHF separately. Q2 is the UHF amp. Q1 is the VHF amplifier. While both could be bad, if the amp works in VHF but not UHF, change only Q1. If it works on UHF but not VHF, change only Q2.

[Tower Guy]

Q3 appears to be a voltage regulator for Q2. If the voltage on inductor L16 is not 3 volts +/-, change Q3.