ALS500 Mobile Amplifier Issues

The ALS500 amplifier has two significant issues that are easy to correct.

The ALS500 with remote jacks has what are essentially unfused small wires exiting on remote jacks. Because current is not limited to safe values for wire size, remote wires can become red hot. It is fairly easy to prevent this.

While into the amplifier, it is best to inspect all of the solder connections. This is especially true on the high current leads and the rear power connector.

The second issue is mainly when PP100 transistors are used in place of 2SC2879 transistors. The amplifier has a large current spike. The PP100's have a lower base bias threshold, typically I measure around .51 volts or so, and high dc current gain at idle. The bias circuit has a very brief (a few milliseconds) spike of around .75 volts before it settles to operating bias levels. The PP100's also have a good bit more VHF gain. This causes the amplifier to momentarily oscillate upon relay closure.

The momentary oscillation causes a very large current drain from the high current 12V supply, near 100 amperes and more.

If a 12Vdc current protected power supply is being used, this spike can cause overload or foldback circuits to trip. This oscillation also can cause random unexplained "Fault" trips of the amplifier protection circuits. It seems reasonable to expect this will also shorten PA transistor life.

Both of these issues, the unfused leads and the oscillation and bias surge, are easy to correct. While correcting these problems soldering and lead dress should also be inspected and corrected!

I have a kit that will will correct the bias and safety problems.