BNOS LPM144-10-100 Repair

I have had a B.N.O.S LPM144-10-100 solid state linear amplifier for some time. It brought it at a ham fest and it worked fine. However, when I tried to use it recently, I noticed that sometimes the amplifier would work, but other times there was no output. Due to the intermittent nature of the fault, I knew it couldn’t be the main power transistor (MRF247). The most likely cause was one of the 3 relays. There were also 5 electrolytic capacitors. I decided to change all 8 parts.

The first thing I did was cross correlate what I had with the circuit diagram (click for full size image/download).

The PCB 

Using a desoldering station to melt and vacuum extract the solder, the 3 relays are easily removed from the PCB with no board damage.

Closeup of the 3 removed relays:

Comparing the original PCB photo with the one below, you can see the 3 replacement relays and 5 capacitors.

I used a Finder 12V miniature DPDT 8A relay as my replacements sourced from Rapid Electronics in the UK, but these relays are universally available from different manufacturers. You will need a DPDT relay with 12V coil (not SPDT as this article previously stated [thanks to Keith GM4YXI for spotting this issue]). My current suggestion would be the TE Connectivity / Schrack RT424012 (datasheet).

Below is the amplifier working! Yay!