RM KL503H linear amplifier

Another wonderful exciting day in the play-pen. Warm breeze drifting in, waves lapping at the office door step. Bit worrying as we are 40 Km from the coast. Anyway.

Today’s project was a nice Italian RM KL503HD linear RF amplifier. Single band, HF 25-28 MHz. brought in by a local amateur radio Ham. Knowing this chap is very much a professional operator, I decided to refer to this unit as his CB burner. Understandably, he became upset and violent. After he had calmed down, we discussed the fault.

The amplifier had started to get hot, even when not transmitting. Would not transmit. Also, the built-in receiving pre-amp which gives you an extra 20db or so in receive mode was not working.

A look at the diagram shows a set of 8 MOSFETs in a parallel push pull arrangement, 4 per side. The pre-amp circuit is switched in via a relay when activated. No PTT line, as the amplifier recognizes RF coming in and switches to transmit.

A quick test on the bench with power applied showed nothing odd. However, when an input was applied, it became unstable, and, a few puffs of smoke.

These RM amplifiers are nicely made and reasonably easy to service. After struggling to remove the PCB and heatsink screws, the board was removed. One bank of the MOSFETs were short circuit and various other components faulty, resistors, diodes and so on. With the MOSFETS out, we found that the part numbers had been filed off. The parts list refers to these as RM3, costing 100,000€ each. Why make things awkward? It’s not as if they are made from Moon dust and Gypsy tears. Looking at the diagram, there seemed to be nothing special. We are only talking about 28 MHz, so I popped in some IRF520Ns and replaced all the other blown-up parts.

Now turning to the pre-amp. This consists of a BF199 and supporting components. Various ceramic capacitors were short circuit, the BF199 was dead and a couple of diodes and resistors. The only way this damage could have occurred, in my opinion, was a transmitter was connected to the wrong input and keyed. We discussed this with the owner and held him to ransom, if he didn’t pay up, I would tell the Ham Radio community what had happened. Off to buy my new Bentley.

With all parts replaced and reassemble, all worked well. Happy with the result.

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