Lexicon PCM80 On Fire!

LPT45 Power Supply

Overview One of the downsides of owning vintage synths and FX’s from the 1980’s is that there is a risk of power supply failure, with capacitors emitting magic smoke or worse still bursting into flames. I bought an Emulator I many years ago for just £50 because when we switched it on it created a huge plume of smoke as various capacitors failed.

My approach over the last few years has been to refurbish the power supplies as I acquire the item, because they are usually 30 years old. In my new studio I also have a smoke alarm fitted and never leave items powered on overnight. I have fitted 1U Samson Power Strip PS10’s to supply all the gear, this enables me to turn off the power to all the gear quickly and simply.

Blown Capacitor

A Cautionary Tale  I recently installed my PCM80 into the main studio and powered it on, and it worked fine. However 30 minutes later magic smoke came out of the rack and I traced it to the PCM80 power supply and immediately switched it off. Looking at the top of the old switched mode power supply you can just see a capacitor top pushing up, which is a common failure mode for these old units.

But once I had taken out the power supply module you can see the failing capacitor has cause other components on the bottom of the PCB to burn out and worse still the plastic shield between the PSU and chassis has started to melt. Within a few minutes this could have become a major fire with plastic dripping onto the units below and smoke filling the room.

Make sure your studio has a Smoke Alarm!!

Fire Damage!

Lexicon PCM80 PSU Replacement With the PCM80 (1993) Lexicon moved from fully linear to switched mode power supplies. They also added subsequent linear regulators for the analog rails, to ensure low noise. They used a standard third party switching power module, which is easy to replace with a modern version with the same voltages, current supply and fixing holes.

The Artesyn LPT45 is the model to go for, and it can be bought for £50. This is a lot cheaper than sellers on eBay want for a PCM80 PSU, which is simply a LPT45. The service manual states the power rail voltages and current demand, so it was easy to check this was the correct part. PCM80 Max Ratings are:

• +5V is 5A
• +15V is 2A
• -15V is 300mA

The LPT45 delivers 4A on the +5V rail but the consensus is that is sufficient to safely power the PCM80. Lexicon did a great job on the power supply design with carefully designed analog power conditioning, using linear regulators, inductors and electrolytic capacitors. This circuitry looks to be in good condition, as it is not stressed as much as the switched power supply. However I will replace the power capacitors to be safe.

Tufnol Protector

Fitting the LPT45 The new power module is an exact fit, same connections and mounting holes. The only problem I had was the plastic shield had been melted by the power failure. I replaced it with a 1mm piece of Tufnol which provides the correct electrical insulation and is not going to melt or catch on fire.

The LPT45 is grounded to the chassis ground by two of the fixing holes which have earth pads, I double checked the earth continuity from chassis to the LPT45 ground connector to be sure. I returned the PCM80 to its rack and powered up – everything works and no smoke!

Learning Lesson Whilst I have refurbished the linear power supplies of the vintage gear I own, I have left the switched mode power supplies alone. That strategy has now changed, and I am replacing them ASAP. This means replacing the Emax power supply, where I will design a mounting  PCB for the Meanwell RPT-60B, and the Wavestation where an LPT42 may fit (again with a mounting PCB). I will also complete the design of a switched mode power supply and PCB for my Roland MKS-80.