BAT VK-500 MOSFET Monster Amplifier Repair

A regular customer of mine had an issue with one of the binding posts of his amazing Balanced Audio Technology VK-500 MOSFET amplifier. I fixed it for him, read about the repair below.

Technically speaking, the BAT VK-500 MOSFET amplifier is an amazing design. It uses all MOSFET devices in the driver and output stages. It features 24 N-channel MOSFET devices in the output stage, avoiding the inherent complementary pair mis-matching problem. The N-channel MOSFETs are perfectly matched and faster, allowing a 300V/us slew rate, an extraordinary spec.

  • 250/450 Watts RMS per channel into 8/4 ohms
  • 1100 joules energy storage from 300,000 uF power supply capacitance
  • Zero global negative feedback design
  • MOSFET output devices
  • Balanced XLR inputs
  • Completely dual-mono topology (two transformers and two power cords)
  • 48cms (W) x 58cms (D) x 24cms (H) (Mass) 50kgs

More information about the VK-500 and comprehensive technical explanation can be found here.

The Repair

The binding posts fitted to the BAT VK-500 are pretty good, but the plastic used becomes brittle with age, making it susceptible to fracture. With this binding post, the plastic on the retaining side, underneath the retaining nut, had fractured and failed. 

Repairing this required either a replacement of all four binding posts or a repair of the broken one. I prefer to repair parts like this, because it minimises wastage of good parts and helps maintain the originality of the equipment.

First I desoldered the broken binding post. This required a lot of heat, due to the heatsinking effect of the heavy metal parts and chassis. Once I had the binding post out of the chassis, I cleaned it and measured it up for repair. 

I used some high-quality nylon washers to build a stronger retaining arrangement than the factory part. The washers fit perfectly and are much stronger that the original plastic parts. I was able to torque the retaining nut up onto the new nylon retainer and then resolder the output wire.

The repair looks completely factory from the outside, so I have maintained the factory appearance and avoided the need to modify the chassis to fit new parts, a great result.

The awesome BAT VK-500, showing massive toroidal transformers, stunningly clean layout and mirror-image left and right hand amplifier modules. Note that this amplifier is dual mono, from the twine power cables and mains sockets to the speaker binding posts.
This is the broken binding post, removed from the amplifier chassis. Damage is obvious…
This is where the broken binding post came from, enough room to work in here but only just.
View from the rear with binding post removed.
Binding post reinstalled, with nylon retaining washers in place. This is a very strong repair, the nylon is extremely tough and completely non-conductive.
Finally, binding post with connecting cable resoldered into place. This again required great heat, due to the heatsinking effect of all those metal parts!
Repair looks perfect!

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