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Restoration of VR14 -- 2008 11 17

Jumping back in time a little, while I'm waiting for the artwork to be silk screened onto the board, I decided to embark on the VR14 restoration. I was able to reform two of the capacitors "in place" -- removing them would have been difficult.

The unit came with a dead 5A fuse, so that was already a bad sign. I replaced the fuse and promptly blew it again. Next, I tried to slowly bring it up with a variac, but I blew the fuse on the variac. So, I asked my buddy to help out, and we went through the power supply step by step. The first part was simply to disconnect everything except the mains to the transformer. Good -- the fuse did not blow. Next, we connected the diode rectifier and capacitor assembly -- bang, dead fuse. Long story short, it was the dual diode rectifier on the side of the unit -- it was a dead short! A new one is available as a bridge rectifier (i.e., a quad diode instead of a dual) from Digikey and it's part number MB1505-ND. This is a 15A 100V part -- I actually ended up ordering the 25A part.

VR12 Regulator Board S M L XL This picture shows the replacement (temporary) diode and the regulator board on a piece of cardboard for insulation.

At this point, we had power but the negative regulation was bad -- instead of seeing -22V, we saw -43V. More diagnosing showed that the 2N5302 was bad. Luckily I was able to order 5 x 2N5302 and 5 x 2N4399 (the other power transistor used on the VR14) from eBay for $30. In the meantime, while I waited for that to arrive, I used the 2N5302 from the deflection circuit to get the power supply running. Nice and clean +/- 22VDC :-)

In case you're wondering, taking apart a VR14, while looking like a major pain in the ass, is actually quite doable. The main trick is to disconnect the four power connectors to the power supply board, and undo the two screws holding it into place. It looks to be a normal 2-high flipchip, except that the connectors on the bottom of the flipchip don't go anywhere -- I guess they were used during testing and manufacture.

So, while I wait for my parts to arrive from Hong Kong,I have the following to do:

  • clean and re-silicone the power transistors
  • clean the insides (of course there was a mouse nest!!!)
  • verify the CRT filament is heating up and operational
  • check the power going to the deflection coils and see if that works
  • verify and measure the -400VDC plate voltage

I'm not going to attempt anything with the 10kV power supply :-)

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