Monday 10th January 2011, 09:14 AM Posted by urbancamo

I had a spare half an hour earlier so I thought I'd take apart the VT330 to see what the magic smoke escaped from. Getting into the VT330 was childs play thanks to the excellent resources available online:

Getting in involved undoing a single bolt on the back of the case, then pressing on two tabs at the front to release the cover.

VT330 Cover Removal

Inside there are three main board - the video amplifier module (attached to the back of the tube), the cpu/bitmap module (which can be seen on the left as a two card sandwich in the picture below) and the power supply/monitor module, which is on the base of the monitor. The flyback transformer is the black box on the right of the top with the red wire coming out of the top curling round to connect onto the tube through a sucker cup.

VT330 Internals

It didn't take long to find the problem. As suspected the flyback transformer had toasted itself. In order to extract the power supply/monitor module I had to disconnect connectors to the other modules and two screws. Before this, however, I needed to ensure that the CRT HT circuit was discharged. I did this using a scope probe held with pliers.

Anode Discharge

Once the board was out it was easy to see the damage caused by the broken flyback. It looks very similar to an issue I had with a VT420 back last March, but I'm much more keen on getting the VT330 back on its' feet given that it is such an unusual monitor. You can see on the following image a black patch on the PCB at the base of the flyback transformer, the carbon deposits from the arcing that I experienced when I turned the monitor on has extended to a nearby resistor (the nearby lightblue cylinder).

VT330 Fly Back Transformer

Also, looking at the state of the transformer, it's clear that it was probably going to go pop sooner or later, regardless of whether it had been bought in from the cold. There is rust on the spring clip the runs up the side and along the top of the transformer.

VT330 Fly Back Transformer - Corrosion

So what's next? The field maintenance print set contains a full set of schematics, engineering drawings and part lists.

VT330 Field Maintenance Print Set

From the parts list I need a 16-26873-01.

VT330 Fly Back Part

Previous recommendations regarding the VT420 from helpful members of the comp.sys.dec group have suggested that it makes sense to replace the high voltage components associated with the transformer - this includes any high-voltage capacitors, transistors and diodes. The schematic, in combination with the PCB, will provide enough information to get most of these parts sorted. In the section shown below the flyback transformer is marked by the dashed line labelled 'T201'.

VT330 Schematic

I found somewhere in the USA providing the transformer and one of the driving transistors, I've placed an order, apparently there will be a week or so delay before shipping, so it may be that RetroChallenge runs out before I get a chance to implement a fix.

Of course, I could replace the entire board, and there is a DEC part number for that: 54-16905-01, VT330 MONITOR BOARD PSU. Unfortunately, the only quote I got back was £ 200, which doesn't include VAT or delivery. I can't justify spending that amount of money, and anyway, where is the fun in that?

The VT420, although it didn't result in a fix, provided me with enough information about how these things work (take a look at the repair faq if you'd like to read more).