Journal Alioth's Journal: Transformers - inductors in disguise 1
Last week, at last, I got my hands on a Vectrex. A really nice one, too - it looks hardly used (and when I took the case off, not a speck of dust inside). If you don't know what a Vectrex is, it's an early 1980s game console. It's about the size of the original Macintosh, and has a black and white vector CRT. Yes, vector. Just like the original Asteroids, or the original Battle Zone, it uses vector graphics, rather than a bitmapped raster display that everything else uses.
However, after 5 minutes of playing with it, it died. It didn't take me long to trace the problem - the primary winding on the main transformer had failed. I'm surprised there was no magic smoke. The problem: I can't even get to where the wires are soldered onto the primary windings, even after prising off the transformer's metal case. It looks like it was manufactured by winding the bobbins with the leads soldered directly onto the transformer wire, buried in the bobbins, then the iron laminations put on afterwards - so it can't be disassembled.
The transformer carries no useful markings, and the Vectrex service manual doesn't even tell you the specifications. All I know is that it's probably got a 115-0-115 primary (with the 115 ends connected, to make the primary 230v), and a suspected 9-0-9 secondary. I say suspected because the rectified lines say 9V DC on the power board, but 9V AC secondaries will give about 11 volts once rectified (RMS * 1.4) with no load. (But then again, many nominal 9V unregulated DC circuits powered by a transformer typically are a bit higher when unloaded, for example, my Spectrum's transformer is rated as 9V DC out, and all the DC inputs are labeled 9V but in reality it gives about 12V, unless you load it down).
So I'm not entirely sure what lump to replace it with. It looks like it'll need a >30VA rated secondary (I have already found a 50VA transformer in the Farnell catalogue which will physically fit). The real trouble is that on talking to someone I know who owns a Vectrex, his suggestion was that no ratings for the transformer were given because it's a "critical safety component" and the wrong voltage could produce X-rays from the CRT. Indeed, I looked at the HV circuit in the Vectrex service manual, and the picture tube's HV circuit is unregulated. Well, it's regulated insofar that it will hold a *relative* voltage, but there's no voltage reference in the regulation circuit, so if you feed it a higher input voltage, you'll get a higher output voltage. (Incidentally, the flyback circuit's oscillator is 555 based, and not all that far removed from some of the power supply ideas I played with for powering nixie tubes, although the output voltages there were 170 volts, not 6800 volts).
I don't want to turn the console into a cheap medical X-ray scanner, so I'm going to have to really get to the bottom of this transformer issue. It turns out half of the primary still works, so I may try and feed that half 115 volts and see what comes out of the other end, if it's 9VAC, then I can order a new lump of iron off Farnell. I would also like to get a HV voltmeter to test the tube anode voltage (which itself is going to be an interesting thing to accomplish, I imagine the HV output is very high impedance, since effectively hardly any current will flow, and so it'll easily get loaded down by a meter if it's not high enough impedance). If not I guess all I can do is stick the machine somewhere else and leave it turned on with a piece of photo paper in a black bag against the screen for a few hours and see if it gets fogged.
The other alternative is to look out for a "spares or repair" Vectrex and steal its power transformer.
Vectrex (Score:2)