Making flyback transformers?

[Tony Duell]

> Oddly enough, vintage TVs using pictures tubes of the same size as the 

Strange... I've found the detailed characteristics of flybacks from 
different monitors to be very different.

> monitors you are trying to repair often have flybacks that could be used 
> with a bit of tinkering. You need only a few voltages out, and if you 

And if you're not lucky, you need a supply of output transistors ;-)

> know if the original monitor was tube or solid state then you know 
> somewhat what vintage TVs to hunt for (thrift stores/flea markets). You 
> also need to find one that has similar horizontal and vertical yoke 
> resistances to your monitor, as well as the B+. If you find these then 
> flybacks are somewhat interchangable with some mucking around to get the 
> pinout to match your monitor requirements. Due to the frequency these 
> are running at a CGA monitor yoke I would expect that the impedance

Alas not all classic computers run at TV rates (GGA). For example I was 
using a composite video monitor yesterday with a 22kHz horizontal scan 
rate. I have no idea where I would get a replacement flyback for that.
 
> should be similar to the impedance of a similar sized TV yoke with 
> similar resistance windings to your monitor.

I remember (just) whem there were sort-of generic TV flybacks that could 
be used in may sets by connecting them correctly. And the ont thing that 
was emphasises was that it was the inducatance, not the resistance, that 
mattered (both of the flyback and of the yoke). In any case the DC 
resistance of most ohizotnal deflection coils used in transistorised 
monitors is a lot less than 1 ohm, and therefore diffiuclt to measure 
with any accuracy.

> 
> Schematics of your monitor and the TV you are swiping the flyback from 
> would be a big help...

Scheamtics are not normally the problem...

> 
> Flybacks design is available (perhaps at a cost) from places like:
> 
> http://www.innovatia.com/Design_Center/mag_desn.htm

As I think is well-known on classiccmp, I prefer to do things myself 
rahter than to get somebofy else to do it. Not just for cost and 
enjoyment reasons, but also because if nobody carries on this sort of 
thing, the knowledge could be lost.

> 
> Which I found that as an early choice using the following search term 
> "flyback transformer theory" in google. A bit further down there are a 
> few PDFs on desinging Flybacks.
> 
> You can test flybacks using Bob Parker's Blue Ring Tester (sold at a few 
> places - like my shop) or you can make your own ring tester using a 

I have a homemade one based on a design published in 'Television' 
magazine in the UK (back when said publication actually had useful stuff 
in it). IIRC it's a 4011 chip running as an oscillator driving a power 
transistor that energises the primary winding of the flyback which has a 
capacitor connected across it to resonate it. You look at the riinging 
waveform with a 'scope (_any_ 'scope). A good flyback rings, one with 
shorted turns is massively damped.

This is, alas, not a complete test. It will not pick up shorted HV 
rectifiers inside the flyback. And some colour monitor flybacks have a 
'diode split overwind' (the EHT winding is in several sections with 
diodes conencted between them). It will not detect shorted diodes or 
inter-widing shorts there. But it does a good job most of the time.

> scope and an audio signal generator - simply pump into the flyback a 
> couple of volts at the flyback frequency and see what comes out of the 
> lower voltage windings...should look fairly clean if the flyback is OK.
> 
> I have some older B&W flybacks for Motorola B&W monitors as used in 
> video games in the 1970s as well. These tend to be open frame flybacks.

THose can sometimes be repaired. Alas many of the machines I work on have 
totally encapsulated flybacks (sometimes, i nthe case of colour monitors, 
also including a thick-film unit with potentiometers to set the A1 (G2 to 
you) and focus voltages. That's the sort I am wondering about how to 
replce. In some cases, they never were available as an individual spare 
parts (only as the complete PCB), so finding a spare is going to be 
impossible. I would have to make one.

-tony