Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Mon Nov 3 16:08:02 CST 2014

> Others have likewise suggested that 'something' is loading the circuit. But
> so far, I can't seem to find anything that doesn't act the way it should..
>  caps, diodes etc. seem to be fine. But I'll give it another close look.

There are basically 2 common reasons for a HOT to run hot (!). One is overcurrent. This generally means
something is loading the flyback too heavily. Remember that this is a high voltage circuit and components
may test fine on the DMM at a few volts and break down when in the actual circuit at a few hundred volts.

The other common reason is that the device is being turned on or off too slowly. The horizontal output
stage is a SMPSU (with extras). The transistor should be hard on (saturated) or cut off. If it takes too long
to switch between those states it will disipate too much power and run hot.

> > It is not uncommon for HOTs to have a very low beta.
> >
> I've more or less gotten that impression, but.. do me a favor and take a
> look at something. Go to http://nerp.net/~legendre/osborne/ and grab the
> Osborne service manual. There's an appendix with full service data for the
> +other+ 7" monitor that they used - a Motorola unit. Unless I'm wrong,
> isn't the HOT in there a Darlington device? Aren't Darls. almost
> universally high-gain devices?

I should have been clearer. A power transistor, such as the HOT, often has a low beta. If the circuit
requires a component with a high beta then often a darlington pair is used. It would be useful to know
what the original HOT was. Have you tried testing the B-E junction of the old part with the diode test
function of you DMM? what forward voltage do you get ? If it's 2 diode drops (about 1.4V), it's very
likely to be a darlington device.

> > > circuits - base drive was +much+ higher (like 10X) level than the output
> > at
> >
> > Hang on, the base drive should be something like 0.6V The emitter is
> > grounded, after all.
> >
> > Normally you get HV spikes on the collector.
> >
> If I re-install the old HOT I can re-check it, but IIRC the base drive
> waveform was like 10X that at the output. This is what led me to try
> subbing it out in the first place.. why would an amp (configured for gain)
> have a lower output than input? And since it was clear to me that the opt.

Remember that voltage gain isn't the whole story. You might have similar voltages on 
input and output but a larger current in the latter circuit. That would be an ampifier, giving
power (and current) gain.

> I don't know squat about analog video, or how to perform / interpret a
> 'ringing test'. Will try searching on that one, but I may need some
> instruction on the subject.

Basically you resonate one of the flyback windings (out of circuit) with a capacitor. Apply a pulse signal
to it (often a little oscillator and a HOT-type of transistor is used). And look at the waveform across said
widing with a 'scope. You should see a decaying sine wave. If it decays very quickly (maybe not even one
cycle) then something is taking energy from the resonant circuit, most likely a shorted turn in the flyback

> Again, if I just had some dang specs I could ohm-out the flyback, know the
> actual specs of the orig. HOT, and all that other great stuff that I can't
> do.

You will never find a shorted turn by DC resistance measurements. The change in resistance is less than the 
manufacturing tolerances (due to the wire diameter not being exact), contact resistance, etc.


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