Re: TIS98 and TPS98 transistor questions

Alrighty, then, let's look at the schematics for your monitor. Q701 &
Q702 is a differential amp powered by Q703, a constant current supply.
Q703 is being run at about 3.6 mA (.65 volts through 180 ohms). Q701
and Q702 normally share this current equally, so about 1.8 mA average
through each of those.

Maximum voltage drop across Q703 (assuming +35 and -35 power supplies)
would be 20.74 volts (35 - .65 through R704 - .65 volt drop at Q701/Q702
emitters - 12.96 volts drop across R740). Power dissipated in Q703 is
about 75 mW. So Q703 needs to be able to handle 20.74 volts, 3.6 mA,
and .075 Watt. Good design practice would be to have Q703 handle the
ENTIRE power supply of 70 volts (but for that much voltage to be present
there would have to be multiple failures happening at the same time ---
unlikely to happen, 35 volts is sufficient but would also represent
failures happening elsewhere), at least double its operating current -
7.2 mA, and at least double its operating power - .15 watt. So for our
purposes, anything that can handle the voltage rating will do. Even if
it can't handle the voltage R740 will protect it from destruction.

Now for Q701 and Q702. Current is 1.8 mA average, 3.6 mA peak. Voltage
peaks out at 35.65, averages 33.85 (1.8 volts dropped through collector
resistor). Average power is 60 mW. Once again, current and power are
insignificant and the main spec here is voltage.

Speed of Q703 doesn't matter, but you want something fairly fast in the
Q701 and Q702 positions.

What does it all mean? Use a fairly fast (over 1 MHz) NPN transistor
with good gain in the 1-5 mA range and a voltage rating of at least 40
volts (the more the merrier). Whatever you use USE THE SAME THING FOR
BOTH Q701 AND Q702. Why? You want these two as close to matched as you
can get. Most important matching is emitter to base voltage. If these
aren't the same you get something called "offset voltage" which will
cause centering to be off. With no offset voltage, zero voltage from
the game board will give you zero current through the yoke (and that's a
good thing!).

Phil Morris wrote:
>
> At 04:59 21/01/2002 -0600, you wrote:
> >Goody. Once you get to PN2222A you can just go to Radio Shack and grab
> >one of those handy 10 packs!
>
> All these different numbers and staring at data sheets is doing my head in. :)
>
> On looking up data sheets for all these replacements (be they NTE123AP,
> PN2222A, etc) and comparing to a data sheet for the TIS98, none of them
> match up exactly in what I thought were the 'important' areas such as Vcbo,
> Vceo, Ptot and IC. I thought any replacement should at least equal the max
> rating of the transistor being replaced, yet some replacements seem to fall
> short in certain areas. See here for a comparison of the TIS98 and the
> 2SC2240 I'm currently using in the G05-801's deflection board:
>
> TIS98 2SC2240
>
> Vcbo 80v 120v
> Vceo 60v 120v
> Ic(max) 200ma 100ma
> Ptot 360mw 300mw
> hFE 300 200~400
>
> Heck, my 2SC2240 *exceeds* the Vcbo and Vceo of the TIS98 by quite a
> margin, yet falls short on IC and Ptot. Yet it works ..........
>
> But I guess what I'm driving at is whether it's differences mean that other
> components might be getting put under more pressure, whether they will last
> as long as should should and whether maybe, just maybe, it and it's buddies
> are responsible for my slightly jagged diagonal lines, etc. Probably not,
> but my ignorance precludes me from being 100% certain.
>
> Cheers,
> Phil
>
>
>
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Received on Mon Jan 21 13:55:37 2002