Hi Tom and all the List,
>Sorry to be the bearer of bad news, but you still have a circuit fault.
The video
>circuit of classic Macs (and Mac Classics) uses the transistor as a simple
>inverter. Its dissipation is normally very small, and a 2N2222 would do
just fine.
Oops!. I knew I needed the schematics ;-)!
<interesting possible faults snipped>
>A more ominous possibility, given the toasted 220ohm resistor, is that
your crt has
>an internal grid-to-cathode short. Frequently, these are undetectable when
the crt
>is cold.
Well, taking the schematics at your 'plus_analog.pdf' file as a reference,
resistor which was toasted in my unit was R27. Q7 was shorted so R27 got
+30V directly to GND through it --> smoke!
I would expect R20 to die if there was a grid-to-cathode short... An open
CR12 seems to me as a possible culprit.
>... If that turns out to be the problem, email me, and I'll describe a
>procedure for fixing it.
Please, let me know the procedure. Although I am in electronics for years,
CRT circuits are not my speciality (although some weeks ago I fixed my 20"
ColorSync horizontal deflection trouble :-) ). Thanks!
>... In any case, once you've fixed the fault, you can use a
>2222 or 3904 or ECG123 or any general purpose medium power NPN with a
reasonable
>speed (say, ft above 100MHz) and a breakdown voltage bigger than, say, 40V.
Well, I need to check the 2222A I put as a replacement. I am not sure if it
died or not (it was a metallic cased transistor), although it probably did.
I am sure a BC182A (think it was that type) smoked; case was cracked.
<snip>
>Even though the schematic is for the Mac Plus, the other b/w all-in-ones
share very
>similar (in some cases identical) video circuitry.
Yes, thanks both for your great work!. I have it already in my Mac folder :-)
Regards,
JOSE
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La Canyada - Valencia (SPAIN)
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