Re: SG502 Schematic, Component Layout


On 26 Jul 2017 03:07:55 +0000, you wrote:

Thanks David and Dave for the responses (and reassurance that I'm not off topic). I looked for identifying markings on the board and the chassis, with little success. The only numbers anywhere are the copyright date (1971) and "R0039E" on the upper right side of the board. Other than the front panel graphics, nothing identifying the unit on the chassis either.

I've never learned to read component date codes, so that will take a bit of research before I can report back.

I'll also have to do some reverse engineering in order to identify Q235, as the area around the coaxial jack looks different than the component layout drawing. In particular, there's a TO-style can device with 8 leads that I don't see anywhere on the layout.
That 8 pin TO-99 can is an LM710 comparator made by Fairchild in the
36th week of 1974 so the design is significantly different from the
schematics we have. Internally the LM710 is a two stage differential
comparator so it neatly replaces differential comparator stage Q200
and Q210 and differential comparator stage Q220 and Q230.

It looks like there is a single transistor after the output of the
LM710 because it can only drive a couple milliamps so it requires a
buffer to drive a 600 ohm load.

The LM710 is pretty picky about the supply voltages that it runs on.
Usually these will be +12 volts and -6 volts and it produces a TTL
compatible output at pin 7 of about 0 volts sinking 2 milliamps and
+3.2 volts sourcing up to 5 milliamps. I see a couple of diodes which
are probably zener diodes used to lower the +20 and -20 volt supplies
down to +12 and -6 volts.

All we really need to see is the circuit from pin 7 through that
transistor to the coaxial cable to figure out what is suppose to be
happening. A close up photograph of the front and back of that area
of the board should be enough to figure it out.

It may be that what you are observing is normal. You might try
measuring the output with a 470, 560, or 680 ohm resistor between the
signal and ground.

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