Hi All,
Thank you all for the very helpful suggestions!
I will try to reply to you all in this one post.
I opened up the A12 assembly and was happy to find that the lamp,
windows, and reflector all appear to be in good physical condition.
There is some very light corrosion on the aluminum reflector, all of
the windows are clear, and the photodiode is very visible at the bottom
of the oven.
The Rb bulb looks very slightly darkened, maybe like a pair of very
lightly polarized sunglasses.
I measured the resistance of the 20 VDC supply line to the lamp
oscillator and it is 3350 ohms, just as Corby said and as I see it
should be from the schematic.
I measured the current to the lamp oscillator and it is only about 13
mA. Paul mentioned that if it was about 15 mA it was probably not
working. And, Poul-Henning mentioned that it should draw about 3W when
working, which at 20 VDC would be 150 mA.
I powered up the lamp oscillator with it on the bench and I do not see
any light from the bulb. I am assuming that it should be visible, but
I'm not sure! What is the spectrum from these lamps like? Visible,
UV???
I measured the resistance of the small coil wound around the bulb, it is
about 0.3 to 0.4 ohms, which seems reasonable.
(It looks like someone has been in this assembly before. There is
solder flux on the transistor leads, the 20 VDC connection, and the bulb
coil leads.)
One odd bit is that the resistor in the transistor emitter leg measures
about 350 ohms (this is R3 in my schematic). The schematic says it
should be 10 ohms. This resistor is on solder posts, so it looks like a
'selected at assembly' item. Corby: it this the 12 ohm resistor you
mentioned?
The voltage across this resistor when the power is on is about 1.6 V.
The base voltage is about 2.2 V (this is across CR1 also) and shows
"flat-lined" on my scope with the board powered.
So, I'm betting that the collector of the transistor is cooked. (The
base-emitter drop of ~0.6 V probably means the base and emitter paths
are still OK.)
But, it is also possible that the R3=350 ohms is way out of line??
Comments welcomed.
Corby: the serial number for the unit is 0968A00302. It has a 105-6012
quartz oscillator, and a mechanical PP clock. The A12 unit is a series
1220, and is green in color. (The OCXO and the bands around A12 are
blue.) The A12 unit has a warranty expiration of 30 April 1978. It has
had its "Mag Filed" and "Thumbwheel" blacked out with marker and new
values written in.
Cheers,
Scott
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