Mighty Monarch of The Air

[vts1134]

Originally Posted by old_coot88

Well, while waiting for the heavy artillery, if you're game to try the 'quick & dirty' method of signal injection, here's how i would do it -
Tie onto the 6.3V heater supply just like you did earlier for the vertical, except this time put a cap in series with the clip lead. Value isn't critical, anywhere from .01 to .1 is OK. This'll give you a DC-isolated 60 cycle signal for injection.
Start at G1 of the CRT (pin 2). The raster should show a mild hum bar. Now hit the plate of the video output stage (pin 6 of the 12AU7). The same hum bar should appear. Now hit the grid (pin 7). The hum bar should show quite a bit of amplitude gain. Now hit the plate of the 1st video stage (pin 1 of the 12AU7). It should show the same amplitude gain.

Now to inject at the grid of the first video stage, it's a good idea to attenuate the signal so as to not overdrive the video stages. So let's utilize the 8.2K resistor (R27) that's already in place between the grid (pin 2) and ground. Add an 82K resistor in series with the cap on your clip lead, so that this new resistor is now the "probe". When touched to the grid (pin 2), it's also in series with R27 and forms a voltage divider giving a little over 10X attenuation. So the grid will be seeing about .59 VAC of signal. Of course you can reduce the value of the new resistor to give less attenuation if desired. {Those two coils L10 and L11 can be considered zero ohms in this situation.}

As a precaution, i would leave the diode M4 disconnected during the procedure.

So if you lose the signal at any point while working back stage by stage, that will isolate where the failure is at.

oc

Can I use the signal generator for all of the above steps? Starting with G1 of the CRT and working back to the detector diode. I'm guessing that anything after the video amp will not give me anything like a picture, but should give me some change in the raster, and anything before the video amp (assuming the failure is upstream) will give me the full pattern. Am I right?