Ludlum Mod 12S Micro R meter

[James Luck]

Hello guys i was wondering if i could get some advice / help on a new project.

If any of you have kept up to date on ebay auctions you may have seen a Ludlum model 12S go for cheap out of Hawaii, here in the states.
well needless to say it is a non functional mod 12s, and i maybe the new owner of said unit.

Well i was hoping it was going to be a easy fix with the idea that likely it was the scintillator IT WAS NOT well sh**.
So after rebuilding the detector which it was certainly in the need of, and putting the meter back together i was let down with no signal to the dial.

Now the high voltage works, and so does the battery test function. I am just clueless on where to go from here, especially since ludlum does not include test points for trouble shooting. this is a older meter, produced in 88 but last calibrated in 03.

I have a strange suspicion that one of the IC's went. Although i am not ashamed to say im quite ameture on my electronic knowledge, honestly i havent a clue where to start checking and what to check. i will say there isn't a cracked component nor a burnt component.

Anyhow any help or advice would be greatly appreciated, and i'm not too against just sourcing new IC parts and replacing them all, but id like to pinpoint which might be my issue.
-James

[kotarak]

James,
did you manage to find the schematics for your unit? I looked at the Ludlum's site and they have schematics drawn in 2000's. and are for newer models - I have no idea how they changed over the years.
Do you have oscilloscope or access to one?
If you have confirmed that you have HV bias to the PMT, then check if the PMT produces pulses, then I'd start with the amplifier's output and based on the findings (pulses or lack of thereof) I'd move forward of the output or check the output of the next section (Discriminator) and so on...these are not complicated units so it should not be very difficult to find where you are loosing the pulses) but you do need a scope to see what is happening through out the different blocks.
For an example - if the schematics didnt change much - I'd check pin2 of U2 (TLC372) this is the input of the discriminator and the output of the amplifier. If pulses are present and amplified, then check Pin 1 of the same IC (U2) - this is the output of the discriminator.
Hope this helps.

[James Luck]

James,
did you manage to find the schematics for your unit? I looked at the Ludlum's site and they have schematics drawn in 2000's. and are for newer models - I have no idea how they changed over the years.
Do you have oscilloscope or access to one?
If you have confirmed that you have HV bias to the PMT, then check if the PMT produces pulses, then I'd start with the amplifier's output and based on the findings (pulses or lack of thereof) I'd move forward of the output or check the output of the next section (Discriminator) and so on...these are not complicated units so it should not be very difficult to find where you are loosing the pulses) but you do need a scope to see what is happening through out the different blocks.
For an example - if the schematics didnt change much - I'd check pin2 of U2 (TLC372) this is the input of the discriminator and the output of the amplifier. If pulses are present and amplified, then check Pin 1 of the same IC (U2) - this is the output of the discriminator.
Hope this helps.

Thanks for the reply Kotarak,
ive tried a few scintillators on the meter to see if its the scints issue.
looking through the schematic in comparison to my model technically the schem in the manual is for a 12S-A and the circuit layout is changed to match the addition of a Over-Load indicator circuit and a central wiring harness connector.

i figured i would probably have to take a gander at it with the O-scope, but that'll wait till the following day. ( yes i may still be awake but sleep is near)

looking through some forum posts around the web i do believe my issue may lie in the amplifier as it will appear the 12s is quite similar to the layout of a older mod 12, but without a exterior connector.

-James