Re: 453 calibrator

Richard Knoppow
 

On my schematic BA and BB are shown right on the edge line of the board. They look like they are connected together but are not. BB goes to ground and to the shield of the coax that goes from R1274 to R1275. BA goes to the center conductor of this coax from R1274. The ground at BB grounds one end of the shield. Its probably grounded that way to prevent a ground loop. No current can flow through the shield. The output from the collector of Q1274 goes through some resistors and the coax to more resistors, through the test loop and to ground. Different circuit than is grounded at BB. The BNC connector is fed from a dropping resistor in the line from Q1274. It is not grounded at the BNC, however, the shield of the BNC goes to ground. Again, this ground is separate from both the coax shield and the loop ground.
The whole line from the collector of Q1274 should show some voltage and some resistance to ground up to the test loop, which will measure ground (or a very low resistance at the R1278-R1277 end. The center conductor of the BNC should read about 200 ohms with the calibrate voltage switch in the 1.0 V position and about 20 ohms in the 0.1 V position. Again, the coax feeding the BNC is grounded at one end only.
If you are measuring a short to ground at the emitter of Q1274 I would be suspicious of C1251. I think that would also pull down the +12V line or pop the 4.7 ohm resistor at R1251.
The PDF I have is from BAMA and is numbered 070-00478-00
I am not editing this so as to allow some context.

On 5/9/2018 12:24 PM, Mike Merigliano wrote:
Why would either BA or BB be a dead short (at ground) to Q1274's emitter? There is a 2.19K resister between the emitter and either BB or BA.  I guess what I'm missing is how someone could see a short on the schematic, even if they mis-interpreted the line representing the board's boundary as a wire or trace. I am not an expert, so when someone sees something on a schematic that I don't, I want to know why.
Connection BB is probably not needed when everything else is connected anyway, as there are two other ground returns -- the one for the 1 volt BNC output looks redundant to BB, and another 180 ohms kicks-in for 0.1 output. The other involves the current-test loop, and there is another 220 ohms involved.
On 5/9/2018 12:29 PM, Chuck Harris wrote:
No, you aren't missing anything.  What Jeff was saying
is if BA is truly at ground, then you cannot have a signal
out your calibrator.  It may appear to be grounded if you
use an ohmmeter that is on too high a scale... due to the
160, or so ohms that the circuit inherently has to ground.

-Chuck Harris

Mike Merigliano wrote:
The signal is not shorted to ground -- it goes through a 2.19K resistor in series
first, so there is no short. My Tek 453 has a calibrator signal, at specification. My
ohmeter and voltmeter check measurements between/around Q1274, point BA, R1274, and
point BB  match what is on the schematic.

Maybe I am missing something.

--
Richard Knoppow
dickburk@...
WB6KBL

Join TekScopes@groups.io to automatically receive all group messages.