Fluke 8840A/AF Mysterious parts, Need help.

Thread Starter

Sampler1

Joined Apr 9, 2020
12
Hi Folks,
I purchased an 8840A/AF. When I turn it on it cycles on and off with the relay clicking. First thing I did was to check the voltage rails and they all seem ok with the exception of the
-8.2Volt rail. It reads 5.2volts. I ended up checking everything from the Bridge to the test points in that section and all was good. Two mysterious discoveries were made..
R601 is supposed to be 560 ohms and it had a factory installed 1 kilohm resistor? Next I discovered I cannot find anywhere CR617 (8.2v zener)?? I have searched Hi and low and it does not seem to be in the circuit, in fact it is not even listed on the parts location??
ANY Ideas?

I am also trying to figure where this -8.2 volt rail goes and where it is used.

Any help would be appreciated.

I have verified all the capacitors, zener diodes, (except CR617), replaced the regulators, resistors in the section of TP603,604,605,606.
I realize most was not necessary because the parts were ok.

Help Help ( the manual can be found all over the net , it includes schematics) 8840A supply.jpg
 

Thread Starter

Sampler1

Joined Apr 9, 2020
12
Mr chips, would you happen to know where the -8.2V rail feeds? Thanks for the info on CR617..I can let that RIP. So strange there is nothing on that page to reference it not being used. I suspected it to be an error.
Thanks again!
 

Thread Starter

Sampler1

Joined Apr 9, 2020
12
I just found the -8.2V. It feeds U303 pin 11 and U304 Pin4.
To be continued , time to sleep..
Night all, Stay safe..U303_U304.jpg
 

AlbertHall

Joined Jun 4, 2014
10,417
If the -8.2V is at +5V then CR614 is open circuit or disconnected but that is not the cause of the fault.
Check for a short between the -8.2V and the +5V.
 

Thread Starter

Sampler1

Joined Apr 9, 2020
12
Today I was back at it. I removed the -8.2V( actual -5V). It feeds U303 pin 11 and U304 Pin4. I wanted to make sure nothing was loading down the supply. After I isolated this part of the power supply I measured again and I was surprised to see it was still at -5volts and not -8.2V. I measured the voltages and Ill post a pic here. Im scratching my head on this one! All in yellow was verified???volts.jpg
 

AlbertHall

Joined Jun 4, 2014
10,417
If those voltages are with everything disconnected from the -8.2V output there are only three possibilities.
1. R602 is much higher than 1k.
2. C60 is leaky.
3. CR614 is faulty.
 

Thread Starter

Sampler1

Joined Apr 9, 2020
12
AlbertHall, Ill check it out in the morning and report back.. I was thinking I have aprox. 21volts before the regulators, would it make a difference if that 21V was too low? I think ill take out the bread board and replicate the circuit, see what gives.
Appreciated!
 
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