Author Topic: Agilent 34450a repair help  (Read 413 times)

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Offline phantom1

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Agilent 34450a repair help
« on: February 16, 2019, 10:39:42 am »
Hello everyone need some help on this bench multimeter agilent  34450a  I got this from Ebay with a few problems
I have been trouble shooting as. Best as I can with no schematics other then what is d/l from keysight
I have everything working on this meter except AC. Meter passes self test just fine I have lots of time on this meter and just cant seem to find anything obvious I thought I would reach out to the forum for help lost please help
I could use maybe if someone has a picture of the board layout top and bottom maybe there is a component missing like a small capacitor, resistor not sure at this point thanks in advanced
Ps...if this is not in the right place please forgive me .
 

Offline Kleinstein

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Re: Agilent 34450a repair help
« Reply #1 on: February 16, 2019, 11:32:13 pm »
The 34450 is relatively new - so usually no schematics available.

The TRMS function is likely through a analog converter chip (e.g. AD636 or similar). Chances are one could identify that chip on the board and check the signal at that chip.

There may be a few unpopulated passive components. Especially with SMD parts it's not likely they fall off by chance. Usually such meters have a decent solder quality. 
 
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Offline phantom1

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Re: Agilent 34450a repair help
« Reply #2 on: February 17, 2019, 11:59:53 am »
Thanks for the reply
Yes it's a Ad8629 chip I have replaced it but still no change there are relays on the main board and thought if one of them my be bad but it passes the self test and you can hear the relays clicking so not sure its crazy I feel like I am just missing something simple just cant seem to find it I have checked the mov, ptc and change the resistor network as well
But I found a bad switch ic uln2003 that was blown I have changed the c072c,and Adg412br as well and the ds75 just for good luck still no ac readings,  let me explain it switches over the that mode and I get the random digits  but not any correct response it acts like the signal isn't strong enough to register if I increase voltage there is a response up or down just not in the ball park it's all millivolts  example  120vac I get 8.6 millivolts  hope that explains  some of what is going on with this meter
 

Offline phantom1

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Re: Agilent 34450a repair help
« Reply #3 on: February 17, 2019, 12:20:05 pm »
Sorry forgot one other thing it is all part of the AC frequency seems like I say this because I can put the meter in frequency mode and it reads square waves just fine low to high but if I inject ac wave its wont read it , but if I get into the 100khz then it will strange  maybe that can help a bit more ...all of the other functions work great spot on with my other test gear
 

Offline Kleinstein

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Re: Agilent 34450a repair help
« Reply #4 on: February 17, 2019, 09:57:53 pm »
The AD8629 is an AZ OP - not sure if an where it is used in the AC part.

Even without a schematics the TRMS chip should be a good test point, as the datasheet should tell the input and output pins and approximate levels. So there should be the scaled AC signal visible and the resulting DC. So one check the RMS chip itself and usually see if the problem is with the AC signal before or the DC part after it (which is less likely).

The other point to check is the first amplification stage and protection, as this is  the exposed part, that might be damaged from ESD or similar spikes.
 
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Offline phantom1

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Re: Agilent 34450a repair help
« Reply #5 on: February 18, 2019, 04:45:36 am »
Ok thanks I did look at this chip when I first started working on this problem it looked ok at the time but haven't checked since  so I will check again and get back to you by the way it's a Isl21090bfz12 1.25v Reference  ps.. when I try to upload any pictures it tells me I have a 1k limit so pics wont upload  any thing I can do besides resizing them thanks
 

Offline phantom1

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Re: Agilent 34450a repair help
« Reply #6 on: February 18, 2019, 10:47:39 am »
Ok I have check the Isl21090bfz12 and it is 1.248v so looks like it is within spec I will attach some pics maybe it can help someone else and hopefully me just wish I have some info as to what to check next at a loss at the moment
« Last Edit: February 18, 2019, 10:50:42 am by phantom1 »
 

Offline phantom1

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Re: Agilent 34450a repair help
« Reply #7 on: February 18, 2019, 10:55:16 am »
Here are some more pics
 


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