[Solved!] Agilent E3648A Output 1 not working  - Page 1
Forum section: electronics / repair

author: Gluteniz
author_rank: Newbie
timestamp: November 20, 2024, 07:05:58 pm
content: Hi,Today when turning on my Agilent E3648A the output 1, only shows -00.38V 0.154A Unreg and can't be adjusted, the normal clicking sound when adjusting is working but no change on voltage.Little info:Bought used worked for around five months then blew "GBU8J / IRF540 / IRF2807" underload.Replaced "GBU8J / IRF540 / IRF2807" on both outputs and after the repair it worked for over a year, but today this problem.Output 2 works as intended.Self test passes.Tried replacing "GBU8J / IRF540 / IRF2807" on Output 1 even though they looked good, but as guessed problem still the sameany guesses, ideas or pointing in the right direction is very appreciated.

author: IanJ
author_rank: Unknown
timestamp: November 20, 2024, 07:59:54 pm
content: Big photos ouch!Have a look on my YT channel, I have repaired quite a few of these PSU's, a couple with symptoms like yours.https://youtube.com/playlist?list=PLFTNjqbH_TxkiYsF76ttjXmxgQeXewjSQ&si=sNvLhOM2El-gUDQGIan.

author: Gluteniz
author_rank: Newbie
timestamp: November 20, 2024, 09:05:06 pm
content: Quote from: IanJ on November 20, 2024, 07:59:54 pmOopsThank you, will check them out!

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 22, 2024, 04:42:44 pm
content: Check the DAC voltages on the test points mentioned in the schematic in the service manual. Also, test the power supply voltages. Follow the path of the DAC signals to the error amps and look for voltages that are not within specifications.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: November 23, 2024, 01:10:56 am
content: I have also done repairs on sister models of this supply, I have a playlist on my YouTube channel which you may want to check out in case there are clues there for you.https://www.youtube.com/playlist?list=PL2z3V9RkHQE2Twk6VoEBURx88TDXPKVSPOne thing to watch for is that the service manuals have many mistakes in them with values, part numbers, identifiers etc., so always double check everything and compare, as the circuit diagrams are not always correct for these.

author: Gluteniz
author_rank: Newbie
timestamp: November 23, 2024, 10:02:56 pm
content: Thank you very much for the help and useful info and videos!Today I took some measurements of the BIAS supply and noticed the biggest difference being the +15v is 14.15v and not sure if this is out of spec?Also followed the path and found that U136 on pin 5 I can see the voltage change from -0.334v to 1.034v when adjusting from the front panel but the output pin 7 on the U136 always stays at 9.21vso I replaced the U136 (TL074CD) but it did not solve this problem.Attached a few pictures of my measurements.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: November 23, 2024, 10:15:58 pm
content: It has been several years since I worked on these myself so my memory is pretty poor on them, but U132 and U135 control the output as well, if I remember rightly they are used for over current, over voltage etc, so if they see a fault they shunt the gate drive to shut it off, you will notice all three device outputs are connected together through diodes.I remember looking at those in my repairs years ago trying to trace a no output fault, I think in that case it ended up being a bad supervisor IC that checks the 5V supply rail, I showed that in one of the videos.

author: IanJ
author_rank: Unknown
timestamp: November 24, 2024, 12:41:47 am
content: Common problem with these PSU’s are the green coloured SMD resistors, they often go open circuit or high value. They are affected by humidity.For your fault you need to break it down.1. Check all power supply rails.2. Check the user adjustable voltages for I and V after the DAC, also including the setpoint signals.3. Check where these signals are split off for the display, the ADC.4. Check where the main signals come together and are basically OR’d to drive the main pass transistor.The manuals for these often have quite good repair guides.Ian

author: Gluteniz
author_rank: Newbie
timestamp: November 29, 2024, 12:40:20 am
content: Problem was found, a failed TL072AC.Thank you very much to all of you that took the time to reply with great info and also videos!now fully calibrated and back in the bench ready to serve

author: derZockerMP
author_rank: Regular Contributor
timestamp: November 29, 2024, 07:05:33 am
content: Good job

author: IanJ
author_rank: Unknown
timestamp: November 29, 2024, 11:28:39 pm
content: Great to hear.