Very weir problem in power supply - Page 1
Forum section: electronics / repair

author: andreal1967
author_rank: Contributor
timestamp: January 27, 2025, 08:18:08 am
content: I am repairing an very old Kantronics KAM+ modem i use in ham radio.I have a very strange problem:As you can see in the schematic, an LT1054 invert the 8V from an LM2937 regulator and then drive a 79L05.my problem is that the LT1054 does not start ...if i heat the IC, it works starts and drive the 79L05 but when it return to ambient temperature it does not oscillate anymore. As the IC is on socket i changed it with a new one that has exactly the same issue. I have also removed the socket and soldered it directly on the board with the same results.I have checked the condensers neat the IC , also changed Q10 but without any results..I hope someone can give me some hints...

author: Slh
author_rank: Regular Contributor
timestamp: January 27, 2025, 08:53:34 am
content: Perhaps a broken track? Or maybe dry solder joint. I have an tek scope which would occasionally fail in the power supply and eventually traced it to a wire link with a dry solder joint! The wire tin coating had corroded so it couldn't be wetted and I had to replace the wire!

author: andreal1967
author_rank: Contributor
timestamp: January 27, 2025, 09:23:04 am
content: i have checked all the tracks ! cannot found any issue.

author: magic
author_rank: Super Contributor
timestamp: January 27, 2025, 09:37:00 am
content: At a quick glance, maybe a problem with C71 or something funky connected to the FB/SD pin?

author: ondraN
author_rank: Regular Contributor
timestamp: January 27, 2025, 09:46:13 am
content: I would try heating each component in the LT1054 system individually. Most likely a capacitor (high leakage..., C71 the most suspicious) or a diode is faulty.

author: indeterminate
author_rank: Regular Contributor
timestamp: January 27, 2025, 10:07:18 am
content: Another possibility is c49The LT1054 requires 2uf on vcc if c49 has failed that would be enough to stop it starting.

author: andreal1967
author_rank: Contributor
timestamp: January 27, 2025, 11:39:13 am
content: C71 as you can see in the schematic is "boxed" and not installed in the board since new. Even in the list of the components C71 i marked as "not present" , i have already replaced C49,C50,C51 with new 10uf electrolitics.... it's a real mystery. Only if i heat the IC it starts to oscillate.

author: ondraN
author_rank: Regular Contributor
timestamp: January 27, 2025, 12:10:17 pm
content: What about pin1 ShutDown? It is not connected on the schematic. Is it at the correct level when it floats?

author: IanJ
author_rank: Unknown
timestamp: January 27, 2025, 12:31:44 pm
content: Try biasing base of Q10 with a resistor to help startup maybe.

author: andreal1967
author_rank: Contributor
timestamp: January 27, 2025, 03:13:47 pm
content: i tried with a 12k resistor from pin 1 to pin 8 to help...... but no help

author: ondraN
author_rank: Regular Contributor
timestamp: January 29, 2025, 11:20:45 am
content: And Vcc is correct and clean? Fast rissing edge?

author: cnqhdszq
author_rank: Regular Contributor
timestamp: January 29, 2025, 12:26:40 pm
content: As you had changed the IC , then I guess there must be something wrong with the board. Can you upload a photo of the board .may be help .