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Old 4th Jun 2019, 8:00 pm   #16
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 1,685
Default Re: Marconi 2024 not powering up

Yes, a great result... well done!

This failure mode is well worth knowing for the future and I suspect quite a few of these PSUs will fail like this.

I don't have images of the colour codes for these resistors for my Marconi 2024 PSU but my PSU failed because the big 100uF 250V cap (with the red caution sticker on it next to your failed resistors in your image above) decided to vent itself quite spectacularly. I was surprised just how much white steam was then vented out of the back of the sig gen by the internal fan! This went on like a mini smoke grenade for many seconds. Amazingly, the sig gen (display) was still working when this was happening and also it was fortunate I was in the room when it happened so I could cut the power.
Inside the sig gen the cap had burped out a lot of hot oily stuff and it was all over the insides of the PSU cover.

I replaced all 4 caps and I keep meaning to measure the ESR of the three (healthy?) survivors because I think this may be important when looking for replacements. I plan to measure at 100Hz, 1kHz, 10kHz and 100kHz but I guess the 100Hz value is the most important. I did try and find low ESR replacements but I fitted them in haste and forgot to measure them first!

Note that the clear gloop that holds these caps in place on the PCB is very difficult to remove. I had to slowly and carefully cut it away with sidecutters and it took a long time.
Regards, Jeremy G0HZU
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