... The goo you mentioned is probably just glue to hold the capacitor down and take the stress off the pins.
... The goo you mentioned is probably just glue to hold the capacitor down and take the stress off the pins.
A bit off-topic but this is something I have been wondering about for long. Why is such goo necessary, or why are these caps so susceptible to excessive stresses that they need such means of protection? Just because of their size? Appreciate any insights that would help clear my wonders. Cheers.
... The goo you mentioned is probably just glue to hold the capacitor down and take the stress off the pins.
A bit off-topic but this is something I have been wondering about for long. Why is such goo necessary, or why are these caps so susceptible to excessive stresses that they need such means of protection? Just because of their size? Appreciate any insights that would help clear my wonders. Cheers.In an assembly where there's vibration (and especially in subwoofers) it is possible for the solder joints to fatigue and crack. By stopping movement of the body of the cap relative to the PCB you prevent this. It's a genuine problem, not just a belt-and-braces thing. I've seen it lots of times.
Almost certainly the small electrolytic on the Vcc rail to the chip (pin 6) has dried out. Number one problem on switch mode PSUs. The goo you mentioned is probably just glue to hold the capacitor down and take the stress off the pins.
Thanks. That makes sense. So that's pretty much about mechanical reasons such as vibration and perhaps thermal stress, etc., not something electrical or anything else. Largely I wanted to know if there are any considerations other than mechanical. I understand from personal experience that repeated/prolonged exposure to something that looks trivial may have an effect far greater than what we would imagine from intuition.
Almost certainly the small electrolytic on the Vcc rail to the chip (pin 6) has dried out. Number one problem on switch mode PSUs. The goo you mentioned is probably just glue to hold the capacitor down and take the stress off the pins.Great! Managed to remove the glued in cap (heat shrink broke, glue was strong as new), measured 100 uF (labelled 470 uF) ESR 40 Ohms.
Rummaged through my old caps box and found a matching one, measured OK, but had visible leakage signs. I think I should simply throw all >10yr old electrolytics in the bin. I already attempted this once, but there are so many different voltages and capacities that purchasing a replacement assortment seems to get quite excessive. Found a (new) Chinese low ESR one and it works!
I think one could even have seen some darkish contamination on the - trace, see detail image.
Would you also replace the primary bus and secondary output capacitor (glued in)?
Not sure what to do with the startup resistor chains. These are pretty burned up, but values are OK and the traces lift off if you barely touch them. I guess I let them be. A shame these heat all day long although they are needed only a few seconds at startup.
Interestingly there seems to be a circuit with an 800 V BJT, a PTC and 3k series resistor near the input, probably to discharge the massive input X capacitor (so you don't get zapped when you touch the plug terminals after unplugging mains).
Thanks a lot, you have been a great help!
What stands right out: There are lots of bulky/heavy components on the PCB, why did they only put glue on two (out of three) electrolytics and nowhere else?
My conclusion: This is expensive industrial equipment, built and tested to certain standards. They have requirements for vibration resistance, put their PCB onto the shaker to verify it and it failed at those two components. They put glue on it, passed the shaker test, bingo.
I once worked with electronics to be mounted on automotive engines and gearboxes. Vibration testing was fun. Heavier components like PQFP would disintegrate seconds after shaker was switched on.