CentreCOM MR820TR Hub Repair
Warning: Be careful when working with power supplies, they can contain dangerous voltages even when in powered off state.
I found a broken "Allied Telesyn CentreCOM MR820TR" network hub in the trash:
When power was applied the power LED would just blink rapidly. The internal PSU is supposed to supply +5V and +12V on two rails, but when I measured these with the logic board connected they were just +1.6V and +8.1V. Disconnecting the logic board showed +4.8V on the +5V rail and variying voltage between +7.3V and +7.6V on the +12V rail, so likely something wrong with the internal PSU.
I measured the ESR of several electrolytic capacitors in-circuit and one of them (C06) in the center of the board had bad readings. I unsoldered this one and measured it again, and it was still bad:
I replaced it with a new one:
This network hub is now working again:
