If the capacitor is defective the unit will still vibrate. It is only used to create 12/13.6 VDC . the Transformer will still vibrate but remember I SAY REMEMBER the transformer is a 6 VAC transformer, and if the capacitors faulty you will still get out put but at the 6 Volt level. . the uni is a 6 Volt system with a voltage doubler circuit .
The output stage charges the capacitor on the positive cycle and the adds that to the next positive cycle giving you the 12/13.6 vdc output.
The Univolt charger is supplied from the 120VAC line voltage through the circuit braker panel and then a fuse in the charger and thus to the transformer. ACHEAP AND DIRTY SYSTEM, BUT USUALLY TROUBLE FREE.
It also has the abliity to limit the current to the battery when it is fully charged. If you write to Unitvolt they will send you a schematic
If you need I can draw the circuit and e-mail it to you if you need. via JPEG format
Rae Baker VE3CZN