When trying to figure out what is causing a particularly stubborn plating problem, and checking out the chemistry seems fine, you know that you sometimes have to check out the electrical area of the process. These are a few things you can do with a VOM (volt-ohm meter).
First of all, be sure you have a good VOM that has built in capacitance and not a cheap “battery tester.”
Checking for AC Ripple from Rectifier
If the rectifier has recently dropped a diode or has degraded filter capacitors, it may not be filtering out the unwanted AC current properly. When this happens, the AC ripple is increased and can cause plating problems. Usually 10% is a generally tolerable amount for most applications. Chromium plating is particularly sensitive to AC and should NOT be above 10%. Electrocleaners, on the other hand, can tolerate much more ripple.
The other reason that you may have high ripple, if nothing is wrong with the rectifier itself, is the amount of capacity of the size rectifier you are using. If you are using only, say 100 amps/2 volts out of a 2000 amp/9 volt rectifier, you will have high ripple percentage. So this is why it is important to have the correct size rectifier for the application.
Now, you don’t need an oscilloscope to obtain the percent ripple. Take the VOM and check the DC voltage. Then switch to AC and check the AC voltage. The percentage ripple can be calculated:
(AC VOLTAGE divided by DC VOLTAGE) X 100 = % AC RIPPLE
Now you will probably have slightly less AC ripple at the tank than right out of the rectifier due to some natural filtering along the turns of the bussing.
Shorts to Ground
Sometimes, a short occurs with the cathode or the anode legs from rectifier to tank. Take the VOM and check voltage of the cathode to ground and then the anode to ground at different places along the two bussing legs. The ideal situation is that the sum of the anode to ground voltage and the cathode to ground voltage should equal approximately to each other and the sum should be the total voltage the rectifier is set on. Let’s say you’re running at 9 volts on the rectifier and you check cathode to ground and get 4.5 volts then anode to ground and get 4.5 volts, this is ideal and there are no shorts. You probably will not find anything ever this ideal but should be a fairly sized portion of the shared voltage on each. But if you get, say 1 volt on the cathode leg, you probably have a short on that leg. You should check along the bussing route to see any metal (pipes, catwalk, etc.) touching the cathode buss and then insulate it from the bus.