andrefranc wrote:
JnTX said he had a bad power transformer once and he disconnected the bad side of the HV winding and ran the set off of just one half of the HV winding through a full wave rectifier. My transformer was arcing and smoking and turned out that half of the HV winding was bad so I disconnected that half and then made a full wave rectifier like JnTX said and it works.
There are two different ways that a transformer winding can go bad. It can either develop one or more shorted turns or it can develop an open winding. These are two fundamentally different problems only one of which can be addressed by continuing to use 1/2 of a winding.
In the case of one or more shorted turns you have the equivalent of a soldering gun inside of your transformer. Perhaps to understand this better, a soldering gun is actually a transformer with a shorted winding with its narrowest point being at the tip which therefore will get the hottest. A shorted winding in a power transformer is exactly the same thing.
Shorted turns in a power transformer cannot be bypassed or worked around by not using the shorted winding. No matter what you do externally that shorted winding will continue to heat up, melt down, fry, and smoke the transformer.
The only case in which you can sometimes work around a failed transformer is in the case of an open winding in 1/2 of a center tapped winding (as long as the open connection is not the center tap itself).
In that case you can disconnect the half of the transformer that is open. Then replace the full wave rectifier by a bridge full wave rectifier operating on the remaining 1/2 of the original winding. This is a little sketchy for one reason, but often works. The problem with this approach is that in a full wave center tapped transformer 1/2 of the winding is not supplying ANY power 1/2 of the time so it has half of the time to cool off during each cycle of the AC.
When you hang a bridge full wave rectifier on that 1/2 winding it is now required to supply power continuously and gets no cooling relief during a 60 cycle sine wave. It is doing twice the work that it was originally designed for (carrying current for every half cycle instead of every other half cycle). Often that is fine and the wires can take it, but sometimes it will exceed the capability of the wire and the remaining winding will subsequently burn out as well.
I will not go into the fact that the current is not actually continuous in any power supply transformer. So that even in the bridge rectifier case there is cooling, but it is still half of what it would have been. This is not intended to be a discourse on power supply design.
When someone has a power transformer that is consuming .8 Amps at 120 volts while connected to nothing (all tubes pulled), this is the sign of a shorted winding and is like having a 100 Watt soldering iron buried inside of the transformer (.8 A x 120 W = 96 Watts). This is not a candidate for disconnecting a wire and adding a bridge rectifier as a solution. The solution is to repair or replace the transformer (find and repair the internal/external short and hope that the insulation is not overly compromised by the heat damage, rewind the transformer, or replace the transformer).
Curtis Eickerman