Re: La2a fuse problem
« Reply #20 on: July 21, 2020, 12:33:13 PM »
 When I  was the head bench guy at a pro audio company,  the biggest issues we would see in our  gear would be related to power.

 This is where having a variac and amp meter can really do wonders. See  could measure and watch the current on the incoming  AC.  When powered normally we would see a nice jump and then it would settle  and work as supposed to.  This is why we used slow blow fuses and so forth. But when ever there was a problem, we could use the variac to sweep up from 0 to mains voltage and see what the unit was doing current wise,  would make it easy to tell if and when it was going to pop a fuse.

From there w would do the following.
1. isolate the PSU from the rest of the unit. Due to power supply design, we could have a working power supply without  it being connected.
2. isolate the mains transformer
3. test mains transformer by itself.  We had  the secondaries on a molex connector which made for quick disconnect to test it and see if it was working.
4. reconnect transformer and test PSU bringing the variac up slowly. When we got to the bare minimum that it would function we would touch parts to see if any was getting to hot.  More often then not a failed bridge rectifier would  be warm to the point touching it was uncomfortable. It was a great way to isolate it.
5.  If the transformer and the rectifiers were good, then we would start looking further upstream but with the variac at bare minimum to get the unit to operate so that we could eventually find the short.
Most common problems for us blowing fuses in order were usually the bridge rectifiers, short elsewhere in the power supply(usually a blown cap), short in a circuit in the unit(blown cap or other part), transformers.

From the sounds of things if you are pop a fuse  with the transformer wired to mains, but without a load on the secondaries, then you probably have a short in the transformer or you have the wrong fuse type and value in place. If you knew the transformer specs, you could run it on a variac and measure the AC on the secondaries to see what is what.  If you have an amp meter, would help in further pointing to where the problem lies.
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