The jumpers are for the transformer primary of the on board transformer. The transformer has two primary windings allowing it 120 or 240 volt primary operation. All the jumpers do is place the primaries in parallel (120 VAC Operation) or in series (240 VAC Operation). The output is eventually rectified filtered and run through a LM7812 regulator to provide 12 VDC low DC system voltage. I really do not see where anything there would cause the off board 70 VAC transformer to cook. It will be something related to the HV DC which begins with that transformer.

Something worth a try or a look is set your DMM to measure resistance. Following the bridge rectifier you have the two large filter capacitors. Measure across them and measure both ways (reverse the meter probes) let's see what the higher voltage DC bus looks like and obviously do this with no power connected.

Ron

 

So I chatted with the guy who engineered this kit and sent him some pictures. He thinks the transformer its self may have had faulty winding and internally shorted out.

I have another transformer, would it be safe to try plugging it in and if the fuse blows again to immediately pull the plug?

 

I suggest starting with an old fashioned tungsten filament lamp (About 50 Watts) connected in series with the primary of the transformer. Start without the secondary connected to anything. It may glow dimly. This is OK. Then try again with the secondary connected connected to the board. If the lamp glows brightly quickly measure the AC voltage across the secondary and the DC voltage across C45 and report the results. I cant see on then schematic what this power supply feeds so I don't know how much current it is expected to supply. I suspect that it is feeding the modulator section. Can you confirm if this is the case by tracing the tracks on the board ?

Les

 

So I chatted with the guy who engineered this kit and sent him some pictures. He thinks the transformer its self may have had faulty winding and internally shorted out.

I have another transformer, would it be safe to try plugging it in and if the fuse blows again to immediately pull the plug?

I won't say it can't happen but everything Hammond I ever bought was quality including their transformers.

"So I chatted with the guy who engineered this kit and sent him some pictures. He thinks the transformer its self may have had faulty winding and internally shorted out".

So if this one smokes will he pick up the tab? I would do as was suggested above. I would also toss in a 2 Amp slow blow fuse just to make sure. The transformer won't immediately smoke or destruct.

Ron

 

I would also toss in a 2 Amp slow blow fuse just to make sure. The transformer won't immediately smoke or destruct.

I need to repeat my earlier question because there was no response. What does putting a fuse on the secondary output do to protect the transformer? Every thing I've ever worked on had the fuse in the primary winding not the secondary like this board/project has. That way if something is wrong power is shut down completely, protecting the transformer.

 

Because the kit makers can put a fuse on the board. They can't put one on the transformer. There's nothing to stop you putting a fuse on the primary.

 

A fuse on the secondary does achieve some protection, especially for large well designed high efficiency transformers. For smaller transformers the protection will be less, and the fuse rating will be more difficult to specify.