Grommes LJ3 help needed...

[bfotk]

It's been quite a few years since I played with this charming Little Jewel. I just plugged it in and when I turned it on, the 5Y3 rectifier flashed brightly and the fuse blew. A second fuse did the same. I went out and got more fuses...

I pulled out the 5Y3, put in a new fuse, and turned it on. The output tubes and the adjacent 12AX7 lit. The 12AX7 that's covered by a metal shield did not.

Not having another known good 5Y3, I inserted a 5AR4 (which I believe but cannot prove is good). I got the flash and blown fuse again.

Suggestions (aside from putting it up on eBay as-is)?

 

[maxhifi]

Something is shorted, causing the tube to blow. Given the symptoms, I would check the filter capacitors with an ohm meter

 

[bfotk]

Fair enough. Do I need to disconnect all connections to the filter caps to check?

 

[gadget73]

Sounds like filter caps to me. They're not liable to read properly with an ohm meter though, or at least I have never once gotten any useful measurement with an ohm meter on a capacitor.

 

[century tek]

With an ohm meter, if the capacitor is shorted, it will show zero ohms or a direct short like touching the probes together. If the capacitor is open, it will show to infinity just like a burned out light bulb or fuse.

Usually when the main filter capacitor shorts, it blows fuses and produces the flash in the rectifier tube. If there is no fuse like in a lot of old stuff, it will take out the power transformer as well.

If the main filter capacitor is open, you will get a loud hum but it should not blow the fuse and cause the rectifier tube to flash.

Leakage, full voltage and other tests, need a capacitor checker and/or a scope.

I would disconnect the can capacitor so that it is not in the circuit when doing the above mentioned tests or you will just be testing everything else in the circuit connected to the capacitor.

 

[maxhifi]

I've had shorted caps read as 0 ohms, especially if it's so bad that it will immediately blow a fuse, or ruin a rectifier. I've had it happen in both tube and solid state equipment. If the cap wasn't dead shorted, it would probably just get hot instead of acting how it does.

You don't need to take it our of circuit to check for a short, but if you find one, you will need to take it out to verify its the cap which is bad and not something else.