AM864 keep blowing fuse::: HELP!!

[Deepdark]

Hi there.

I just finished a fed. AM864 for a client. So far so good, it was pretty straight foward. First thing first, I redraw the psu to a RC filtering topology with diodes, instead of the LC filter with the tube rectifier, as I just linked. This was done to simplify things and keep cost down. Transformer is Edcor XPRWR012, 350V C.T. at 10ma, 6.3V C.T. at 2A. Fed. AM864 is rated 30ma, so we know this is a good transformer for the task. Heater was connected the classic way, and not as on the schematic.

First time I fired up, teh voltages was low (175V after the rectifiers). I diagnose and found I forgot to connect the psu ground (the ground taking all 47uf caps.). Stupid me. Trouble came after I connected that ground. Now the fuse blow each time I fired it up.

I began to search over the circuit, testing continuity to be sure their was no missing wire somewhere, etc. I found that 6sk7's pins 1 and 6sq7's pin 1 weren't connected to ground (in fact, I used the federal manual's schematic which contain, as I found later, some error like that), and their were a missing link between R1/C1 and input transformer C.T. / R5. Once connected, I fired up and, fuse blown again

So now that I ran the entire circuit and everything was allright, along the good schematic, I suspected the trouble to came from the psu. So I disconnected the links to R6 and to the circuit, to isolate the psu alone. Fired up and fuse blown.

So we know the trouble sit in there. To be honnest, I'm a little in the dark right now. What can cause the fuse to blow. Next step I'll disconnect the heaters to isolate the trouble. I just checked the heaters conections and they are all good.

Fuses are 1A, from the top of my memory. 

Do you guys have any cues or anything else to check, any ideas?? Thanks folks.

 

[Deepdark]

that is the psu board with the caps and R6. HT connect underneath the board, that's why we don't see anything on the back of the diodes, same with the ground wire. It is underneath the board, at the last cap and go to the IEC star bolt

 

[Deepdark]

general connections. Red/black wire is the ht center tap and go to IEC star bolt, same with the white/Brown wire (heater c.t.). the tiny green wire is the psu ground

 

[Deepdark]

interconnection little strip board. Brown wire are the heaters and they then go to tubes with the green and yellow wires.  Red wires are the HT and they go underneath the psu board with the orange wires. The green wire on the center pin is the ground

 

[Deepdark]

I began to think of a shorted diode, or a shorted filter cap,  probably due to the previously missing wires. From objective electrical theory, a centered tap full wave rectifier circuit with an shorted diode, the fuse will blow. Maybe it's what I got here. Do we test the filter cap the same way we test diode?

 

[mjrippe]

Are you using time delay or "slow-blow" fuses?  A fast acting fuse will probably blow due to initial inrush current as the filter caps charge.

 

[Deepdark]

sloblo fuse. What I think is that because I forgot the ground psu wire the first time, and then the pins1 ground wire from the tubes and the wire between R5 and R1/C1, maybe their could be have had a current buildup and the result is that finally, one diode or both have shorted.

 

[Deepdark]

by the way, is it ok to test the psu alone? I mean, not connecting the psu to the circuit, just for the sake of troubleshooting!? Since their is no load, and it's not a regulated psu, maybe it's not a good idea???!!

 

[Deepdark]

Little update. I disconnect the heater circuit and no more blown fuse. So now that i partialy found the culprit,  i would like to k ow a thing. When looking at the schematic i linked (psu s hematic) is there a trouble the way i connect the heaters? What if i reverse the heater connection at one tube? I don't see any trouble but, we never know. What do you yhink can cause the fuse to blow? Bad tube? Since i heatshrink all tube socket pins, i d 't think the problem is There. Is a tube can cause a short circuit? I could put out all tubes, reconnect the heater circuit, and put 1 tube at a time to see where's the trouble sit. Or maybe someone have another idea/way of testing it?

Thanks

 

[Deepdark]

I think i found the culprit. One 6sk7 looks to be bad. I'll make some more test but it looks to be the trouble. Another cue is that when shaking the bad one, it sounds like something is loose inside, jyst like a broken lightbulb. Maybe it has been damaged in the shipping