Demon 250 amplifier blows fuses

[klj2497]

I have a Demon 250 amplifier. I have not used it for awhile because it blew the fuse in the rear and i lost it I tryed a 20amp 32v that blew i tried a 20amp 250v fuse when i turned it on i thought it made a strange noise so i turned it off im wondering if anyone knows the fuse size for the Demon 250 Thanks

 

[Dmans]

klj2497,
The schematic I have for the Demon 500 (has 2-6JG6 driving 4-6LR6 tubes) shows a 15a primary fuse. I do have a schematic for the Demon 250 (has 1-6KV6 or 6JG6 driving 3-6JG6 tubes) but there is no "call out" for the "F1" fuse.

I would think the 15a fuse would be overkill for this amp but would allow you to properly measure the amp draw and install the correct size fuse.

73's
David

 

[klj2497]

Thank You Dave. Now i have to figure out what is blowing the fuse. I removed all the tubes and put a 20 amp fuse in. it still blew the fuse. It starts for a few seconds, and the fan starts to spin then it blows.

 

[ExitThirteen]

I'm willing to bet that a filter capacitor has likely dried up and failed short, which will pop the fuse. I would install new filter capacitors and check all the HV diodes, before attempting another power-up.



~Cheers~

 

[543_Dallas]

Thank You Dave. Now i have to figure out what is blowing the fuse. I removed all the tubes and put a 20 amp fuse in. it still blew the fuse. It starts for a few seconds, and the fan starts to spin then it blows.

Rectifier diodes would be the next place I'd look. You might as well put a new set of filter caps and bleeders while you're at it.

 

[nomadradio]

Here's the diagram I have for the 250. Kinda short on detail.

C10 and C11 are the HV filter capacitors. If they are original, they should have failed years ago.

When they do fail, they tend to short. This will overload the rectifier diodes D1-D4. Odds are that they are damaged from that overload and are now shorted. If you'll find the transformer wire connected to the center lug of the High/Low switch, take it loose and insulate the end with tape. Now try to power it up.

If the fuse blows, the power transformer has failed. The fuse in this amplifier is sufficient to protect your house from a fire, but too large to protect the components in the amplifier from damage.

And if the fuse holds, the fan spins up and the tube filaments light up, you need to replace the HV rectifier diodes and the filter caps. We always replace the two bleeder resistors at the same time. Old ones can change resistance value and cause trouble.

Tubes are always a question. If you have access to a tube tester, making sure that none of the tubes have broken down and shorted inside will improve your odds of getting it back on the air.

You will probably need to use a series-connected pair of 1kV-rated rectifier diodes for each of the four single diodes you see in the diagram. Each of the four diodes would have to have a rating of 1500 Volts for this amplifier. Those are had to find, but the 1000-Volt 1N5408 recitifer is easy to find, especially on Fleabay.

Best of luck and 73

 

[Shadetree Mechanic]

If you keep putting fuses in it you are going to ruin something. When a fuse blows it is possible that it has weakened over time. If that is the case then replacing the fuse will get it going again. If it blows again then it is time to stop and do some checking. I have seen people do this with circuit breakers until it won't reset. Then they say the breaker is bad because it won't reset.........

 

[klj2497]

Thanks everyone i was really stumped now I have some good places to start.

 

[klj2497]

LOL shadetree mechanic

 

[klj2497]

I did some measuring, and one of the large capacitors is shorted, and the resistor next to it is shorted too. Most of the 8 diodes in series are also shorted out It looks like some repair has been done in the past from the number of diodes

 

[klj2497]

i think the resistor is R5 and the big cap, i think it is C10

 

[nomadradio]

It needs both filter caps and all eight rectifier diodes.

And both bleeder resistors, as well.

Nothing to be gained by trying to replace just some of them.

But try taking the transformer wire loose from the high/low switch before you spend money to repair the damaged caps and rectifiers and make sure the transformer doesn't pop the fuse with that wire taken loose.

73

 

[klj2497]

Thank you for the help. I Ordered the two big capacitors 100uf 450v. two resistors 2watt 390k ohm. and eight 1n5408 diodes. Ill put those in and see what happens

 

[nomadradio]

I still recommend trying it with that wire pulled loose from the center lug of the High/Low switch before installing parts.

The question for me is how many fuses can you blow before the transformer sustains damage and shorts out inside.

Maybe one. Maybe two or more?>

If it pops a fuse with that wire pulled loose and a piece of tape wrapped around the loose end this means the transformer is bad.

73

 

[klj2497]

I have to go to the hardware store tomorrow and buy some fuses. I only had one extra fuse. Then ill take the transformer wire loose from the center of the high low switch and tape it then put a fuse in i guess i will use another 20 amp 32v fuse i do not know the right fuse size