Loud HUM that increases in intensity

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axeman
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Re: Loud HUM that increases in intensity

Post by axeman » Sat Mar 26, 2011 4:18 pm

For shits and giggles connect an alligator clip on the bus ground wire and the other end to ground.

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Re: Loud HUM that increases in intensity

Post by page361 » Sat Mar 26, 2011 4:31 pm

Greast suggestions thank you! I'll dig in right now and let you know...

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Re: Loud HUM that increases in intensity

Post by page361 » Sat Mar 26, 2011 5:12 pm

Hey Toner...to answer your question;

"Has the HT fuse blown with new tubes AND the white rectifier wire reconnected?"

...the answer is YES, the HT fuse blew when the white wire was disconnected and the original tubes were installed. I have since reconnected the bias voltage feed (white) wire, installed a new set of power tubes, and still the HUM from HELL...

Working on getting a new pair of grid resistors installed :)

MIke

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Re: Loud HUM that increases in intensity

Post by axeman » Sat Mar 26, 2011 5:52 pm

You mean sreen resistors 1k 5 watt. I have tons of them.

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Re: Loud HUM that increases in intensity

Post by page361 » Sat Mar 26, 2011 6:52 pm

Yes - I mean the 5W 1K screen resistors...it's been a long day!

OK - here's what I have so far...

Checked AC and DC voltages at the rectifier and both were good...338V VAC and 459 VDC respectively. Checked the heater wires on output tube sockets - they read 3.31v AC...so looking good so far. With that I am going to assume that both the PT and rectifier diodes are OK...

..still need to do the ground check per axeman's suggestion...

Before I risk blowing another my last fuse I need to run to Fry's and buy some more...I'm plum out of 500ma fuses. Will be back in a few. Thats so much again for everyone's help so far...

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Re: Loud HUM that increases in intensity

Post by toner » Sat Mar 26, 2011 7:11 pm

Before turning it on again, drain the filter caps and test both screen resistors for ohms (should be 1k ohm). To further test, remove the power tubes and power fully on. Then take voltage readings on pin 6 and 4 of both output sockets. They should be within a couple of volts of each other. If you don't get any voltage on pin 4 or if it's low, replace that screen resistor.

If the screen R's test okay, something is still pulling too much current and causing the HT fuse to blow. I'd leave the output tubes removed and start testing voltages at V3 and work back towards the preamp.

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Re: Loud HUM that increases in intensity

Post by page361 » Sat Mar 26, 2011 10:29 pm

Axman - what do ya know...it was indeed the ground bus bar for the pots that was causing me grief..BUT...this is the exact method I used in my Metro JTM-45....and I have no issues whatsover...must admit I'm stumped.

Can someone suggest to me how to come up with a better bus bar using the PEC pots? Obviouly what I have is not going to work :)

THANKS AGAIN EVERYONE...WOO HOO!!!!!! I GET TO GIG WITH MY JMP 50 NEXT MONTH...WOO HOO!!!!!

Mike

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Re: Loud HUM that increases in intensity

Post by axeman » Sat Mar 26, 2011 10:38 pm

You actually did a good job with the bus ground wire, you just forgot to grounded. I would leave it alone and ground it correctly with solder. Know go jam.

Good Luck

Art

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Re: Loud HUM that increases in intensity

Post by page361 » Sat Mar 26, 2011 11:04 pm

So you suggest just running another bus wire from the bus bar to say one of the chassis bolts or something lik that???

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Re: Loud HUM that increases in intensity

Post by axeman » Sat Mar 26, 2011 11:49 pm

No, solder a black wire from the bus wire to ground/nut. Pick one.

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Re: Loud HUM that increases in intensity

Post by page361 » Sun Mar 27, 2011 12:46 am

Yep did exactly that..THANK YOU!

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