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Post subject: 1968 drip rail bassman head AB165 blowing power resistors
Posted: Mon Jan 14, 2013 7:32 pm
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hi, i am trying to get a '68 bassman amp to run. the 1K resistor (brown black red, i think) in the cap tray keeps cooking to death. the filter caps are all original. pulled the tubes and it still cooks. so it's not a shorted power tube. help...........thanks.


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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Mon Jan 14, 2013 7:39 pm
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Have you checked the output of the power tranny's secondary windings? That's where all of the amp's high voltage comes from. If all is copasetic there, take a look at the diode stack on the rectifier board.

HTH

Arjay

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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Mon Jan 14, 2013 8:18 pm
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hi arjay, it shows 320V AC on the power tranny side of the diode board (both red wires). the other side of the diode board is 297V DC. i think that's to spec. will a dead filter cap or two smoke the power resistor that's attached to them?


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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Mon Jan 14, 2013 8:59 pm
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A distinct possibility.

Power-rail resistors work just as hard as those electrolytics. Whenever I'm "under the doghouse" for a filter-cap job I always check them as well for peace of mind.

Arjay

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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Tue Jan 15, 2013 1:36 am
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Hi Duke, the 165 should be seeing around 400+vdc,after the first cap reservior. Following the choke the same. Across the 1K resistor we're looking at a 10vdc drop between the first 20uf cap and the next 20uf cap. What are you seeing at this point? .... the other guys might have a better handle on this!!!!! I'm inclined to think the power supply filter caps are in need of an R&R, same for the resistors on the power rail. JMO, Art

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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Tue Jan 15, 2013 11:08 am
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two friends tell me i have a direct short to ground somewhere. i lifted all the filter caps and put the meter on them. way out of spec but they seem to be working. how do i track down a short?


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Post subject: Re: 1968 drip rail bassman head AB165 blowing power resis
Posted: Tue Jan 15, 2013 12:58 pm
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raulduke52, maybe check to see if the choke (TR2) has a short. With amp off, check the resistance from each end (usually two black wires)--- to the choke's frame. Should be very high. Like near infinite DC resistance. If the readings are in the hundreds of ohm range or lower --- prolly best to fit a new choke.

Check to make sure the filter caps (esp those 20mfd/525VDC ones) are properly placed. With (-) to ground (chassis).

Also, if amp has been worked on, sometimes hot wires can accidentally get poked through the tagboard and onto the chassis. With amp off & filter caps discharged, check the resistance of the positive or B+ side of the power rail to ground. Should be close to infinity, again.

If these suggestion do not find the issue --- you prolly need new filter caps. Best to replace ALL of them at one time. Be sure to rebias the output tubes after ANY mods to the power rail (caps and resistors).

http://www.webphix.com/schematic%20heav ... _schem.pdf

Good luck! Keep us posted.


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