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Topic: broken pot shaft |
Tommy Boswell
From:
Virginia, USA
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Posted 23 Apr 2015 5:33 am |
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My Fender Bassman 70 has a broken shaft on the bass channel bass tone control. My schematic shows it as a 250K-J (all the tone pots are the same), and 021857, which I assume is a part number.
Where can I get the part? And can I do it myself? I can do basic soldering. And I know you have to be careful when you go into a tube amp, stuff in there can kill you. |
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Lane Gray
From:
Topeka, KS
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Posted 23 Apr 2015 7:05 am
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If you ask me, a Radio Shack pot should be adequate, since it's not a volume pedal.
I'd bleed the capacitors through a light bulb just to be safe
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Bill Terry
From:
Bastrop, TX
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Bill A. Moore
From:
Silver City, New Mexico, USA
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Michael Maddex
From:
Northern New Mexico, USA
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Michael Brebes
From:
Northridge CA
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Mark Fowler
From:
Minnesota, USA
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Tommy Boswell
From:
Virginia, USA
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Posted 23 Apr 2015 4:58 pm
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| I knew you guys would come through, thanks! |
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Tommy Boswell
From:
Virginia, USA
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Posted 23 Apr 2015 5:01 pm
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| Lane Gray wrote: |
| I'd bleed the capacitors through a light bulb just to be safe |
I assume that's because glass does not conduct electricity? Also, does the light bulb glow until the capacitors are discharged? |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 23 Apr 2015 5:20 pm
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The light-bulb acts as a current-limiting resistor, and gives a visual indication of residual charge.
_________________
Too much junk to list... always getting more. |
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Lane Gray
From:
Topeka, KS
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Posted 23 Apr 2015 10:00 pm
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Bingo. Draining them with a screwdriver can kill them
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Ian Worley
From:
Sacramento, CA
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Posted 24 Apr 2015 1:03 pm
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| Lane Gray wrote: |
| ... I'd bleed the capacitors through a light bulb... |
Can you explain this technique? I'm not planning on going inside my amp anytime soon, but inquiring minds want to know... |
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Lane Gray
From:
Topeka, KS
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Posted 24 Apr 2015 2:06 pm
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The wires going to the "doghouse" holding the caps? Touch one end of the wire with the light bulb in it to ground, and the other end to the point where they meet the board.
I think I'd read that turning the amp on while unplugged will discharge them also.
Or, better yet, here's a handy video on amp safety, including bleeding filter caps. DIY: Amp Safety 101: http://youtu.be/DkEc58-vWc4
Down in the comments, one guy suggested taking an extension cord, cutting off the blades of the power plug and leaving the ground conductor, and plugging the amp into that grounding cable, and plugging that dead cord into the wall.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Bill A. Moore
From:
Silver City, New Mexico, USA
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Blake Hawkins
From:
Florida
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Posted 24 Apr 2015 4:49 pm
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What Bill Moore posted: I've used a similar resistor with clip leads for years to discharge the caps.
A note here: All well designed power supplies in
tube type amps include a bleeder resistor to discharge the caps automatically when the amp is turned off.
Most of the time, this does the job. But, when an amp has a malfunction you don't know what is going on, so manually discharging the caps is an important safety procedure.
I do it on every amp I open up. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 24 Apr 2015 5:06 pm
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| Lane Gray wrote: |
Down in the comments, one guy suggested taking an extension cord, cutting off the blades of the power plug and leaving the ground conductor, and plugging the amp into that grounding cable, and plugging that dead cord into the wall. |
I don't see how that would help... you still have a cord, plugged into the wall... is it the grounded-only one? And what does grounding the chassis do for you? Not much, if anything.
On another note, turning the amp on to discharge caps doesn't work... no current flows unless the heaters are hot, when cold a vacuum tube is a perfect insulator... and electrolytic caps can act like batteries and regain a charge electro-chemically. Some caps can charge up just sitting around... a television picture tube can store up thousands of volts from cosmic rays hitting it.
Some amps shouldn't require discharging... the Fender amps (Twin etc) that have stacked caps in the PS have splitter resistors across them, these bleed the voltage down when the amp is off... but the resistor can fail. All the small Fender amps don't have this, you can get bit real hard by a Deluxe or Princeton.
When in doubt, short it out!
_________________
Too much junk to list... always getting more. |
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Lane Gray
From:
Topeka, KS
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Posted 25 Apr 2015 8:33 am
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Stephen, the idea is to send all those electrons to ground. I don't think it's a bad idea but it does seem like overkill. Like belt and suspenders.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 25 Apr 2015 9:39 am
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| Lane Gray wrote: |
| Stephen, the idea is to send all those electrons to ground. I don't think it's a bad idea but it does seem like overkill. Like belt and suspenders. |
Lane, when you have the chassis open, and you're messing around inside, having it grounded does nothing for your safety... you will still shock the crap out of yourself if you touch the chassis and B+... but now, you will be able to shock yourself by touching B+ and any other grounded equipment. I can't think of any scenario where this idea helps at all... sorry.
_________________
Too much junk to list... always getting more. |
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Lane Gray
From:
Topeka, KS
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Posted 25 Apr 2015 10:45 am
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Good point.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Michael Maddex
From:
Northern New Mexico, USA
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Posted 25 Apr 2015 4:22 pm
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| Lane Gray wrote: |
| ... Down in the comments, one guy suggested taking an extension cord, cutting off the blades of the power plug and leaving the ground conductor, and plugging the amp into that grounding cable, and plugging that dead cord into the wall. |
It sounds to me like this person has his equipment mixed up. It's been a while, but IIRC, sometimes a tech working on a chassis or PCB from a PC will want to ground himself and his board (that is, bring both to the same potential) to minimize the chance of a static discharge which can damage some of the chips used in computer circuits. This is, obviously, a very different situation from working on tube amps or the power supply for the PC.
FWIW, M2¢W. YMMV.
_________________
"For every expert, there is an equal and opposite expert." -- Arthur C. Clarke |
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Blake Hawkins
From:
Florida
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Posted 25 Apr 2015 6:36 pm
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Just a note to what Michael said. When working on static sensitive solid state equipment. The technician should be using a grounding wrist strap which includes a current limiting resistor.
That is the safe way to do it.
Additional comment to my previous post. I checked over my Fender diagrams. Steven is 100% correct about Fender amps. Other than the resistors he mentioned, Fender does not have bleeders in their power supplies. |
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