Blackface Bassman blowing fuses on startup

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Blackface Bassman blowing fuses on startup

Postby Yoda on Thu Apr 21, 2016 4:31 am

Hello all,

I have a BF Fender Bassman with the AA165 circuit, I just performed a 3-prong conversion, and replaced all electrolytic caps (filter, bias, and cathode bypass). When I plug it in, the fuse blows as soon as I turn on the power switch. If it helps, I did the 3-prong conversion first and then the fuse blew after I tried to test the 3-prong conversion so I decided to proceed with replacing all the electrolytic capacitors and trouble shoot everything afterwards. Everything looks good to my eye but perhaps those of you with more experience could shed some light on what I can look for as far as troubleshooting.

Thanks in advance for any help.
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Re: Blackface Bassman blowing fuses on startup

Postby jaywalker on Thu Apr 21, 2016 1:26 pm

Welcome to the forum. A high resolution picture would be useful. They can't be directly posted in the forum. Use a host site such as Photobucket then use the "IMG" code to place them in your post here.
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Re: Blackface Bassman blowing fuses on startup

Postby OldSchoolDave on Thu Apr 21, 2016 3:42 pm

Yoda wrote:When I plug it in, the fuse blows as soon as I turn on the power switch. If it helps, I did the 3-prong conversion first and then the fuse blew after I tried to test the 3-prong conversion so I decided to proceed with replacing all the electrolytic capacitors and trouble shoot everything afterwards.


Best to only change one thing at a time when troubleshooting... Are you certain the 3 wires are properly connected?

If you pull the power tubes, does the fuse still blow?

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Re: Blackface Bassman blowing fuses on startup

Postby Dean Jr. on Thu Apr 21, 2016 7:10 pm

PIcs of the AC coming in, please. Do not turn the amp on again, imho. You have the AC wired incorrectly somehow. Here is a tutorial. Note that the white wire goes directly to one PT primary lead. The black wire goes through the fuse to the switch to the other PT primary lead. Most of us remove the death cap....that .047 mfd cap going to ground from the polarity switch.

http://www.unclespot.com/2to3prongconversion.html
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Fri Apr 22, 2016 2:02 am

Ok, here are pictures of the of the main power coming into the amp:

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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Fri Apr 22, 2016 1:49 pm

My vintage Fender power is wired up the same. Here is an example of my71 Super Reverb:

Image

I'll guess on two things you can check. Use your DMM set to continuity beep. Check the green ground wire in the chassis is actually wired to the ground tip on the end of your plug at the wall.

One of your caps may have been installed backwards. You may have mixed up your polarity. Have you double checked that?
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Fri Apr 22, 2016 1:57 pm

Show us a pic of you caps under the dog house. First two caps should be in series l I believe. Are your first two caps rated 70 uf at 350V? Which equates to 35 uf at 700V total.
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Re: Blackface Bassman blowing fuses on startup

Postby OldSchoolDave on Fri Apr 22, 2016 3:01 pm

It's not wise, IMHO, to keep the "Convenience" outlet and the Ground switch in the circuit when wiring a vintage Fender to a proper 3 wire grounded cord.

Your ground switch is probably jumping hot to ground in its present position. Flip it the other way and see if the fuse still blows.

But - in any case - get it out of the loop!

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Re: Blackface Bassman blowing fuses on startup

Postby Dean Jr. on Fri Apr 22, 2016 4:27 pm

Re: the 'ground switch'....I agree that it is best perhaps to nt utilize it in any manner when rewiring one of these old Fenders. But....the death cap has been removed. IF the switch is in proper working order, there is no problem with the way it is wired. The switch contacts here are merely being used as contact points. That switch does nothing when switched when the death cap is removed. Those two contacts on the ends are not affected at all in its present situation....as long as the switch itself is in proper working order. IF the switch has broken parts in side, then it is a problem.
KeithB7, fwiw, your pic does not show the same wiring. I prefer to rewire those Fenders that came with grounded power cords....I take it to the arrangement shown in that Uncle Spot link. That grounded Fender arrangement is not as 'correct' as it could be.

Yoda, a multimeter will tell you the condition of that switch. IF the switch is in proper working order, it is 'inactive'....those two contacts have no way of interacting since that death cap if out of the picture. Those points are simply contact points. IF the switch is broken inside and there are loose components, then a resulting inadverdent contact could be made there.....and blow a fuse. You could simply move the PT primary that is contacting the white AC neutral at that switch to the AC courtesy plug contact where that white neutral is contacted....remove the yellow cloth covered wire at that point. Run a wire from the contact for the black hot AC wire from the courtesy plug to the end of the fuse. Some people avoid wiring to the courtesy plug since that established an ungrounded power source. Twist----twist it properly---- the white wire and a PT primary and use a wire nut to secure that connection. You can still make a contact for the black wire on that plug and then wire to the fuse directly. The courtesy plug will be dead.
I don't like what is going on at the fuseholder because I cannot see both contact points there. You could lift the PT primaries and fire the amp up to see if there is a short prior to the PT primary. IF the fuse holds in that situation, reconnect the PT primaries, lift all of the PT secondaries and see what goes on. IF the fuse blows then, your PT primary is bad. IF the fuse holds, reconnect the secondaries one winding at a time to find the problem area. I would have all tubes out of the amp for all o f this work.
Note: build yourself a current limiter and use it for firing up an amp after this type of work and for troubleshooting fusing problems. Very inexpensive and useful tool. Do a search for 'Uncle Doug current limiter' youtube.
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Re: Blackface Bassman blowing fuses on startup

Postby Dean Jr. on Fri Apr 22, 2016 4:29 pm

KeithB7, fwiw, that wiring in your 1971 amp is NOT the same as what is shown in that link to Uncle Spot. That Fender scheme is not as 'proper' as it could and should be, imho and ime. I rewire those even though they are grounded from the factory.
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Fri Apr 22, 2016 5:32 pm

Sorry folks the image I posted was done so in error. I meant to post different photo. Currently I am not able to post the pic I had intended to.
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Fri Apr 22, 2016 11:34 pm

Here's a pic of what's going on at the fuse holder, along with pics of the filter cap board. I could not find Sprague Atoms in 70uF so I used 100uF caps of the same voltage.

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Image
Image
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Fri Apr 22, 2016 11:38 pm

The pictures appear chopped on my phone, but the pictures of the filter cap board should be of the whole board, top and bottom, so let me know if they're displaying correctly so I can fix it if there's a problem.
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Sat Apr 23, 2016 12:28 am

Am I dreaming or are those first two 100 uF caps installed backwards? Power goes in at second cap, negative side (red wire) and grounded at positive end of first cap here. I think.

This is a drawing I made of a blonde Twin circuit I was working on. This is not a bassman, but do the same principles not apply here?
I missed drawing in the two resistors on the first two caps, but the wiring and polarity is more what I want to point to.

Image
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Tue Apr 26, 2016 10:45 pm

To the originator: Were you able to figure this out? Any updates?
I am curious and like to learn from examples this this. Thanks, Keith.
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