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Post subject: Blowing Fuse - What's the Problem?
Posted: Tue Oct 14, 2014 3:29 pm
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Aspiring Musician
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While playing my 1967 Fender Bandmaster (AB763), the 2 amp slo-blo fuse blew. I removed the two power tubes (6L6's), to eliminate them as a possible cause, and put in another 2 amp slo-blo, which also blew immediately upon turning on the power. Is there perhaps something specific to zero in on to determine the cause? :cry:

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Tue Oct 14, 2014 8:22 pm
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I always check power tubes and rectifier tube. Did you check the caps where they ever changed?


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 12:53 am
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Lenny1716 wrote:
The 2 amp slo-blo fuse blew. I removed the two power tubes (6L6's), to eliminate them as a possible cause, and put in another 2 amp slo-blo, which also blew immediately upon turning on the power.

:cry:


Lenny1716, you prolly have a short in the power supply portion of the amp. this amp does not have a tube rectifier. Rather, it uses 6 solid-state diodes. Most likely, it's a bad main filter cap under the doghouse. Were these ever replaced? Maybe time for a cap job.

Good luck!


http://bmamps.com/Schematics/fender/ban ... _schem.pdf


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 1:42 am
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You write fuse blow when you turn Power On . You did'nt put Standby on "Play " position right ?

Check all the parts before the Stanby switch.

Look at schematic

There is the Power Transformer , rectifier diodes and 2 filter caps and 2 resistors .
It is include tube's heater circuit ; remove all tubes in case of short heater

All other parts after the Stanby switch are good this include Power Outout Tubes if you write what your amp really do.

Never put more power full fuse , always the original specs.


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 4:12 pm
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Aspiring Musician
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BMW2002Ti wrote:

Lenny1716, you prolly have a short in the power supply portion of the amp. this amp does not have a tube rectifier. Rather, it uses 6 solid-state diodes. Most likely, it's a bad main filter cap under the doghouse. Were these ever replaced? Maybe time for a cap job.

Good luck!


http://bmamps.com/Schematics/fender/ban ... _schem.pdf

I had some caps replaced some years back, but now don't know which ones. That will require a little further investigation on my part to see if these were what was replaced or perhaps, bad...thanks for the feedback.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 4:25 pm
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stratele52 wrote:
You write fuse blow when you turn Power On . You did'nt put Standby on "Play " position right ?

Check all the parts before the Stanby switch.

Look at schematic

There is the Power Transformer , rectifier diodes and 2 filter caps and 2 resistors .
It is include tube's heater circuit ; remove all tubes in case of short heater

All other parts after the Stanby switch are good this include Power Outout Tubes if you write what your amp really do.

Never put more power full fuse , always the original specs.


YES, the Standby was in "Play" position... :(

I am to understand, I should remove all the tubes and power up in the "Standby" mode...?

Thanks for the feedback...I'll try that, using the spec'd fuse type (2-amp slo-blo).

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 4:42 pm
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Test I suggest is for blow fuse when amp is in Standby .

Test if amps blow fuse when Stanby position , not play position like you do

If it not blow fuse in Stanby ,and fuse blow when in Play position you have to test all components use in Power Supply after the Stanby switch .


You follow me ? :lol:

Check filter caps , Output tubes , Output transformer .....
Filter caps is #1 to check , Power tube ; you do


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 15, 2014 4:51 pm
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Lenny1716, if the amp popped a fuse with the 6L6GC pulled and standby in the play position --- you prolly do not need to do another test with all tubes pulled. I have never seen an amp popped fuses because of a shorted 12AX7 or 12AT7. These are the only remaining tubes, after you pull the 6L6GC's.

The issue is 99% before the standby switch. Which are the main filter caps. The main power tranny. Or the main power supply diode rectifiers.

Can you take a photo of the main filter caps, under the doghouse? If any of those caps (including the 20mfd/500VDC ones) are shorted or completely dried out --- the main fuse will pop --- since, they are all hooked up in parallel. My bet, is these are the problem. You need to rule them out first. As electrolytic caps have a limited life expectancy that can very from brand-to-brand and amp-to-amp. The two 35mfd/350VDC in the first filter stage being the usual suspect. Though, all should be replaced --- at the same time.


Good luck on the hunt!


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 9:35 am
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BMI...Thanks for the info. I will see what I can do to post pics as suggested.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 3:43 pm
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If you need electrolytic cap replacement, I suggest doing ALL 'lytics. Main PSU, Bias Supply, and bypass caps on the gain stages. There are "cap kits" available on the Net.

I recommend using F&T in the main PSU and Bias supplies. 25mfd/50VDC Sprague Atoms (if possible -- higher voltage rating will tend to last longer) in the bypass.

Here is an example of the kit (not related to seller):

http://www.ebay.com/itm/RECAP-KIT-for-V ... 2c585b9370


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 5:18 pm
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BMW2002Ti wrote:
Can you take a photo of the main filter caps, under the doghouse?

Good luck on the hunt!


OK, I guess I understand less than I thought as I am not familiar with the term "doghouse"........ :oops: what am I looking for? I have the chassis removed.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 7:25 pm
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If your unsure you might want to take to a tech but if do buy caps I would not buy from fenderjunkie73


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 7:27 pm
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The main filter caps are under the rectangular aluminum cover, behind the tubes, in this photo.


------

Image


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 16, 2014 8:43 pm
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Aspiring Musician
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BMW2002Ti wrote:
The main filter caps are under the rectangular aluminum cover, behind the tubes, in this photo.




OK, and thanks. Believe these pics disclose some suspect conditions....

http://i1306.photobucket.com/albums/s567/Lenny1716/Filter%20CAPS/DSC_0002_zpsb96e4657.jpg

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Am Std 60th Ann Comm
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USA Prof HSS
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Epiphone Std Plustop PRO
Tele AmOrig 50's
Tele Elite
Takamine Acoustic/Electric EG-531S
Blueridge Acoustic/Electric BR-J33CEO


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Fri Oct 17, 2014 12:03 am
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At least three caps out of entire bunch show signs of "pimples." All should be replaced.


Good luck! :D


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