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Post subject: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 7:57 am
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http://www.mediafire.com/i/?ly7v447y7vg8dyb

This amp has done this before and I had it repaired thinking it was just a freak thing that'd happened. Here I am again though, and I want to know what's causing this so I can repair it myself and take some preventative measures. I was thinking of installing a 120mm computer fan to help with the heat.


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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 8:36 am
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Are there any symptoms in the amp just before the resistor burns? Noise? Distortion? Loss of volume?

See Rev A of the schematic:

http://support.fender.com/schematics/gu ... ematic.pdf

R47 is the first resistor in the power supply filter chain. I suggest you measure the B+ voltage. You may have a problem in the rectifier, the filter caps, or the transformer itself that is causing the B+ to be too high (should be 329 VDC according to the schematic) and causing the resistor to overheat and burn up.

Edit: Corrected schematic reference.

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Last edited by bluesky636 on Wed Nov 23, 2011 10:27 am, edited 1 time in total.

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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 9:18 am
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+1 bluesky636, I think that it is rev A, second page? It does look like R47 is next to the power supply filter caps.

If that is the correct schematic, R47 feeds the output tubes screen grid resistors (DC supply Z), so I agree that filter caps and/or output tubes are suspect. Since R47 is the first resistor in the supply, the problem could also be in the other supplies "down-stream", which again indicates power supply filter caps, and/or preamp/PI tubes as suspect.

If it's an older amp, might be due for filter caps and output tubes. :idea:

Sometimes, not always, the caps can show bulging, or even have crusty-looking stuff at the positive end, both indicating bad caps.

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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 10:38 am
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Oh. Oh. The problem and solution just jumped out and bit me. :lol:

The schematic says R47 should be a 2 watt resistor and that R48 and R49 are 1 watt resistors. Look closely in the OP's photo at the three resistors and the parts outlines on the PCB. All three resistors are the same physical size, but the outline for R47 is considerably larger. It appears that whoever repaired the amp replaced R47 with a 1 watt resistor instead of the required 2 watt resistor! Over time the current flowing through R47 slowly cooked it, or a one-time surge fried it on the spot. Perhaps a tube failed and the excess current took out R47. A 2 watt resistor may not have fried.

Replace R47 with the required 2 watt resistor and check all the listed voltages. Check the grid circuit for any signs of an overload. Maybe replace the output tubes to be safe. If everything checks out OK, I bet the problem never occurs again.

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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 4:24 pm
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bluesky636 ,
On your schematic I see R47 a 470 K at the reveb pan output, not in power supply . But in the photo it look close to power supply

What 's wrong ?


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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Wed Nov 23, 2011 5:08 pm
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stratele52 wrote:
bluesky636 ,
On your schematic I see R47 a 470 K at the reveb pan output, not in power supply . But in the photo it look close to power supply

What 's wrong ?


In their infinite wisdom, Fender has included TWO different schematics and chassis layouts on their site:

049301 Rev D dated 2/18/99 and 0057275000 Rev A dated 4/23/01.

The drawing that matches the OP's amp and the one you should be looking at is 0057275000 Rev A dated 4/23/01.

I had the wrong drawing listed in my original post. Shimmilou caught my mistake and I corrected it.

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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Thu Nov 24, 2011 3:14 am
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Thank's bluesky636, I found it.


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Post subject: Re: Blues Jr - Burnt Circuit Board
Posted: Sun Nov 27, 2011 7:30 am
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I wonder what happened to the OP?

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