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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Fri Oct 17, 2014 2:11 am
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BMW2002Ti wrote:
At least three caps out of entire bunch show signs of "pimples." All should be replaced.


Good luck! :D



+1

Check polarity !


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 18, 2014 12:43 pm
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I appreciate all the feedback given so far, and am looking for replacements. I saw on one website the recommendation as an alternate to the 70uF 350 VDC, would be to use an 80uF 450 VDC or 100uF 350 VDC. Any thoughts on alternate values for the 70uF, or even the 20uF 500VDC?

How would (or does) using alternate values affect the performance of the amp?

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 18, 2014 12:49 pm
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Fot filter caps ;

Most of the time , close value work ok , 80 MFD instead of 70 is ok

Watch for size more MFD and/ or higher voltage = bigger size


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 18, 2014 6:20 pm
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Lenny1716 wrote:
I appreciate all the feedback given so far, and am looking for replacements. I saw on one website the recommendation as an alternate to the 70uF 350 VDC, would be to use an 80uF 450 VDC or 100uF 350 VDC. Any thoughts on alternate values for the 70uF, or even the 20uF 500VDC?

How would (or does) using alternate values affect the performance of the amp?


I've used both F&T 80/450 and 100/350 cap in R&R'ing the main filter caps of these Fender amps. Both fit fine, under the doghouse. Use the 80/450. A bit more voltage rating will give you a little bit better life, under heated conditions.

Final capacitance of two 80/450 in first stage = 40mfd/900VDC.

Final capacitance of two 100/350 = 50mfd/700VDC.

Be sure to change the 20mfd/500VDC caps. And check the resistance of all resistors under the doghouse. Esp those two 220k-ohm, 2 watt resistors. You may want to change them out, for piece of mind. Use 5% tolerance, 220k-ohm, 2-watt metal oxide resistors.

Here's a photo of a Super Reverb main cap job, I did. Be sure to double-check the polarity of the caps. Note how the position of those two caps, in the first filter stage.


Good luck! Hope this solves the blown fuse issue. You may also want to check the bias, after the job is completed.


-----


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 18, 2014 9:06 pm
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BMW2002Ti wrote:
Lenny1716 wrote:
I appreciate all the feedback given so far, and am looking for replacements. I saw on one website the recommendation as an alternate to the 70uF 350 VDC, would be to use an 80uF 450 VDC or 100uF 350 VDC. Any thoughts on alternate values for the 70uF, or even the 20uF 500VDC?

How would (or does) using alternate values affect the performance of the amp?


I've used both F&T 80/450 and 100/350 cap in R&R'ing the main filter caps of these Fender amps. Both fit fine, under the doghouse. Use the 80/450. A bit more voltage rating will give you a little bit better life, under heated conditions.

Final capacitance of two 80/450 in first stage = 40mfd/900VDC.

Final capacitance of two 100/350 = 50mfd/700VDC.

Be sure to change the 20mfd/500VDC caps. And check the resistance of all resistors under the doghouse. Esp those two 220k-ohm, 2 watt resistors. You may want to change them out, for piece of mind. Use 5% tolerance, 220k-ohm, 2-watt metal oxide resistors.

Here's a photo of a Super Reverb main cap job, I did. Be sure to double-check the polarity of the caps. Note how the position of those two caps, in the first filter stage.


Good luck! Hope this solves the blown fuse issue. You may also want to check the bias, after the job is completed.


-----


Image


Is there a specific reason for the sleeving (insulation) on some of the leads. And again, thanks for all the information.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Mon Oct 20, 2014 6:49 am
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The leads with the Teflon insulation are (+). Just an old habit to help prevent shorting when placing the cover onto the caps.


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 23, 2014 1:38 pm
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I have ordered caps ((3) 20uF and (2) 70uF); however, upon measuring the forward bias of each diode in the two parallel branches at transformer TR1 output, one leg offers virtually no resistance. It would therefore seem, the real cause of the blown fuse.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 23, 2014 3:18 pm
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That is diodes should work , they conduct voltage/ current in one direction = 5 ohms

You reverse your meter probe ( polarity ) , diode stop voltage / current = high resistance .

All diodes are OK

It is not the cause of blowing fuse


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 23, 2014 4:31 pm
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Been watching this thread. I was going to suggest checking the diodes(but you guys seemed to be finding the problem). I've found this failure, a number of times in mid-sixties amps. You and Stratele52 are on the same page !!! A meter with a diode check positon is the easiest way to check each diode(but the resistance check also works). If it were me, I'd R&R all of the diodes on the rectifier and the bias boards. I'd also lift and check underneath the top wafer board and make sure the lower insulating board is doing it's job(no errant stalagtites)!!! For a Twin, I'd usually go with (2) hexfreds in each rectifier leg, but that's just a personal preference. BMW2000Ti should have a better suggestion for the diodes!!! JMHO, Art

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 23, 2014 6:42 pm
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I like UF4007 or UF5408 in the main power supply. And UF4007 in the bias power supply.

-----


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 23, 2014 8:45 pm
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Yep Yep!!! I'd go with Beemer's suggestion. The Freds aren't built for this set-up. So installation is a hassle and they are very sensitive to the soldeing heat. You'll want to use heat sinks no matter which diodes, you choose!!! Between the diodes, filter caps(bias cap), and substantial resistors in all of these sections.... the R&R should put the amp on solid footing!!! JMO,Art

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Fri Oct 24, 2014 1:51 am
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Lenny1716

If diodes where bad , fuse will blow before you put Standby switch in Play position.

If the amp must be in play position with Standby switch ( for blowing a fuse ) , circuit and parts before the Stanby switch are alright .

I write it before.

Diodes are before the standby switch


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Fri Oct 24, 2014 8:22 pm
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Lenny, I agree 100% with Stratele52 !!! It's tough trying to solve amp problems on a forum. Stratele did ask for a test without tubes and the amp in the "standby" mode. Test results??? I'm not sure that I'm interpreting the diode resistance test correctly. In the top(pictured) leg the readings are about 2meg ohm per diode and infinity with your meter probes reversed, right? And the lower leg readings are < 5 ohms ; and what are the readings when the meter probes are reversed on these diodes??? Don't mean to make more work for you. But, it is usually a good idea to identify the problem before throwing parts in the amp. Good Hunting!!! Art

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 1:08 am
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You know, the cost of diodes (even Fairchild or Vishay) ultra-fast are so cheap --- maybe just go ahead and change them. Rarely does OEM diodes goes bad. But, I haven't had any problems with new issued UF diodes. Gutiar or hi-if amps.

Just be sure to clip a lightweight heat sink (like alligator clip) ontot he diode you are soldering. Too much heat can damage them.


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 1:44 am
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Test show diodes look good , more chance to overheat new diodes and have one more trouble with the amp .

If fuse don't blow with amp switch to ON only. Diodes are OK .


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