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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 11:47 am
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Once again, thanks to all for your feedback. As part of my effort to isolate the potential source of the blowing fuse(s), with both 6L6's tubes (still) removed and the Standby switch in "standby" mode, I applied power, On-Off switch, "On". The 2-amp slo-blo fuse does blow. I also have recorded the measurements taken across the diodes in the "suspect" branch reversing the meter leads to bias the diodes in each direction. The result is noted in the below photo.

Image

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 1:32 pm
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Sorry I may not understand your fist diodes ohms reading :oops:

Your diode look bad ,

BMW002Ti is right , replace them all .
My apologies BMW :oops:


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 3:07 pm
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stratele52 wrote:
Sorry I may not understand your fist diodes ohms reading :oops:

Your diode look bad ,

BMW002Ti is right , replace them all .
My apologies BMW :oops:

What I was attempting to show in the annotated picture, was the resulting reverse and forward bias measurements of each of the three diodes with the multimeter set on RX1 scale. The first diode had a reverse bias measurement of 20-ohms, and forward bias measurement of 9-ohms. The second and third diodes each had the same reverse bias (4-ohms), and forward bias (3-oums) readings. Though the absolute ohmic value is not relevent, it is presumed to demonstrate that when reverse biased, none provide sufficient opposition to current flow, resulting in a short condition.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 3:47 pm
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Lenny1716 wrote:
stratele52 wrote:
Sorry I may not understand your fist diodes ohms reading :oops:

Your diode look bad ,

BMW002Ti is right , replace them all .
My apologies BMW :oops:

What I was attempting to show in the annotated picture, was the resulting reverse and forward bias measurements of each of the three diodes with the multimeter set on RX1 scale. The first diode had a reverse bias measurement of 20-ohms, and forward bias measurement of 9-ohms. The second and third diodes each had the same reverse bias (4-ohms), and forward bias (3-oums) readings. Though the absolute ohmic value is not relevent, it is presumed to demonstrate that when reverse biased, none provide sufficient opposition to current flow, resulting in a short condition.




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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sat Oct 25, 2014 7:33 pm
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Yeah, Lenny, you had the culprit nailed, sometime ago!!! Good job !!! Sorry for making you go through the hassle of convincing us with the extra tests. But, that's just the way it is, on these forums. Thanks for the indulgence. BTW I also have a '67 Band Master(since 1980) that has seen many gigs in a Blues band. Great amps!!! Art

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sun Oct 26, 2014 11:09 am
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aclempoppi wrote:
Sorry for making you go through the hassle of convincing us with the extra tests. But, that's just the way it is, on these forums. Thanks for the indulgence. BTW I also have a '67 Band Master(since 1980) that has seen many gigs in a Blues band. Great amps!!! Art

NO problem at all.. :D ...that's my reason for the post and value of this forum. I did not want to jump to any conclusion and appreciated the scrutiny. Originally, I had not considered the diodes to be suspect. So, I will be replacing the caps and diodes. I may also replace the resistors associated with these parts, though I have measured each, and found them to be OK. I will post my progress or results once I have all the parts in hand and installed. And BTW too, I have my '62 Concert that keeps me company while the Bandmaster is in the repair mode.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Sun Oct 26, 2014 5:05 pm
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stratele52 wrote:
Sorry I may not understand your fist diodes ohms reading :oops:

Your diode look bad ,

BMW002Ti is right , replace them all .
My apologies BMW :oops:



No problems, at all! Stratele. :mrgreen: :D



BTW... Lenny, I like the ultra-fast diodes. Quieter operation and only a few pennies more, than plain 1N4007 or 1N5408. I like using UF5408 in the main power supply. They are near equivalent to what Fender used (usually RCA made BY255, which are no longer available from reputable manufacturers).

Both BY255 and UF5408 are rated for 3-amps forward current. BY255 at 1300 reverse voltage. UF5408 at 1000 reverse voltage. Both over-rated for the application in this amp --- to insure reliable, long life. Try to get Fairchild or Vishay from a reputable source. Many reports of cheap Chinese made counterfeits on the Net, esp EBAY --- which are not made to specs.

Use the UF4007 in the bias supply. Where constant voltage is around 40-50VDC. And current in the tens to low 100's of milliamps. Overkill again, to help insure long trouble-free bias voltage --- very, very important for long 6L6GC life.

You prolly already know this... replace the cap(s) in the bias supply, too! F&T's 100mfd/100VDC is a fine single replacement item. Note polarity!


Good luck on your project! :D


http://www.mouser.com/ProductDetail/Vis ... quNRV/YA==


-----


Redone main power and bias power board in a 1968 Bassman.



Image


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Mon Oct 27, 2014 10:05 am
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BMW2002Ti wrote:
stratele52 wrote:
Sorry I may not understand your fist diodes ohms reading :oops:

Your diode look bad ,

BMW002Ti is right , replace them all .
My apologies BMW :oops:



No problems, at all! Stratele. :mrgreen: :D



BTW... Lenny, I like the ultra-fast diodes. Quieter operation and only a few pennies more, than plain 1N4007 or 1N5408. I like using UF5408 in the main power supply. They are near equivalent to what Fender used (usually RCA made BY255, which are no longer available from reputable manufacturers).

Both BY255 and UF5408 are rated for 3-amps forward current. BY255 at 1300 reverse voltage. UF5408 at 1000 reverse voltage. Both over-rated for the application in this amp --- to insure reliable, long life. Try to get Fairchild or Vishay from a reputable source. Many reports of cheap Chinese made counterfeits on the Net, esp EBAY --- which are not made to specs.

Use the UF4007 in the bias supply. Where constant voltage is around 40-50VDC. And current in the tens to low 100's of milliamps. Overkill again, to help insure long trouble-free bias voltage --- very, very important for long 6L6GC life.

You prolly already know this... replace the cap(s) in the bias supply, too! F&T's 100mfd/100VDC is a fine single replacement item. Note polarity!


Good luck on your project! :D


http://www.mouser.com/ProductDetail/Vis ... quNRV/YA==


-----


Redone main power and bias power board in a 1968 Bassman.



Image


Based on recommendatios, I have ordered the UF5408 and UF4007 type, from Newark.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Mon Oct 27, 2014 6:59 pm
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Good luck! Keep us informed on your progress. :D


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Tue Oct 28, 2014 9:42 am
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Caps done, just waiting for the diodes.....
Image

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Tue Oct 28, 2014 10:39 am
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Good job! For the diode board --- I feel it's best to remove the leads and work on the board, away from the rest of the amp. Better room. Just keep note of where the lines attach.

You may need to open the eyelets for those UF5408 (depending on the diode manufacturer). Fairchild uses big leads. Just use a small Dremel bit at slower speeds on the brass eyelets. Put in all new components, first. Clean & treat with flux. The solder will flow easily on those heat sensitive solid-state devices.


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Tue Oct 28, 2014 11:08 am
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BMW2002Ti wrote:
Good job! For the diode board --- I feel it's best to remove the leads and work on the board, away from the rest of the amp. Better room. Just keep note of where the lines attach.

You may need to open the eyelets for those UF5408 (depending on the diode manufacturer). Fairchild uses big leads. Just use a small Dremel bit at slower speeds on the brass eyelets. Put in all new components, first. Clean & treat with flux. The solder will flow easily on those heat sensitive solid-state devices.


Yes, I was concerned about the tight quarters and had given thought to removing the board too. I also noticed how large the lead diameter appeared in the photo posted of the reworked board. Will see what the situation is when I get my parts. Thanks for the info.

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Wed Oct 29, 2014 8:23 pm
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DONE. Received and installed the (6) UF5408's and (1) UF4007. Yes, accident with soldering iron coming in contact with the wire insulation, required trimming reattaching. Reassembled amp head and powered up...it work. Thanks to all for the information that enabled me to accomplish this. Special thanks to BMW, for the details regarding the diode installation, especially, opening up the eyelets to deal with the large UF5408 leads.

Image

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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 30, 2014 5:50 am
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Very good work! One "solder hickey" permitted. :lol:

You may want to check the idle bias current passing on the 6L6GC's. Recheck the plate voltage, too.

Enjoy! :mrgreen:


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Post subject: Re: Blowing Fuse - What's the Problem?
Posted: Thu Oct 30, 2014 4:06 pm
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BMW2002Ti wrote:
You may want to check the idle bias current passing on the 6L6GC's. Recheck the plate voltage, too.

Enjoy! :mrgreen:


Something I have never done before... :( ..but probably need to do.

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