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Post subject: FENDER BLUES DEVILLE 4X10 PROBLEMS NEED HELP PLEASE!!!
Posted: Fri Oct 15, 2010 7:25 pm
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So I bought a MIM Fender blues deville 4x10 off of craigslist for a great deal a couple of weeks back. The groove tubes inside started sounding pretty bad (lot of rattling) so I decided to replace them with 2 electro-harmonix 6l6eh hard duet power tubes and 3 tungsol 12ax7 preamp tubes which I heard had less rattle and low microphonics. I put them in and everything was sounding ok but I decided that I would try to bias the tubes.

I grabbed my multimeter and put the ground on the chasis and the test point on the circuit board near the first 6l6 tube. I pretty much followed this how to on youtube http://www.youtube.comwatch?v=19IUadZJf-c Moving the bias pot clockwise would only reach a reading of 52. I heard the bias should be from 60-80. Anyway I couldn't get mine to 60.

So after playing around with the blue bias pot to double check that it wasn't reaching 60, upon turning the amp on I saw a dial/resistor (R71) start sparking directly above the two 350v capacitors. I immediately turned the amp off; I hope it didn't do too much damage. Now I am afraid to turn the amp back on and I can see some residue on the circuit board from where the resistor was sparking.

I thought it might be some kind of loose soldering but I couldn't find anything on either side of the circuit board. I looked at the schematics here on fender.com but the schematic on the blues deville looked different than my circuit board, mine was similar to a fender hot rod deville except for the more gain switch and some other things.

Were the tubes I put in bad for this amp? I grabbed the hard 6l6 tubes because I wanted more headroom for fender's beautiful cleans.

Here is a picture of my circuit board followed by a close up of where the dial/resistor started sparking. (luckily I had the circuit board exposed when I turned the amp on or else I wouldn't have noticed the spark and probably would have kept the amp on longer frying the rest of my board).


Any thoughts or ideas anybody? It will be greatly appreciated! I don't have too much money to take it to a tech nor is there a good music store or an amp repair guy around here :/. Maybe the dail resistor (R71) needs to be replaced?


Image

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Image

Thank you! :)


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Post subject:
Posted: Fri Oct 15, 2010 7:38 pm
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http://s45.photobucket.com/albums/f89/s ... C00618.jpg

http://s45.photobucket.com/albums/f89/s ... C00617.jpg

http://s45.photobucket.com/albums/f89/s ... C00616.jpg

Me think it's repair shop time.



http://www.bpesolutions.com/atechnical/ResistorQV.pdf


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


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Post subject:
Posted: Fri Oct 15, 2010 8:12 pm
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Thanks for the links! Ya I'm going to probably have to take it to a tech but that's my last case scenario :?


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Posted: Fri Oct 15, 2010 8:19 pm
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Hi mydeadblues,

Looking at the schematics for both, one of the differences between the Hot Rod Deville and the Blues Deville may be that the BDvl has two test points and the HRDvl has only one. If that is the case, you should only have about 35 mV at either test point (not 70). So you might have red plated your tubes and/or burnt a resistor or two on the circuit board, if you went to 52 mV on one test point. Maybe even burnt a trace on the circuit board. Check your amp closely and see if you have 2 of the 1 ohm resistors, 1 for each test point. Check the traces closely, for burnt and also cracked at the solder joints, especially around both 5-watt resistors by the bias adjust.

I couldn't see any damage on the pictures, not sure where you saw damage, or was it just sparks. R71 may be in the B+ power supply, which could also indicate that the tubes were red plating. Just some things to check. Nothing wrong with the type of tubes that you used. :idea:

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Posted: Fri Oct 15, 2010 9:05 pm
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Thanks for the reply!

I'm trying to look through this thing with a fine comb but my eyes aren't trained too well for this (at least not yet). The tubes may have been red plating how do I know if the BDvl has 2 or 1 test points? I don't think I did any damage to the circuit board yet I saw the (R71) spark only and turned the amp off immediately so hopefully no damage was done. The two white 5-watt resistors by the bias control look to be intact (no disconnection or discoloring from burning/sparking). I also checked soldering points throughout the back of the circuit board for connection and they all seem to be intact as well. Should I try turning the bias control down and turn the amp back on? What do you mean by the r71 is in the b+ power supply?


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Post subject: BDvl problem
Posted: Fri Oct 15, 2010 9:53 pm
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Well it seems that the posted schematics are wrong for the BDvl, so I thought that it might have 2 test points. The test point on the HRDvl is a 1-ohm resistor that is common to both tubes. A mV reading at that test point would be divided by the 2 tubes. Some amps have 2 test points, so there will be 2 of the 1-ohm resistors, 1 for each tube. In the case of 2 test points, you would only read half of the total mV at each test point. Look at the resistor on the other side of the test point that you used on the circuit board (1-ohm), and see if there is another one for the other tube. There may only be one, I don't know. I didn't watch the youtube video, was your test point on the smaller circuit board near one of the output tubes? Was the video for the HRDvl or the BDvl?

The R71 on the HRDvl schematic is in the power supply circuit for the tubes (B+). If the tubes red plated, they could have caused damage in the power supply circuit. It could also be a bad bias adjust pot. If that pot goes bad it can also cause the tubes to redplate. Usually components that spark are suspect, they don't usually spark and survive. If you can pull the R71 resistor and check its value, and also pull the bias pot and check it closely. If they are good, put them back in and resolder.

You can pull both output tubes and then turn on the amp and standby switches, and check the C- voltage and see if it varies smoothly through the entire travel of the bias adjust pot (check DC volts from wiper of bias pot to ground). The HRDvl schematic indicates a factory setting of around -54.6 VDC at C-. I just can't imagine that you had sparks and nothing was damaged. I would be leary of trying the same output tubes, if everything else checks out OK.

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Posted: Sat Oct 16, 2010 1:22 pm
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Ya I was thinking that the BDvl schematic was wrong. I looked closely at the smaller circuit board and after looking at the HDvl schematic, the BDvl also has two test points. After closer inspection I may have been testing at the wrong test point. I think I was testing at the smaller resistor next to it :shock: The video was for the HDvl and I used the same test point he was using in the video.

So I should pull the output tubes and use the same test point when testing the c- voltage?

After that should I try the amp with different output tubes?


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Post subject: Re: BDvl problem
Posted: Sat Oct 16, 2010 1:45 pm
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shimmilou wrote:
Well it seems that the posted schematics are wrong for the BDvl, so I thought that it might have 2 test points. The test point on the HRDvl is a 1-ohm resistor that is common to both tubes. A mV reading at that test point would be divided by the 2 tubes. Some amps have 2 test points, so there will be 2 of the 1-ohm resistors, 1 for each tube. In the case of 2 test points, you would only read half of the total mV at each test point. Look at the resistor on the other side of the test point that you used on the circuit board (1-ohm), and see if there is another one for the other tube. There may only be one, I don't know. I didn't watch the youtube video, was your test point on the smaller circuit board near one of the output tubes? Was the video for the HRDvl or the BDvl?

The R71 on the HRDvl schematic is in the power supply circuit for the tubes (B+). If the tubes red plated, they could have caused damage in the power supply circuit. It could also be a bad bias adjust pot. If that pot goes bad it can also cause the tubes to redplate. Usually components that spark are suspect, they don't usually spark and survive. If you can pull the R71 resistor and check its value, and also pull the bias pot and check it closely. If they are good, put them back in and resolder.

You can pull both output tubes and then turn on the amp and standby switches, and check the C- voltage and see if it varies smoothly through the entire travel of the bias adjust pot (check DC volts from wiper of bias pot to ground). The HRDvl schematic indicates a factory setting of around -54.6 VDC at C-. I just can't imagine that you had sparks and nothing was damaged. I would be leary of trying the same output tubes, if everything else checks out OK.



Though I am not looking at the schematics now , I thought there was something fishy about it. Are there any differences between the original and Reissue?


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Post subject:
Posted: Sat Oct 16, 2010 2:17 pm
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R71 in my deluve schematic refers to //resistor to C32. There is no reason to damage it even if you overbias the amp. In the picture I saw a kind of small black hole at the pin to body junction of the resitor. This is a known failure mode for such a resistor, often due to mechanical dilatation due to thermal effect causing arcs between both parts.
But the bias is not normal and I don't understand because in the schamatic there is only 1 one-Ohm-resistor for bias measurement.

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Posted: Sat Oct 16, 2010 2:22 pm
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Thanks Tissan do you think R71 just needs to be replaced?


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Posted: Sat Oct 16, 2010 2:50 pm
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A check of the supply by a tech sounds better to me. In this amp there is 450V DC that can be fatal for human. These resistors help to balance the voltage on the supply capsand and discharge them when you swith off. Take care.

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Post subject:
Posted: Sat Oct 16, 2010 5:30 pm
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Sorry about your amp problem.

I tell people all the time to leave the bias alone.

If the amp is working right, don't mess with the bias, you're likely to mess things up. Replacing tubes doesn't mean you have to change the bias. If everything was working right, before you changed the tubes it should be in the proper range and okay. There is no magic number setting, but there is a operating range.

Why did you think you had to adjust the bias?

Now I bet you wish you had left it alone.

You have to know the plate voltage to know where to set the bias voltage.

There is a reason so many high end amps don't even have a bias pot.

Anyway now I think you better seek a good amp tech and have him check things out.

Hope it doesn't cost you much.


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Posted: Sat Oct 16, 2010 6:38 pm
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Ya I'm kicking myself for touching it now. I was browsing through some videos on microphonic problems and saw this video on biasing a deville. It looked easy enough in the video (step by step and everything). Assuming that the hot rod and blues deville had the same bias adjustment range was my fault and now I have to pay for my mistake. I'm hoping it won't cost me more than $100.


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Posted: Sat Oct 16, 2010 8:50 pm
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Hey man, I can't see a problem with a person getting their fingers dirty trying to work on their amp ! As long as they don't electrocute themselves ! The amp didn't burst into flames and burn the house down, right ? I started out the same way, curiousity and necessity. I've learned more from failures than successes. I've followed this thread, but the lack of an accurate schematic, kept my mouth shut. This is a real problem, if you or others want to understand your amp. A contact with the Fender customer support dept, might be in order. At this point, I 'm not sure that you caused the problem. Art

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Posted: Sun Oct 17, 2010 12:18 am
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Hey man thanks! Ya I was definitely curious and I do want to learn more about the electrical circuitry so I dove right in ha. I'm hoping I'll find an amp tech soon and I'll ask him if I can overlook as he diagnosis the problem. Hopefully I can get a basic understanding and develop my knowledge of the circuitry. I am also going to e-mail fender and ask for an updated schematic or for the reissue schematic because it differs from my amp.


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