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Thread: Fender Blues Deluxe Reissue bias doesn't match schematic, has intermittent jump in bias voltage

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    Fender Blues Deluxe Reissue bias doesn't match schematic, has intermittent jump in bias voltage

    Hi there-

    I have a Blues Deluxe Reissue from 2017 that has a strange problem in the negative bias circuit. I searched the forums and found a thread from 2012 where someone seemed to have a similar problem, but their's was constant not intermittent, and it wasn't totally clear if they resolved the issue. (https://music-electronics-forum.com/...es+deluxe+bias) Other than that I can find little-to-no info about this anywhere on the web.

    My problem is much like the one in the above referenced thread. I too see around -105V at pin 5 of the power tubes, but only intermittently. Always at start up, never cuts out or changes if it successfully turns on.

    I also have a negative bias circuit that does not match the most recent available schematic (2014, REV A, page 2 (https://www.thetubestore.com/lib/the...atic-Rev-A.pdf). I have contacted Fender in an attempt to see if they have a "REV B" or some circuit update, and the customer service rep told me he didn't have access to anything newer than REV A.)

    Specifically, the negative bias supply has C37 and D10, which should not be in the Deluxe. Also, some values are changed. C37 and C43 are 47uf/160v (should be 100uf/100v), R76 is 82K (1.5K in Deluxe, 10k in Deville), and R77 is 68K (should be 100K). R83 is not installed.

    It has the proper power transformer for the Deluxe with the two brown wires, no center tap.

    So, that finally brings me to my questions:

    1) does anyone know anything about this circuit change? Is this the new normal? If so, is there any sort of schematic/docs regarding it?

    2) anyone have any idea why this newer circuit might fail, permanently like the above referenced thread, or intermittently like the one I am looking at?

    3) anyone have any idea why this circuit might be better than the one on the schematic? And,

    4) is there any reason not to put it back to the wiring on the schematic? It seems to double, then divide the voltage. Any reason to just not double it?


    Thanks in advance for any insight anyone is able to offer.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by MichaelNuzum View Post
    ...... I too see around -105V at pin 5 of the power tubes, but only intermittently.......
    First, -105 is way to much negative voltage. Second, what does that statement mean? -105 is intermittent, then it settles to what? Or, bias is completely lost? Or, other? Can you better explain the intermittent part? What happens to bias? The fact that something is intermittent makes me think connection or solder joint problems.

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    Don't rewire it to match a different schematic.
    Can you post some pictures of the board, including the bias circuit area and any board number markings?

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    Yep, -105 is way too much, but it is what is generated at the junction of D15 and C43. Then dropped through R76 down towards the more normal range of -55-ish. It does that in the DeVille bias supply, but isn't supposed to for the Deluxe according to the schematic.

    The intermittent refers to, let's say every fourth or fifth time it is turned on, the power tubes see the full -105 at pin 5, so they never conduct, no sound. Turn it off. Next time, turns on just fine, sees around -45-ish at pin 5 like it should.

    Bias is never lost, it just is too much negative bias voltage so no current at bias probe point.

    I too suspected solder, but ran it several times and probed with chopstick on all components in the negative bias section, and never saw the voltage waver from physical manipulation.

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    g1- I don't have a good camera with me tonight. I will post pics tomorrow. From what I can tell, everything looks stock, original soldering, etc.

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    Supporting Member The Dude's Avatar
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    I'm wondering if you're losing circuit ground. Check ground connections. What does the filament winding AC do? Is it staying constant across P15 and P16?

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    Lifetime Member Enzo's Avatar
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    I would suspect that whatever configuration you have, aside from the glitch, the amp works fine, the bias is fine. SO the thing to do is repair the amp, not redesign it or modify it to some other circuit.

    Most manufacturers state that circuits can be changed at any time without notice.

    I doubt there is a problem specifically with the bias circuit, more likely connections. If the voltage leaps to -100, then it sounds to me like the voltage divider is losing its ground connection.

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    I would agree, it sounds to me like you're losing ground reference somewhere.

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    Don't forget the joker g1's Avatar
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    Sounds like this unit matches the Hot Rod, not the Blues.
    Here is Rev.B which shows the items you mentioned:
    Attached Files Attached Files

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    Lifetime Member Enzo's Avatar
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    DeVille/Deluze series - hot rod and blues - developed over time. You could follow them as they worked to consolidate the models. Using the same board for several models, just install different parts into them, or like the schematic, wire jumpers for one model but not the other etc.

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    Well, you guys are right, it seems to be an intermittent loss of ground!

    I pulled the board and was horrified by the condition of some of the solder joints. I have found several questionable joints around the bias circuit, the bias pot and R77, specifically. I removed the crappy looking solder with chip-quick and solder braid, then laid in fresh clean solder. Before, when it would see excessive negative voltage, I could trace voltage all the way to the bias pot, but nothing at R77. Now I see the proper negative voltage on the leg of R77 that ties to the bias pot.

    So now I have good negative bias voltage, but I have too low voltage on the low voltage (+-16v) section. The power dropper resistors (R78,R79) are seeing +-48 volts at input, but putting out something like +-2.6v at the output end.

    The weird thing is, all the components in the low voltage supply tie into the same ground as R77. All of them show good continuity to ground. It had fine low voltage power before I reflowed the solder joints, so it is something I have done! Kills me because I have been very careful with the ribbon cables, and I have pretty good solder skills, so I wasn't expecting to make anything worse. Man, this amp hates me!

    I will keep plugging away. Obviously I will re-check any joints that I hit, hopefully that's all it is. I will update when I figure it out.

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    Lifetime Member Enzo's Avatar
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    I of course have not seen it, but think of this:

    Voltages were OK before, but then you restored a ground, and now that allows some other defective part to conduct current to ground, dragging down the supplies. Maybe?

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    Enzo, I totally hear what you are saying, and will check out along those lines, thanks.

    The only rub I have with that logically is, I didn't restore a ground that was never there, I restored a ground that was usually there, and when it was there, everything worked fine. All I did (hopefully) was take a solder joint that was working 90% of the time and make it 100% (as far as the bias ground reference). When it was working that 90% of the time, so was everything else. Channel switching and everything that runs off the low voltage worked.

    I have to believe that as a result of my re-soldering, I messed something up. No solder blobs or connected traces or anything like that. As I mentioned I have decent soldering skills. Nothing sloppy or messy, but something went sideways.

    I checked all the joints I re-soldered and they all look very clean. Everything leading in and out from them has conductivity where it should. But the low voltage section is whacked.

    It is being fed proper voltage to test points 42 and 43. When I measure across R78 and R79 they both say they are dropping 46-ish volts. Then after the dropping resistors, I only see 2.5-ish volts. Weird part is, both diodes (D13,D14) and caps (C40, C41) show good conductivity to ground (measured around circuit board and at chassis).

    I'm going to give it a little more time tonight. Hopefully something will reveal itself.

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    What does your meter say on diode check across those zeners?
    There's not a lot running off those low voltage rails. 3 IC's and a couple other items. Easy enough to de-solder the IC supply pins (4 & 8) and see if the voltages come back up.

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    g1- I read .671 forward, .753 reverse, and .641 forward, .788 reverse on diode mode. Under power they drop 2.1 and 2.5 volts. They both have good ground continuity. I haven't pulled them to test out of circuit. Should I?

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    What baffles me is all I did was rehab a few mostly working solder joints. When they were working, so was everything else. Nothing was changed or modified, no parts removed.
    Click image for larger version. 

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    I re-flowed one leg of R76, both legs of R77, two of the legs for the bias pot, and one leg of R78. All have been checked for continuity to traces and components upstream and downstream, and they seem to check out. Being a double sided board I checked to see if there were any component side traces I might have missed, there are none.

    But, apparently in doing that, I somehow negatively affected a previously working low volt system. I don't understand how I might have taken out some other component downstream, but I admit, there is a lot I do not understand.

    I checked continuity on ribbon cables just to make sure one didn't go funny, but they check out.

    I am at a bit of a loss right now. Complete head-scratcher.

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    The zeners are suspect. Check them out of circuit and repost. FYI, you can just unsolder one leg of each and test. Also, there are other things in parallel with the zeners, so it could be something else loading the supply, but I would check the zeners out of circuit first anyway.

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    Hey all, had to go out of town for a week, but I am now back at it. I pulled one leg of one of the zener diodes and got .658 one way, open the other.

    I don't have benchtop power supply to test for voltage drop. I was hoping to get two fresh 9v batteries and make an 18v test supply. I would then wire the batteries, a 27 ohm resistor and the diode (2v drop/75mA test current = 26.66). Then I should see 16v drop across the diode. Did I do the math for that right?

    Is this a reasonable way to test, or is there a better way? Can one leg remain on the circuit board for the voltage drop test, or should it come fully out ? Thanks again.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by MichaelNuzum View Post
    What baffles me is all I did was rehab a few mostly working solder joints. When they were working, so was everything else. Nothing was changed or modified, no parts removed.
    Click image for larger version. 

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    I re-flowed one leg of R76, both legs of R77, two of the legs for the bias pot, and one leg of R78. All have been checked for continuity to traces and components upstream and downstream, and they seem to check out. Being a double sided board I checked to see if there were any component side traces I might have missed, there are none.

    But, apparently in doing that, I somehow negatively affected a previously working low volt system. I don't understand how I might have taken out some other component downstream, but I admit, there is a lot I do not understand.

    I checked continuity on ribbon cables just to make sure one didn't go funny, but they check out.

    I am at a bit of a loss right now. Complete head-scratcher.
    Looks like the one Zener nearest the chassis mtg hole isn't soldered well to Ground. Over time, the resistor leads on those 5W bathtub resistors AND the zener diode terminals become quite oxidized. I've had to remove them, scrape away the oxidation to get to fresh metal tin them and then reinstall them. Your PCB looks to be in excellent shape....not seeing the usual long-term discoloration from the 5W dropping resistors. Look at an equivalent Hot Rod series PCB, and you'll appreciate what you have here a lot more.....PCB quality on the Hot Rod series leaves a lot to be desired.

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    Well, it's not the zeners. I broke down and purchased a bench top power supply. I put one of the zeners on it, and it dropped 16.16V. I suspect the other will be similar.

    The way the power supply is dropping symmetrically makes me suspect one of the ic's. So far, I pulled U1 and no change. There are three others, I'll see what happens there. Hopefully pulling one of them will make a difference. At least help me isolate it to one leg of the system vs another.

    I really don't like doing it this way, but I am out of ideas at this point.

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    Supporting Member The Dude's Avatar
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    Unfortunately, when there's a short on a supply line, not much else you can do except unsolder connections until you find it. PITA, but sometimes necessary.

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    Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!

    So now I have to figure out if it is the ic or something else in that leg the circuit. I just went ahead and ordered replacement ic's so I have a good new one to throw in there. Hopefully that will fix it and let it be happy once again.

    Thanks again for all the help, y'all.

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    I am still left wondering what the heck happened? I am careful to discharge the power caps before I work, I use a solder station that claims to be esd safe, and I never arced or sparked or noticed any sort of shorting. How in the world did I take out the reverb drive circuit?

    I am someone who likes full knowledge of their mistakes to learn and avoid a repeat. I am big enough to admit when I am at fault, acknowledge what I did incorrectly and grow from there. But with this one, while I have to believe I am at fault for the failure, I can not for the life of me, figure out what I did incorrectly. Hard to learn from your mistakes when you are not sure what the failure action was. Kinda frustrating.

    Anyway, thanks again for all the help and logic. I do this by myself out in the middle of nowhere and have no local resources/peers here to bounce ideas off of. You all provide invaluable feedback and ideas and I am truly grateful for folks like you that feel inspired to share your knowledge. Much appreciated.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by MichaelNuzum View Post
    Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!

    So now I have to figure out if it is the ic or something else in that leg the circuit. I just went ahead and ordered replacement ic's so I have a good new one to throw in there. Hopefully that will fix it and let it be happy once again.
    I hope you're installing sockets for those IC's, in case you ever need to change one again.

    Murphy's law, part 672 subparagraph "n," states, if you don't socket, you WILL have to unsolder that IC again sometime. OTOH, if you use a socket, that IC will never go bad again. Most of the time...

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    Quote Originally Posted by MichaelNuzum View Post
    Yep Dude, total PITA. I unsoldered all four ic's, and wouldn't you know it, the last one finally brought the voltage back up. Four ic's x 8 legs each = 32 solder joints (really 64, it's a double sided board), and not one lifted pad!! Told you my solder skills were pretty decent!
    Good job, but a tip for future reference:
    since you are only concerned with removing the short from the supply rails, you only need to desolder the power pins on those IC's, so 8 connections in total. Double sided is more tricky, but you can do resistance checks to make sure the pin is disconnected.
    If you do them one at a time, you probably won't have to do all 4 unless it turns out the last one is the bad one (usually my luck ).
    Sometimes, when the unit is much more complex, you end up cutting traces to isolate sections, then reconnect them later. It is not ideal, but if it ends up saving hours of labour, it ends up being in the customers best interest.

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    Hey g1- thanks. You know, I was curious about that. I wasn't sure if just un-soldering the power feed leg would work or not. I was concerned that because it is a double sided board with through-plated holes, just removing the solder might not break the connection (and I was hoping to avoid cutting traces). So, I erred on the side of being certain. Maybe too much work, but with this amp's failure being so confusing for me, I wanted to be absolutely sure I found the fault.

    And yeah, of course, it was the last of four ic's that was the guilty party! Oh well.

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    Just out of curiosity, did you read any voltages on the pins of the ICs before removing them? Did any of them seem to be getting warm?

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    52 Bill - I just checked that the ics were seeing the low voltage. I didn't think to check the temp. I just recently got an IR laser thermometer gun, it seems fairly sensitive. I have honestly only used it as a quick way to look at power tubes to make sure they are all conducting. Would an over-consuming ic heat up enough to show up on something like that?

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