Results 1 to 21 of 21

Thread: Fender Auto bias circuit

  1. #1
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10

    Fender Auto bias circuit

    A pretty new Fender Bassman 100T bass amp came to me with their Auto bias circuit, which I have never seen. It allows the user to step between cool, normal, and warm bias setting, and it biases each individual tube separately on an ongoing basis. It has four LEDs, one for each tube, green is good, red is bad, and orange means that tube in the pair with the bad one is shut off. When it came to me, one pair was lit red and orange, and the amp didn't have full power. While I was plaing it, it fizzled out to nothing, which was the complaint. Indeed I discovered all four LEDs were either red or orange. It has the original tubes in it, so wanting to determine if the tubes were actually bad, or if the auto bias circuit was at fault, we opted to start with a fresh quad of 6L6s.

    While I am waiting for that order to arrive I decided to take a look at the circuit, and came to an impasse. I see were raw C- comes in, and I see the white arrows for bias V 4-7, but I don't understand how this circuit works. Does anyone have any experience or insight with this?

    I suspect a problem with the circuit because I swapped the tube that showed a fault with one that showed green, and after several minutes of all 4 showing green, it went back to showing a fault in the same socket as before, and the tube that had showed red is now showing green on the socket it got swapped to.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  2. #2
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,380
    Thumbs Up/Down
    Received: 2,032/24
    Given: 5,182/11
    Rep Power
    24
    If you have the schematic, please post it.

    0 Not allowed! Not allowed!
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  3. #3
    Supporting Member nevetslab's Avatar
    Join Date
    Apr 2010
    Location
    Burbank, CA
    Posts
    1,993
    Thumbs Up/Down
    Received: 732/1
    Given: 1,734/1
    Rep Power
    11
    Quote Originally Posted by g1 View Post
    If you have the schematic, please post it.
    I have a Super Sonic schematic that has that Automatic Bias circuit

    Super-Sonic_100_2011_schematic_Rev-B.pdf

    0 Not allowed! Not allowed!
    Logic is an organized way of going wrong with confidence

  4. #4
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    "If you have the schematic, please post it."

    Sorry, I thought I had done so.

    http://www.fmicassets.com/Damroot/Or..._schematic.pdf

    1 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  5. #5
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    751
    Thumbs Up/Down
    Received: 85/0
    Given: 4/0
    Rep Power
    9
    Randall,
    P23 obviously feeds the autobias circuitry but not shown on your posted schematic.


    From the Super_Sonic schematics (assuming they are the same) then the cathode voltages (from the current sense resistors) feed some Opamp circuits which generated sum an difference voltages per output tube push pull pair. These feed analog to Digital Converter inputs on a microcontroller.
    The microcontroller uses Pulse Width Modulation (PWM) outputs (one for each tube) to control the bias. These would be adjusted by the software to get the sum to a selected value and the difference to zero, and set the LED color for that tube appropriately.

    I would suspect one or more bad 6L6s but be sure to check that the current sense resistors (shown as Fp1) ?Fire proof 1 Ohm?, the grid "fuse" resistors (shown as Fr22) ?fire resistant 22 Ohms? and the 470 Ohm screen resistors.


    Cheers,
    Ian

    0 Not allowed! Not allowed!

  6. #6
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,074
    Thumbs Up/Down
    Received: 609/3
    Given: 314/0
    Rep Power
    28
    Unfortunately Fender did not include the bias board.
    P23 and the associated circuitry does appear similar to the Super Sonic 100 P11 connector.

    Looks to me as if the whole ball of wax is microprocessor controlled.

    Fender auto bias circuit.pdf

    1 Not allowed! Not allowed!

  7. #7
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    All the suggested resistors check OK, except for one small issue. The 470R 5 watt 10% screen resistor on V6 measures 456R, well within tolerance, but the farthest away from 470R than the other three. It is also the tube socket that is showing a failed tube with a red LED. I just put in a fresh matched quad of JJ 6L6. The LEDs are indicating the same as with the originl tubes. This is looking like a microprocessor board swap to me at this point, I have no chance of fixing that.

    edit: I have some more data on this. I believe all the components around V6 are good. But I have -44v bias on V6 and -53v on the other three, which are at ~30mA. V6 is at ~ 60mA. Plate and Screen voltages are the same for all four tubes. After a time, the auto bias puts -76v bias on V6 and it mate V5, turning them off.

    SO, which is it, do I have one tube drawing too much current due to another issue, and auto bias is catching this and turning that pair off, or is the auto bias responsible for the lower bias voltage at V6?

    The bias voltages are this way in standby, so I am working in this mode for now. I have -80v RAW C- at one side of R265 and -44v on the other side where BIAS V6 connects. I find that if I measure the bias voltage in standby at V6 pin 5 it is -44v, but if I unplug power to the amp it jumps up to -60v, where the other three are also at, and slowly fades down along with them. So, this suggests to me that auto bias is regulating the bias to V6 incorrectly, and then shutting it down when it detects a too high current draw.

    The question is, how to remedy this without rolling the dice on a replacement smd processor board?

    0 Not allowed! Not allowed!
    Last edited by Randall; 03-01-2019 at 11:50 PM.
    It's weird, because it WAS working fine.....

  8. #8
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    After chasing this for a while, I did a reset on the bias board (which, btw I had done before), by pressing both up/down arrows for 3 - 5 secs, and the amp has now stabilized. All green LEDs with V6 pullling the same current as the others, and the bias voltage being the same as well. So, I guess this is not the most reliable circuit. I know I don't care for it. I would not be surprised based on what I have read, for this to happen again. Unfortunately, this will be one of those cases where I will have to tell him there is not much else I can do for him, unless he can get Fender to send him a new board to try. I have read of several cases where Factory authorised shops tried several repairs only to ending up replacing the amps, so Fender is certainly aware of this.

    edit: It ran all green for two hours, so I powered it down and let it cool off. When I powered it back up, after about 10 minutes it indicated V5 red and V6 orange, the opposite of before. So, that points to the microprocessor board.

    0 Not allowed! Not allowed!
    Last edited by Randall; 03-02-2019 at 06:49 PM.
    It's weird, because it WAS working fine.....

  9. #9
    Twobie
    Join Date
    Jan 2018
    Posts
    2
    Thumbs Up/Down
    Received: 7/0
    Given: 0/0
    Rep Power
    0

    Fender Auto Bias - Bassman 100T - Issues RESOLVED

    Hello all - I realize this is an old thread but I have found a solution to the autobias issues related to the Fender Series of Amplifiers with this feature. The amp model that came to my shop was a Fender 100T Bassman - an excellent sounding amp and well designed WHEN IT WORKED. My customer bought this amp second hand - so out of warranty (no original receipt & past warranty expiration anyway). I mention this because my experience dealing with Fender Tech support was not at all good - I am not an authorized service center so I didn't expect the best support but the support I did receive was awful. In essence, exploring my options - I was told I could buy the bias board assembly if need be and to my surprise it was listed on the Fender website parts list @ what I thought was a reasonable price of around $60. But when I tried to put it in my checkout basket message popped up that it was no longer available - after several calls and emails back and forth I was told that they removed it from on-line parts and ONLY authorized service centers could purchase it - when I asked for some help determining if it was in fact the cause of the problem (had my voltages and component readings all ready) they wouldn't help me - and when I asked what they did with others who had this problem which is common - they told me that if the amp was in warranty they would just replace it if not customer was out of luck. More to come....

    Anyway - the problem was similar to what is described here - one of 4 6L6s would bias up then eventually most times within 5 minutes - one of that pair would turn LED orange, then entire pair would fail. Swapping know good tubes would not fix problem moving suspect "bad" tubes to other sockets did not have problem follow tube - it stay with socket - I spent hours monitoring voltages and checking every component (except ICs on bias board) in the bias circuit - most of the resistors that are feeding the bias control circuit are high quality precision resistors and were for the most-part well within tolerances for that component.

    Resetting the bias-board cleared the problem - the first time it cleared after I re-tubed the amp - I thought that the board was just glitched and that this corrected the issue - I let the amp burn in (outside the case) for 3 days straight - last 36 hours with 1K tone running through the amp / speaker cab just to be sure - then powered up & down several times - ran another 24 hours and called customer to pick up amp. After putting amp back in cabinet and having customer test amp while he was there picking it up - it failed - very embarrassing indeed!!! - What changed putting it back in cabinet????? First thing I thought was heat - and why wasn't this cooling fan ever turning on was this the problem. - After reading temps with infrared HQ temp gun - no... Fan I later reasoned out was not for general cooling of the amp - it was for the big power resistors which are only used when amp is in silent mode (headphone or pugging head into mixing board) - that's when these large power resistors are used to simulate speaker load - the sensor did work at the rated temperature (tested that with heat gun and temp reader - & it operated as designed) - although as an added bonus I installed an LED lit 2 way mini toggle so that customer can bypass thermostat and power fan on anytime - in the off position is reverted back to factory spec using thermostat (dual color LED - Red =on, blue = factory/off).

    So what was problem - well there are these ribbon cables connected to the bias daughterboard and main motherboard providing the PCBs operating voltages as well as the voltages from the cathodes of each output tube and to the output tube connection where the negative bias voltage would be applied to bias the tube (based on the cathode current of that tube in idle). I noticed that the cabinet gently pushed that cable down - so I started moving the cable around while monitoring the tube bias LEDs - (BTW Fender used cable ties on these ribbon cables crunching them and causing tension on the connectors - these are computer grade cables that are not designed for that kind of abuse) - low and behold if the cable was moved or squeezed the amp would lose bias or bias voltage causing one or more of the bias LEDs to go crazy. So I called Fender for a part number so I could replace the cable - again they said they couldn't or didn't know or it came as part of a kit with the board - no help at all - after some trial and error and educating my self on the different type of ribbon cables (pitch, pin count and the various different connector head sizes and types) I found only one supplier that had the exact replacement for this cable - MOUSER (note that the cable connector on one end is standard but on the other end is a low profile head - two low profile heads will work). I am certain that considering the number of failures with this bias circuit that it was either a bad batch of ribbon cables or it was the poor installation method used (cable ties that literally pulled one or more wires off the head especially those wires on either end by taking a flat cable and making it round with a too tight cable tie. I verified the fix 100% and after 3 months - customer uses his Bassman 100T almost everyday with no return of the bias fail problem. Mouser Part # 200-TCSD10D05.0001N Mfg. Part # - Type IDC Ribbon Cable - TCSD-10-D-05.00-01-N.

    NOW KEEP IN MIND - this was determined after traditional amp tech troubleshooting - eliminating possible other causes - like loose or dirty output tube socket pins, loose or broken output tube solder connections to board, all resistors / caps from all pins on output tubes checked for continuity, values, bad or broken solder joints, all other cables ribbon and other types criss-crossing the board - sockets, pins and loose connections. After all that using the "chop-stick" test - SAFETY FIRST - with amp out of chassis and speaker load connected - power on and using a non-conductive probe (e.g. wood chopstick) monitor voltages listen to speaker output - touch wires, connectors apply gentle pressure, touch key components in output tube circuit feeding bias board to see if anything is loose or intermittent -

    If the ribbon cable is proven to be suspect turn off power and discharge filter caps - carefully remove glue used to hold cable head on motherboard may need to use an exacto knife or similar - CAREFUL not to cut or damage circuit board or other components / wires - IU found problem to be in ribbon cable head connected to motherboard - the one connected to the bias board was OK. No need to remove either the bias board or the motherboard to replace cable - clean area around connector where u needed to remove glue install new cable gently don't bend pins - and then test amp before gluing down ribbon cable. I use E-6000 for gluing components to PCBs (e.g. capacitors etc..) and also for loose fitting cables like these ribbon cables - they do not have any kind of locking mechanism and must be secured using glue. You can use silicone (softer, longer cure, not as strong) or this self leveling E-6000 - very strong use just a small amount - ensure cable head is fully seated all around and perfectly flat to the board not cockeyed - this stuff dries quickly and the odor is strong but completely gone after it cures for about 24 hours. I don't recall if both ends were glued or just the connector on the motherboard (which required that mini head to fit) Just base this decision on how old cable is glued.

    Besides the fan bypass on toggle DPDT w/3mm dual color LED mod, and securing cables in a non-damaging way - the only other mod I Strongly recommend for this amp is changing the lobster claw tube retainers for the 4 output tubes with the spring type Marshall style tube retainers because they hold those upside down heavy tubes better and keep them from shifting especially if you are a working musician moving the amp around = shifting can cause bad pin connections and arcing - which can quickly ruin your tubes - and BTW output tube selection can make a big difference in tone - I personally like the Tung Sol Brand 6L6-STR & KT66 for this amp although I also discovered and tried The Tube Store's Premium line 6L6GC - a bit pricier but I was floored by the tone - much like the now defunct Winged "C" 6L6GCs - see link https://www.thetubestore.com/preferred-series-6l6gc . Tung Sol is my favorite for both the 6L6 & KT66. The Kt66 does require a different Spring Retainer than does the 6L6/EL34 - the spring length is the same but the end "cup" that secures the rounded end of the tube is wider on the retainers for the KT66 because the tube is much fatter than either the 6L6 or EL34.

    So I repair any new or Vintage tube amplifier any pedals especially the vintage ones - I stock many vintage / period correct parts for both amps and pedals and save my NOS tubes for vintage amps as I see no reason to install vintage tubes on any new production amps - 20 years ago maybe - but today there are so many great choice in new stock - if you have a favorite I can get them but I stock Tung Sol and JJ and now a couple of sets of these Premium 6L6s (which look and sound like Blackplate RCAs - haven't noticed any reproductions that actually look like an RCA tube. - I can be reached via text @ 908 400 2760 - I am located in New Jersey. - Here is a pic of the LED/Fan mod.


    Chris


    [IMG]D:\From Nora PC 1_25_20\Chris Stuff new Laptop\Amp_Schematics\Fender\Bassman_100T\20190902__Bass man_100T_Fan_LED_Switch_Blue_On[/IMG]


    OBVIOUSLY I haven't figured out how to post a picture from a file on my PC - I don't understand why we have to use these HTML programing codes to upload a picture or file - (like a schematic or drawing) - maybe I am missing something..

    7 Not allowed! Not allowed!
    Last edited by Chriscolt; 02-21-2020 at 08:04 AM. Reason: apology for not knowing how to post a picture from my PC

  10. #10
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,380
    Thumbs Up/Down
    Received: 2,032/24
    Given: 5,182/11
    Rep Power
    24
    Great post Chris, and welcome to the forum.
    Not sure how you tried to attach the image? If you click on the 'insert image' icon, you should be able to select your file, then click on 'upload'. Or you can select 'go advanced' below the reply box, then use 'manage attachments' to attach your file.
    Lastly, it's possible you may not have sufficient privileges yet as you only have a couple posts.

    0 Not allowed! Not allowed!
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  11. #11
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,845
    Thumbs Up/Down
    Received: 587/1
    Given: 513/2
    Rep Power
    18
    Or upload photos to an external hosting site, eg imgbb, and provide a link.

    0 Not allowed! Not allowed!

  12. #12
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    Great job, and clever fix, but at some point shouldn't we be asking ourselves at which point is this entirely too much work and time for what we are paid? I know I do, especially when it keeps me from working on other amps and generating billable time.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  13. #13
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,983
    Thumbs Up/Down
    Received: 2,274/7
    Given: 0/0
    Rep Power
    55
    It is like any complex procedure. Will we be seeing it, or was it a one-off? My first PV Classic 30 was a pain, and I spent too long on it. But once done, the next one took just a few minutes. My local friend was struggling with one recently, and I came over and demonstrated how to get it apart simply. SO if we struggle with this Fender mightily and discover some cure that will be used often. next time we know it.


    I am concerned that I see more and more people deciding to reengineer circuits instead of repairing them.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  14. #14
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,685
    Thumbs Up/Down
    Received: 1,653/21
    Given: 1,367/0
    Rep Power
    14
    Quote Originally Posted by Randall View Post
    Great job, and clever fix, but at some point shouldn't we be asking ourselves at which point is this entirely too much work and time for what we are paid? I know I do, especially when it keeps me from working on other amps and generating billable time.
    Fender way over-engineered the bias system on this amp, then depended on cheezy ribbon cable & accompanying connectors to make it work. Good thing they're not bio-engineering, we'd have people with seven elbows on each arm, and three knees on each leg. To the "engineers" I say "DECAF boyz, DECAF! Stop fixing problems that don't exist. Keep It Simple, Stupid!"

    It's been making my head spin, reading this thread. Thanks Chriscolt for popping in and detailing the fix. I hope nobody shows up at my place with one of these. "It's got tubes in it, you're s'posed to be able to fix tube amps, right?" Grrrrr.....

    1 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  15. #15
    Senior Member
    Join Date
    Mar 2008
    Posts
    396
    Thumbs Up/Down
    Received: 8/0
    Given: 0/0
    Rep Power
    13
    "It's got tubes in it, you're s'posed to be able to fix tube amps, right?"
    Well, the internal combustion engine is all the same from the beginning but they keep adding all kind of electronics around it and I don't see anyone to object their new cars so maybe we'll have to live with it.
    I was wondering if anyone could kindly save the software from the uCU (if it's readable and not locked)?

    0 Not allowed! Not allowed!

  16. #16
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    3,566
    Thumbs Up/Down
    Received: 258/0
    Given: 130/1
    Rep Power
    18
    Quote Originally Posted by GainFreak View Post
    Well, the internal combustion engine is all the same from the beginning but they keep adding all kind of electronics around it and I don't see anyone to object their new cars so maybe we'll have to live with it.
    I was wondering if anyone could kindly save the software from the uCU (if it's readable and not locked)?
    It's not an issue yet, but when one of those little computers in your car fails, you might not be able to get a new or rebuilt one from the manufacturer. Even if you could find one at a wrecking yard, it won't work in your car until it gets programmed at a dealer so that the other computers in your car recognize it's serial number. Otherwise, you probably won't be able to drive your car or even get it started.

    In the case of one of these amps, it will probably need to be modified to run without the bias board, or just scrapped.

    3 Not allowed! Not allowed!
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  17. #17
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,685
    Thumbs Up/Down
    Received: 1,653/21
    Given: 1,367/0
    Rep Power
    14
    Quote Originally Posted by GainFreak View Post
    Well, the internal combustion engine is all the same from the beginning but they keep adding all kind of electronics around it and I don't see anyone to object their new cars so maybe we'll have to live with it.
    Don't see anyone? YouseTube star Scotty Kilmer speaks for us who don't appreciate the so called "evolution" of cars that are more computer than car. "They're bound to turn into endless money pits." New Mercedes have no fewer than 75 computers. Not a typo. 75 computers, in one car, to make it work. Same is happening with farm equipment. When it won't work without all the computers happy & in sync with each other there's no choice but to have the dealer work on it. Expensive, time consuming, and did I mention extremely inconvenient. If your dealer doesn't have the part, or isn't hip to the latest kink in the software & how to fix it, you're up the creek. Car owners, yeh maybe you can get a rental or maybe if you're lucky a loaner from the dealer. But if you're a soybean farmer stuck with 1200 acres to harvest, and your dealer's 200 miles away, and doesn't know f@^#-all about how to deal with your John Deere's computers & software, well brother you just lost the farm. The days of fixing it with bailing wire & chewing gum are over. Why a simple item like a bass amp must be subject to the same nonsense is beyond my understanding. Keep It Simple Stupid!

    3 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  18. #18
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,983
    Thumbs Up/Down
    Received: 2,274/7
    Given: 0/0
    Rep Power
    55
    I think it is simple. You can still buy a used Deere A and keep it running. We have people thinking they have to adjust their bias every few minutes or the world will end, and people cry when some amp comes out without an adjustment. So they put in an automatic bias feature. Don't like that? Fine, buy a used old Bassman head and a cab. No one says we HAVE to buy the latest high tech thingy.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  19. #19
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,685
    Thumbs Up/Down
    Received: 1,653/21
    Given: 1,367/0
    Rep Power
    14
    Quote Originally Posted by Enzo View Post
    I think it is simple. You can still buy a used Deere A and keep it running. We have people thinking they have to adjust their bias every few minutes or the world will end, and people cry when some amp comes out without an adjustment. So they put in an automatic bias feature. Don't like that? Fine, buy a used old Bassman head and a cab. No one says we HAVE to buy the latest high tech thingy.
    That may be true with vintage amps. Today's youngsters are less and less interested in them. Unless you have a smash-bang "collector's item" like a Vibroverb just like the one Stevie Ray played through, the general run of oldie but goodie amps are losing value. When young Guitar Johnnie can plug his guitar into his computer and generate sounds to his liking, what need does he have for vintage. Fender's newest all solid state Twin replacement looks like it's taking the world by storm. Surely smaller & more portable versions will appear soon.

    With cars & farm equipment & similar items, it's a different story. The supply of good used equipment is dwindling due to the usual action of rust, accidents, and other attributes of entropy. I was lucky to find a well-maintaned 2001 Jeep Cherokee a couple years ago to replace the '96 version that had served me well for 14 years. But how many other people are going to be content driving a 20 year old car? In farm gear, there are some astonishing improvements that can maximize production through applications involving GPS. But all that takes - computers. Modern gear involves a ton of computers just like those new Mercedes cars. Those who can afford it, of course they'll take advantage of it and prosper. Except when it breaks down - Mean Time Before Failure tells us, the more complex the system the more likely it is to break down. If you don't have a couple of spare harvesters in your barn, and your neighbors are all busy with theirs, you're up the creek. Call the bank, tell 'em to take the farm, move to the city & live under a bridge, there's your future Mr Farmer.

    0 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  20. #20
    Senior Member
    Join Date
    Mar 2008
    Posts
    396
    Thumbs Up/Down
    Received: 8/0
    Given: 0/0
    Rep Power
    13
    With cars & farm equipment & similar items, it's a different story.
    You're right. We're talking only one uCU here and even if it's obsolete and you can't find it you can always throw the bias board and make a regular bias board. Not so with a car.
    However if the bias board uCU is baked you can always replace it but without the software it's useless. That's why I suggested to save it if possible (although Fender must have taken appropriate measures) in case you need it in the future.

    1 Not allowed! Not allowed!

  21. #21
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,704
    Thumbs Up/Down
    Received: 868/1
    Given: 661/1
    Rep Power
    14
    We can only speculate at the reasons for Fender having this board. I would guess it's attractive to the user to know that if a tube fails he'll know which one to replace and also that he'll be able to do it himself. So they're here to stay so might as well have a strategy to deal with them rather look at it in horror. Here are a few ideas that spring to mind to get the ball rolling so feel free to add or amend.

    Reseating and cleaning relevant connectors is always a good starting point. Especially as these rely on accurately sensing a few mV. A bad ground will wreak havoc.

    Try swapping the tubes around to see if the problem follows the tube, or just try known good ones. Don't forget about potential bad contacts or dry joints on the sockets.

    Simplify. It's pretty easy to disconnect the board and lash up a conventional bias arrangement to see if things are stable without the board. I heard of a case where an intermittent speaker cable caused the board to misbehave and this would show up more easily with the board out of the picture.

    Check the sensed voltages on the inputs to the board and then the processor. Get some super fine needle tipped probes for your meter. They are a god send. If those are wrong there's no hope for the poor little micro.

    Look and see what is happening with bias voltage outputs. If any are off look to see if it's correct coming from the micro. This is often a pulse width modulated signal but some e.g. Bugera use a multi channel DAC. If the micro is dead you'll need to replace the board unless you have a copy of the binary and have a programmer and can replace just the processor.

    Micros seldom misbehave of themselves, they just react to the inputs and that includes the power rails so check them too. Bad decoupler caps can cause unpredictable behavior. Bad micros are most often completely dead.

    2 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Fender super bassman auto-bias & tube matching issues
    By nevetslab in forum Maintenance, Troubleshooting & Repair
    Replies: 0
    Last Post: 07-03-2018, 10:32 PM
  2. Super Sonic auto bias failure
    By lowell in forum Maintenance, Troubleshooting & Repair
    Replies: 2
    Last Post: 05-03-2018, 04:07 PM
  3. Mixed auto/fixed bias in amp
    By GainFreak in forum Theory & Design
    Replies: 14
    Last Post: 03-23-2017, 12:45 AM
  4. Need help changing an auto bias circuit to adjustable Crate BV50
    By gtrplayr1976 in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 05-18-2011, 02:20 AM
  5. Tube amp death (auto bias-trouble)
    By ThatNaysayer in forum Tubes (Valves)
    Replies: 6
    Last Post: 09-19-2009, 08:52 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •