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B&K ST-140 Service Manual and/or advice needed (low bias adjustment problem)

Discussion in 'DIY' started by TheBrownNote, Feb 10, 2018.

  1. TheBrownNote

    TheBrownNote New Member

    Messages:
    17
    I'm looking for a service manual and/or circuit description for the B&K ST-140 or ST-140M. I already have a few low resolution versions of the schematic, the minimalist owner's manual, and a one-pager on adjusting bias that applies to multiple amps. I'm not sure if they ever produced a model specific service manual but I'm hoping somebody can help me out. I've split a stereo version of this amp into two monoblocks, and on one of them the bias is low and will not adjust up when trimmed. Looking for insights into the circuit so I can address that. Thanks in advance!
     

     

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  2. elnaldo

    elnaldo Addicted Member

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    Hello. Can you link to the low resolution schematic? Is it legible at least?

    You can compare voltages at both amps to see what's happening. Reading the voltage drop at the emitter resistors (at the driver, pre drivers, bias transistors) you have and idea if they are conducting. Try to compare the voltages at the resistors connected to the emitters, at both amps. Be careful , proceed step by step, take notes, you'll find a difference. Narrow your search around the point where the voltages get different.

    Measuring resistance across different points, or from different points to ground, in circuit, can also detect some difference between the good and bad amp.
     
  3. ovaeasy

    ovaeasy The Fifth Resident

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    You're way ahead of everyone else around here if you have those documents. Questions concerning these usually go unanswered, and believe me when I say I have an interest as my son has both the stereo and monoblock versions. My lack of experience with these MOSFET amps precludes tearing into them, so we treat them with the utmost respect.

    I too have an interest in seeing your schematics. I also have a SUMO Polaris and the documentation for that amp is also somewhat extinct. Must be a glutton for punishment.

    Good Luck
     
  4. Binkman

    Binkman AK Subscriber Subscriber

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  5. TheBrownNote

    TheBrownNote New Member

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    Elnaldo, thanks for the advice. I recently came across a better resolution, full frame version of the schematic, here it is. Also, I have a working stereo version of the amp hooked up to my main system now that's pretty much stock except for some capacitor replacements, and I could bring that into the workshop to poke around and get comparison values. I guess I just need to roll up my sleeves and get into it.
     

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  6. TheBrownNote

    TheBrownNote New Member

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    17
    Thanks! I posted the good ST-140 schematic I found recently, so grab it if you need to. I also have the schematic for the EX-442 dual mono model because I was interested in seeing the differences in the circuit. If anybody needs that please let me know. One thing I haven't seen is the a schematic for the ST-140M monoblock version, and I doubt there's an official version because they're virtually identical. I cannabalized an ST-140M that had allegedly been convertedfrom stereo by the factory roughly eight years ago (not sure if this means by B&K or by Stargate Electronics, who seemed to be handling their service for a while). Anyway, the original circuit board that I kept unmodified has no changes in terms of component values besides a 500 ohm pot instead of a 200 in the P2 bias pot position and a 1K resistor in place of the final 17.4k resistor specified in the schematic. Other than the extra pair of output mosfets, missing board for one side, and the full use of the power supply by one side, these are the only differences I noted between the stereo version of the ST-140 and the ST-140M.
     

     

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  7. TheBrownNote

    TheBrownNote New Member

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    Thanks Binkman, I do have that document and it seems that's all they produced. This is the method I've been using to adjust the bias, but it just stays at low values around 30 mA DC and is totally unresponsive. I replaced the stock bias trim pots with multiturn ones, matching the 500 ohm value to the value of the stock monoblock board I have (not the value on the stereo schematic, see above for details). Solder joints and connections all seem good, signal and power are going into the board, I'm feeding it a signal and measuring per B&K's recommendations. I was hoping it was an obvious thing but I've checked all those by now, so I'll just need to drill down when I have time. I appreciate your continued helpfulness! On the Mosfet matching front we had talked about before, I decided to go with factory matched Exicon replacements from Profusion in the UK, but I need to make sure everything else is working before making that ~$160 purchase.

     
  8. TheBrownNote

    TheBrownNote New Member

    Messages:
    17
    Follow-up on differences between ST-140 circuit diagrams in terms of P2, the pot that adjusts bias. The good resolution version of the schematic linked to above uses a 500 ohm pot with a 475 ohm resistor paralleled across it, but the version attached here has a 200 ohm resistor instead. Doing the math, the resistance is only slightly higher with the resistor and trim pot combo vs. the 200 ohm pot, so they should be roughly equivalent even though the taper of the trim pot when tweaked might be different. Right? As stated above, the monoblock I parted out that once worked properly and sounded decent version has a 500 ohm pot in this position so that's what I used. Unfortunately it seems there's no way to confirm through documentation that this was the correct decision.
     

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  9. elnaldo

    elnaldo Addicted Member

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    OK, from this schematics, bias procedure monitors current at the negative rail fuse holder.

    Low current there, means something fed by that rail is not conducting.

    I'd start checking: R11, value (no power) and voltage drop across the resistor when the unit is powered.

    Then, R9 and P2, value (no power) and voltage drop across the ends of the resistors when the unit is powered.

    Where those JV+ and JV- go? whatever is connected to those "JV" draws current from the same rail you are monitoring, so if something is not working on those connectors, less current flow at the fuse holder.
     
  10. TheBrownNote

    TheBrownNote New Member

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  11. TheBrownNote

    TheBrownNote New Member

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    Elnaldo, those were insightful comments, and although I never got to checking the components you suggested since the problem was more obvious than that, they did help me think about the problem. To answer your question, the JV+ and JV- are just test points to check the rail voltage. The JP and JM on the schematic, though, are relevant to the last problem I'm having now and with continued issues throughout this project. These are where the gates of the second set of N and P mosfets are connected to the amp when in monoblock configuration, with jumpers over to the second spot on the heat sink where the other channel's board would normally be mounted. As you probably know, there needs to be a 1k resistor in front of each gate to keep the output Mosfets stable, so that's why they're not just paralleled like the sources and drains are. In one of the photos you can see these five jumpers coming off the back side of the board and on the other the brackets on the side of the heatsink without the board. They were using what they had to work with in terms of the existing heat sink, but it's clearly a makeshift and awkward arrangement with unnecessary lengths of wire connecting output devices, some silly bare wire stock to connect the source pins of each Mosfet mounting bracket and provide a bit of structure, and multiple places where the arrangement could flex and connections could fail. To add to these problems I rewired the entire signal path with some super stiff Kimber TCX cable to improve the sound, but this made stuffing the whole thing in and out of the heatsink multiple times nerve wracking, and I had to fix broken connections a few times. Disregard the strange wire colors that don't match the circuit, I had to deal with the colors they had available and use tape to make the wires distinct. Anyway, the only remaining problem is with the amp in the pictures, and the it occurs only when I add the second pair or output devices and screw them in. What happens is it pulls one of the red wires taut and flexes the board toward the unoccupied side, creating a shorted solder joint somewhere in the two Mosfet brackets that attach to the board. When I remove the second set and nudge the board right, it goes back to normal with the ability to adjust bias and offset. The intermittency of the problem made in infuriating to diagnose. The problem on the second amp was a bad solder trace going to the upper leg of bias pot P2 that looked OK to the eye but lacked continuity. After I soldered in a clipped off leg of a resistor to that section it worked. During the adjustment process, though, I discovered that the bias is set lowest when the trim pot is fully clockwise, which is the opposite of the usual arrangement. Including this for anybody reading this thread in the future that wants to replace the low quality one turn unsealed pots the amp comes with with precision multi-turn pots: turn the bias trim pot fully clockwise before starting the bias adjustment procedure on the B&K ST-140, not the other way around! At any rate, once the trim pot was actually adjusting this amp was fine, and the other will be fine once I reflow all the solder joints on the connections between the board and the Mosfet mounting brackets. Thanks again for your help on this, Elnaldo and thanks as well to all of you that responded. Looking forward to stopping work on these and just listening to them! The two boards sounded good in the same box with the component changes I made, but I'm hoping the extra pair of output devices and added, dedicated power will create some audio magic.
     

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  12. elnaldo

    elnaldo Addicted Member

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    Glad you got it working !! Nice caps you have there!

    (Shorts when assembling the unit ! I faced that once. Very hard to trace !! with the unit disassembled, everything was fine. Final assembling, check, one channel dead silent. $·%$/%(/%, disassemble again to check, everything worked $·%$&/%)/, assembled again, channel disappeared... Decided to give up, very frustrated, looking to replace a burned light bulb, reached to see the lamp socket was too close to a solder point. Checked that point, it was the signal I was loosing, shunt to ground by the lamp socket when tightening the screws ·%&$&/&%/ )
     
  13. TheBrownNote

    TheBrownNote New Member

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  14. TheBrownNote

    TheBrownNote New Member

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    It is very frustrating, and I guess the only good part about it is that you develop the ability to identify in advance areas where things could go wrong and prevent or identify them sooner. I hope so, anyway. Thanks again!
     

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