SVT3Pro Bias Setting

Discussion in 'Amps and Cabs [BG]' started by arnocl, May 25, 2008.


  1. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Hey Jerrold, good to see you on here!

    Farace, I'd bet that you fixed it. If it noises up again, I'd bet a second cleaning would fix it for good. Pretty excellent detective work!
     
  2. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    Still no noise; the loudest sound in the room was the refrigerator, even with the volume cranked, and no noise from tapping components. I buttoned her up and called her good to go.

    Thanks to everyone for all the help, and again, apologies for this being in a biasing thread (but I really thought at first it was going to be that).
     
    B-string and JimmyM like this.
  3. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    If you don't ask, you'll never know!
     
  4. B-string

    B-string Supporting Member

    This helps to define what symptoms cold bias causes and what it does not. No apologies needed IMO.
     
    farace likes this.
  5. Slapin

    Slapin

    Oct 9, 2016
    I had the fuzz distortion on my 2003 svt3 pro, got rid of it now just fine tuning to 25mV. Now it seems one MOSFET is considerably higher than the others. At this stage im currently at:

    Back
    3.1 5.5
    18.5 3.2
    2.3 6.5
    1.7 10.0

    is this difference too high (18mV to 10)?
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Double-check the resistance of all of the resistors that you are measuring the voltage across. One or two may have shifted up in value giving a false reading.
     
  7. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Something as simple as cleaning connectors as part of a regular maintenance program can go a long way. Good to hear that it worked out.
     
  8. learj

    learj

    May 18, 2011
    This was a great thread! Got my amp
    sounding sweet!
     
    B-string and Goingdownslow like this.
  9. learj

    learj

    May 18, 2011
    Can someone put me in the right thread. Been trying to find one for my svt 4 pro. When I power the amp up I get static noise even with the mute button pushed in with nothing plugged in to the amp. Driving me crazy!!
     
  10. learj

    learj

    May 18, 2011
    Changed tubes all went away
     
  11. On my SVT-6 Pro, exact same setup, I did actually adjust the lower bias pot. I went too far, admittedly, and I got a nice puff of smoke from the heatsink. After opening up the heatsink and taking the upper circuit board off, I realized there was an additional blue pot, that looks identical to the first bias pot. So now I'm confused as to which pot should have been adjusted. I'll be able to test and repair the circuit board. It doesn't actually look like anything got significantly cooked. I'll simply have to test each component.

    I'm thinking now that I should have adjusted the upper blue pot to change the bias. So when I adjusted the lower pot, it increased the wrong circuit flow causing the overheat. Is that correct?
     
  12. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    You know, this may sound crazy, but if it were my amp and I was bound and determined to work on it myself, I would at least find out what the trimpot I'm about to adjust does before I adjust it. This is a textbook example of costing yourself more money by trying to save money.
     
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Good luck.
     
  14. True, but let's focus on the solution, not the problem. If I had the money, I'd take it to someone, but I don't. There's no other option.
     
  15. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Herein lies the problem... it's virtually IMPOSSIBLE to focus on a solution if you can't correctly identify the problem. This is "troubleshooting 101" in a nutshell. I agree with Jimmy 100% on this one.
     
    JimmyM likes this.
  16. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    The SVT-6PRO is complicated. Unfortunately, you'll need test equipment to service this amp.

    It operates in bridged mode only but the two halves can be un-linked for servicing and testing. There are two bias pots AP1 for channel 1 [+]half, and AP2 for channel 1 [-] half. The pots should be labeled so you can see which one you turned. There are ten IFRP-240R power transistors in each half. A lot to check out.
     
    KoolCharisma likes this.
  17. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Bridge mode amps require even more involved test procedures, and sometimes there are offset and symmetry controls depending on the specific circuit topology used. On these kinds of amps, often even qualified experienced techs will refer back to the paperwork to avoid accidental destruction.
     
    beans-on-toast likes this.
  18. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    I can totally relate to saving money, but you will spend more money trying to fix it yourself than taking it to a tech. You already smoked it once, and that may not even be the problem.
     
  19. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    or the only problem...
     
    JimmyM likes this.
  20. This is exactly the information I needed AND it collaborates with my findings. I've done my checking. The 5 MOSFETs on the Blue Dot AP1 side were fried and the causation of the smoke. All other components are working. It's a $20 fix.

    So after replacing these, it's obvious that AP1 will adjust The 5 Blue Dot IFRP's. AP2 I'm sure will then adjust the Red Dot side.

    Essentially these are the two identical SVT-3 amps, which once bridged, make the SVT-6. So for further diagnostics, which resistors and I'm checking to identify the individual bias amount? Would it be the The pairs of green ceramic resistors on each side? There are paper resistors running between the MOSFETS, which is what I originally tested. I think that was my problem. The current traveling from these bridged resistors was giving me the AVERAGE current between both sides. So the 21mV rating, while seeming tolerable, could easily reflect that one side might be working optimally at 25mV, but the other side might have been running at 16 or 17mV, hypothetically. Who knows, but I'll have to wait for the MOSFET's to come in before I can know more.

    Can someone please confirm my hypothesis that these paper resistors running between the two sides are simply used for bridging and would therefore reflect an average current AND that testing the pairs of green ceramic resistors on each side will give me the reading of the two bias adjustments independently?
     
  21. Primary

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    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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