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My BH420 isn't a Thunderfunk, but it sure is funky... schematics, anyone?

Discussion in 'Amps and Cabs [BG]' started by Frank Martin, Jul 8, 2005.

  1. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    And not just in a good way...
    It's working, and sounds cool, and even got through a 4-5 hour session without any problems (though it did get hot..).

    Since it's an old amp, and I got it used, I asked my Dad to have a look at it, to check if everything's alright.
    well, when we opened it, we saw someone's been funky with the electronics... o_O

    There is a fan added. However, it's not secured to the chassis anymore, the glue gave in. And the way it was fed power... :rollno: Also, it's not blowing from one side to the other, it just blows air at the power section... a bit illogical, but it seems that was the only place where they could fit a fan.
    There are some nasty resolders. I have never soldered anything in my life, but I bet even I could do better than that...
    The power amp section also has some extra wires hanging around, with those ugly solders...
    Some of the jacks on the back panel are not connected. For example, the fx send.
    And some more...

    So, does anyone happen to have the schematics for the AMP BH420 (World version: switchable 100-120/220-240V)?
    It would be much appreciated.
  2. Quadzilla

    Quadzilla Supporting Member

    I know that Dave at Thunderfunk started working on a schematic, but it's not complete. I'm getting ready to send my BH-420 to him for a going over. His rates are very reasonable!
  3. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    Adding shipping rates for me would be a bit unreasonable :eek: :D
    my Dad is an electro-engineer, so luckily he can handle things like this :cool:
    Thanks for the heads-up!
  4. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    Fremont, Ohio
    Editor-in-Chief, Bass Gear Magazine
    I'd be interested to hear what he charges for that.

  5. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    I don't have a schematic for the preamp and it is different from the amp I'm building. The power amp seems to be the same, and I can email you that scheamtic.

    About the fan though. I use a 24v low-speed fan run at about 18 volts. The voltage is easily gotten with a 1K5 5-watt resistor from the diode bridge (+75v).

    TIP: The fan won't do anything to cool the amp unless you get the heat into the air. The original design uses a thermal mass heatsink that puts the heat into the chassis by conduction. I add large TO-220 heatsinks on top of the power transistors. (You'll need to drill them with two holes to mount on the TO-3P 2-hole transistors).

    Mouser #532-504222B00


    You don't need to blow air at them. You only need to stir the air inside the chassis. This is how convection works. Blow the heated air off the heatsinks to raise the temperature differential for maximum heat transfer. The air at the heatsink surface is hotter than the air inside the box. (Your dad will understand this). The warm air will find it's way out by itself. This will drop the temperature of the transistors form 100C to 65C. I also use Boron Nitride putty tape to mount the transistors (made by Thermagon) which will drop the temperature to 62.5C.

    Email me and I'll send you a PDF of the preamp board layout, and the power amp scheamtic.

  6. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    I have the full BH-420 schematic set but have been too lazy to scan it (someone else asked for a copy a few months back).

    It came with the owner's manual (yeah, I bought mine brand new almost 20 years ago).

    Stay tuned.
  7. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    Thank you, Dave!
    The schematic would be very helpful. It looks like only the power amp part was hacked, the preamp section is more or less intact.
    As for cooling, I did learn physics some time ago, so I understand this, too ;) However, the way this was done is a bit irrational: it doesn't have enough space behind it to suck the air from, and it basically just stirs the air a bit, but the hot air is stuck inside, with the large power transformer block (ugh! it's heavy!) blocking airflow on one side, and the cables and the pcb on the other side. Blowing towards the openings on the side - and clearing the space at the openings - would be much more logical.
    Since the heatsinks are on the top, we though about perforating the top of the chassis above the power amp section, and putting the fan there, above it, to suck the hot air out.

    Anyway, it works, and hopefully, with your help it will work even better :hyper:
  8. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    :D That's cool
  9. e.maynard


    Nov 11, 2004
    Afton, Virginia
    I would love to get a copy of this if possible. I'd be happy to PayPal you some money for your time if you'd be willing to scan yours.

  10. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL

    that's freakin' service. :cool:

    btw, dave, i'm legally marrying my TFB550 @ city hall this spring. i'll be sure to send you an invitation. :D
  11. rok51

    rok51 Supporting Member

    Sep 2, 2002
    Crawfordville, FL
    It certainly doesn't get better than that-way to go Dave!

  12. jurroppi1


    Feb 18, 2008
    If you've scanned this and have it handy, I'd sure appreciate a copy of it.

    I know there hasn't been any action on this thread for a long time, but I was hoping you could help.

  13. robboy

    robboy Supporting Member

    Jul 13, 2006
    Boca raton Florida
    I'd also like a schematic if you ever scan it. I'd be happy to pay you for it.
  14. illidian


    Jul 2, 2004
    For those of you asking about the schematic, you may be better served PM'ing brianrost than posting on this topic.

    Also, Dave Funk may have completed his by now?

    Just trying to help, these amps are wonderful. :bassist:
  15. Beej


    Feb 10, 2007
    Victoria, BC
    I've got a PDF of the original users manual that also has a couple of schematics in it. Feel free to PM me with your email and I'd be happy to email it to you...
  16. jurroppi1


    Feb 18, 2008
    I appreciate the heads up on this.

    Dave sent me a copy of the schematic and user's manual. I'd like to see another copy, because it looks like there may be more than one rev - just to be sure.
  17. Beej


    Feb 10, 2007
    Victoria, BC
    Any other takers? I'm happy to help...

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