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Help! Sudden death of an AMP BH420 head !

Discussion in 'Amps [BG]' started by tonequester, Feb 4, 2014.


  1. tonequester

    tonequester

    Joined:
    Feb 4, 2014
    Greetings.

    I apologize for the long post that follows, but I'm new here and I must start out begging for help. If anybody is familiar with the AMP BH420 head, I sure could use some advice in troubleshooting. I'm pretty much a "tube guy" and have worked on a number of them, and done lots of mods to them as well. I once fixed my solid state Sansui stereo(blown power transistor), and I think that I have much the same problem here with the BH420.

    My brother asked me to take a look at a friend's bass amp. being stupid, I assumed it was a tube head. The problem with it was that it occasionally "cut out" while being played. I figured it was probably a loose jack or bad power cord. However, when I plugged speaker and bass into it, and turned the power on, the red indicator lights only lit up for a split second, and the amp died. I had the cover off so I could observe the circuitry, and sure enough, the instant it "died", I noticed just a tiny "whiff" of smoke or fume come from the board/heat sink, that has the big power transistors mounted to it. I got the schematic for it at : www.epanorama.net/sff/Audio/Musical/Products/BH-420.pdf. I should have never fired it up because I noticed right off, before firing it up, that R 65, and R 86(both 800 Ohm, 5Watt) had been overheating enough to "char" the board underneath them. They set right behind the tone balance control, opposite side of the output section, in the head.

    I guess that my question is: Does anybody have any such experience with this head, that could narrow down my search for what is evidently open on the power board/heatsink. The only thing I can think of to do, is to remove ALL transistors form this "board" and test. I've already checked the 4 diodes on that "board", and they are good to go.

    Any tips or advice is greatly appreciated in advance !

    tonequester.
     
  2. WillInDenver

    WillInDenver An ounce of perception, a pound of obscure. Supporting Member

    Joined:
    Mar 9, 2005
    Location:
    Denver, nee Austin
    The guy who will know is Dave Funk, who runs Thunderfunk amps. He purchased the AMP IP from Gibson years ago, and his Thunderfunk line are the modern descendants.

    I'm not affiliated with Thunderfunk at all and I don't know Dave, but his email is at david.funk@thunderfunk.com. Website is www.ThunderFunk.com.

    Good luck.
     
  3. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Well those two resistors are the voltage drops for the +/- 15vdc regulator supply. The output high voltage rail seems to be fused at the output devices. Possible you have a short in the 15v regulator, cap most likely. Isolate the "HV" from the regulator at those resistors to isolate where the short is.
     
  4. tonequester

    tonequester

    Joined:
    Feb 4, 2014
    Greetings WJGreer and B-string.

    Thanks for your timely replies guys ! I truly appreciate the info you have given me, and will check those things you have suggested. After playing guitar for 30+ years, I just got my first bass, with intentions to teach myself what I can. Unfortunately, before I even got the chance to fire it up for a learning experience, I got tangled up with this bass head. Even though it's untimely "death" was NOT my fault, I feel obligated to try to fix it for the guy. Although I know my way around tubes pretty well, my solid state experience is very limited to kit builds/effects. The inside of this "legendary" head is "complex" to say the least, when compared to an old Fender Princeton.

    Thanks again, and I hope I can be of service in return. tonequester.
     
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  6. tonequester

    tonequester

    Joined:
    Feb 4, 2014
  7. tonequester

    tonequester

    Joined:
    Feb 4, 2014
    Greetings Bstring.

    Please pardon unorthodox post, I'm new to the game here.

    You mentioned that my problem might be related to a shorted cap,in conjunction with the power resistors R65 and R86 on this BH420 head. In looking at the schematics, I see 2 electrolytics across the HV rails(47uf). Would these be likely candidates for the faulty components ?

    I did email David Funk per the advice of WJGreer. However, while I wait in the hope of his advice, I might as well check your opinion out concerning the shorted cap. I'm not so sure of myself with solid state equipment, as I am with tube type designs, that have always been much less complex, and less powerful.

    I removed the power transistor board/heatsink, and began to check components out of circuit here, due to the fact that I did notice just a trace of fume when the amp "died". This "board" was easily removed, but the burned area that I referred to is located on the board that is situated at the front of the head, and includes most of the controls. It will be quite a task, just to get to the printed circuit side. It does seem that this SHOULD be examined further. If you have an opinion about the mentioned caps, I would like to hear it before I proceed.

    Thanks again for your help thus far. It's just my luck that BEFORE I can begin to learn how to play my new Ibanez short scale(don't play bass yet.) I would have to dive into a powerful bass amp first. I don't mind the learning experience, but much rather it to be on my own equipment. Perhaps, I should learn to "just say no" ! tonequester.
     
  8. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Yes C36 and C37 47uf. Being old caps they may have dried up and shorted. You also have two zener 24v diodes on the supply legs before the 15V regulators. If you can lift one end of each of those resistors it will isolate the 15volt supplies and preamp before you dig further. If the unit stays powered up with the 15v +/- disconnected, you can start checking that 15 volt supply. If it still won't stay "lit" then find the fault in the HV supply first or you are spinning your wheels.
     
  9. hennessybass

    hennessybass Supporting Member

    Joined:
    Oct 11, 2008
    Location:
    Austin, TX
    Just came across this thread and don't think I'll be much help, but really interested as I also have a AMP BH420 that I am considering making some home repairs to.

    I owned an AMP back in the day, and after gigging it for quite a while, it died on me. Took it to the shop and they did some repair having to do with "power" and everything was great after that. That's about all I remember.

    Fast forward to now, and I bought another AMP. This one has a pretty loud hum that sounds electrical to me. It hums with no signal and volume turned down. Have also gotten some pretty nasty pops and crackles when playing it.

    I opened up the amp to do a visual inspection, and noticed a potential problem in what sounds like the same place as the OP...

    There are two, what I thought were caps on mine look like they have something funky going on. That is, they look like they are leaking. There are two similar caps on a different PCB (a vertical one) that look in even worse shape. I was going to try to replace those and see what it did. The two on the vertical boar would be pretty easy, but the two on other board would be quite a chore to get to as the OP also has said.

    I'll post up some gut shots of mine when I get home. Hope I'm not hijacking the thread. Just got excited when I came across this thread.
     
  10. tonequester

    tonequester

    Joined:
    Feb 4, 2014
    Greetings Bstring.

    Thanks once again for ALL of your help ! However, I can't see how I can lift a leg of those Zeners(which I have found) without disconnecting that whole front panel board, so I can get at the p.c. side of the board. I'd have to desolder some wires, and unplug others, and HOPE that I could flip the board over. I can do that, if it's necessary. However, I'm just wondering IF there is anyway I can check Voltage at some point just before the Zeners, or a way to check HV non-invasively here ? It's beginning to look like this may be way over my head. All of a sudden, I now have a lit green power indicator, where before it would not light up ? I have no problem with taking that board out and checking the caps, without power hooked up, IF there is a possibility that a bad cap is the only thing wrong, but it's going to be a b---- to set it up with power, and still get at the P.C. side of the board(maybe much jumpering......maybe much "lighting me up" !).

    Man, I'm working with DMM, screwdriver, and soldering iron here, and not much else. You and I both know what's MOST lacking. If I jerk that board, and test the caps(replacing if bad), then can I check Zeners reliably "out of circuit"....no power ?

    I apologize for stating/asking the obvious. I'm not quite as stupid as I may seem, but I AM a "gutless wonder" when it comes to working on somebody else's cluster---- ! This crap always happens to me. I accept an amp that's "sposed to be intermittent contact problem, real rare at that. I plug in and it "blows it's bowels" instantaneously! I ought to send the damned thing back with a big.... SORRY ! Too soft a heart/head, and not wanting to give up, leaves me here asking for help. If this was an all tube head, it would already have been "smokin" musically, instead of "smokin" electronically !

    Anyway, please advise, and sorry for the Whining. I'd just like to be able to check the HV before I go through all I'll have to, in order to remove that board. If this thing is going to be a constant cascading meltdown, I need to reconsider.

    Best regards...tonequester.
     
  11. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    hennessybass, chances are excellent all those caps are the source of the hum and in danger of shorting.
     
  12. hennessybass

    hennessybass Supporting Member

    Joined:
    Oct 11, 2008
    Location:
    Austin, TX
    Thanks, B-String. Like I said, I will post up some pics before I start tearing the amp apart. Like the OP said, removing the PCB is going to be quite a chore.
     
  13. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    There should be two wires carrying the higher voltage going to the board with the low voltage regulator? Disconnect those, unless the zeners are shorted they are best checked for in circuit operating voltage.
     
  14. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Pics GOOD. I always take pics before anyway so no surprises when reassembling (like where did that wire go).
     
  15. hennessybass

    hennessybass Supporting Member

    Joined:
    Oct 11, 2008
    Location:
    Austin, TX
    Hey Guys,
    Here are some photos of the inside of my AMP BH420 detailing the repairs I think need to be made. Again, I am hearing a loud buzzing with no input into the amp, and the occasional crack and pop sort of thing when playing. My limited knowledge makes me think this is because of bad caps that need to be changed. I feel like I can do the repair myself as I'm pretty hand with a Soldering Iron (have made a ton of cables, assembles pedal FX from kits, etc). Goes without saying to unplug the amp, but should I be worried about getting shocked. My understanding is that tube amps can hold voltage even when unplugged.

    First, a shot of the inside to put things in perspective...
    [​IMG]

    Two caps on the main PCB (right behind the controls)...
    [​IMG]

    Close up of those two caps...
    [​IMG]

    Four caps located on the vertical PCB...
    [​IMG]

    And close up of those caps...
    [​IMG]

    Looks like all of the caps are leaking to me. Any thoughts?

    Where is a good place to source some new replacement caps?

    Thanks for any help, and again, sorry to hijack the thread!!
     
  16. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Those are power resistors. Please have a good tech do the work...;)
     
  17. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Those are not caps, those are resistors. I wouldn't worry about changing those unless they are way out of spec.

    The big blue cans with the screws on top are caps, as are the yellow ones that say things like "47uf 50v or 1000uf 10 volt" and have black band with the "-" symbol. Those big blue ones can hold quite a big charge! New caps that big should be powered up slowly.

    tonequester, looks like from the pics you can remove the red and grey wires from the big blue caps to isolate the +/- 15 volt supply. Be sure to retighten the remaining wires.
     
  18. hennessybass

    hennessybass Supporting Member

    Joined:
    Oct 11, 2008
    Location:
    Austin, TX
    Thanks guys. I wonder what's causing the noise? Looks like it's off to the tech.
     
  19. Quadzilla

    Quadzilla Supporting Member

    Joined:
    Feb 11, 2002
    Location:
    Just north of Baltimore, MD
    Just ab FYI, I recently bought a BH420 that had almost no input signal and a lot of hum and crackle. it turned out to be a severe case of a dirty pots. it took a couple of hours of blasting with cleaner and twisting of knobs to get them right
     
  20. tonequester

    tonequester

    Joined:
    Feb 4, 2014
    Greetings to all.

    Ain't no hi-jacking my threads....ALL are welcome. Yea, hennesseybass, me thinks we have similar problems, However, on my craphead, there is dark brown burned board you can't miss. The power resistors look the same, but I checked them, and they are within tolerance. B-string my friend, I took out that mother-of-a-board, and neither cap was shorted, AND they were even real close to 47uf, by my new LCR meter. The Zeners were fine. I did fix the bad transistor that I found blew out on me when I fired this hunk of junk up. I dared to turn the thing on again and I have NEVER heard the noise that this head made. Think AK47 on rapid fire mode, and a revving chainsaw......this, with the volume at zero. My problem now that I'm pretty much back to square one, is that I haven't ever torn into a power supply before, and don 't know where to start. Any advice here would be appreciated. Oh, hennessybass. it doesn't matter tube amp or solid state, when you have those great big blue filter caps, you can get bit real good, if you don't discharge them to ground first thing. Also, I second the motion about these old heads needing their pots cleaned. This one was horrible., and I cleaned them first off. B-string, would you try to work from the power cord toward the output, or from the caps that tested good, backward toward the plug ?

    Thanks for ALL of the comments guys. I appreciate it. tonequester.
     

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