Help with bass amp PS problem?

Discussion in 'General Electronics Chat' started by Mike33, Mar 10, 2011.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Hi, thanks for looking! The bass player in my band said he had some sort of 'power surge' (ok, he doesn't know what that really means. I think he had opposite polarities on his amp and the PA....touched bass to mic stand, and his amp went poof). I'm trouble shooting it for him. It is an Ampeg B2R, solid state. No power at the preamp yet, so obviously my issues are (at least) in the power supply. Transformer is ok, as is the recto, and filter caps.

    I found & replaced a few really toasty power resistors (R67, 68, 69, 70), and the power transistor that switches in his fan (heat sensing circuit, not shown) was LOOSE in the case, so replaced it. I am getting B+ out to the junction of those resistors, and it just dies after them, at the zener diodes (D23 & 24). I pulled the zeners and THEY ARE NOT DEAD, and no shorts to ground or anything. But now I have to order new ones (tragic removal...). The caps after them seem ok, too. So, any ideas why this thing suddenly loses its + and - voltage at that point? The ground is continuous from CT transformer to the case, btw. To the left on this schem is the 3-wire transformer. All seems good w/the bridge recto, all the way up to the beginning of R68 and R70.
    Any thoughts?? Thanks!!!

    [​IMG]
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Is it possible that you changed the zener diodes and put them into the circuit backwards? Keep in mind that a zener diode operates in the reverse bias direction and acts like a typical diode in the forward biased direction.

    hgmjr
     
  3. nomurphy

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    Aug 8, 2005
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    Note that 32V across 180 ohms is over 5W, is R68/R70 getting hot? If so, then something is conducting to GND (double-check zener installation). Otherwise, there is an open.

    If the zeners had opened, then there was +/-32V going to the amp section.

    If you measure pretty low resistance between the junction of R68/D23 and GND (and similarly on the negative side), it is likely that you have blown the output xstr(s) or other devices in the amplifier section. Also, without speakers attached, ohm between the PS rails and the output common point (which goes to the + spkr) to check for shorted output devices.
     
  4. nomurphy

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    Aug 8, 2005
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    If the zeners had opened, then there was +/-32V going to the amp section.

    May have blown all the op amps.

    See attached:
     
  5. Mike33

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    Feb 4, 2005
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    Thanks, guys. The diodes are out of circuit now, I removed them to be sure they weren't friend (gave every appearance of being dead). But when I tested them with another PS, they are fine - but I cut them out and don't have lead length to reinstall the same ones. Geez. So, gotta order 2 more.

    I'll try some resistance checks between rails and output, nomurphy....

    OK, from that junction with the zeners (NOT in circuit), I am reading low 100's of ohms to the output jack tips. And, from that junction to ground =.9K and rising (as caps charge, I figure...). So that seems good.
    May I assume the output trannies are not burned? So far, other than the fan transistor, no other semiconductors come up as blown...random checks on 1N914s here and there show them normal.
    Just odd that the voltage 'dies' right there, as if there were a short to ground. Might be the caps following those diodes? C28 etc.?
    I should probably disconnect the output of this (the points marked +16 and -16 are at a harness, and go to the preamp - which I haven't even begun debugging yet, ugh).
     
  6. nomurphy

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    Aug 8, 2005
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    Check IC1 regulator which is on the +16V and is related to the fan circuit. Also the offset/relay circuit that is voltage related (TP17 etc).

    But DON'T run it without the 16V zeners properly in place.
     
  7. Mike33

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    Feb 4, 2005
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    Will do, nomurphy (probably too late, tho, it's been on without them, doh...even tho I know better!). Still, no voltage was getting to the preamp with/without those zeners *see next paragraph

    I pulled the ribbon cable, separating preamp from poweramp.....now that "missing voltage" is gone - sitting at 32v all the way to the cable. So, I am thinking something was already shorted/blown in the preamp (one of the TL074s, perhaps....) and pulled the voltage way down at those zeners.
    Measurements after disconnecting preamp show 64 v +/- on the power trannies, and 3.5v +/- at their emitters, and 32v, 32v- on the power rails out to the ribbon cable.

    I'm going to delve into the regulator this evening.
    And check out some stuff on the preamp. If he did arc between the bass and mic stand, there are questions re. blown stuff due to polarity reversal, for sure. This thing uses 4 TL074s, which I now suspect....tried tracing an input signal right off when I opened this up, and it was exhibiting sounds of 'no ground'/short right from the input jack......
     
  8. Mike33

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    Feb 4, 2005
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    Hmm, yeah. I'm not very familiar with the LM35, but I am getting 36v at the 16v V+ pin, .5v (out), 0v(ground) on its 3 pins. Schem says 16V, but without those zeners, this is probably due to those high rails...if I leave the power amp on, and sitting in a position to get 'hot', the fan will go off and on automatically - so I think the LM35 might be ok. I will recheck that voltage once I get some new 16v zeners!
    No more messing around until I can get an order in.

    TP17 and 18 are within normal parameters... I'll just passively scope out the preamp - at this point, I suspect the ICs there were damaged when the spike hit. Might hook up a +/- 9v test power supply just for grins, to see if the opamps do anything (no power amp connected, of course!).
     
  9. nomurphy

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    Aug 8, 2005
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    In taking a closer look at the schematic I attached, I see that this amp employs a three-prong power cord and has chassis GND connected to board GND. There is no input polarity switch. This means that the guitar & amp both should have been at ground potential and nothing should have happened to this amp.

    If your friend was doing something to override or bypass the 3-prong power cord and screwed up the grounding, it was - and is - really stupid.
     
    Last edited: Mar 11, 2011
  10. Mike33

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    Feb 4, 2005
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    Yeah, I completely agree, Nomurphy. I wasn't there (before I'd met him), but it seemed odd. He said he turned on, simply touched his bass strings to the mic stand and then something 'arced' - fuse blew, and amp never worked again.
    But when I first got it, the fuse was not blown. That TIP31C in the fan ckt was rolling around in the case, and no voltage goes beyond those 2 zeners (preamp short most likely). I suspect a bad power strip (defeated, most likely), or could have been something wacked in the outlet it was plugged into (in a club, no less).....
    Problem is likely in the (4) TL074s up in the preamp. If I had pulled the connector first, I'd have seen +/- 16V I bet, which then shorts in the preamp.
    Getting closer, tho :eek:) Still worth trying to do. Changing 4 TL074's isn't too bad.

    AFAIK, the other semiconductors are ok (diodes are, at least....crossing fingers for the mosfets!!!!).



    Anyone know any quick and dirty tests, in-circuit, for the LM35, MOSFETS and BJTs?? I guess 'shorted' would be clear, as would "open"....thanks!
     
  11. Mike33

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    Feb 4, 2005
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    Ahh - removal of preamp board shows a VERY toasted trace up there. Preamp ground bus. At last, a visible PROBLEM! Fixed that, and am removing the 4 chips. They show continuity between power pins....junked. There are 4 transistors up there I need to diagnose. No other semiconductors, tho! The diodes there are fine. Progress is good. :)
     
    Last edited: Mar 12, 2011
  12. Spence

    Member

    Apr 23, 2010
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    The bass guitar (amplifier input) and the microphone (amplifier input)should be protected from high voltages by virtue of their design. Whatever you do, get both amplifiers checked by a professional, you might be surprised to know how many guitarists and singers have been electrocuted and have converted a "live" performance into a "dead" performer. Get a residual current detector too.
     
  13. Mike33

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    Feb 4, 2005
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    Thanks, spence....yup, I am quite familiar with folks like Keith Relf of the Yardbirds, who were electrocuted due to reversed polarity power supplies!!! As far as understanding blocking caps and whatnot, I'm about as good for that as anyone's gonna get in the area I live in (central maine - even an amp tech in this area will tell you there's little danger!! YIKES!! Foolish). I like a kV-rated blocking cap, even on a tube amp input. And hey - the design did its job - HE was safe - the amp was not, however. The ground bus in the preamp was BURNED thru. But nothing got to HIM.

    What tends to happen (for those who don't know the story) is, sometimes a jerk miswires an outlet by reversing hot and neutral. And another outlet on the same or another circuit is wired correctly. You plug the guitar amp in one, the PA in the other....when the 2 meet :BOOOM: you complete the path and can be dead. A few milliamps at over 30V is enough. Minimally, you pop the semiconductors, as in the case I'm discussing here.

    I use an outlet tester at MY gigs, in fact. Any new venue is checked. The bass player "assumed" the place he was at was safe. I'd bet a dollar to a dime he plugged into a defeated power strip (ground broken off) that went to a reversed outlet. People fail to check these things and then there is a problem :(
    Had there been a proper ground, his amp might have been saved, as well. Just what the "WHOLE" story is, who knows? It was before I met him.

    I do understand much about amp repair and building, it just takes me time to 'grok' each one I open up since when I dig in, each is constructed differently. And it's usually a year or 2 between them. I have built a few tube amps, so safety is a pretty clear issue for me.
    No worries, no design changes will be made :D

    I WILL suggest he have the PA amp looked at, tho: the problem COULD have arisen THERE!! Since nothing in this amp suggests "Dangerous" to my eye (or DMM).....
     
  14. Mike33

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    Feb 4, 2005
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    ***UPDATE***

    Ok - after more digging, I determined that a spike had toasted the ground bus on the preamp board (separate from the power supply/power amp board on this amp). Needed to be removed to see it... I subbed in a copper wire for the burned trace. It was clear that the (4) TL074's were toast (smoke residue on the board by them!), so I replaced those as well. Two new regulator diodes (for the +/- 16v supply) were soldered in, and I had replaced a few 'fried looking' power resistors. The amp is now fine! Cost, about $20 for parts incl. shipping. Note - disconnect preamp from poweramp next time, thereby isolating faulty units from each other! Would've made testing this much easier. :D

    More inquiry proved that the guy was using this amp in a location known to have reversed power outlets. I'm going to give them a call tomorrow, when I can get an actual manager instead of a bartender. I've played there myself, and complained, but they didn't listen. This is a potentially lethal situation! So - musicians, use an outlet tester to be sure all your equipment is on the same polarity circuit! This could've been MUCH worse...they pay an electrician $200 to fix it, or risk killing someone, having a fire, etc. How much would THAT cost them? Cheapo's....rrrrrrr :(

    Thanks for the tips, guys! :cool:
     
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