Mesa Nomad 45, very low -V on op amps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by EpiglottisDynasty, May 8, 2019.

  1. EpiglottisDynasty

    EpiglottisDynasty Member

    Messages:
    36
    Joined:
    Jan 6, 2015
    Location:
    El Paso, TX, US
    I'm working on an amp for a friend, and I'm a bit stumped. I'm really not all that knowledgeable on solid state circuits. The reverb is dead, both the in and out. I suspect the headphone out, which shares the same power supply is too. Both are powered by 5532 op amps, powered by +15v on pin 8 and -15v on pin 4. At least in theory. The power to pin 4 is only about -2v. Same goes for the headphone out chip.

    I've already checked the power supply, and it seems fine. If I disconnect the chips, the voltage rises up to -15. Pull the Zener and it will hit -40. I've tried bypassing all RC filter resistors, trying another rectifier diode, removing the Zener. If I remove power to one chip (either the headphones or the reverb) from the circuit, it will rise up to about -3.5v, but still no sound either out from the send or in from the return. Both chips seem to be shorting down the negative side significantly but not the positive. Swapping the chips entirely is very much not easy to do because they are soldered to a PCB that would take a lot of work to remove. Pins 3 and 5 are indeed grounded as expected. Pins 2 and 6 (the + on the op amp circuit) are sitting at about -.025v. I have no idea if that is reasonable.

    I'm just trying to find a non-power supply related failure that would take down both circuits and drawing a blank. Any ideas? Something could have caused both chips to fail short? Apparently the reverb circuit has never worked since he owned it.

    Also, is there something as easy to understand and comprehensive as ValveWizard for solid state circuitry? I really need to get this down better.

    I found the schematic here: https://schematicheaven.net/boogieamps/boogie_nomad45.pdf
    The reverb circuit is in the bottom of page 4 (in the PDF) and the power supply is the bottom right of the last page.
     
  2. teemuk

    teemuk Member

    Messages:
    2,921
    Joined:
    Apr 21, 2009
    The chips are shot and load down the supply. Just replace them.
     
  3. PushedGlass

    PushedGlass Member

    Messages:
    481
    Joined:
    Sep 26, 2014
    I think that's the answer. Are they getting hot?

    Might consider socketing them while you're at it; pay it forward for the next guy.
     
  4. AR-305

    AR-305 Member

    Messages:
    970
    Joined:
    Dec 20, 2007
    Location:
    NW North Carolina
    Shorted chip. Replacement inevitable. Adding chip sockets is a very very good idea.
     
  5. Jeff Gehring

    Jeff Gehring Silver Supporting Member

    Messages:
    4,999
    Joined:
    Jan 17, 2008
    Location:
    Kansas City area
    Maybe the chips can be reformed by slowly bringing them up on a light bulb limiter and a variac?

    ... Nah. The trash can awaits all dead silicon. Socket the new ones.
     
    ironman28 likes this.
  6. EpiglottisDynasty

    EpiglottisDynasty Member

    Messages:
    36
    Joined:
    Jan 6, 2015
    Location:
    El Paso, TX, US
    Alright, thanks everyone. If everyone agrees it is chips, that's where I'll go next. I was just skeptical that both would go like that and it isn't going to be fun to do. It isn't just desoldering chips, there is so much disassembly (and desoldering elsewhere) required to even flip the PCB over, near as I can tell.
     
  7. teemuk

    teemuk Member

    Messages:
    2,921
    Joined:
    Apr 21, 2009
    I feel you. Unfortunately some amplifiers are not designed with easy serviceability in mind.

    Tip (In case you didn't know it already): Entirely clip off the chips before any unsoldering. Then, when you start to unsolder, you only have the small thermal mass of those little pin "leftovers" to heat up. They will come off easily and you risk less damaging the PC board tracks by heat.
    Don't try to desolder and remove the entire chip as is. It's dead anyway so you don't have to care about preserving it. Even if "good" its a generic opamp and costs just a few pennies.

    Sockets can ease serviceability but can also be THE cause for intermittency-type faults by themselves. e.g. they get dirty, corroded or may in time just rattle some of the contacts loose when the amp vibrates. A lot depends on the quality of the socket. In practice, chips don't fail THAT often to warrant socketing and I see the point of socketing only with special chips that cost a lot and benefit from preserving during removal. "Hardware" updates by changing the chip (e.g. EPROMs) are another application that comes to mind. Just my two cents about this topic.
     
    Last edited: May 9, 2019
  8. Tron Pesto

    Tron Pesto Member

    Messages:
    745
    Joined:
    Feb 18, 2016
    Location:
    Mahwah, NJ
    Just had this in a Vox AC15 - the silly thing about the AC15C1 is that the first (of 5) IC carries the entire audio signal - the reverb and dry signal are split AFTER the first of the dual op amps in the JRC2047D chip.

    That first chip died/shorted and thusly, the amp was completely dead. The smoking gun, like yours, was that the +/- VCC from the transformer through the zeners was the correct +/- 27V, but at the chips, it was something like +/- 1.5V. After pulling all the chips (and installing sockets), I was able to use one of the three reverb circuit ICs in that first spot and the amp was back up and running fine (albeit wihtout reverb as I was a chip short).

    I ordered ICs from Mouser, got them and installed the rest and all is well - the amp is 100% now.

    Interesting side note: when I ordered the chips, although they were not in stock, there was no minimum order and they apparently had to be fabricated as it was a 14 week lead time. No problem, I waited and it was more like 6 weeks anyway. However, now if you look on mouser, there is minimum order of 2,000 now! I guess they didn't like making the "scant" 50 that I ordered!
     
    EpiglottisDynasty and pdf64 like this.
  9. EpiglottisDynasty

    EpiglottisDynasty Member

    Messages:
    36
    Joined:
    Jan 6, 2015
    Location:
    El Paso, TX, US
    When trying to pull the zener to test, it split in half. I suspect it had already broken and was working intermittently, which allowed voltage to go way over -15V and fry both chips. With the chips in place, it was actually pulling down voltage straight off the recto to under 10 and under 4 by the first filter caps. I must have troubleshot everything in the circuit before the chips themselves.

    I have the circuit board of some old bass amp here, full of a couple of TL072CPs. Different voltage ratings (higher than 15, so should be good) but same pinout. Dropped one in and a new zener, and the reverb is alive again! I did actually manage to get the reverb chip in from the top by cutting the legs and then clearing the holes with a cap lead, and then flowing a bunch of solder down. That trick probably won't work with the headphone circuit because the inductor next to it impedes soldering on that side. I'll probably ask if I should even bother. That circuit has obviously been dead as long as the reverb and he never noticed. I did remove the chip to keep it from shorting the power supply down.
     
    Tron Pesto likes this.
  10. PushedGlass

    PushedGlass Member

    Messages:
    481
    Joined:
    Sep 26, 2014
    Good advice, this.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice