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I think I broke my older GB amp

Discussion in 'Amps and Cabs [BG]' started by geoff_in_nc, Aug 6, 2012.


  1. geoff_in_nc

    geoff_in_nc

    Dec 13, 2008
    Raleigh, NC
    I've got a Genz Benz ML200-210T which I like pretty well. One interesting thing about it - which will be important in a minute - is that it has two speaker outs on different power amps - so 2x 100W, but can also be bridged into a single output for 1x 200W.

    Typically at our gigs, since they're small, I run just the combo, not my extension speaker and NEVER past 3 on the master dial.

    I gigged with it last Friday and it did great. It stayed in my car until Sunday night... I went out of town directly after the gig. I didn't turn it back on until tonite (Monday), and it now sounds terrible. Very distorted, weak, farty. The output level is fluctuating greatly - it's lost tons of volume, but notes here and there are louder than others. I've just now run it bridged into the internal speakers, bridged into my extension speaker cab (8ohm 4x10), and separated power amps into the internal speakers plus extension cab at the same time. They all sound the same - it's not the speakers themselves.

    When I plugged it in tonight, I evidently bumped the bridged/separate amp switch with the power cable and turned on the separate amps mode. For probably 2 minutes I rooted around trying to figure out what was wrong, turning the volume knob (but not up past 3 or 4 ever). Finally I figured it out, powered it off, set the switch to bridged, and that's when I found out it sounded like whale farts.

    So my question is did trying to make noise for 2 minutes or so with the select switch on separate and nothing plugged in to those outputs, damage the power amps? Is it possible the preamp could go south somehow and do this and the power amps are just reproducing what's fed to them?

    Sigh....
     
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Ok, let's slow down and troubleshoot this slowly and logically. The ML-200 has a long and solid track record of reliability so first let's see what's going on.

    1. switch the amp to seperate (dual) mode and using a tested good speaker cable and a known good bass (with good battery... did you check this?) see if both outputs sound distorted.

    2. If both outputs sound distorted, next take a short 1/4" patch cable or instrument cable and plug from the effects send to effects return bypassing the effects loop. Does it sound ok? If so, switch the speaker mode to bridged and plug into the bridged jack and see if this now sounds normal. Then remove the effects loop cable and see how this sounds.

    3. Try plugging your bass into the effects return jack and see if this sounds normal (but at a much lower volume).

    4. Is it possible that you broke the input gain control? Does it rotate smoothly and normally? A broken control element will also have this symptom.

    5. Let's see what these results bring and then we will figure out what has happened. We still have all the factory parts available and support your amp fully.
     
  3. geoff_in_nc

    geoff_in_nc

    Dec 13, 2008
    Raleigh, NC
    Thanks very much in advance Agedhorse for the troubleshooting help. Apologies if the formatting is messed up here, I'm trying to do this via phone.
    I need to get a new multimeter so cables not tested per se, but two cables that were good before, both result in the same distorted output. The bass I'm using is passive, but the sound does resemble when my (now sold) Cirrus's battery died

    Will test tonight.

    Will test tonight.

    Rotates smoothly (with all the detents of course) didnt get to try gain variation when powered on yet.
     
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Try the above stuff and let's see what it shows.
     
  5. geoff_in_nc

    geoff_in_nc

    Dec 13, 2008
    Raleigh, NC
    Checked my cables both signal and speaker - 2 signal ones tested essentially zero ohms on my cheesy meter. 3rd I didn't test - didn't plan to use it. Speaker cable also was essentially zero ohms.

    Did this test - no changes in sound, still sounded bad. Had to use 3rd junky cable mentioned above, 1st two (the nicer ones) didn't fit the jacks it seemed. 3rd junky cable evidently worked fine.

    Here I fiddled with the gain control, and as I dialed it back and forth the distortion pretty much went away, and output level got closer to expected. This was actually before I bypassed the effects loop I think.

    Yes, it responded this way, and at this point the distortion was already gone.

    Like I said above twiddling the gain control got the amp sounding good again. I thought it was rotating smoothly, but maybe there was oxidation I couldn't feel? Whatever it is I don't particularly like this as a final solution.

    Coming from a semiconductor background, I know intermittent problems never heal themselves. They just hide until they reappear at the worst times.

    What I'm thinking - and I'd like your thoughts as well - is that I have some intermittent issue and should take it to a pro to diagnose/fix. There's one in town I've heard really good things about and he's a GB warranty provider as well I just found out.

    Once again, thanks in advance for taking the time to help... it's a testament to Genz Benz's folks that you do this.
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Is it possible that you had the master volume way down and the input gain very high and you were just overdriving the input amp or eq section? I think this is the most likely scenario, or possible a bit of grit/debris got into the pot and you cleared this?

    There really haven't been any intermittent problems with this amp. I would hate to see you chasing something that isn't really there.

    Another thought... by any chance were you driving the amp hard with the fan switch turned off? If the amp gets hot, the protection circuitry can cause this symptom too.
     
  7. geoff_in_nc

    geoff_in_nc

    Dec 13, 2008
    Raleigh, NC
    I varied the master between probably 0.5 to 4 trying to get something coming out, and while the volume did go up as the knob was turned it was still pretty darned quiet - much quieter than it is now. The gain stayed around 5-7 which is where I typically keep it.

    It could have been some dirt in the pot I guess.

    The fan was switched to "on". The amp was running on about 2-3 on the master during the gig, and when it started making the farty sounds it was a few days later when it was first turned back on, and was accidently switched to dual mode for about 2 minutes with no sound coming out (the speakers were plugged into the bridged jack) and then switched back properly to bridged when I figured out what I had done. During that time the master was initially turned down some but may have been as high as 4 or so at times as I tried to figure out what was happening.

    I guess I could just see what happens going forward - wait and see. I have access to another amp if I need it if I have to put this one in the shop.

    I've got to say I really like the tone from the ML200, and its nice and simple which I really like as well.
     
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Check your cables for intermittent shorts... should read open between tip and sleeve even when wiggling the ends.
     
  9. geoff_in_nc

    geoff_in_nc

    Dec 13, 2008
    Raleigh, NC
    Ah, good point, thanks, I'll do that!
     

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