Need help troubleshooting effects loop in Carvin x100b reissue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guitargeek6298, Dec 7, 2016.

  1. guitargeek6298

    guitargeek6298 Member

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    Hello all. I bought about a month ago a Carvin x100b head from someone on Reverb. So far I've really liked the head and is serving its purpose, but last week I started having trouble with the loop, where it's very quiet when switching it in. Bypassing the loop goes back to the normal volume. It doesn't do this every time but enough where I suddenly can't rely on it.

    I know it's a solid state loop, so it's not a tube issue or something simple like that. I've tried flipping the switch for the signal level and it doesn't seem to make a difference.

    Is this a known issue with these amps? Mine is a 2014, which should be well after they addressed the volume issue with these amps, which actually was a problem with the loop from what I understand.
     
  2. hdahs143

    hdahs143 Supporting Member

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    I would try cleaning the FX loop jacks with some Deoxit. This typically will take care of volume/signal loss when the loop is engaged.
     
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  3. guitargeek6298

    guitargeek6298 Member

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    Thanks for the suggestion, I'll definitely try that. I think this might be more complicated than that because of the amount of volume loss, but I'll definitely start there.
     
  4. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Are you actually running effects in the loop? What loop gain setting do you use?

    There is not a lot of potential jack-induced switching problems with this circuit, since there is only one tip shunt jack used (on the FX return). To eliminate the jack entirely as a problem, connect a patch cord from FX out to FX in. Then see if you can get the gain loss problem to re-occur as you switch the loop in and out.

    If the problem re-occurs with that patch cord in the loop, and the volume loss happens with the loop in, you might have some sort of problem with the 'electronic switch' FET Q10 that passes the loop signal to op amp A5B. Q10 and Q9 are selected with opposite polarity control signals, so either Q9 is on/Q10 is off and the loop is bypassed, or Q10 is on/Q9 is off and the loop is in the circuit.
     
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  5. guitargeek6298

    guitargeek6298 Member

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    I'm running a reverb and delay in the loop. I've tried both effects individually in the loop and swapped cables but it's still happened. I haven't however just tried the patch cable alone, so I'll have to try that tomorrow when I'm at rehearsal.

    Thanks for the help!
     
  6. guitargeek6298

    guitargeek6298 Member

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    So I was just able to try running just a patch cable in the loop: Unfortunately, it's still doing the same issue, large volume drop. Not all the time though, sometimes it sounds just fine but it's totally not reliable.

    Another issue developed today that I just noticed as well, and I'm not sure if it's related. When I play hard on the clean channel, I get a nasty kind of breakup, like a transitor-y kind of sound. If I play normally or quietly, the clean sounds great, but if I really dig in at all, there's this ratty-ness that I hear. It's almost in the background though, kind of weird. I wasn't sure if that was the bad FET in the loop distorting the signal or if it was something else. I put new preamp tubes in when I got the amp, JJ's and a Tung-sol for V-1.
     
  7. guitargeek6298

    guitargeek6298 Member

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    Reading this again, I think it could be a problem with the opamp; when I get the volume drop, it's also rather fizzy sounding I was noticing today.
     
  8. PremiumPlus

    PremiumPlus Member

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    On the original X-100B, there is a three position slide switch on the back of the amp that controls the gain of the effects loop. Check it and maybe clean the switch with Deoxit.
     
  9. guitargeek6298

    guitargeek6298 Member

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    Thanks. I have checked that, haven't used Deoxit yet. I've switched it to every position and it was still doing it. I couldn't get it to work correctly by jiggling the jacks, sliding the switch, etc.

    I emailed Carvin to see if I could purchase a new board. If that's not too expensive I might just go that route to make the swap easy. I'm short on time for this sort of stuff these days.
     
  10. PremiumPlus

    PremiumPlus Member

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    Wow, that could be pricey. I've worked a lot on those old X-100B's and if I recall, the opamps are socketed. Looks like A4, a 4558 dual opamp, could be the problem. A6, a 4049, is in the switching control and could be suspect too...but the odd thing is that you say it works but it's at greatly reduced volume, if I understand you right. I'm curious if the LED in the footswitch is as bright as it should be, that would tell us something about the condition of the power supply. Q8 would be something to check too. These are cheap parts, but you say you're short on time, so that's a problem...good luck.
     
  11. guitargeek6298

    guitargeek6298 Member

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    It's not an old x100b, it would probably be easier to fix if it were. It's a reissue from 2 years ago. I did have the chassis out last night, the components were all facing the chassis so it was tough to see without taking the aux board that the effects loop is on, but I'm not sure that the opamps are actually socketed. It was definitely cheaper construction, the board wasn't very high quality, etc. But at least the effects loop board is separate from the main board, and the EQ board, so all I'd have to do is unhook a ribbon cable, disconnect the sockets, install the new board, connect ribbon cable, done.

    Initially I did think that it sounded fine, except just very quiet. But I sat down last night just to try to diagnose it better, and noticed a fuzziness to the sound when the loop was engaged and it was having the issue. Kind of a rough transistor like distortion, like the input of a solid state amp getting hit way too hard. I also noticed that the clean channel, even with the loop bypassed, had a similar kind of distortion to it when playing very hard (which I typically don't do on the clean channel, so hadn't noticed it previously). That's why I'm wondering if the two issues are somehow related, because I know that this clean channel shouldn't breakup like... ever. It's very clean.

    I'm not great with schematics, I can read some basic ones like for old Marshalls and Fenders, but once I'm looking at channel switchers I start to get confused. But I was wondering if there's an opamp in there after the preamp (perhaps for a buffer?) that is in the signal path when the loop is engaged that's causing the distortion in the loop, and is bleeding through the clean channel as well. I haven't noticed it on the dirty channel, but obviously it's going to be harder to pickup on that channel.
     
  12. guitargeek6298

    guitargeek6298 Member

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    Oh, the LED's appear nice and bright too. But I was wondering if it was a power supply issue or maybe even heat related, since it isn't consistent. I have noticed that it seems to work right after I turn the amp on, but after that it's hit or miss, which made me think about the old fuzz pedals that change sound as the temperature changes.
     
  13. PremiumPlus

    PremiumPlus Member

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    Sorry, I was looking at an older schematic so disregard my parts numbers. Carvin has a ton of schematics and I'm not seeing a call out for a reissue. Would it be the X-100B/X212B Rev C? On that schematic the opamp that drives the effects loop is A10, a 4565 dual opamp. One side is for the send, the other for return. Q1 and Q110 are also in the signal path...and more. Its not a trivial circuit, and I'm not even sure if I'm looking at the right schematic. I'd give Carvin's service department a call. They used to be very helpful, but I haven't called them in 20+ years, so YMMV. Hope this helps.
     
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  14. guitargeek6298

    guitargeek6298 Member

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    I actually do have a call into Carvin but haven't gotten a call back yet. I haven't been able to find a schematic for the reissue specifically yet, I'm not sure about the Rev C.
     
  15. guitargeek6298

    guitargeek6298 Member

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    I actually received a schematic from Carvin to aid in diagnosing the problem, but they don't actually sell replacement boards (which I find strange). I'm not sure how I could upload it here though for a point of reference.
     
  16. guitargeek6298

    guitargeek6298 Member

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    For posterity I'll just leave an update here:

    Emeric at Carvin recommended cleaning the ribbon cables. I was skeptical that this would really help, but tried it anyways, and so far it seems to have completely solved the effects loop issue. I've got a good hour and a half on the amp since cleaning, and I've had zero issues with the effects loop.

    My clean channel issue seems to have been solved as well.

    Emeric did say that because the contacts aren't gold plated, they may have to be cleaned from time to time.

    Thanks to everyone who responded.
     
  17. Floyd Eye

    Floyd Eye Member

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    I was going to mention that. Ribbon cables seem to be a recurring problem with Carvin amps. Including their bass amps.
     
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  18. guitargeek6298

    guitargeek6298 Member

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    Yeah, the guy from Carvin actually said that. I told him I didn't think it would be ribbon cable, and he mentioned that he's seen brand new amps that needed the contacts cleaned. Seems like the gold plating would be worth it.
     
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