Bogner XTC 20th problems...please advise

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GreenRust

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Hello - haven't posted much on here, but have gained a lot of wisdom from other members by reading their posts - also parted with lots of cash thanks to you guys, but hey, that's part of the fun right? :)

I have had a Bogner 20th XTC for about 7 years - been working great until recently. LOVE the amp. I really hope you guys can help me out - it's my favorite amp and I can't really play for more than 20 - 30 minutes (see below)

The problem:
  • Becomes much more shrill sounding - much more treble, cuts out bass, loses the Bogner 'chew' and harmonics - kind of sounds like a very poorly modded Marshall - no body, balls, none of the good stuff - sounds anemic. Sort of sounds like I'm turning up presence and treble all the way, bass all the way down - but even worse.
  • Intermittent - the issue comes in and out - sounds good, then bad, but probably converges more toward bad over a few minutes (i.e. shortened periods of good then all bad)
  • the issue only occurs after the amps has fully warmed up - about 15 to 30 minutes of playing - which leads me to think it has to be heat related
  • the intermittent issue used to get resolved (for a few minutes) by twisting the presence knob - but since I've cleaned the pots this no longer solves it - now the issue is just more persistent
  • impacts all 3 channels
What I've done to try to solve the problem:
  • fully re-tubed it twice - new pres and new power
  • replaced all cables
  • cleaned the board, connections and pots
  • replaced a couple mojo dijon caps (they were looking a little out of spec - amp was bought second hand and I've had it for 7 years)
  • cleaned all input jacks and fx loops jacks
  • tried different speaker cabs and cables
  • ran it without the loop / fx - same issue
  • Even cleaned my guitar jacks (just for completeness)
None of the above have resolved it.

As it impacts all 3 channels, in addition to the presence knob twist 'fake solution', leads me to believe it's in the power amp somewhere - maybe some of the filter caps. I'll be the first to admit, I'm ignorant with tube electronics.

I've gone out to Bogner and Jorg mentioned that it could be the big blue caps on the chassis and gave me a method (scope or something in loop jack) to test. I'm going to take it back to my tech and would really appreciate some options from you guys for him to test and validate.

Sorry for the long post - would really appreciate your input. Trying to convince my wife that I need an XTC Pandora to replace my 'broken' one probably wouldn't go down too well :). Honestly, I love the 20th though, does everything I want and puts a big smile on my face (before the problem happens of course). You guys get it. Looking forward to your thoughts. Thanks!
 
No idea what the problem is but you could always try to start narrowing down where it might be in the signal path by elimination. Run another preamp or pedal straight into the poweramp and see if the issue persists. If you have a load and another amp somewhere, try running the 20th's preamp into another poweramp and see if the issue persists.

This might at least let you confirm whether it's happening in the preamp or poweramp.
 
I don’t think it’s the caps. Filter caps that go bad bring in ghost notes and they aren’t intermittent to that extreme like that. They fail mechanically by bulging or venting. Electrically they allow more ripple current into the DC supply.

Is the bias circuit stable? Check to make sure the bias voltage is stable and not drifting. Check to make sure the resistor to ground is soldered well. If that resistor isn’t soldered well, the amp will go from class A/B to hard class B and into cutoff making it sound like that.

When you’re experiencing it, feel the sides of the filter caps to see if any of them are hot to the touch. Don’t touch the tops of them as you could get electrocuted. If you have a thermal heat scope it’s even better.

I personally think you’re chasing a bad solder joint in the power section somewhere. Have your tech do a continuity test and chopstick the power section.
 
Start at Input jack/s. Trace towards output. Cheap / easy stuff first. Loop ? Glpg80 suggestions then output trans.
 
Thanks a lot for your thoughts above guys - very much appreciated.

@Great Green - good idea on isolating the power and pre sections. I don't have a load box but I have a power amp I could run into then run 'wet / dry' , but 'double dry' w/o fx (perhaps Extra Dry - refreshing) and adjust the volumes to test the issue.

@glpg80 - great thoughts. Good catch on the resistor to ground. I think it's in power section as well. I'll suggest the continuity test and chopstick to my tech and also have him check the bias drift - XTCs have always taken time to 'settle' bias wise, at least for me.

@Rocker68 - good thinking and thank you; I feel like I've eliminated most of the cheap / easy stuff already but will see what I can eliminate again.

Do you guys think it could be a faulty solder joint on a pot by any chance?

Thanks again for all you help and guidance!
 
when you say you replaced some dijon caps that " looked " out of spec .... how do you mean " looked " ??

visually they didn't look good ..... or the measurements taken on them didn't look good / out of tolerance ?
 
GreenRust said:
I'm going to take it back to my tech and would really appreciate some options from you guys for him to test and validate.
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Hang on - has it already been to your tech or not? Sounds like you did all the troubleshooting yourself.
 
griff10672 said:
when you say you replaced some dijon caps that " looked " out of spec .... how do you mean " looked " ??

visually they didn't look good ..... or the measurements taken on them didn't look good / out of tolerance ?
Click to expand...
Measured out of spec - tech replaced them - I don't open amps since I zapped myself when I was younger.
 
ZEN Amps said:
Hang on - has it already been to your tech or not? Sounds like you did all the troubleshooting yourself.
Click to expand...
Yeah already to tech for rebias and some trouble shooting - did some work etc, but still not resolved - it's a funny one because the problem doesn't really present itself unless you play through it for about 20 minutes at decent enough volume - the tech should of course do this / validate etc. but I can't stand behind the guy with a whip - though at this point I want to :D Techs that are willing to do this are rare enough around me - let alone competent.

I did all the other trouble shooting myself - anything that doesn't involve opening the amp. When I've had 'amp problems' in the past it's typically a bad jack somewhere (dirty FX loop) or just needs new tubes or some cable acting up somewhere along the line etc. - simple fixes. This is the first issue I've had like this in about 20 years of playing - it's a PITA, but hey it's the journey that's fun right fellas?
 
Just guessing here, but if messing with the presence control temporarily fixes the issues, it could have something to do with the NFB circuit and or the OT/output speaker connection. If there's a cold solder joint somewhere from the NFB —> OT/Speaker jack circuit then this could cause the symptoms you're describing. This includes the NFB resistors, caps, depth/presence pots, caps, switches, OT itself, impedance selector, speaker jacks, etc.

Conversely, it could be something as simple as cold solder joint on a ground connection. Without the amp in front of me, it's impossible to say.
 
Have you tried it out at a different location and were able to repeat the problem? As an electrician I can tell you never underestimate what a structure's power quality can do with electronics. Especially equipment like vacuum tube amps.
 
What do you mean by " I have gone out to Bogner"? Have you taken the amp to the shop? If not, take it there, or send it in if you are not in this area.
 
FourT6and2 said:
Just guessing here, but if messing with the presence control temporarily fixes the issues, it could have something to do with the NFB circuit and or the OT/output speaker connection. If there's a cold solder joint somewhere from the NFB —> OT/Speaker jack circuit then this could cause the symptoms you're describing. This includes the NFB resistors, caps, depth/presence pots, caps, switches, OT itself, impedance selector, speaker jacks, etc.

Conversely, it could be something as simple as cold solder joint on a ground connection. Without the amp in front of me, it's impossible to say.
Click to expand...
Thanks for your thoughts FourT6and2 - good thinking.
 
Soundstorm said:
Have you tried it out at a different location and were able to repeat the problem? As an electrician I can tell you never underestimate what a structure's power quality can do with electronics. Especially equipment like vacuum tube amps.
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Yes, I have tried this - such a great point though. We moved house since this issue appeared and it occurs in both places, plus non of my other amps act up in any way.
 
Jdub said:
What do you mean by " I have gone out to Bogner"? Have you taken the amp to the shop? If not, take it there, or send it in if you are not in this area.
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I emailed them and Jorg replied - to be fair, Jorg was as helpful as he could be given the situation, i.e. not being able to examine it and having to reply on my description of the issue.

Regarding shipping - I live in Ireland. My post office will only ship up to 20KGs (boxed amp weighs a little more...amp weighs 23.2KG) and even then with insurance, etc. it would cost about 600 - 700 euros to send the amp to Bogner in LA. Would prefer to avoid doing this.

If I can't get this resolved locally though, sending it to Bogner is probably the next step and I would have to sort out shipping with a courier - FedEx would probably cost a fortune. Again, would prefer to avoid this if possible.
 
Try to record a video/audio of it happening. Including how messing with the presence control fixes it.
 
GreenRust said:
I emailed them and Jorg replied - to be fair, Jorg was as helpful as he could be given the situation, i.e. not being able to examine it and having to reply on my description of the issue.

Regarding shipping - I live in Ireland. My post office will only ship up to 20KGs (boxed amp weighs a little more...amp weighs 23.2KG) and even then with insurance, etc. it would cost about 600 - 700 euros to send the amp to Bogner in LA. Would prefer to avoid doing this.

If I can't get this resolved locally though, sending it to Bogner is probably the next step and I would have to sort out shipping with a courier - FedEx would probably cost a fortune. Again, would prefer to avoid this if possible.
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Gotcha. I did not realize you were in Ireland. That certainly complicates the matter with regards to having Bogner fix it. Best of luck getting this resolved as quickly and inexpensively as possible.
 
I wish you well in finding/fixing the issue
I have two of these and hope this issue is something I won’t have to deal with in the future
 
FourT6and2 said:
Try to record a video/audio of it happening. Including how messing with the presence control fixes it.
Click to expand...
Plugged in over the weekend...granted did not have a lot of time ...but played for a good 30 minutes, at high volume - volumes at c. 10 o'clock and the problem did not present itself at all. Nearly makes it more frustrating, there's no way it went away.

Also, the presence knob 'fix' used to work before I cleaned the pots etc. Now it doens't seem to work, or didn't before. I'll keep playing it and capture a clip at least - could be helpful regardless. My gut tells me it's in the power section - perhaps NFB etc.
 
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