ADA Depot - A Forum To Support Users of ADA Amplification Gear
ADA Power Amps => Microtube 200 MT-200 => Topic started by: davec69 on March 13, 2016, 12:02:27 AM
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Need some help please. For the past few weeks, my MT200 has been experiencing a slow death (apparently), until last night it finally gave up the fight. (In the middle of a gig no doubt).
The symptoms started a few weeks ago, with occasional crackling and lowering of volume on the B channel. The A channel appeared fine. By quickly raising and lowering the B channel volume, I could get the volume to return. My first thought was the tubes, so I pulled out a backup set last week, and replaced both tubes. To be completely honest, I'm not really sure that the replacement tubes were 100%. The symptoms would come and go for the last few weeks, until finally last night at a gig, I started to get this loud crackling sound in the middle of a song, sort of similar to the sound you get when plugging and unplugging a guitar into a Marshall stack repeatedly, with the amp volume maxed. Eventually, it just became a long loud noise, and the volume of my guitar just faded away. I reached over to turn down the volume on both channels, but the noise didn't stop. Eventually, I had to turn off the power switch. Fortunately, I was also going direct out of my Kemper amp, into FOH,so I could keep playing.
I'm going to try a new set of tubes tomorrow. Any other ideas, as to what I should check while inside the amp?
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The pots are just after the input jacks so it could be a lot of things.Did you power off the amp of did you put it on standby?
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It happened really fast. I want to say that after turning down the volume did not work, I turned the amp to standby first, and the noise kept going, but I'm not sure. I may have just hit the power switch.
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Well try the new tubes first and we'll take it from there.
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Ok, so I swapped out the two 12AX7 tubes today, and the noise still persists.
I have attached an mp3 recording, that may help. (WARNING: Lower your volume, prior to listening to the attached file) It's basically just buzzing, clicks and pops. At the 24 second mark, I put the amp in standby, and the noise stops, I then took it off standby, and you can hear even more noise return.
The meter lights on channel B are lighting in sync with the pops and clicks. The meter lights on channel A are not lighting. The volume knob has no effect on the level of the noise.
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Does this happen on both channels or only on channel B?
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Only channel B it seems.
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Ok my guess (and i can only guess) it's something in the input switching system.So my first guess is U202 (it's a 4053 chip) is busted or not driven correctly.
Since the MT200 can be used as a mono amp this chip (through the mono input and bridge output jacks) controls if the amp is in stereo mode or mono mode.
Now another thing to try but i doubt it will help but you never know,is turn the amp off !! ,take a jack and insert it into the mono input a few times (plug it in and out of few times) and do the same with bridge output.These jacks have switches inside that controls the U202.Maybe there is something wrong (bad contact).Try this first.
You could also try too hook it up in mono and see if you still have this problem??(using the mone input and bridge out)
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Ok, so a little more testing today.
1. I tried repeatedly inserting and removing a plug into the mono input and output jacks, and then testing both channels in stereo.
Result: I'm now only getting sound, and metering from channel A. Channel B has no audio or metering activity now. Only a solid buzzing sound. Similar in sound to an amp that's not grounded correctly.
2. I unplugged all the cables, and then inserted one input and speaker output into the mono jacks.
Result: Only channel A had any activity (I guess this would be normal in mono?) I did hear the guitar coming through, and the metering was working on channel A. In addition I also got the buzz. (from channel B I assume)
3. As a last test, I swapped the two output cables, just to make sure it wasn't a speaker issue.
Result: I got the same result as test 1. This time with the speakers reversed. This time, though, I did get the clicks and pops on channel B. Maybe just as a result of turning the amp on and off.
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Well that seems to tie it down to channel B, not metering on B makes me think input related. Have you given the input and output jacks a good clean with contact cleaner ?
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I'll give the jacks a cleaning tomorrow. I can pickup a CD4053 IC at a local shop. Should I try swapping it?
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Well it probably wouldn't hurt (if it's not too expensive) and will rule it out, but pots and jack cleans are easy.
As an aside (and I have no idea if this will fix it, but certainly won't hurt), I had very similar (sound wise) crackles in one channel of my B200s, after 2 times to the tech with no improvement MJMP suggested re-flow the solder joints around the power supply (he'd had the same issue (so apples with apples here)), but then he said may as well re-flow the whole amp (ie re melt all the solder joins) which I did (and added new solder to the joins here and there where needed, but mostly just re-melt them all). Worked a treat, my B200s is now back in business. As I said, it may not fix it but it won't hurt (as long as your careful not to wreck a trace etc). I wasn't getting the buzzing though, just the crackles (so loud I was worried for the speakers..).
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Well a CD4053 is cheap (like 1 $ or less) and it should be in a socket so it's easy to replace.
But it seems the input jacks aren't so good anymore either.
Maybe a bad connector or like R says a bad solderjoint could also be the problem.Try to disconnect/connect those small red headers.These carry the audio signal from the inputs to the pcb's.
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Success!!! :banana-rock:
My MT200 is alive again. Not sure which task actually repaired the problem, but I went through all the suggestions posted in this thread.
1. Replaced the CD4053 IC. Picked up 5 locally for .35 cents a piece.
2. Removed the I/O jack board, and sprayed all the jacks down with Caig Deoxit 100% Cleaner
3. Used a can of compressed air, clean out the inside, with particular focus on the fan area.
4. Reflowed the solder on every visible joint that I could reach, with dissasembling.
5. Put the original Svetlana 12AX7s back in the amp.
Put everything back together, fired it up, and glorious tone has returned again....My MT200 has never sounded better. Even sound like I gained a little extra volume. Just in time for our San Francisco gig this weekend. Gonna give the amp a workout at rehearsal tonight.
Thanks to everyone who chimed in on the thread. If anyone needs a CD4053 IC, I have a couple extra. Let me know.
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Hey fantastic news :thumb-up: :whoohoo!:
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Nicely done,glad it works again. :thumb-up: