Let's Get Technical > Power Amp Tech
ADA Microtube 200 Help Needed
davec69:
I think I may have killed my MT200. Over the years, I have been lucky in keeping it up and running, but this time I was not so lucky.
So, I recently started having some trouble with the MT200 coming out of protect mode. A few years ago, I had a similar problem. After reading through threads here, I found that it may be a neon amp issue, so I ordered some replacements. When they arrived, I swapped out the lamp, and I was up and running again. (BTW, I also replaced every electrolytic cap at that time, to give the amp even more life) The amp has worked great since.
Fast forward, 3 or 4 years later, and I again started seeing signs of lengthy protect mode startups. So, I thought I'd try replacing the lamp again. All seemed to go well, but on the first start up after the lamp swap, I heard a very faint crackle sound. So low, I thought it could have been my imagination. After warm up though, I am now not getting any sound from either channel. The amp still powers up as usual, and everything appears to be working, but I get no audio.
Today I opened up the MT200 again, to look for anything obvious, but the only thing that I found which may be a problem, is a resistor, that could possibly be blown. The resistor is found on the PSU Loading Chart schematic, and is labelled R42 (200ohm /.05w). (FYI. The actual resistor on the board is 270ohm/.05w) I'm putting a picture below. As you can see, the center of the resistor looks a little brown. Before replacing it, I thought I'd post here for thoughts.
rnolan:
Hey davec69, Something is not right (obviously LoL), there's also brown colouration on top of some of the caps :dunno: . MJMP will have a better idea than me. The output transistors look ok. Do the tubes light up ok when you power on ?
Dante:
I don't know the first thing about this stuff, but those are some of the best damn close up pix I've ever seen, WOW :thumb-up:
Sparker:
Hi Dave.
First off, re-check your neon replacement work and make sure there are no solder splashes anywhere,
broken wires, etc, anything else that may be giving the fault.
R42 is part of the -15V supply rail, it sets the current to the 15V Zenner Diode D26 (1N4744A).
This -15V supply rail feeds the input buffer opamps (as well as other things), so if missing could
block signal through both channels, even though the protect mode is not activated / on.
Other components involved in the -15V supply are: D25 (1N4007), C36 (470uF/35V) and C37 (0.1uF/25V).
As Richard said, C36 looks a bit tarnished (i.e. could be dodgy/short). So, as a quick hit, without
taking any measurements, you could just replace all these components ... and try it (but buy 3 of each).
If it still fails, then you will need to dig deeper and start isolating circuits & taking measurements.
HTH
Mike
MarshallJMP:
What parker says is a good way to start.You could measure over D26 (which is next to D25 which in turn is next to R42). See if you have -15V there.
Another problem could be R34 (220k/0.5W) they also go up in resistance until the high voltage gives up. That could also be the problem (had this before with these).See if you have +170V on that red connector.
Or a blown fuse (3A SB) on both amplifier pcb's (which would be strange).
How to check the other voltages on the 4 pin red connector on both amplifier sections.
Pin1= +65V
Pin2= -12V
Pin3= Ground
Pin4= +170V
-15V you can measure over D26.
Let us know what you can find out.
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