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Author Topic: ADA Microtube 200 Help Needed  (Read 12102 times)

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davec69

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ADA Microtube 200 Help Needed
« on: Time Format »

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.







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rnolan

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Re: ADA Microtube 200 Help Needed
« Reply #1 on: Time Format »

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 ?
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Dante

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Re: ADA Microtube 200 Help Needed
« Reply #2 on: Time Format »

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:
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Sparker

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Re: ADA Microtube 200 Help Needed
« Reply #3 on: Time Format »

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
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MarshallJMP

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Re: ADA Microtube 200 Help Needed
« Reply #4 on: Time Format »

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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rnolan

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Re: ADA Microtube 200 Help Needed
« Reply #5 on: Time Format »

Excellent, here come the experts  :whoohoo!: . They'll get you sorted  :thumb-up: . And as mike says have a good check around the neon.  My gut feel (for what it's worth) is this isn't the issue but rule it out as it could be and that's what you've changed.
@ Dante, they are good pics  :thumb-up: , MJMP also takes some good ones though LoL.

@MJMP, is the brown ring/tarnish on the resistor a cause for concern ?
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MarshallJMP

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Re: ADA Microtube 200 Help Needed
« Reply #6 on: Time Format »

Well those resistors that are used with zener diodes can get hot.So not really a concern but it's best to check it out anyway.
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davec69

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Re: ADA Microtube 200 Help Needed
« Reply #7 on: Time Format »

Thanks Rodan.  The marks on top of the caps were made by me with a orange highlighter, when I replaced the caps.  It was my way of keeping track of which caps I had already replaced. 

I haven't checked whether the tubes light up, I can do that tonight, and report back. 


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 ?
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davec69

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Re: ADA Microtube 200 Help Needed
« Reply #8 on: Time Format »

Another question for the experts.  When working on the PSU board, do you remove it, or just tilt it up for access?  I could not find an easy way to remove the board, which made it difficult to replace the lamp.  It's entirely possible that the solder was not good.
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davec69

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Re: ADA Microtube 200 Help Needed
« Reply #9 on: Time Format »

Ok.  Here's what I found, from a little further testing tonight.

Firstly, the neon lamp appears to be blinking when the amp is on, with or without stand by.

Yes.  The tubes appear to be glowing, though the glow is very faint.

Voltage measurements

D25 - 23.36/ on side nearest transformer - 14.86v other side
D26 - (-14.78v)
R34 - (177.8v)

Voltages for the red plugs going to the preamp boards were pretty much the same for both channels.

Of the 4 wires  Both the outside wires read roughly 11.5v. The two inner wires both fluctuated in the 5mv range.   

Below are a few video links, in case it helps. (Sorry for the phone ringing)

1.  General Overview of the inside of the MT200 - http://www.tandemmedia.com/images/mt200/adamt200.mp4
2.  Faint Glowing Preamp tubes - http://www.tandemmedia.com/images/mt200/adamt2001.mp4
3.  Blinking neon lamp - http://www.tandemmedia.com/images/mt200/adamt2002.mp4


« Last Edit: Time Format by davec69 »
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MarshallJMP

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Re: ADA Microtube 200 Help Needed
« Reply #10 on: Time Format »

How did you measure the voltages on the red connector? You always need to measure from pin 3 ground to the other pins ,so between pin 3 and 4 you need to have 170V, pin3 to 2 -12V and pin3 to 1 65V.

The neon lamp doesn't blink on and off is it? it's just sort of moving around inside?

On D25 do you mean you measure 23.36 V on the kathode and 14.86V on the anode?
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davec69

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Re: ADA Microtube 200 Help Needed
« Reply #11 on: Time Format »

I went back and checked again.  I checked the 4 pin connector going to the board that has the tube and fuse. 



Pins 1&2 =11.86v
Pins 1&3 = 11.87v
Pins 1&4 = 0.626v

Pins 3&1 =11.87v
Pins 3&2 = 0
Pins 3&4 = 11.25v

I should have said flickering on the neon lamp, not blinking.  It doesn't go on and off, it just flickers as shown on the video.

I re-measured D25, and I'm getting roughly -24.08v on both sides.






I mesure
« Last Edit: Time Format by davec69 »
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MarshallJMP

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Re: ADA Microtube 200 Help Needed
« Reply #12 on: Time Format »

That's the small connector with the shielded wires, there is another one with a big connector, this one carries all the voltages.
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davec69

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Re: ADA Microtube 200 Help Needed
« Reply #13 on: Time Format »

Ok.  Here's the measurements from the other connector on the preamp boards.  There are four wires.  Red, Green, Black, White.  I used the black wire as the ground.

Red / Black = 3.736v
Green / Black = 11.89
White / Black = 092.2v

The measurements for both channels were pretty much the same.
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MarshallJMP

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Re: ADA Microtube 200 Help Needed
« Reply #14 on: Time Format »

On the red/black you should have 65-75v that powers the power amps.If you only have 3V then it's normal your poweramps don't work.So there's something wrong in the power supply.
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