ADA Depot Too - A Forum To Support Users of ADA Amplification Gear

Let's Get Technical => Power Amp Tech => Topic started by: davec69 on January 21, 2017, 04:03:34 PM

Title: ADA Microtube 200 Help Needed
Post by: davec69 on January 21, 2017, 04:03:34 PM
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.

(http://www.tandemmedia.com/images/mt200/mt2001.jpg)

(http://www.tandemmedia.com/images/mt200/mt2002.jpg)

(http://www.tandemmedia.com/images/mt200/mt2003.jpg)

Title: Re: ADA Microtube 200 Help Needed
Post by: rnolan on January 22, 2017, 05:26:31 AM
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 ?
Title: Re: ADA Microtube 200 Help Needed
Post by: Dante on January 22, 2017, 07:36:44 AM
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:
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on January 22, 2017, 08:57:00 AM
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
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 22, 2017, 02:41:57 PM
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.
Title: Re: ADA Microtube 200 Help Needed
Post by: rnolan on January 23, 2017, 04:24:28 AM
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 ?
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 23, 2017, 06:18:56 AM
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.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 23, 2017, 10:26:05 AM
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 ?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 23, 2017, 12:12:17 PM
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.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 23, 2017, 06:51:33 PM
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 (http://www.tandemmedia.com/images/mt200/adamt200.mp4)
2.  Faint Glowing Preamp tubes - http://www.tandemmedia.com/images/mt200/adamt2001.mp4 (http://www.tandemmedia.com/images/mt200/adamt2001.mp4)
3.  Blinking neon lamp - http://www.tandemmedia.com/images/mt200/adamt2002.mp4 (http://www.tandemmedia.com/images/mt200/adamt2002.mp4)


Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 24, 2017, 03:37:10 PM
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?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 25, 2017, 06:09:56 PM
I went back and checked again.  I checked the 4 pin connector going to the board that has the tube and fuse. 

(http://www.tandemmedia.com/images/mt200/mt2004.jpg)

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
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 26, 2017, 05:21:49 AM
That's the small connector with the shielded wires, there is another one with a big connector, this one carries all the voltages.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 28, 2017, 04:51:28 PM
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.
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 28, 2017, 07:27:56 PM
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.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 29, 2017, 11:59:55 AM
Any ideas on how i should proceed, what I should check or replace?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 29, 2017, 12:22:46 PM
Well, I think I've found the source of the problem.  I pullout the pcb (this time, all the way), so I could replace the neon, and was greeted with a mess on the back of the pcb.  Looks like there was a rubber foot on the back, which was placed right under the C37,D25,D26,R42, which melted/disinigrated, and created a mess.  (See pics)

I used some goof off to clean off all the black goo, then sprayed some Deoxit to remove the corrosion.  The PCB is now clean again.  I also cut out a new card to shield the PCB from the metal enclosure.

I also since I had the pcb out, I replaced R42 (200ohm/.5w).

Same symptoms still persist.  Unit powers on, protect light off, no sound when taken off standby.

(http://www.tandemmedia.com/images/mt200/mtpcb1.jpg)

(http://www.tandemmedia.com/images/mt200/mtpcb2.jpg)

(http://www.tandemmedia.com/images/mt200/mtpcb4.jpg)




Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 29, 2017, 02:09:03 PM
What happends with the voltages if you disconnect 1 or 2 of those big connectors carrying the voltages, do they stay the same?
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on January 29, 2017, 02:13:03 PM
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.


Can you measure the PSU voltages again, using the Green wire as the 0V/Ground ref as follows -

1 = White = +75V
2 = Black = -12V
3 = Green = 0V
4 = Red = +170V

Let us know
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on January 30, 2017, 05:34:13 PM
New measurements below. 

I also tried it with one of the connectors, disconnected, and the reading was the same on the other channel. 
FYI.  I did notice on the front panel, that all the leds for one side were lit, while the connector was disconnected.

1 = White = +75V (80.5v)
2 = Black = -12V (11.88v)
3 = Green = 0V
4 = Red = +170V (08.10v)

Let us know
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on January 30, 2017, 11:53:25 PM
Looks like there could be a problem around Q9 (TIP48) in the PSU - this is part of the HV regulator (buy a couple). This might not be the only fault though, but I would start with trying to get that +170V rail back up first. Would also be a good idea to get a couple of the Q10 as well - MPSA42.
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on January 31, 2017, 03:13:43 PM
Also check R34 should be 220k, sometimes these go up in value.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 07, 2017, 01:44:42 PM
So, I swapped out the TIP48 last night, and noticed that one of the legs was broken on theold part.  After the swap, I still get the same voltages on the 4 pin connector.

Tonight, I will replace R34, and the MPSA42, and report back.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 08, 2017, 07:28:10 PM
Tonight I swapped out the 220K R34 resistor, and Q10 (MPSA42).  No change.  I'm still getting -8v between the Green and Red connector.
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 08, 2017, 11:12:21 PM
Just to make sure, this is with both PSU connectors pulled from the power amp boards and not in standby?
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 09, 2017, 08:36:20 AM
Can you measure the voltage on the standby switch, one side to ground (green), when the standby is off both sides should have around 220V
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 09, 2017, 06:14:42 PM
I checked the voltage to the two connectors on the standby switch, to the green ground on the 4 pin connector. 

First connector on standby switch to ground = 6.9v
Second connector on the standby switch to ground = 50v

When I flipped the standby switch on, I measured 6.9v on both connectors.

Also, I am now hearing a crackling sound from something.  Very faint, and sporadic crackling.  I couldn't track it down.
Title: Re: ADA Microtube 200 Help Needed
Post by: rnolan on February 10, 2017, 01:57:35 AM
Hmm crackling often = bad solder joins, a good thing to do (regardless) is re-flow (remelt) all the solder joins particularly around the power supply (was suggested by MJMP and fixed my (crackling) B200s :thumb-up: ).
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 10, 2017, 03:45:12 AM
Can you measure the voltage over R30 and R40?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 10, 2017, 05:59:43 PM
Measurements from R30 & R40 were a little weird.

Amp powered on in standby, and I got:

R30 = 182v (1st measurement) then I checked again and got 19v.
R40 = -7.92v

Measurement when not in standby

R30 = 7v
R40 = 7.92v

Not sure if it matters, but the led in the power switch also flickers on and off on occassion.

Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 11, 2017, 11:55:34 PM
And your meter was set for DC Volts (not AC)?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 12, 2017, 07:42:11 PM
My trusty Radio Shack meter was in voltage auto mode when I checked.

I did go back tonight and check again, Immediately after first power on, I got

R30 = 176v (in standby)   6.96v (standby off)
R40 = -6.95v

Waited ten minutes with power on, I got

R30 = 12.6v (in standby)   7.92v (standby off)
R40 = -7.92v

Turned power off, waited 10 minutes, then powered up and immediately tested,

R30 = 22.5v (in standby)   6.94v (standby off)
R40 = -6.94v
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 13, 2017, 04:57:02 AM
Check R40, voltage drop is way to low.Should be around 220V.Also check C28 and C29.Like sparker said make sure it's on DC!!!There's no auto setting for AC or DC.You always have to set it manually.
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 14, 2017, 05:08:46 AM
Can you show us a picture of your meter? Does it have the SELECT button underneath the display? If so you may need to press this to get the DC volts reading.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 14, 2017, 05:46:39 PM
Below is a generic pic of the DMM that I'm using.  I also have a super duper GreenLee DMM with all kinds of buttons, but the Radio Shack DMM is much easier to operate.  Sorry for the huge picture.  I found it online.  I set it the meter the same as shown in the pic, for testing voltage.  It defaults to Auto range and DC setting.  Not sure if it matters, but I connect the red lead to the other red connector on the DMM.

Last night, I replaced R30 and R40, and possibly made a little progress. 

Voltage now reads a consistent 235v at R30. 
R40 reads 235v on the side closest to R30, and on the other side, R40 read 177v

I hooked it up and tested for audio last night, but still no sound.

The reading on the both 4 pin connector between green and white is still unchanged at roughly 6v-7v


(https://cdn.shopify.com/s/files/1/0953/5270/products/02200075_00_f21b8df0-ff92-44e5-8341-bf446b9093bb.jpeg?v=1446834229)
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 15, 2017, 09:53:25 AM
So now you have +170V at the end of both power connectors from the PSU board - when both are pulled from the amplifier boards (and not in standby)?

If so, connect power to 1 amp board at a time to see which one is faulty.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 15, 2017, 11:53:45 AM
Sorry if my last post was confusing.  I don't have 170v at the 4 pin connector yet.  Still at 7-8v or so when measured between green and red pins.

I have managed to get 177v at R40, after replacing R40&R30.  Previously, it was 7-8v. 

There must be some other problem between R40 and the 4 pin connector.

BTW, my measurements have been while the 4 pin connectors are connected.
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 16, 2017, 08:19:13 AM
How did you measure those voltages? Can you just measure over those resistors, not to ground?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 16, 2017, 06:22:08 PM
I went back and measured across R30, and I get nothing, when I place a DMM lead on opposite sides.

R40 gives me a measurement of 238.9v

Just to see what I get, I checked R10 as well, and I'm getting 170v.

R42 gives me 8.9v
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 16, 2017, 10:56:20 PM
The voltage over R30 should be very small since there's no real current draw.R40 voltage also looks good.

R10 and R42 are not part of the high voltage??

Can you measure the voltage on the base of transistor Q10?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 18, 2017, 05:57:29 PM
When I test Q10, I'm just getting fluctuating millivolt readings.  I place the black probe on the center leg, and tested the two outside legs.

I tried the same thing on Q5, and got 5.38v
And again on Q3 and got 2.50v

On Q10, just fluctuating mv readings.
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 19, 2017, 11:24:34 AM
The base of Q10 is the middle connection and you need to measure it to ground.  Standby switch off. On the collector you should have the same voltage as over R40.

https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&sqi=2&ved=0ahUKEwjwveGf8ZzSAhUsKsAKHdszDToQFggaMAA&url=https%3A%2F%2Fwww.onsemi.com%2Fpub%2FCollateral%2FMPSA42-D.PDF&usg=AFQjCNGoBE5qzDEe4yFKaGD-4uJU08QfsQ&sig2=1vPiQTQ2woJNI7EnOk0TpA&bvm=bv.147448319,d.d24

==>datasheet of the MPSA42.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 19, 2017, 04:31:04 PM
When measured to ground, standby off, I'm getting:

Emitter= -8.10v
Base= 7.82v
Collector= -7.5v

I checked R40 also, and it's at:

246.4v (Measured with one probe on either side of R40)
7.43v measured with red lead on R40 and the black lead on the green cable of the 4 Pin connector.
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 20, 2017, 02:06:10 AM
Q8/R31/D24/D23 and R34 form a 1mA reference current. This should give around 180V over R35, try measuring over R35. Also keep both power leads out of the amplifier boards (both of them) when measuring, as this could affect readings if one of the amps are duff.
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 20, 2017, 10:48:09 AM
Yep so over R31 you should have 6.2V.The voltage over R35 will be a bit lower then 180V since a part of the current also goes thru R33,Q10.

Don't get it how you can measure negative voltages. Seems to be a ground problem.If you measure over R40 you have the correct voltage but if you measure to ground you don't.One side of R40 is connected to ground so this should give the same result.

Can you do something else? Shut down the amp and let the caps bleed out.Then set your meter to ohms and measure between the ground and both sides of R40, one side should be zero ohms, the other side should be 220k ohm.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 20, 2017, 05:35:26 PM
Can you do something else? Shut down the amp and let the caps bleed out.Then set your meter to ohms and measure between the ground and both sides of R40, one side should be zero ohms, the other side should be 220k ohm.

Tried this, and I get no movement on the meter, for either side.  Once I turn on both switches though, I'm still getting roughly 249v across R40.

Q8/R31/D24/D23 and R34 form a 1mA reference current. This should give around 180V over R35, try measuring over R35. Also keep both power leads out of the amplifier boards (both of them) when measuring, as this could affect readings if one of the amps are duff.

So I measured across R35, with both switches on, and I'm only getting .0010v.

Tried these measurements with both of my DMM's

Not sure if it's related, but my power switch also seems to be a little wonky.  After flipping the power on, sometimes the amp will go off, and I'll need to flip it a couple times.  Sometimes it will go off, after I flip the standby switch off.  Almost seems like a bad switch.
Title: Re: ADA Microtube 200 Help Needed
Post by: rnolan on February 21, 2017, 03:44:55 AM
( Almost seems like a bad switch), if only it were that simple ? it may well be  :dunno: (I hope so  :wave: fingers crossed) sometime it is that simple....
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 21, 2017, 06:38:33 AM
Can you measure the voltage over D24? You should have the same voltage over R31.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 21, 2017, 05:48:12 PM
Can you measure the voltage over D24? You should have the same voltage over R31.

With both switches on, I get 1.03v across D24, or -0.643v, if I swap the two leads.
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 23, 2017, 09:49:43 AM
Lol - Frickin' lovin' this! Can you get another meter and try the same?
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 23, 2017, 01:28:19 PM
Yeah that would be a very good idea because you get some really strange results   ::)
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 24, 2017, 12:47:50 PM
I've tried this with two different meters.  Maybe I'm placing the probes on the wrong points.  Where should the probes be placed to measure D24?
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 24, 2017, 01:54:26 PM
Just on both sides of D24, it can't be you measure 2 different voltages when you swap the probes.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 24, 2017, 06:02:59 PM
Not sure if it mattered, but I replaced the batteries in my meter before testing. 

Today I'm getting 0.685v across D24.

Also, The power switch is definitely getting worse.  Today, it kept cutting out, every time i flipped the standby switch.  After a few flips, it briefly started making a crackling sound.  Eventually, it stayed on.

Any idea where I can get a replacement power switch?
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 25, 2017, 01:33:22 PM
Are you sure you are measuring on D24 and not on D23?
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 25, 2017, 02:59:03 PM
Yes.  I'm measuring the diode adjacent to R31.
Title: Re: ADA Microtube 200 Help Needed
Post by: davec69 on February 25, 2017, 03:31:53 PM
Don't know if it helps, but I went through and tested forward voltage drop for all of the diodes in the area.  Here's what I found:

D20 .483
D21 .514
D22 .674
D23 .552
D24 .694
D25 .543
D26 .523
Title: Re: ADA Microtube 200 Help Needed
Post by: MarshallJMP on February 27, 2017, 10:30:41 AM
D26 is a zener diode of 15V and D24 is a zener diode of 6.2V so you should have those voltages over these.
Title: Re: ADA Microtube 200 Help Needed
Post by: Sparker on February 28, 2017, 10:03:37 AM
Were these readings measured using the Vdc setting on the meter (with the amp fully powered) - not using the diode check function (diode symbol) with the amp power off?