ADA Microtube 200 Help Needed

[Sparker]

And your meter was set for DC Volts (not AC)?

[davec69]

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

[MarshallJMP]

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.

[Sparker]

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.

[davec69]

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

[Sparker]

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.

[davec69]

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.

[MarshallJMP]

How did you measure those voltages? Can you just measure over those resistors, not to ground?

[davec69]

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

[MarshallJMP]

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?

[davec69]

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.

[MarshallJMP]

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.

[davec69]

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.

[Sparker]

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.

[MarshallJMP]

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.