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Old 7th March 2014, 05:44 AM   #31
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Hey guys,

Tauro - Q105 Collector has 38.2V on it, meaning it's not conducting!

I took your advice sbrads and placed two 220R (it was all I could find in my parts box!) resistors in series with the +-47 rails before the power caps as I couldn't find an easy place to put them. This has causes me to have +-42V straight of the bridge as opposed to +-47V, is that to be expected?

I have then shorted Q122 so the amp stays powered on and have found something interesting I think...

As in you noted in your previous post sbrads, where is the 0v connection on the inputs? When I first starting measuring I had 32.2V on TP101 (and all connections that are in parallel with it), however this steadily started dropping with my multimeter connected. It got all the way to 19V when I powered the amp off however I am sure it kept going.

My take on this: C109/C110 are powering this up until those caps become charged. I measured one the + side of these caps, the top side of C105, C/E of Q101 and I have a constant and stable voltage there.

So I think CB101 must have a ground connection somewhere on the other board that this needs.


Thoughts?
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Old 7th March 2014, 01:27 PM   #32
sbrads is offline sbrads  United Kingdom
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42v is fine for safer fault finding. It also means your amp isn't drawing loads of current which is also good.

It's looking more and more like you have a 0v connection problem. If you have another 220R resistor (again, for safety rather than a dead short!), connect it between the 0v right at the reservoir caps and the bottom of R101. See if that amp's output voltage drops to near 0v. If that works, try the same on the other amp. R101 and R102 are a bit high really, so the output may still have a small offset voltage.

Last edited by sbrads; 7th March 2014 at 01:30 PM.
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Old 15th March 2014, 09:32 AM   #33
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Hey guys,

Finally got around to getting some time to do some more testing, I grounded a 220R on the bottom of R101 and this is what I have found (attached).

My observations:
1. I think Q101 is now conducting as I now have a voltage drop across R107 and I have 0.76V on both sides of R109, (it was 14.4V on the left and 17.2 on the right when I didn't ground out R101)
2. I have a stable 26.5V at the output of the left channel and 26.45V at the output of the right channel. Measuring across the outputs from L to R I have 50mV...
3. I still have 0.7V at R165 meaning protection still kicks in.
4. It's still mirrored (my measured voltages from the diagram) onto the R channel as well however measuring from the RHS of R119 to R120 I have 58mV. This could just be because of component tolerances I guess...
5. I have the expected voltage drop still across the B/E of all the transistors, so I'm thinking it's not one of those?

What do you guys think? Should this be grounded through CB101 normally or?
I'm really not coming up with much else...

Thanks again!

Felix
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Old 15th March 2014, 10:58 AM   #34
sbrads is offline sbrads  United Kingdom
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The left side of R109 should be at 14.5v fed from D109 zener. This would affect both channels. If it's really 0.76v it may also indicate other supply problems in that D109 section.
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Old 15th March 2014, 04:25 PM   #35
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Hi,
I think Q105 it is not conducting. Base it is 38.4 and the emitter it is 39 volts If it is conduct it will make the collector negative. That would bias transistors Q109 & Q111 base to 1.1 volts
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Old 16th March 2014, 11:37 AM   #36
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Hey Guys,

Sbrads I think you are onto it, looking around D109 I had 0.75V when I had R101 grounded, however 14.5V when it wasn't. I then noticed the top of the Q133 FET has a 25v supply, coming from the rear power amp. I plugged that board back in and it gave me back my 14.5V and a lot more sensible voltages on all of the transistors (-1.1 on Q105 for example). So, I think it turns out this does have to be connected however I now have 150mv on both channels at the output and -0.7V at R165 meaning circuit protection still kicks in. Now I am getting proper readings though I will be able to do some better diagnostics!

Regarding CB101, I'm pretty sure the +-15v there are supplying the other board, however the earth connectors I think are actually needed. It would be a little tricky to plug this back in and still easily test, so would it be safe to scrap the 220R (going to ground from R101) and directly wire the earth pins of CB101 to 0V? This way I wouldn't have the 500mV around there and I can get precise readings... I still have the 220R in with the bridge so I shouldn't be able to draw too much current... thoughts?

I should hopefully get some more time tomorrow to get some better readings!

Thanks again!
Felix
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Old 16th March 2014, 12:07 PM   #37
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by FelixAUS View Post
Hey Guys,

Sbrads I think you are onto it, looking around D109 I had 0.75V when I had R101 grounded, however 14.5V when it wasn't. I then noticed the top of the Q133 FET has a 25v supply, coming from the rear power amp. I plugged that board back in and it gave me back my 14.5V and a lot more sensible voltages on all of the transistors (-1.1 on Q105 for example). So, I think it turns out this does have to be connected however I now have 150mv on both channels at the output and -0.7V at R165 meaning circuit protection still kicks in. Now I am getting proper readings though I will be able to do some better diagnostics!

Regarding CB101, I'm pretty sure the +-15v there are supplying the other board, however the earth connectors I think are actually needed. It would be a little tricky to plug this back in and still easily test, so would it be safe to scrap the 220R (going to ground from R101) and directly wire the earth pins of CB101 to 0V? This way I wouldn't have the 500mV around there and I can get precise readings... I still have the 220R in with the bridge so I shouldn't be able to draw too much current... thoughts?

I should hopefully get some more time tomorrow to get some better readings!

Thanks again!
Felix
It should be safe to link 0v to CB101 E pin, but I always believe in safety first so I would be tempted to put 10R in series, measure the voltage across it and if the current (ohm's law and all that) is sensible then you can short it out. Sensible current? Experience is everything there, but <100mA should be OK and I would expect it to be much less than that.
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Old 18th March 2014, 11:16 AM   #38
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Hey Guys,

Did as you said sbrads and only had around 1mA, so straight grounded it and everything is fine. Getting MUCH more sensible voltage readings now. (Finally, it feels like we're getting somewhere!).

So, looking at my new measurements I have around -29mV on the output of the left channel and -27.2mV on the output of the right. Is this too much negative bias? Measuring all the junctions of the transistors I have the correct voltages as I should (I think) except for this smallish negative bias.

Looking at R165 I still have -0.74V, so protection still kicks in. How can this be? Is this because of the 1.07V I have on the bottom of D103? (the muting circuit being fed from the IC?).

Let me know what you guys think, nothing obvious is hitting me...

Cheers!
Felix
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Old 18th March 2014, 07:41 PM   #39
sbrads is offline sbrads  United Kingdom
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The -27.2/-29mv readings are fine. Still pondering over the shutdown voltages, bit strange at first sight.
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Old 18th March 2014, 09:50 PM   #40
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Hi,
Output should be withing 4 mv.
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