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Old 2nd September 2012, 02:50 PM   #11
Ronj is offline Ronj  Singapore
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Join Date: Jun 2010
Location: Singapore
I have reached the point of no return. Removed all the driver transistors, power transistors, emitter resistors and the bias diodes. Also removed the transformer.

Click the image to open in full size.
Click the image to open in full size.

There are few interesting findings. transformer has two secondaries. One measures 35VAC while the other is 7VAC. I was surprised that, the only use that 7VAC had was to power that small power-on/pilot bulb. Nothing else. It doesn't go anywhere else. Well... except that one of the lines(of the two tapping that gives 0-7V) is connected to the system ground.

The two yellow wires in the picture are 7V lines. While the two white lines in the second picture are 35V lines.

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


Another interesting thing is the bridge has used two different type of diodes. What seems to be a higher current rating diode is used on the negative part of the bridge while the positive part does away with two 1N4001 type diodes.

And for powering the pre-amp section; it seems to be using just a resistor based voltage divider. Which measures +/-17V against the common ground. My comprehension of this stuff is not that good either, but at least that's what I can see...

Some more pictures showing the diodes and what I consider to be a voltage divider....
Click the image to open in full size.

Now the question is should I go for an unregulated power supply, with a 12V transformer which would give +/-16VDC after rectification(using the on board ones after removing the voltage divider) or should I use a L7815CV and L7915SCV (I have them) and give it a regulated +/-15V and take the rectification of board?

For the 1st option, I readily have a transformer with two 2A 12V secondaries with me... The second tapping, I can use to power my T-Amp regulated with a LT1084-12CT(Linear Technology LDO 12V fixed 5Amp capable).

For the second option again I have a +/-18V 30VA toroidal transformer which I can use for the pre-amp section regulated with 7815/7915 pairs. And I will have to put in an additional 12V transformer from my parts bin for the T-Amp though with regulator as in option1.
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Old 3rd September 2012, 04:18 AM   #12
Ronj is offline Ronj  Singapore
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Join Date: Jun 2010
Location: Singapore
Before coming to work this morning, I just checked and it looks like +Ve terminal of those blue caps are connected to the emitter resistors. and -Ve terminal is connected to the speaker out. That means they are not filter caps for the pre-amp section but coupling caps for the power amp stage?

Looked in a hurry but that how it looked like. I will re-reconfirm. But yeah the bridge rectifier is the same and someone must have replaced the diodes. But then what a strange design. Filter cap sits 1mile away from the bridge on separate wires. As circled here..

Click the image to open in full size.

And that leaves me with a bigger mess as it means the pre-amp section works on unregulated 47V. And I can't do away with the original transformer then. And I don't think I can regulate it down to 12V for T-Amp as well. So I may have to add a second transformer!
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Old 3rd September 2012, 05:09 AM   #13
Ronj is offline Ronj  Singapore
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Join Date: Jun 2010
Location: Singapore
Just thinking aloud! How about I use the center tapped 18-0-18 toroidal transformer I have in hand and do a full-wave bridge rectifier discarding the center tap. I think I will end up with 50V DC then. Then use a simple 47V zener in a N-P-N Emitter Follower regulator? What do you think about that? That way pre-section will have cleaner power. Typically how much current pre-sections like these need? any idea?
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