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Old 10th June 2013, 08:34 AM   #11
AJT is offline AJT  Philippines
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Quote:
Originally Posted by still4given View Post
AJT

Yes, that is the same way I will do it after I attach the power transistors and D1-D4. That is basically the same procedure I used for setting the bias on my Low TIM,
i build my amp completely, connecting everything up and then i test....i have been building these amps since the 80's...

Quote:
Originally Posted by still4given View Post
Do you think it is bad to have them gathered together once the amp is running properly?
Please explain your method for setting the bias. Sounds interesting.

Blessings, Terry
you can tie them up together to tidy up your amp once your amp is up and running....

the 10 ohm resistors that i use is for protection...they are to be replaced with regular fuses once amplifier set-up and adjustment is successful

to know that your amp is working as it should, the output offset voltage needs to be less than +-100mV...
then you should be able to adjust bias with the turn of the 5k trimpot, otherwise even if offset voltage is 0 volts, if bias can not be adjusted at all, then there is still something wrong with your amp...
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Old 10th June 2013, 10:48 AM   #12
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from the site
Quote
Testing the Circuit Boards

After you solder the parts to the circuit board, it is tested using the same procedure specified for the Low TIM circuit board. First, you must solder the short circuit jumper across Q7 and you must solder the 100 ohm 1/4 W resistors from the loudspeaker output to the emitters of Q16 and Q17. If you don't have a bench power supply that puts out plus and minus 85 to 93 V dc, you can test the circuit board at a lower voltage. I would prefer test voltages of at least plus and minus 50 V dc. An option is to connect bench power supplies in series to obtain the plus and minus 85 to 93 V dc. I have routinely connected two 40 V Hewlett Packard power supplies in series with the positive and negative outputs of a Hewlett Packard 50 V dual power supply, and I have never had any problems. To protect the circuit boards, you might want to put a 100 ohm 1/4 W resistor in series with the plus and minus power supply leads for the tests. The current drawn by the circuit should be low enough so that the voltage drop across these resistors is less than 1 V if nothing is wrong on the circuit board. There are 2 ground wires from the circuit board. Both must be connected when testing the boards.

I can't stress how important it is to be careful in testing a circuit board. Even simple errors can cause the loss of many expensive transistors. I always use current limited bench power supplies to test a circuit board before and after connecting the power transistors. I also bias an amplifier using current limited power supplies in place of the amplifier power supply. When I initially power up an amplifier with its own power supply, I always use a Variac variable transformer to slowly increase the ac input voltage from 0 to 120 V rms while observing the amplifier output on an oscilloscope with a sine wave input signal. If I see anything wrong on the oscilloscope, I turn the Variac to zero and try to diagnose the problem using the bench power supply. I never use a load on the amplifier for these tests.
Unquote

That is the procedure for testing the circuit boards so that your assembly is not drawing heavy current. This prevents damage to the power transistors and is not to be followed beyond, for any further adjustments.

After you found that your boards pass the above testing, assemble all as required to its full requirement. Also connect the RC across the output, but do not connect the speakers.

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It is not necessary to divide the diodes between the two heat sinks because both heat sinks will operate at the same temperature. I recommend setting the voltage across Q7, i.e. the voltage between the collectors of Q22 and Q23, so that that amplifier is biased at 120 mA. This will give the same quiescent power dissipation per heat sink as in the Low TIM Amplifier.
Unquote

The amp is to be adjusted for idle current of 120 mA.
That means 60mA approx. has to pass through each resistor (R45 to R48) on ver2.1a

Using ohms law which is V=IR, calculate the voltage across R45 wherein I=60mA.
When it is mentioned 'voltage across', it means you have to put both the probes on the component mentioned.

This is done by adjusting P1 on a fully assembled amp.

Before going into the adjustment of the amp, the P1 preset has to be set to the position as mentioned on the site.

It is better you refer to the lowTIM (Leach)explanations as they are more beginner friendly.

Gajanan Phadte
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Old 10th June 2013, 11:09 AM   #13
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Your readings at the beginning are of no use as they are not for a fully assembled amp. By looking at the voltages of Q7, I thought that there was something wrong there.

Even after removing the shorting(at Q7), please keep the series resistors in circuit for the first powering.

Gajanan Phadte
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Old 10th June 2013, 12:27 PM   #14
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Quote:
Before going into the adjustment of the amp, the P1 preset has to be set to the position as mentioned on the site
this pot is set to maximum resistance, 5k, reason being at this position Vbe's collector voltage is lowest and therefore the idle current of the output stage is also at its lowest...
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Old 10th June 2013, 06:56 PM   #15
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Thanks to all of you who are participating. You are exactly who I was hoping would show up. I am remembering a lot of this as I get back into it. I remember going through all of this back then.

Quote:
i build my amp completely, connecting everything up and then i test....i have been building these amps since the 80's...
That is exactly how I did this amp the first time I built it. Unfortunately, when I brought it up to power, I didn't get any signal on the outputs. IIRC, I also burned a couple of resistors. I believe I got the resistor problem sorted out back then but never got it to work. That is why I decided to follow DR Leach's advice and test the board first this time and try to get the boards working first.

This morning I decided to hook both boards up side by side so they would have the same rails and the voltages would more meaningful. I got about half way into checking things when I noticed that on one of the boards, the voltage on Q12C/Q22E was jumping rapidly between 42V-58V. I had some extra devices so I swapped them out with a matched pair. When I checked again, it is still doing the same thing. I can't find anything else fluctuating. The other thing of note is that the Collectors of Q22/Q23 on this board are only about .8V and on the other board they are 2.0V. Perhaps this has something to do with it.

I am going to complete taking readings so I can have a more comprehensive chart.

Thanks again!
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Old 10th June 2013, 08:35 PM   #16
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PCB picture is not very clear, but maybe there is partial short-circuiting between voltage amp to-92 transistor and pre-driver black anodized (scratched) transistor heat-sinks ?
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Old 10th June 2013, 10:57 PM   #17
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while i was in Russia, i walked two guys, one in Manila and another in Hongkong, we did manage to make their amps work.....the guy in Hongkong is now selling SuperLeach amp kits on Ebay....

we did it thru emails.....
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Old 10th June 2013, 11:56 PM   #18
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Quote:
PCB picture is not very clear, but maybe there is partial short-circuiting between voltage amp to-92 transistor and pre-driver black anodized (scratched) transistor heat-sinks ?
I did check for that. There is no continuity between the devices and the heatsinks nor are the heatsinks touching any circuits on the board.

Here are a couple of more recent pics of the boards.

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

Quote:
Originally Posted by AJT View Post
this pot is set to maximum resistance, 5k, reason being at this position Vbe's collector voltage is lowest and therefore the idle current of the output stage is also at its lowest...
So that I'm clear; My pot goes from 0000.2 to 2.2K. 0000.2 is maximum resistance?

Quote:
Originally Posted by AJT View Post
while i was in Russia, i walked two guys, one in Manila and another in Hongkong, we did manage to make their amps work.....the guy in Hongkong is now selling SuperLeach amp kits on Ebay....

we did it thru emails.....
Please don't think I am questioning your experience. I am having to refresh my memory at every step. It has been 8 years since I fooled with any of this stuff and I was a novice then.

I did get my Low TIM working so at least I knew then how to bias it. Please believe me, I appreciate your help and hopefully other will learn from this experience.

Blessings, Terry

Last edited by still4given; 10th June 2013 at 11:59 PM.
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Old 11th June 2013, 12:11 AM   #19
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Originally Posted by still4given View Post

So that I'm clear; My pot goes from 0000.2 to 2.2K. 0000.2 is maximum resistance?

Blessings, Terry
2.2k is your maximum
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Old 11th June 2013, 03:43 AM   #20
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Thanks.

I found a couple of mistakes today. R21 and R22 call for 30 ohm and I had 30K in there. I didn't have any 30ohm so I stuck 33ohm in it to see if it would help. Well it stopped the fluctuation but now I have some voltages that are pretty far off between the two boards. I wish I knew what the voltages were supposed to be, at least on a few of the devices. At least I would have something to shoot for. I was typing up a new chart. Now I have to start over becaaus a few of the voltages changed.

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