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Old 1st March 2010, 12:30 AM   #7201
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Please help me!!!!!!

I justed finished my first diy amplifier f5! But the one channel has a problem, across the R12 (0.47ohm) I have 23mv with the 5k pot to zero and the other R11 gives 0V(0.47ohm) with the 5k pot to zero, so when I try to raise the pot of R11 the voltage across R12 also rise, in addition the 100ohm resistors R6-R5-R7-R8 get quickly very very hot....

Now trying to setup only the one channel!

Thanks
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Old 1st March 2010, 01:02 AM   #7202
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follow the procedure written by NP.

also you will need to adjust one pot and then the other say in one turn or half turn increments... ****are you using a 20 turn pot ?????????
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Old 1st March 2010, 08:45 AM   #7203
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Quote:
Originally Posted by prooptiki View Post
Please help me!!!!!!

I justed finished my first diy amplifier f5! But the one channel has a problem, across the R12 (0.47ohm) I have 23mv with the 5k pot to zero and the other R11 gives 0V(0.47ohm) with the 5k pot to zero, so when I try to raise the pot of R11 the voltage across R12 also rise, in addition the 100ohm resistors R6-R5-R7-R8 get quickly very very hot....

Now trying to setup only the one channel!

Thanks
Its a bourns 3296 multiturn. The truth is that before powering the amplifier, I turn all the trimpots to zero by turning them anticlockwise (following the procedure written by NP), but when I fired the amplifier across the R11&R12 I had aboyt 1.2v!!!, As a new one in diy it passed a little time to realize what happened so I closed the amplifier quickly when I realised that the R11&R12 are burning with a little smoke!!! After that I turned all the trimpots full clockwise, and I fired up again, as I told you the one channel measures ok along R11 & R12 (when I get 0.50mv the output gives 0.37mv) I left it for 30minutes (only the one channel) and I didn't get any different voltages, not even the amplifier heatsinks get hot at all, is this ok!!!

What else I have to check or measure so as to help me?
(by firing the problematic channel the 100ohm resistors get really hot....)

Last edited by prooptiki; 1st March 2010 at 08:49 AM.
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Old 1st March 2010, 09:14 AM   #7204
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when you fire the problematic channel what voltage do you get on the output ? also the bias on the 0.47 Ohm resistors is meant to be 0.59 or 0.6 Volts (not milli volts..)

if you take a photo of the board close up i can help perhaps

or take a look at mine:
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Old 1st March 2010, 09:21 AM   #7205
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Sorry for my mistake volt is correct!

the working channel : when I get 0.50v along R11-R12 the output gives 0.37v
the problematic :as I said R12=0.20v and R11=0v when I setup R12=0.59v then R11=0.15v (I didn't count the output dc offset)
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Old 1st March 2010, 09:24 AM   #7206
AndrewT is offline AndrewT  Scotland
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Hi,
if the variable resistors smoked, then replace them. The current capacity of these items is tiny.
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Old 1st March 2010, 09:31 AM   #7207
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As I said the R11&R12 burnt for a while (but they measure ok)! The trimpots are working ok I beleive, becase they are changing the volts across R11&R12 with the big difference R12=0.20v and R11=0v when I setup R12=0.59v then R11=0.15v!
I am afraid the problem is at Q1 &Q2 (2sk170-2sj74), in addition as I said the four 100ohm feedback resistors, when I firing the amplifier, in a second gets really hot....

Last edited by prooptiki; 1st March 2010 at 09:35 AM.
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Old 1st March 2010, 09:57 AM   #7208
AndrewT is offline AndrewT  Scotland
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Hi,
if the variable resistors are set to minimum then their resistance should be zero ohms.
This will ensure that Vgs of upper and lower output FETs is set to zero. That in turn ensures there is zero output current.

If all these zeros don't happen then the variable resistor is not doing it's job.
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Old 1st March 2010, 10:13 AM   #7209
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How can I check a variable resistor that it is in zero ohms, there are three legs! full clockwise or anticlockwise?

Click the image to open in full size.


Quote:
Click the image to open in full size.
This is my layout, so connecting the polymeter to the legs 1-2 (together) and the 3 pin I must get zero ohm correct?

Last edited by prooptiki; 1st March 2010 at 10:25 AM.
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Old 1st March 2010, 10:30 AM   #7210
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This is my layout:

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