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Old 22nd April 2008, 10:50 PM   #141
hoka is offline hoka  Netherlands
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You are right the silkscreen orientation of Q 5 is indeed wrong and must have been the cause of the blowup.

I have not been able to bring them back to life.

Shame
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Old 24th April 2008, 03:28 PM   #142
cdswift is offline cdswift  United Kingdom
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Default DC Offset output voltage

Has anyone experimented to solve the DC offset output voltage problem. Mine was about 70 mv with Q1 & Q2 as 2N5551 (the supplied transistors). These have an hfe of about 195. I have swopped them for BC550 which have an hfe of about 600 and the offset drops to below 20mv.

The only problems that I can see with the BC550 are lower max VCE voltage (50 versus over 100) and frequency (100 MHz versus 200 - 300).

Anybody else had any experiences here, or indeed any alternatives for Q1 & Q2 ?

Regards

Chris
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Old 24th April 2008, 03:37 PM   #143
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Chris,

Try 1.5k as R6, that might solve your problem, 1k seems low for the 500uA tail current.
Beta-selected Q1/Q2 would be a good point too.
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Old 24th April 2008, 03:57 PM   #144
Kan is offline Kan
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Default Re: DC Offset output voltage

Quote:
Originally posted by cdswift
Has anyone experimented to solve the DC offset output voltage problem. Mine was about 70 mv with Q1 & Q2 as 2N5551 (the supplied transistors). These have an hfe of about 195. I have swopped them for BC550 which have an hfe of about 600 and the offset drops to below 20mv.

The only problems that I can see with the BC550 are lower max VCE voltage (50 versus over 100) and frequency (100 MHz versus 200 - 300).

Anybody else had any experiences here, or indeed any alternatives for Q1 & Q2 ?

Regards

Chris

20mV is good result.


BC550 is OK for +/-40VDC operation!
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Old 28th April 2008, 09:04 PM   #145
ratchet is offline ratchet  Netherlands
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Finally

One of the two apms, waiting for the PSU to be ready (at least ready enough for testing).
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Old 29th April 2008, 07:00 PM   #146
ratchet is offline ratchet  Netherlands
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Connecting the transformer

Does anyone have any ideas if it makes a difference how you connect the transformer, method A or method B?
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Old 29th April 2008, 07:24 PM   #147
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"A" os OK, "B" wouldn't work at all!

Anyway transformer gives AC voltage not DC! (50/60 cycles of -/+ in a second.)
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Old 29th April 2008, 07:37 PM   #148
ratchet is offline ratchet  Netherlands
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Thanks. I know about transformers working only for AC. I just economized on my basic and largely inadequate drawing skills. :-)

But I was wondering, after the rectifiers converting to DC of course and after the caps doing their business, if there is a preference for either option.
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Old 2nd May 2008, 05:26 AM   #149
ttan98 is offline ttan98  Australia
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Quote:
Originally posted by ttan98



A few suggestions:

1. use 1-2amp fuses on the +/- 35Vdc power supply.
2. use a variable transformer at the main and slowly increase the voltage, eg apply say 1/3-1/2 of the main voltage and measure relevant/key bias voltages on resistors and transistors. that way you can quickly detect incorrect orientations of transistors or incorrect values of resistors.

cheers.
Recently my power blew up, and took the power
transistors as well.

I will outline the procedure below I took to fix it, similarly it can be followed when a power amp kit is first power up. It avoids replacing expensive power transistors if wiring or soldering are incorrect performed or defective components.

method 1
======
less dangerous, put a fast blow 2A fuse on the +/-power supply voltage. If there is a short the fuse will blow.

method 2
======
I use this method, if you don't take precaution(can be dangerous) don't try it. I put 2*100ohm(*), 1/4W carbon resistor on the +/- power supply terminals on the PCB. If there is short, the carbon will burn quickly hence protecting the power transistors. You got to be quick, on power up if there is a short, the resistor will burn and quickly turn power off. If there is no short, the supply voltage on the PCB will be much less than the voltage at the power supply because of the drop across the 100ohm resistor, eg power supply is 45V and PCB is about 20V.

* values depend on the total quiscent current the amp is taking.

Now my amp works.
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Old 4th May 2008, 08:26 PM   #150
ratchet is offline ratchet  Netherlands
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Default Help

I hope this thread is still alive. I assembled the two boards and connected them to a temporary PSU (just transformer, rectifiers and 2x2 10.000uF caps). The PSU measures just fine, about 44,3V plus and minus. I did connect the signal ground to the PSu ground.

But, no matter how I adjust the bias pots, it seems to make no difference. Q4 gets rather warm (not hot) and both amps sort of stabilize around 25-30mA in the V+ lead. This happens with both boards, ruling out (I hope) the more simple explanations like a transistor that got to hot or an error in soldering resistors etc.

Any ideas any one?
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