100W Ultimate Fidelity Amplifier

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prasi

prasi

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camelator said:
thanks for answer,
I have 4R7 resistor on each rail.
I think the measured value is in phase with the fact that SK1058 snd SJ162 stay cold
I measure +17mv on the 4R7 resistor located at the + rail
and 25mv on the 4R7 resistor located at the - rail
as far I remember the pot R18 is about 100ohm (but not sure)
Turning the screw change the pot resistor value to 177 ohm but not the voltage at the 4R7 resistor
I also checked all the diods and resistors and if I didn't made a wrong reading, all values look fine.

thanks for your comments
Click to expand...

Pot as per design should be 1k, or atleast a 500R. I ended up something like 220ohm for 130mA bias (from memory).
 
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camelator

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Joined 2013
Pic of my board
 

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xrk971

xrk971

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There's not much to go wrong on that amp. If bias adjust doesn't respond usually it's a bad transistor (BD139/140) or a bad resistor(s) feeding the bias circuit. I would use a multimeter and use diode mode voltage drop check between the base and emitter pins. Should be about 0.6v. If not the transistor may be dead. Use same method and check to make sure none of the 1N4148's are dead. Then check all TO92's with same method. You will need to look up pin outs for each kind of transitor to check.
 
Last edited:
C

camelator

Member
Joined 2013
xrk971 said:
There's not much to go wrong on that amp. If bias adjust doesn't respond usually it's a bad transistor (BD139/140) or a bad resistor(s) feeding the bias circuit. I would use a multimeter and use diode mode voltage drop check between the base and emitter pins. Should be about 0.6v. If not the transistor may be dead. Use same method and check to make sure none of the 1N4148's are dead. Then check all TO92's with same method. You will need to look up pin outs for each kind of transitor to check.
Click to expand...

Hi XRK971,
many thanks for your help,
I already checked the resistors and diodes.
I amt very confident about testing transistor while power is up but I'll do that this evening.
If I don't find the wrong one => I'll double check the resistors & diodes.
 
C

camelator

Member
Joined 2013
xrk971 said:
Should be about 0.6v. If not the transistor may be dead. Use same method and check to make sure none of the 1N4148's are dead. Then check all TO92's with same method.
Click to expand...

Hi all,
I've checked all transistors (Base-Emitter),
I have around 0.6mv for the smalls...

But for TO92 :

BD140 is 0.7v
and BD139 is 0.25 V??????

I think it should be higher??? If my understanding is correct it should be around 1V for these ones

Do you think it can be my issue?
 
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prasi

prasi

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Joined 2014
sites.google.com/site/analogdesign1234/
Paid Member
camelator said:
Hi all,
I've checked all transistors (Base-Emitter),
I have around 0.6mv for the smalls...

But for TO92 :

BD140 is 0.7v
and BD139 is 0.25 V??????

I think it should be higher??? If my understanding is correct it should be around 1V for these ones

Do you think it can be my issue?
Click to expand...

Measure on spare BD139 and replace the one on PCB.
After powering, check for overheating of any transistors and also for the 4.7ohm zobel resistor.
If you use a heatsink (small alu. plate on Sonal's PCB) for BD139/140 insulate their backs.
 
Last edited:
C

camelator

Member
Joined 2013
prasi said:
Measure on spare BD139 and replace the one on PCB.
After powering, check for overheating of any transistors and also for the 4.7ohm zobel resistor.
If you use a heatsink (small alu. plate on Sonal's PCB) for BD139/140 insulate their backs.
Click to expand...


Hi prasi ,

thanks for your comments,
I think there is no room for heatsink. Or I have to look for something very thin
 
xrk971

xrk971

Founder of XSA-Labs
Joined 2012
etsy.com/shop/XRKAudio
Paid Member
camelator said:
Hi apexaudio,
many thanks for your attention,
indeed, I put a A92 instead a MPSA92....
Higher gain, and higer voltage saturation :(

Ok hope I have MPSA92 in stock :(

many thanks for this comment
Click to expand...

A92 is same as MPSA92. Just double check pins if using 5401.

For BD's you can mount them on bottom and bent 90 deg to use heatsink or a very thin piece of aluminum with two holes drilled. Use insulator as others mention to keep tab of heatsink from touching and shorting.
 
C

camelator

Member
Joined 2013
Well not good news...
Changing the BD139 didn't solve the issue.
my bias resistor is measured at 5.0ohm and DC is 24.9mv
without power diodes look good.
=> I decided to check diodes with my PSU connected.
I red few mv on diodes, and very randomly, but the last diode I checked, maybe I touched something wrong, JFET temperature rised, and I blowed a 10R resistor....

:( It worked the first day, and now, I think I should change all diodes to be sure. I have 8 PCB left so...
 
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