R40 Too Hot to Touch for more than 2 seconds.

[hduncan]

Hello All. I've been searching around but just cant figure out whats wrong so I am afraid I need to start another one of these threads...

I turned R39 all the way CCW until I heard slight clicking on rotation, powered it all up according to the instructions, kept my finger on R40 while doing the preliminary tests and it never even got warm..

Then I started doing the adjustment on R39 to get the Transistor Case for Q6 to 22.2V. I was successful but then I noticed R40 was BLAZING hot. Instant burn when I touched it.

I have since moved my POS probe from the transistor case to the "Connector-Side" of the resistor and adjusted R39 until I read 4.8V as I read to do in other threads.

Now it doesnt burn my finger instantly, only after 2 seconds.

Is this normal operation?

I have N72 Revision 3.0

[hduncan]

Side Note: While checking all of my work, I noticed that the BOM states R44 as "Red-Violet-Red-Gold-Gold" and I could only find a "Red-Violet-Red-Gold". Mine has one less gold band than what the BOM says I should.

However, I tested the one I have and get 2.65 and the BOM states that the Gold-Gold is 2K7 so I went ahead and assumed it was a typo.


Also, Here are some pictures I took of the unit in general.

[tpryan]

I have since moved my POS probe from the transistor case to the "Connector-Side" of the resistor and adjusted R39 until I read 4.8V as I read to do in other threads.

What thread is that? Adjust for 3.8 volts across R39. That will give about 83mA through the transformer.

[hduncan]

I bet it was just by brain malfunctioning. I probably read 3.8 and *remembered* 4.8

Thanks for the clarification, I am going to try it now and give you an update in several minutes.

[hduncan]

Ok I adjusted the screw to exactly 3.80V and the resistor now does not feel hot at all, unless I press my finger on it - then it is JUST approaching the point where I'd have to remove my finger. It fees much safer now =)

Is there anything I can do to confirm 100% that the unit is functioning exactly as it should? I don't have a golden ear, and probably won't be able to tell if I am not getting the full potential out of it because I dont have experience with great preamps. These are my first, and my first kit as well, so I just want some peace of mind that I did a proper assembly.

Thank you!!!

[tpryan]

If you have all the parts in the right place and all of your soldering is good the module will work to spec. If you suspect a problem, the first things to check are the DC voltages at Q1 through Q6. If you want to test it further you may be able to press your DAW into service as a basic analyzer, but you'll have to figure that out on your own.

[hduncan]

Hey Tim,

Thanks for being on top of this.

As far as "checking the DC voltages at Q1 through Q6", what exactly do you mean by that? I am a total novice so can you tell me what that means in total idiot-proof terms?

Do you mean to check Q1, Q2, Q3, Q4, Q5, and Q6 by touching the pos/neg probes to the ins/outs of each component?

Thanks!

[hduncan]

I'll take anyones advice if Tim is too busy =)

[tpryan]

http://www.allaboutcircuits.com/vol_6/chpt_2/1.html

[hduncan]

Thanks Ryan!

Any news on that SC25?