Hello,
So recently I made this kit and all I neded is to bias. So as I started doing bias my fuse blew. I tought I put a wrong fuse but I didn’t. I replaced the fuse and it blew up also. I don’t know what the problem is. I tried to locate it but I failed. There are no burnt leads on the PBC and there is no “smell” that something is burned. And i havent seen any shortout.
p.s sorry for my english
Did you perform the lightbulb test as shown in the manual?
Most likely there is an assembly or hook-up issue.
Can you provide LINKS to HI-RES and SHARP pics of your assembly?
We can take a look and maybe provide some feedfack.
The light bulb part was smooth, and i replaced it with a 1A fuse. The apm worked fine but then there was fairly loud hum on the speakers. So I tried ajusting the bias and during the ajustment the fuse blew. And I can’t get the amp up and running.
Here are links to the pictures:
fbcdn-sphotos-h-a.akamaihd.net/ … 9e9fce2bce
fbcdn-sphotos-h-a.akamaihd.net/ … 2fe56a7273
fbcdn-sphotos-h-a.akamaihd.net/ … 95226949eb
fbcdn-sphotos-h-a.akamaihd.net/ … 49e77e5b81
We are unable to spot any obvious assembly problems.
Since it has worked for a while, we have to assume that the power transistors have not been swapped around.
Did you adjust the bias while a signal was fed to the amp? (do not do this).
What is the AC output voltage of your transformer (measured, not what is indicated on the transformer)?
Most likely, one or both power transistor are defective and need to be replaced.
It is also possible that transistors of the drive circuit have been damaged or destroyed.
We’d recommend to replace them.
I was ajusting the bias while I was feeding the signal. How can I fix that? By replacing all transistors?
And what can I do to get rid of the humming noise?
Also I am working on a new kit of yours, K8084. One of the TL072 is missing, I can’t reutrn the product beacuse I have bought it last month.
Can I replace TL072 with TL072CP?
Thank you for your time and help! 
We’d recommend to replace all transistors, just to be sure.
Regarding the hum, this is difficult to solve from a distance.
Make sure wiring layout is neat and tidy and according to good engineering practice.
Use shielded wire for the audio input. Make sure there are no ground loops.
Sorry about the missing part. Most likely the part you mention is OK to use.
If you provide an address, we’d be happy to mail a spare IC.
Thank you for your help,
No need to ship the IC because the shipping would be more expensive than the IC
Most likely yes, but we would take care of it.
Hello,
So recently I made this kit and I’ve a problem to adjust bias with trimmer RV1, the minimum voltage on R19 is 0.09V (with the trimmer at the minimum position), is impossible arrive to 0.01V.
I also noticed that the transistor T3 (BC639) becomes hot just after switching (I’ve measured 50°C)
I’ve perform the lightbulb test as shown in the manual and it is ok.
I’ve test the power, I’ve +44Vdc and -44Vdc (the output power of transformer is 33VAC).
There is some control I can do to find the problem?
Thanks
Alessandro
Please note that 30VAC is the abolsute max. supply voltage (see specs).
Can you provide LINKS to HI-RES and SHARP pic of your assembly (both solder and component side?)
At the following link you can download the photos in high resolution:
drive.google.com/folderview?id= … sp=sharing
The transformer used is a Velleman 12030, the measured voltage in continuous is + 45Vdc and - 45Vdc, it is possible that this is the problem?
I also tried to replace all transistors, but the problem remains.
Thanks
Voltage is at the limit but should still be OK.
Sorry, the pictures do not allow us to spot a possible cause of troubles.
We can offer inspection/repair of your kit if you return it to:
Velleman Projects Tech. Dept.
Legen Heirweg 33
9890 Gavere
Belgium
Please include a detailed fault description.