K8060 blew fuse after bias adjustment

Kits Audio Hi Fi Projects
1

Hi,

I did the bias adjustment on my finished amp without any problem. LED came on and adjustment was fine.

I then connected an 8ohm speaker (I also changed the LED for one to go on my front panel), connected a pot and input jack and Earthed the R transformer connector to the chassis. As soon as I powered it on the speaker gave a humming noise (not too loud) and the LED came on but then the 1amp slo blo fuse blew in about 1 second.

I also have your dual 1amp psu kits running off of the same fuse but it’s only supplying a small opamp mixer. Would that likely be drawing too much current? If so what fuse should I use. 2amps seems too much.

Because the bias adjustment went OK does that eliminate the PCB and components?

Also can I try it with the output open circuit safely(ie no speaker)?

By the way I have it sitting in a chassis next to another one which are touching - although the power transistor metal is insulated from the other.

Any suggestions or likely causes are very much appreciated.

Thanks

2

There are a number of issues which you mention that are tricky or not done:

  1. You’ve earthed the transformer. Don’t do this.
  2. Always connect a speaker
  3. Make sure units are completely isolated from eachother. Do not run anything from the same transformer.
3

Thanks for your very quick response.

I have done what you suggested but it still blows the fuse.

I can’t understand why the fuse didn’t blow when I first powered it on to do the bias adjustment. Is there any harm in going back to that stage? In other words disconnect everything but the transformer and fuse and recheck if that works? At least I can start to eliminate things and check some voltages on the board etc. (assuming that works!)

Thanks again - great service