A local musician lost this amp head during a gig. The bassist also blew some fuses in his rig. Everything failed at once. Besides making a great testimony for carrying your own power conditioning equipment, a lot of stuff had to be fixed. The Unbrokenstring Crew goes to work!
There were no signs of life other than the tube amp section appeared to be fine, and, surprisingly, the LEDs in the foot switch assembly lit up. Some Google-ing revealed that Line6 built this amp, which included Line6 modeling technology to implement the reverb and other internal effects married to a Reinhold Bogner power amplifier. Now that I could locate a manual, we could narrow down where the failure occurred.
What’s this? This wire was pinched between the chassis and the amplifier enclosure. The insulation is a little deformed but otherwise OK. This was poked back where it belonged as this was NOT the problem.
We didn’t have the 3.5v and -3.5v power. The schematic showed that this was derived from a single winding of the power transformer, sent through a full wave bridge rectifier, and filtered with a big capacitor. The issue is somewhere around the rectifier. Here, I’ve flipped the Power Supply Unit (PSU) board upside down so I can remove the rectifier to more easily troubleshoot the circuit.
These solder joints were cooked. All the solder was removed and the holes cleared of solder. Now I can see that the circuit board was open-circuit on the trace at the bridge rectifier. The holes in the pads shown in this picture is where the bridge rectifier was. I’ll fix the open circuit board trace with a short piece of wire when I put this part of the circuit back together.
Here’s the view of the board without the rectifier. It’s a little hard to see here, but the bottom oval pad was cracked. That will be repaired. The rectifier is OK, so it all goes back together. That is what you call ‘troubleshooting beyond the component level.’
Thanks for reading all the way to the bottom!
David Latchaw EE