Blackstar HT-Dual valve distortion pedal
How can one little pedal have so may faults?
This came in with a faulty power adaptor. A 16V AC plug top. Replacing this seemed to get things up and running. However soon after, we had an accompanying burning component and board smell. Perhaps something caused the transformer’s death?
With the unit dismantled, we found a burnt MOSFET which had badly carbonized the board where it lived. This MOSFET is in the HT generation circuit feeding the ECC83 valve. The filter electrolytic was burst into two. Also found the two 78xx series regulators were short circuit, along with the rectifier diodes on the AC input. Even the 470 uH choke in the HT circuit was shorted! With all these parts replaced, along with the MOSFET we powered her up. Within a few seconds, the MOSFET got too hot to touch and the filter electrolytics also got hot. Managed to get a diagram and the HT feeding the valve should be 270VDC. A quick check before we had another burn up showed the HT was up at 380VDC. Wow. Long story short, we found an open circuit 1M Ohm resistor in the feedback circuit. This feeds into the VFB pin on the UC3843 switching chip. This must have been the cause of the original problem.
Now try and explain to the customer why it wasn’t a 2 minute job.
