The day was going quite well. A long standing customer, Mr. C came in with this beast of an amplifier. Mr. C used to be a HiFi critique and as a result, was delighted with lots of high end equipment.
He said one channel was dead and he wanted to sell the amplifier and had a buyer. Could I fix it for little €s. Normally I would suggest taking it to the local car mechanic, but given the status of my client, was helpful. I like Hafler equipment, but this was about to change. My mental health reference was about to enter into the negative meter reading.
Upon the bench and connected to the current limiter, with a normal dummy load, power up, it sat there drawing a good 110 Watts. Looking at the way this lump is put together, there was no warm relationship between designer and service engineer.
Indeed, one channel was dead and after ages spent dismantling the silly heatsink assembly to pry the dead channel PCB away to test, we eventually found a dry joint on the – B supply. Only took an hour, given the nature of the intermittent condition. With that sorted, the protection relay kicked in and both channels were on. BUT, channel 1 had a 90 KHz unstable signal present, with no input feed. This was causing the heavy standby current. Now, I have to cut a long story short…. This oscillation seemed to be coming from the mid section of the power amp. There are several non polorised capacitors in the bias and long-tail pair configurations, all of which read okay. As did the other decoupling capacitors. Being a MOSFET output stage and the devices hard to come by, messing about can be a nightmare. The amp is quite complex, one needs to be quite grown up. Now I know I will be challenged on this, but given I was going to struggle to get any income from this job ( I need to eat and support my lavish lifestyle, fast women, loose cars), I decided to decouple the bias circuit with a few hundred pF capacitor. Perfect. Yes, I know, not the best solution, but, the amp works great and both channels are balanced. Budget controlled by the owner!