Hi All,
This is my first post as I just discovered this forum today. I have been in the repair of TV, video and broadcast equipment for the last 30 years or so. Recently a friend asked me to repair a HH MA150 amp which was totally dead. After checking it out I found both 3.15ma fuses were o/c. I found one of the output transistors s/c so I ordered a pair and replaced both. This restore life to the amp but it sounded distorted. I checked both + and - HT supplies, the + and - 15V supplies which all checked out OK. I checked out all the other transistors on the driver board as many are dc coupled to the o/p transistor that was s/c. All transistors checked out ok. The following day when I switched on the amp to have another go at the distortion fault I was presented with a loud hum sound. At this point I checked the 0 line which should read 200mV max. This was totally 'off the radar' so I switched off the power to avoid a puff of smoke and a loud bang. I also discovered that if I switch on and off the main power switch 5 or six times the unit will come on with NO him present. I suspect capacitor trouble so I checked the electrolytic s and found that all have a high ESR reading compared with new ones of the same value. Could this be the problem as the amp is over 30 years old. Maybe a block change of the caps is what is needed.
I welcome any comments from other members.
Thanks in advance.
This is my first post as I just discovered this forum today. I have been in the repair of TV, video and broadcast equipment for the last 30 years or so. Recently a friend asked me to repair a HH MA150 amp which was totally dead. After checking it out I found both 3.15ma fuses were o/c. I found one of the output transistors s/c so I ordered a pair and replaced both. This restore life to the amp but it sounded distorted. I checked both + and - HT supplies, the + and - 15V supplies which all checked out OK. I checked out all the other transistors on the driver board as many are dc coupled to the o/p transistor that was s/c. All transistors checked out ok. The following day when I switched on the amp to have another go at the distortion fault I was presented with a loud hum sound. At this point I checked the 0 line which should read 200mV max. This was totally 'off the radar' so I switched off the power to avoid a puff of smoke and a loud bang. I also discovered that if I switch on and off the main power switch 5 or six times the unit will come on with NO him present. I suspect capacitor trouble so I checked the electrolytic s and found that all have a high ESR reading compared with new ones of the same value. Could this be the problem as the amp is over 30 years old. Maybe a block change of the caps is what is needed.
I welcome any comments from other members.
Thanks in advance.