dyazdani
Active Member
I recently purchased a PA-7 that was advertised to be broken, specifically that it would not power on. The unit was cheap and I figure the chassis would be good for a DIY amp even if I can't fix the original.
Upon receiving the unit, I noticed that the limiter board is pretty well shot. The spark killer was open and the 6.8 Ohm/10 W resistors in the front end of the board were failed. Some of the transistors on one of the amp channels are fried, but that is for another day as I need to get it power first.
I rebuilt the limiter board, but I'm having trouble now. I powered it up through a DBT with everything downstream (i.e. filter caps and onward) of the limiter board disconnected, and all was OK, except that I didn't know what to expect for voltages. Upon taking out the DBT, it fried the resistors again (R401-404). I do have the spark killer jumpered, but everything else but the relay and the inductor L401 is new.
Any ideas of what to check or how to proceed? I thought the limiter board would be easy to troubleshoot, but I'm obviously missing something.
Upon receiving the unit, I noticed that the limiter board is pretty well shot. The spark killer was open and the 6.8 Ohm/10 W resistors in the front end of the board were failed. Some of the transistors on one of the amp channels are fried, but that is for another day as I need to get it power first.
I rebuilt the limiter board, but I'm having trouble now. I powered it up through a DBT with everything downstream (i.e. filter caps and onward) of the limiter board disconnected, and all was OK, except that I didn't know what to expect for voltages. Upon taking out the DBT, it fried the resistors again (R401-404). I do have the spark killer jumpered, but everything else but the relay and the inductor L401 is new.
Any ideas of what to check or how to proceed? I thought the limiter board would be easy to troubleshoot, but I'm obviously missing something.