I have a k150-8 plexi head that is blowing fuses. I built a bulb limiter. I am getting 55v and -55v off the filter caps. the schematic for board 5032 shows 39.5v and -39.5v.
Am I missing something here? Any help would be appreciated before I move foward with trouble shooting the power amp board thanks. Nate
The power supply is normally somewhere around +&- 40vdc, so with a light bulb limiter it should be less than that. The 55 volts that you are getting is much higher than expected.
What is the line voltage coming in from the wall? Is the power transformer original to the amp?
120 ac out of wall. transformer looks original. I have another k150-8 plexi thats a parts amp. I get roughly the same voltages from that power supply as well.
stevem Messages: 4733 Registered: June 2004 Location: NY
Senior Member
Your DC voltages will be high once parts of the output stage blow open , the high voltage is due to the circuit not pulling the current is should .
It's time to start pulling the two output transistors on the heat sink strip and checking them and performing a resistance check on the two 5 watt ciramic box resistors.
stevem Messages: 4733 Registered: June 2004 Location: NY
Senior Member
What are you reading ac voltage wise coming out of the power transformers two purple wires, this voltage is taken across the purple wires, not one at a time to ground.
stevem Messages: 4733 Registered: June 2004 Location: NY
Senior Member
There is something not right with your readings your posting up.
I opened up my k150 and then took readings at the input to the diode bridge I had 95.6 volts ac, and you had 48.7, this sounds to me like your where measuring each ac output leg of the power transformer to ground, not across both legs?
On my amp when I measured those legs to ground, then I got a reading of 48.3 volts ac.
Your high DC voltage readings across each main filter should make for high ac voltage readings also as you check back from the DC portion of the power supply to the power transformer.
Your DC voltage readings on the filters are some 16 volts too high, mine with the amp working at idle are 41.9 and 41.7 on the negative rail .
Thanks for getting those readings! The readings across the diode bridge 91.4 acv. Are my dc voltages too high because the filter caps are not connected to the power amp circuit board?
stevem Messages: 4733 Registered: June 2004 Location: NY
Senior Member
Oh crap yeah!
You never posted up that you had the power amp section disconnected, or I missed it, so yes, your power supply is golden and the issue with popping fuses is in the driver / output stage!
I replaced the driver transistors. The bulb limiter is barely glowing with a 60 watt light bulb. I am getting about 37.5 volts going in to the power amp board positive and negative..I am getting around plus and negative 20 volts dc on both wires going into the pre amp. Any ideas where to start? I tested the output transistors they are fine. I will be able to work on the amp later this evening and will be able to give more accurate voltage readings if requested. Was wondering if anyone had any ideas. Thanks in advance.