I take it you have checked the output transistors?, the ones on the external heatsink - I don't see any mention of those been checked or replaced?. These are by far the most likely transistors to blow.
You mentioned three faulty transistors on the board, which ones were they?.
Hi Nigel,
As the left channel had no issues, I decided to put the working left channel board in the right channel slot (I know that this is not recommended) but it worked, confirming that the fault was on the board itself which is why I didn’t mention the output transistors. As I was working on it today I did check them and can confirm that they are ok.
What I set out to do today was to remove the capacitor that is attached to P5 and found that doing so seem to solve the problem. The fuse did not blow but there was a low-level hum. What I did notice though is that R53 started to overheat and I switched off immediately.
The capacitor that I removed read OL.
Hopefully now if I can get a replacement cap, it should be ok?
No, that capacitor isn't even shown on the circuit - it should work perfectly well without it.
You say it read 0L - what's that?.
R53 is one of the emitter resistors, it's unlikely that one would overheat and the other one (R52) not.
Do you still have the short across the two bases I suggested above - as someone who spent 46 years repairing such things it was my common practice to leave that in place until everything was working, and only then remove it and try and set the quiescent current.
Check the voltages on R53 and R52, each side of both of them - 'should' be around 24V, and the differences between each end of the resistor show the current through it - simple ohms law, 0.5V drop across a 0.5 ohm resistor means 1A flowing through it. Do the tests quickly - get the meter in place, turn it ON, and read the voltage, then turn it OFF. Wait a few seconds, move the meter to the other end of the resistor, and do it again. Repeat for the second resistor, and post the results.
You mentioned a low level hum - which I presume means you have a speaker connected?, again connecting speakers is one of the last things you do when repairing amplifiers, after it's repaired. It's not too bad on an amp of this style, but I still wouldn't do it 😀
Do you have a variac or adjustable power supply? Being able to slowly dial up the voltage is a great troubleshooting tool.
You mentioned blowing fuses.
People repairing SS amps usually put a resistance in series with the AC plug (transformer input) to limit the damage if something shorts. Also allows you to take measurements while the fault is occurring. A 60 w tungsten lamp usually works on low wattage amps, although those are hard to find now. I put mine in a socket, and since the wires to the screw terminal sockets tend to slip off, I have it in a grounded steel box with a circuit breaker. The sockets are designed for solid wire. You need some metal mesh to see if the bulb lights up (indicating a problem).
In badly designed bias control circuits, I tend to clamp the pot wiper to one end with a silicon diode. 1n4004 will do. Limits damage if the pot wiper lifts.
People repairing SS amps usually put a resistance in series with the AC plug (transformer input) to limit the damage if something shorts. Also allows you to take measurements while the fault is occurring. A 60 w tungsten lamp usually works on low wattage amps, although those are hard to find now. I put mine in a socket, and since the wires to the screw terminal sockets tend to slip off, I have it in a grounded steel box with a circuit breaker. The sockets are designed for solid wire. You need some metal mesh to see if the bulb lights up (indicating a problem).
In badly designed bias control circuits, I tend to clamp the pot wiper to one end with a silicon diode. 1n4004 will do. Limits damage if the pot wiper lifts.
Yes, just google "dim bulb tester" or "lightbulb limiter" for 101 ways to make a self indicating and above all, protective current limiting device for testing amps safely - bearing in mind that mains voltages are potentionally lethal and any necessary insulation or enclosure you use in making such a gadget should be the real deal, as an electrician or appliance manufacturer would use for exposed mains power wiring.
Hello! I also have leak stereo 30 plus amplifier, unfortunately both power amplifier boards are missing. Could you take a good resolution photo of the board from the side of the printed tracks so I can solder them myself? Maybe you have a service manual with the layout of the board tracks?
it all depends on the part of the world you live in but you would have found a 30 + broken down for peanuts faster
Or something else that was actually worth restoring. These early Leak SS amps were notoriously poor.