so here is what i think the issue is
all fuses including fuse F4 in and all the power supply voltages are ok with the exception of F6 which has 25v on it(not rectifier side) when F6 is removed.
when you put F6 in the voltage becomes 1.6v, or thereabouts, so there must be a problem with back feed from the 25v side ?
all fuses including fuse F4 in and all the power supply voltages are ok with the exception of F6 which has 25v on it(not rectifier side) when F6 is removed.
when you put F6 in the voltage becomes 1.6v, or thereabouts, so there must be a problem with back feed from the 25v side ?
🙂 I wouldn't like to say how valid a method that is tbh. It just sounds like the output stage is passing/leaking current. Pull Q615 and Q617 out and the output stage can not conduct at all. If it does then there is a problem with the drivers or outputs. If it doesn't then there is a problem with Q615 and Q617 (if it conducts with the base of those two linked).
ok ill have a go tomorrow, but i have pulled everyone of those transistors out and all tested ok.
You have to be methodical. Even if the transistors seem to test OK you need to prove that by seeing if the stage behaves as it should when we pull those transistors out. With them out the drivers are firmly turned off because of the 330 ohm between base and emitter. The stage can not conduct in that state... but does it? If it does then one or more of the transistors is faulty. If it doesn't then one or both of those removed are faulty as long as the stage is still faulty when you link the base of those BF422/423's.
You will find as you get more experience that basic transistor tests aren't always enough and leakage sometime will not show on a diode test. An old AVO8 with its 15 volt battery for high ohms ranges was good at doing that.
You will find as you get more experience that basic transistor tests aren't always enough and leakage sometime will not show on a diode test. An old AVO8 with its 15 volt battery for high ohms ranges was good at doing that.
oh ok, fair enough, ill check back with you tomorrow.i got to gert this one sorted out as its getting on my nerves now 😢
You will and you will keep learning. The fun part with fault-finding is getting to grips with the more obscure problems. Transistors can test OK on a meter but be leaky or have low gain. They can also breakdown when a certain voltage is reached and they can go open circuit when a few milliamps starts to pass. They are all less common issues but something to be aware of.
so i pulled both of these, put the fuse in and no change.
just say for hypethetical reasons i was right about the voltage coming back, and think about what could have happened when i pressed the SLC switch,because up untill i did, all was good and ready to go, well it seemed that way.
F6 definatly has +24v one side and -45v on the other(with it removed)
when F6 fuse is in it is -3.7v
The current in the channel you pulled those transistors should fall to zero.
Check by measuring the voltage across the 0.22 ohm resistors in the channel that you pulled the transistors that you are still seeing a high voltage across those resistors. That would show current is flowing (which it should not be). If there is no voltage across the resistors then we move on and look elsewhere.
so the plot thickens
i have had to pull F6 out for now so i can finish looking at the other channel that i though was ok, but when i checked the centre again it was over 1v,so on we go
i found R651 open circuit to start with and now i have 40v on the rear,😵 so on we go, trouble is im going to devon for a few days in the morning so i am running out of hours
i have had to pull F6 out for now so i can finish looking at the other channel that i though was ok, but when i checked the centre again it was over 1v,so on we go
i found R651 open circuit to start with and now i have 40v on the rear,😵 so on we go, trouble is im going to devon for a few days in the morning so i am running out of hours
Great. So now repeat the test with them refitted and with their bases linked together (which is the same as linking the multiplier out). You should still get a similar result.
so i think i have it
with R651 open circuit there had to be an issue with the output, and One of the outputs had blown along with the driver, so left channel all good
on to the right one now and after checking everything my mind set to work on possibly an earth short.I had already checked all the circuit inside and out and could find nothing, untill i re checked Q623 and the casing/collector was showing down to earth so i took it out and there was nothing wrong with it.
When i looked more carefully the plastic sleeve didnt reach all the way up to the heat sink, and that last be of screw was tousching just very slightly.
so cut a long story short i have both centres at 0v and both idle currents at 6mv, and i also tried that dreaded SLC switch and all is well, THANK GOD!!!
so the question i still cant work out is why? when the switch was depressed it took out some of the components.Was it because R651 was open circuit perhaps?
i never give up, and where lack of experience sometimes shows, determination wins through in the end.🙂
i think i deserve a cup of tea after that lot ☕
with R651 open circuit there had to be an issue with the output, and One of the outputs had blown along with the driver, so left channel all good
on to the right one now and after checking everything my mind set to work on possibly an earth short.I had already checked all the circuit inside and out and could find nothing, untill i re checked Q623 and the casing/collector was showing down to earth so i took it out and there was nothing wrong with it.
When i looked more carefully the plastic sleeve didnt reach all the way up to the heat sink, and that last be of screw was tousching just very slightly.
so cut a long story short i have both centres at 0v and both idle currents at 6mv, and i also tried that dreaded SLC switch and all is well, THANK GOD!!!
so the question i still cant work out is why? when the switch was depressed it took out some of the components.Was it because R651 was open circuit perhaps?
i never give up, and where lack of experience sometimes shows, determination wins through in the end.🙂
i think i deserve a cup of tea after that lot ☕
Aye, a cup of tea time 🙂
So the problem was a physical short, well done
The switch thing is a bit of a mystery because it should not really alter anything that could remotely cause a blow up. Could the short have been in a knife edge and a bit of movement somewhere caused it to appear the two were related.
All you can do is test it and try it.
Feeding time 😉
So the problem was a physical short, well done
The switch thing is a bit of a mystery because it should not really alter anything that could remotely cause a blow up. Could the short have been in a knife edge and a bit of movement somewhere caused it to appear the two were related.
All you can do is test it and try it.
Feeding time 😉