on the attached diagram i have marked the + and - rail voltages.
Is there a reason they are different? (marked a)
also there is a realy high voltage 82.7v (marked b) that i cant work out how it gets to that figure given the surounding voltages
sorry if they are silly questions,but im trying to work my way back from the db LED board that isnt working, checking the voltages and getting 31v either side of this diode
the rest of the amp is working perfectly, its just this LED board that isnt working.
thanks 😱
Is there a reason they are different? (marked a)
also there is a realy high voltage 82.7v (marked b) that i cant work out how it gets to that figure given the surounding voltages
sorry if they are silly questions,but im trying to work my way back from the db LED board that isnt working, checking the voltages and getting 31v either side of this diode
the rest of the amp is working perfectly, its just this LED board that isnt working.
thanks 😱
Attachments
> realy high voltage 82.7v (marked b) that i cant work out how it gets to that figure
Typo for 32.7V. It comes from a 33.1V source. It goes to a 35V cap. It can't be 82V. But a 3 can look like an 8 to an overworked draftsman.
Typo for 32.7V. It comes from a 33.1V source. It goes to a 35V cap. It can't be 82V. But a 3 can look like an 8 to an overworked draftsman.
"Is there a reason they are different "-quote ---yes different values for the collector feed resistors .
120ohms -v- 470ohms feeds .
120ohms -v- 470ohms feeds .
so this is what i have recorded so far
you thoughts would be appreciated as im not far advanced enough yet to work it all out
thanks
paul
you thoughts would be appreciated as im not far advanced enough yet to work it all out
thanks
paul
Looking at the design voltages in black and looking at your tested voltages in pen there is a massive drop in voltage values so there must be a large current increase in Q807 and C811 you've marked =5.04 volts when it should be zero volts as its connected to earth.
Check Q807 for a short --check Q809 zenner for a fault and C807 +C811 for a fault and you would be as well checking the value of R807 --that's not much to check as the rail feed is okay so big current rise due to a faulty component somewhere in that part of the circuit .
Check Q807 for a short --check Q809 zenner for a fault and C807 +C811 for a fault and you would be as well checking the value of R807 --that's not much to check as the rail feed is okay so big current rise due to a faulty component somewhere in that part of the circuit .
This amp is apparently full of fusible resistors - sneaky little devils that were designed with low-melt alloy links to melt rather than burn open circuit so as not to start fires and avoid insurance risks but in reality, they were a royal PITA to track down and replace with the correct spec. fusible resistors and many products were considered uneconomical to repair.
Looking at R508, there is about 31 volts across its 120R @1W rating. That would mean 8W dissipation in the resistor if it were functional so it must be toast but that could just be the the beginning. Assuming your measurements are correct, what caused the fusible to fail in the first place? Age, voltage surge, shorts down further the tinkering about with meter probes or just a random parts failure?
Looking at R508, there is about 31 volts across its 120R @1W rating. That would mean 8W dissipation in the resistor if it were functional so it must be toast but that could just be the the beginning. Assuming your measurements are correct, what caused the fusible to fail in the first place? Age, voltage surge, shorts down further the tinkering about with meter probes or just a random parts failure?
so i started checking some others, starting with R806, and thats odd for starters as it states it should have a 470ohm 1/4w there but whats actualy there is a 82ohm jobby, maybe someone put the wrong one in,??
now i have another of these amps that i brought for spares and that does have a 470 in place, so could this be the issue, or one of the issues??
so i replaced it and i am now getting 33v in and 29.8v out of R806.
i dont have time now today to test the rest as im off out for a couple of well earned(if standing outside getting freezing) beers, ill check the rest tomorrow
now i have another of these amps that i brought for spares and that does have a 470 in place, so could this be the issue, or one of the issues??
so i replaced it and i am now getting 33v in and 29.8v out of R806.
i dont have time now today to test the rest as im off out for a couple of well earned(if standing outside getting freezing) beers, ill check the rest tomorrow
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Thats the failing of virtual fault finding you cant see if somebody has installed the wrong value parts ,at least I was only one resistor number out.
so actualy you were right Duncan, although R805 was 'ok' i made the classic mistake by not checking the value against the amp index of parts, so as with R806, an 82ohm was fitted instead of a 120ohm one, but this one was reading 200+k so it was faulty anyway.
not sure why, someone obviously had a fiddle around with it,so ive replaced those 2 with the correct ones i had out an identical amp i use for spares.
So the amp is working fine with the LED board working as it should now.
im going to give the rest of it a good check over,as i havent check things like the phono output yet, change the main power caps.(i dont change all of them by default anymore)and run it up for a couple of weeks on and off.
I have had it up to full volume this morning(good job the neighbours are out) and nothing appears to be smoking or getting hot.
so cheers for all who helped with this.
Im starting to get the hang of things a bit more thanks to you guys
