So the forensics are starting to come together now 🙂
It sounds like you have fitted Q601 incorrectly at some point and so you have created a direct path... easier to draw...
So Q605 may possibly have suffered (perhaps) but what happened is you put the positive supply across that 6.3 volt cap via the 68 ohm and the 27 ohm in series with the cap. The transistor would have been fitted incorrectly so it functioned either as a forward biased diode or so that the junction was reverse biased and broke down.
So the 68 ohm, the 27 ohm and the cap need checking replacing. Maybe Q605 and of course Q601 needs fitting correctly.
ok i get what you are saying here. the original c1400 was fitted to the board as per the print and this is how i put it back, but i will check as suggested
so done as you suggested.
27ohm checked ok
68ohm replaced(this is the one that burns)
Q605 checked out ok but i replaced
Q601 put back as originaly fitted
C615 is brand new
turned on R643 still burns
27ohm checked ok
68ohm replaced(this is the one that burns)
Q605 checked out ok but i replaced
Q601 put back as originaly fitted
C615 is brand new
turned on R643 still burns
So you need to try and find where the current in the 68 ohm is going, Rule things out one by one.
Put your meter across the 27 ohm and very quickly switch on. If you have voltage (how much if you do?) means that current is flowing in that path.
If that is OK then it looks like you still have a problem around the output stage and current is flowing via some failed junctions. If the output and drivers are good then no DC fault condition can push current down that route.
Put your meter across the 27 ohm and very quickly switch on. If you have voltage (how much if you do?) means that current is flowing in that path.
If that is OK then it looks like you still have a problem around the output stage and current is flowing via some failed junctions. If the output and drivers are good then no DC fault condition can push current down that route.
Messy but you'll get the idea. Each of the four lines shows a possible fault route that would burn the 68 ohm up but these all need at least one failed device in the output stage to do this. If they are all good then no current can flow down any of these paths.
current across r631 is -13mv
voltage either side is about 1v
but blimey this is getting stranger by the min
so i re soldered a few of the resistor joints around R631 and now we are back to the start of things, no burning of R643 but the centre is wandering again like it was to start with, al beit not as bad as before 🙁.Time for ☕ and a re think
voltage either side is about 1v
but blimey this is getting stranger by the min
so i re soldered a few of the resistor joints around R631 and now we are back to the start of things, no burning of R643 but the centre is wandering again like it was to start with, al beit not as bad as before 🙁.Time for ☕ and a re think
So back to how it was and you haven't really done anything. Could there have been a whisker of solder going between pads unseen that a resolder fixed.
If you find the flux on the board looks messy then meths will clean it off. So does iso. It does no harm but it can make things look messy.
Just done that. I added a Penguin to mine, well not in it. Nearly dinner time.
If you find the flux on the board looks messy then meths will clean it off. So does iso. It does no harm but it can make things look messy.
Just done that. I added a Penguin to mine, well not in it. Nearly dinner time.
mmm i like them too.
so 3 days later and im back to where i started
the trimmer doesnt realy seem to have any effect so this is what i have to sort out now, blimey what a mess
now the voltages look right to me now on Q601
base +600mv
emmiter +1.0v-a little high
collector +30v
so it must be on the right track now, i just have to follow the path again and hopefully ill get to the botton of it
one thing i did do was scrape away the shellac on some of the prints and created new solder paths, so maybe this helped, i couldnt see any breaks though
base +600mv
emmiter +1.0v-a little high
collector +30v
so it must be on the right track now, i just have to follow the path again and hopefully ill get to the botton of it
one thing i did do was scrape away the shellac on some of the prints and created new solder paths, so maybe this helped, i couldnt see any breaks though
I think the trimmers effect is very limited when you look how it works. It alters the current in D601 and so alters by a very small amount the fixed voltage that diode provides. It also effect the voltage Q603 sees between B and E and that voltage set the current in Q601 and that in turn has the effect of altering the offset voltage. It all operates rather indirectly on the offset voltage.
Q603 is a classic text book current source. If you fix the voltage between B and E then the voltage (and so current) in the 390 ohm R629 is also then fixed and this current will not alter even if the supply to the transistor changes over a very wide range.
Q603 is a classic text book current source. If you fix the voltage between B and E then the voltage (and so current) in the 390 ohm R629 is also then fixed and this current will not alter even if the supply to the transistor changes over a very wide range.
Scroll down the page to where it says:
https://www.electronics-notes.com/a...ollector current is β times the base current.
Yours is similar but because it uses a PNP transistor it is upside down and works into the positive rail.
Simple stabilised active current source circuit
https://www.electronics-notes.com/a...ollector current is β times the base current.
Yours is similar but because it uses a PNP transistor it is upside down and works into the positive rail.
ill have a look at what you put in the next post.
the range is very small, but i know these and what they should look like as you adjust them and it is a very possitive action that is visible, so when i adjust it and see little movement there is def something amiss, and i will find it.
so i have the centre at 0v now, although i have to have the trimmer nearly all the way clockwise to get that, and that is unusual
but get this, and i cant work it out, the minuite i connect the earth bar to the heatsink R643 starts to burn, how weird?
i have triple checked and there is nothing touching on the underside and also checked there is no current on the heatsink
so connecting both grounds causes an issue
so i took off the new ground wire i used to connect the points we discussed earlier on, and it doesnt burn now
so i checked the other 3020 i have here and that isnt connected to ground either, so maybe its not supposed to after all.
strange🤔
but get this, and i cant work it out, the minuite i connect the earth bar to the heatsink R643 starts to burn, how weird?
i have triple checked and there is nothing touching on the underside and also checked there is no current on the heatsink
so connecting both grounds causes an issue
so i took off the new ground wire i used to connect the points we discussed earlier on, and it doesnt burn now
so i checked the other 3020 i have here and that isnt connected to ground either, so maybe its not supposed to after all.
strange🤔
so quite a bit of success in the last hour
both channels pretty much 0v centre, and this was after putting both original Q601/2 back-weird.
one channel is very low in volume though so i just have to sort that out(update i forgot to put that capacitor back so all good-keep your fingers crossed 😉
both channels pretty much 0v centre, and this was after putting both original Q601/2 back-weird.
one channel is very low in volume though so i just have to sort that out(update i forgot to put that capacitor back so all good-keep your fingers crossed 😉
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so update-works ok on both channels, the only thing now is there is alot of interference noise coming from one of the channels, as if something was loose, or if you blew hard into a microphone(does that make sense lol)
You sound to be doing well with this one. The noise first of all needs pinning down to either power amp or the preamp. I think you can isolate those on the NAD with the jumpers at the back.
If its the power amp then see if responds to physical poking and pressing of the board. If it looks like a component fault then first guess might be another resistor damaged somewhere or perhaps ceramic cap.
Could this earthing weirdness be anything to do with it? All those green circled points have to go to ground. They are ground.
If its the power amp then see if responds to physical poking and pressing of the board. If it looks like a component fault then first guess might be another resistor damaged somewhere or perhaps ceramic cap.
Could this earthing weirdness be anything to do with it? All those green circled points have to go to ground. They are ground.
so not out of the woods just yet.That noise is attributed to the voltage sway i had to start with and appears to return from time to time, so i cant button it up just yet, i dont want to risk blowing the speaker up
I personaly think its the transistors as it comes and goes, one min its all over the place and the next its fine for 30 mins or so, almost like something is breaking down due to current flow and heat maybe?
so cenario
play music for 30mins, then it goes wacko
leave it for a few mins with no output, and settles down again.
I personaly think its the transistors as it comes and goes, one min its all over the place and the next its fine for 30 mins or so, almost like something is breaking down due to current flow and heat maybe?
so cenario
play music for 30mins, then it goes wacko
leave it for a few mins with no output, and settles down again.
I keep suggesting freezer spray. It really could help with this but you need to practice dispensing it literally one drop at a time from the nozzle,
Yes, it will show on a scope but because the amp as whole is one gig feedback loop you will see the noise everywhere. Don't discount small ceramic type caps, they are well known troublemakers generally, and particular the small compressed disc types. The scope would show if the noise was just low frequency stuff or if there was high frequency instability behind it all.
It could be a transistor, it does happen.
Yes, it will show on a scope but because the amp as whole is one gig feedback loop you will see the noise everywhere. Don't discount small ceramic type caps, they are well known troublemakers generally, and particular the small compressed disc types. The scope would show if the noise was just low frequency stuff or if there was high frequency instability behind it all.
It could be a transistor, it does happen.