If you think there is a ground issue showing up under load then make up a suitable 3.5mm plug that has two 33 ohm resistors as a load.
Have you tried a different set of headphones ?
Have you tried a different set of headphones ?
I have tried with a couple of sets of headphones.
Would I make the 3.5mm plug to test for proper grounding? How would I best test the grounding?
Would I make the 3.5mm plug to test for proper grounding? How would I best test the grounding?
You would have to make the lead to simulate headphones. In other words a 3.5mm plug with the resistors soldered onto the tags of the plug.
You would use the body of the plug as 'ground' because that is how the headphones see their ground point. You should then see the mixing you describe if you measure from plug body to L and R tags on the plug.
Assuming you do see the mixing then you would check from plug body to the main O2 ground initially by seeing if there was any AC voltage between the points signifying an open or high ground.
If not then you keep the load and the mixed output connected and then repeat all the tests from the beginning, commencing with the input and working through.
You would use the body of the plug as 'ground' because that is how the headphones see their ground point. You should then see the mixing you describe if you measure from plug body to L and R tags on the plug.
Assuming you do see the mixing then you would check from plug body to the main O2 ground initially by seeing if there was any AC voltage between the points signifying an open or high ground.
If not then you keep the load and the mixed output connected and then repeat all the tests from the beginning, commencing with the input and working through.
Stumbled at the first hurdle!
Oh well, it would appear that my handling of the components or the small size of the board, or both have got the better of me.
Following the steps, striped end of D3 has virtually no voltage with no opamps/comparators fitted, nor is there minus voltage on non striped end of D4.
I have 18v ac at input to board.
I don't think it matters, but I messed up placement of R6 so I tracked down the alternative terminals and placed it underneath the board. You tell me if this isn't ok.
Where do I go from here?
TIA
Oh well, it would appear that my handling of the components or the small size of the board, or both have got the better of me.
Following the steps, striped end of D3 has virtually no voltage with no opamps/comparators fitted, nor is there minus voltage on non striped end of D4.
I have 18v ac at input to board.
I don't think it matters, but I messed up placement of R6 so I tracked down the alternative terminals and placed it underneath the board. You tell me if this isn't ok.
Where do I go from here?
TIA
Attachments
If you have 18 vac input on the board and no DC voltages on D3 and D4 outputs then there has to be a break somewhere.
If you put your meters black lead on the negative end of C2 and C4 (which is the main zero volt line) then you should be able to measure 18V AC arriving at both diodes and around +25 volts DC and -25 volts DC on the output of the diode.
You should also have continuity from C2/C4 (zero volt line) back to one lead of the incoming AC from the transformer.
If you put your meters black lead on the negative end of C2 and C4 (which is the main zero volt line) then you should be able to measure 18V AC arriving at both diodes and around +25 volts DC and -25 volts DC on the output of the diode.
You should also have continuity from C2/C4 (zero volt line) back to one lead of the incoming AC from the transformer.
Thanks Mooly. arriving at both diodes means non-striped end right?
Neg end of C2 and C4 to inlet (non-striped end) of D3 gives 18V AC, but neg of C2 and C4 to inlet of D4 gives virtually no voltage. Outlet of D3 has almost no voltage, outlet of D4 has 18V AC
Thanks Mooly, but in my previous post I said I didn't have those voltages.
Is the inlet side of the diode the non-striped end? Just make sure I'm singing from the same hymn book.
The fact you haven't got those voltages means there has to be a break somewhere 🙂
You said there was 18V AC arriving at the board. That means each diode has to be producing DC at its output. If its not then the AC is getting there.
The inlet side of the diode is different for each rail. The positive rail has AC applied to the anode of D3, that's the non stripey end. The negative rail has it applied to the cathode of D4 which has the stripe.
You said there was 18V AC arriving at the board. That means each diode has to be producing DC at its output. If its not then the AC is getting there.
The inlet side of the diode is different for each rail. The positive rail has AC applied to the anode of D3, that's the non stripey end. The negative rail has it applied to the cathode of D4 which has the stripe.
Ok my fault entirely, I thought my Fluke 16 DMM was auto sensing. It wasn't.
Once I put it to DC it measured 24.2V DC at those points.
Now to the regulators...
No DC at pin 4 or pin 8 (I'm guessing they're the pins opposite to the cutout).
So that's a U2 malfunction?
Here's a pic of the top of board in case anyone wants to pick apart my work
Once I put it to DC it measured 24.2V DC at those points.
Now to the regulators...
No DC at pin 4 or pin 8 (I'm guessing they're the pins opposite to the cutout).
So that's a U2 malfunction?
Here's a pic of the top of board in case anyone wants to pick apart my work
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D1 and D5 have correct voltages, but reading on in your excellent fault diagnosis walkthrough, I have virtually nothing at pins 4 or 8 or between 4 and 8 in either ac or dc on any of the remaining 8 pin sockets
If you have the correct voltage on D1 and D5 then there are only 3 possibilities that I can see...
1/ Those two diodes are fitted wrong way around. If you have the correct voltage on both ends then they are correctly fitted. If you have voltage only on one end of each diode then they are wrong way around.
2/ Something wrong with switch S1
3/ You have break in both tracks carrying the supply to U2 etc.
1/ Those two diodes are fitted wrong way around. If you have the correct voltage on both ends then they are correctly fitted. If you have voltage only on one end of each diode then they are wrong way around.
2/ Something wrong with switch S1
3/ You have break in both tracks carrying the supply to U2 etc.
I may have found another fault.
Look at photo in #5384
Where I had messed up the placement of R6, I had followed the track around and replaced it on underside of the pcb. If you look closely at R6 top connection on my photo, you'll see a break in the track ( just past the black heat shrink of incoming power cables, visibly in the photo).
This connection goes to the small hole at the top of J3, which in turn goes to one connection of D7, which is not lit up.
Thank heaven I bought a second pcb as I can visually interrogate it for track runs.
I'll report back once I've done this repair, here's hoping....
Problems around R6 wouldn't stop the raw -/+12 volts reaching pins 4 and 8 of U2. Work through it all slowly 🙂 Once you have the correct supplies on U2 we can begin and check the circuitry around U2.
Hi Mooly, the instructions don't mention looking for voltages around U2, but the remaining empty ic sockets, that's where I don't have power
Interesting, once I had repaired the missing track link, I now have voltages at 4 and 8 on remaining three ic sockets, and the LED lights up.
I think I'll power down and wait for caps to settle and check with some old headphones, once I've replaced the other ic's
I think I'll power down and wait for caps to settle and check with some old headphones, once I've replaced the other ic's
I thought you were including U2 in the 'no voltages on pins 4 and 8'.
It sounds like you have made progress. There should +12 volts on pin 8 of all the IC's and -12 volts on pin 4 of all the IC's.
The caps should discharge in seconds once power is removed.
It sounds like you have made progress. There should +12 volts on pin 8 of all the IC's and -12 volts on pin 4 of all the IC's.
The caps should discharge in seconds once power is removed.
Many thanks Mooly, I was actually a bit ahead of you there in following your fault diagnosis sheet which I printed out.
A preamp is born!
I couldn't have done it without your help Mooly so a big thank you!
You did say that your expertise is in fault diagnosis on this amp and you were right. Now to hook it up to the ACA and play "The Wizard", maybe Fairies wear boots after...