I think we couldn't do anything with this PCB. I have some mystic error somwhere.
Help me please to repair second PCB. May be if we fix this I will fix the first one.
It's in this state now:
removed R10 and R6 here and the bases of the AD149 transistors. The voltage comes up (there were no voltage before). What should I do next?
Help me please to repair second PCB. May be if we fix this I will fix the first one.
It's in this state now:
removed R10 and R6 here and the bases of the AD149 transistors. The voltage comes up (there were no voltage before). What should I do next?
Hi terrom,
No such thing as a mystic problem, only one that has you stumped.
Okay, so return that board to how it would be if it was working. Write a note detailing what parts were changed and any other work you did, like fixing traces. Do not write down any assumptions or conclusions.
Did you give the cracked areas another look as I suggested?
Pack it up neatly and include the old parts in a bag with it. Ignore it completely until you are ready to work on it again.
Second board.
Examine it closely for cracks and broken solder joints. Take pictures of the entire board, both sides before you do anything.
After the physical inspection, measure the power supply voltages. Report your findings and stop.
-Chris
No such thing as a mystic problem, only one that has you stumped.
Okay, so return that board to how it would be if it was working. Write a note detailing what parts were changed and any other work you did, like fixing traces. Do not write down any assumptions or conclusions.
Did you give the cracked areas another look as I suggested?
Pack it up neatly and include the old parts in a bag with it. Ignore it completely until you are ready to work on it again.
Second board.
Examine it closely for cracks and broken solder joints. Take pictures of the entire board, both sides before you do anything.
After the physical inspection, measure the power supply voltages. Report your findings and stop.
-Chris
First the last question about the 1st pcb. Why on the C3 is a steady voltage of 13V? If there is a crack it should be 0V i think?
Hi terrom,
I'll leave that one to Mooly. I haven't looked at those schematics in many months. I will say that breaks in the copper do not have to be complete, same with solder connections. So you can have a resistive connection. High inrush currents on the capacitor can chop a voltage down farther than you might expect across a resistive connection.
Consider this. The world can be one big gray area where the black and white of absolutes fades to that uniform gray. You are looking at bad connections as either open or short. They can be in between. So, measure directly across the capacitor, not to a ground point. Some regulators work on the ground side, so voltages measured wrt the circuit ground may not show what you expect. So for the first capacitor you ought to measure directly across the capacitor. One clue this is the case might be a positive (but low) voltage on the capacitor positive terminal, and a negative voltage to the negative terminal.
Don't jump to conclusions and list the facts as you find them. Have you been writing notes down? There is a reason for this and I still do it.
-Chris
I'll leave that one to Mooly. I haven't looked at those schematics in many months. I will say that breaks in the copper do not have to be complete, same with solder connections. So you can have a resistive connection. High inrush currents on the capacitor can chop a voltage down farther than you might expect across a resistive connection.
Consider this. The world can be one big gray area where the black and white of absolutes fades to that uniform gray. You are looking at bad connections as either open or short. They can be in between. So, measure directly across the capacitor, not to a ground point. Some regulators work on the ground side, so voltages measured wrt the circuit ground may not show what you expect. So for the first capacitor you ought to measure directly across the capacitor. One clue this is the case might be a positive (but low) voltage on the capacitor positive terminal, and a negative voltage to the negative terminal.
Don't jump to conclusions and list the facts as you find them. Have you been writing notes down? There is a reason for this and I still do it.
-Chris
Lets try and explain this to you in such a way that you might be able to fix it.
Use the point arrowed as your ground for all measurements. This is the point where the middle wire from the transformer enters the board. It is important you make sure that the wire from the transformer connects securely to the board. This is where your black meter lead connects for all the measurements.
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You now measure the voltage on C2 and C3 as shown here. Measure first using a DC voltage setting on your meter and then change to AC voltage setting. You MUST measure on the actual leads of the components and not on the PCB. That is to make sure continuity is perfect all the way to the capacitors.
In BOTH CASES the voltage should be zero (0.00 volts).
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Now set your meter on AC volts and measure the voltage on the bridge. You MUST measure on the bridge leads directly and NOT on the PCB. Again, this is to confirm continuity all the way to the bridge. You should see around 14 volts AC on each point.
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Now measure the DC voltage here. You should have around 20 volts DC. Again, you MUST measure on the bridge lead itself AND NOT on the PCB.
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Now measure the DC voltage on C3 as shown here. It should be identical to the reading you measured above. Again, you MUST measure on the component lead itself and not on the PCB.
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Here the results (everything from the ground):
1)
DC C2+ 0.01mV
DC C3- 0.04mV
AC C2+ 0.032mV
AC C3- 0.035mV
2) AC
bridge ~1 15.2V
bridge ~2 15.4V
3) DC
bridge minus - minus22V
bridge plus - 12.85V
4) DC
C2 minus - minus22V
C3 plus - 12.85V
1)
DC C2+ 0.01mV
DC C3- 0.04mV
AC C2+ 0.032mV
AC C3- 0.035mV
2) AC
bridge ~1 15.2V
bridge ~2 15.4V
3) DC
bridge minus - minus22V
bridge plus - 12.85V
4) DC
C2 minus - minus22V
C3 plus - 12.85V
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So bridge is the first that gives me lower voltage than it should be (12.85 instead of 22V)
So is the bridge affected by the other components or it should give me 22V in any state?
So is the bridge affected by the other components or it should give me 22V in any state?
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Excellent 🙂 now lets see this through if we can.
Based on those readings (very clear, thank you) then either the bridge or C3 appears to be faulty. Now its 50:50 which it could be. The easiest way to prove it might be to eliminate C3 from all this and we can do that by tagging another similar cap across it. Do you have another cap of around that value (anything over 1000uf and 25 volt) ?
Don't remove it or disturb anything if possible, just tag it across C3 and retest.
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Yes, with 14 volts ac going into the bridge it should be giving over 20 volts across C3. If the bridge has gone high resistance then it would cause this and also if C3 was open circuit it would do the same.
So we have to prove which it is now.
Another 'engineers trick' which might give you a clue but it isn't guaranteed is this......
When you switch off, does the voltage across C3 fall away slowly or does it suddenly drop to nothing. If the cap was open it would drop to nothing instantly.
Based on those readings (very clear, thank you) then either the bridge or C3 appears to be faulty. Now its 50:50 which it could be. The easiest way to prove it might be to eliminate C3 from all this and we can do that by tagging another similar cap across it. Do you have another cap of around that value (anything over 1000uf and 25 volt) ?
Don't remove it or disturb anything if possible, just tag it across C3 and retest.
----------------------------------------------------------------------------------------------------
Yes, with 14 volts ac going into the bridge it should be giving over 20 volts across C3. If the bridge has gone high resistance then it would cause this and also if C3 was open circuit it would do the same.
So we have to prove which it is now.
Another 'engineers trick' which might give you a clue but it isn't guaranteed is this......
When you switch off, does the voltage across C3 fall away slowly or does it suddenly drop to nothing. If the cap was open it would drop to nothing instantly.
I have 1000uf cap behind me. Should I solder it to the points of C3 instead of it or in addition to the old one?
It will do no harm to try it. Just lightly solder it across C3 leaving C3 in place and undisturbed.
Not unexpected, but its further proof.
Change the bridge. I'm assuming its not getting hot in all this.
Change the bridge. I'm assuming its not getting hot in all this.
Do you know if I could replace this bridge (AEG B40C 800Si or AEG B50C 800Si or B40 C800) with something modern?
Definitely yes, and there must be dozens of possible replacements.
You need to look for one of at least 100 volts P.I.V. (peak inverse volts) and of at least 1amp capacity. You would have to be careful to get the pins correct as you fit it to the board.
You need to look for one of at least 100 volts P.I.V. (peak inverse volts) and of at least 1amp capacity. You would have to be careful to get the pins correct as you fit it to the board.
Something like this... there are many to choose from,
W02G-E4/51 - VISHAY - BRIDGE RECTIFIER, 1.5A, 200V | CPC UK
AM151 - UNBRANDED - BRIDGE RECTIFIER, 1.5A, 100V | CPC UK
You could also use four separate diodes to make a bridge if you can't get one. 1N4002 or 1N4004 type would be good.
I'll have to leave it for tonight but it looks like we are on to the problem 🙂
W02G-E4/51 - VISHAY - BRIDGE RECTIFIER, 1.5A, 200V | CPC UK
AM151 - UNBRANDED - BRIDGE RECTIFIER, 1.5A, 100V | CPC UK
You could also use four separate diodes to make a bridge if you can't get one. 1N4002 or 1N4004 type would be good.
I'll have to leave it for tonight but it looks like we are on to the problem 🙂
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