Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Definately a startup-only problem then...
I know that you've already checked the resistor chain R121/R155/R122/R156, but it might be worth double-checking by measuring the (AC) voltage accross each resistor with the mains on - make sure the voltage is the same accross all resistors.
Is it possible that something's shorted near R157 that could be pulling the voltage down at the junction of R155/R122?
I doubt that it's important, but what nominal mains voltage are you on - 110 or 230?
Definately a startup-only problem then...
I know that you've already checked the resistor chain R121/R155/R122/R156, but it might be worth double-checking by measuring the (AC) voltage accross each resistor with the mains on - make sure the voltage is the same accross all resistors.
Is it possible that something's shorted near R157 that could be pulling the voltage down at the junction of R155/R122?
I doubt that it's important, but what nominal mains voltage are you on - 110 or 230?
Those voltages look about what I would expect. Can you check the voltages across R106, R107, R109 and R150 while the circuit is trying to start. I would expect the reading to be zero. When you forced the circuit to start were the output voltages correct. (I think about +/- 25.5 on V+ and V- and +/- 10 on +/- VCC) ?
Those voltages look about what I would expect. Can you check the voltages across R106, R107, R109 and R150 while the circuit is trying to start. I would expect the reading to be zero. When you forced the circuit to start were the output voltages correct. (I think about +/- 25.5 on V+ and V- and +/- 10 on +/- VCC) ?
Measure the voltage across the resistors with power applied but with it in the sate were it should start. I do not believe there should be any voltage across them until the supply is running normally I'm just looking for thing that could be drawing current from pin 12 of the IC. Measuring the voltages after you have forced it to start was to see if it was working perfectly once started. I was particularly interested in V+ and V- (The output ends of L201 and L202) This is to see if the voltage regulation of the switched mode part is working correctly. If it is not it may direct us to the start problem via a different route or give us a clue in some other way.
Measure the voltage across the resistors with power applied but with it in the sate were it should start. I do not believe there should be any voltage across them until the supply is running normally I'm just looking for thing that could be drawing current from pin 12 of the IC. Measuring the voltages after you have forced it to start was to see if it was working perfectly once started. I was particularly interested in V+ and V- (The output ends of L201 and L202) This is to see if the voltage regulation of the switched mode part is working correctly. If it is not it may direct us to the start problem via a different route or give us a clue in some other way.
The 80 mV across R106 and R107 must be due to leakage current though the photo transistors in the opto isolators as the LEDs in them cannot be being driven as there is no power at this stage on the low voltage side of the circuit. I think this must be normal as I do not believe both have failed. I do not now where the 400 mV AC across R109 is coming from. (I asked for this test to be sure that ZR101 which you had already tested had not failed in some strange way. This reading cannot be due to leakage through the zener. ) I was expecting V+ and V- to be about 25 volts. Do you have any documentation that says what these voltages should be ? This is to be sure I have not calculated the voltages incorrectly.
I guess that the AC on R109 must be just leakage through ZR101 and/or being sourced from the IC (a bit of pulling up?) through D105... remember it'll be "pulsed" or very ripply DC on pin 12 of the IC, as C118 charges from the mains and is then discharged by whatever our mystery current sink is.
If there's more voltage on R150 (681 ohms?) than on R106, doesn't that indicate a lot of current flowing out of pin 7? That seems very odd. Or could it be flowing through the gate of a damaged Q103?
Your measurements in #23 give a current of about 370uA (RMS) - which should be enough to start the IC, which has a maximum startup current of 250uA, according to the datasheet. I'm probably mistaken here, but shouldn't these add up to 230V, not 130?
I guess that the AC on R109 must be just leakage through ZR101 and/or being sourced from the IC (a bit of pulling up?) through D105... remember it'll be "pulsed" or very ripply DC on pin 12 of the IC, as C118 charges from the mains and is then discharged by whatever our mystery current sink is.
If there's more voltage on R150 (681 ohms?) than on R106, doesn't that indicate a lot of current flowing out of pin 7? That seems very odd. Or could it be flowing through the gate of a damaged Q103?
Your measurements in #23 give a current of about 370uA (RMS) - which should be enough to start the IC, which has a maximum startup current of 250uA, according to the datasheet. I'm probably mistaken here, but shouldn't these add up to 230V, not 130?
If I measure with scope at R156 upper point I get 70V pulse to 50Hz against PCOM. At lower point I get 4V. The measurement at #23 was with multimeter in DC so it´s RMS.
The current of 250uA should be with 10V at pin 12 which are not there...
My idea now is to connet R156 to ground, filter these 70V from R156 and convert to 12V with DC/DC converter, then connect the 12V output to D108 cathode, I think it should work like that.
After looking at the documentation I realised I was wrong about the voltage on V+ and V- The part of the circuit I thought was the feedback was the over voltage circuit.
If Q103 had a gate to source short (Or high leakage.) it would pull pin12 of U101 down as Q104 would be trying to pull the gate positive resulting in higher than normal emitter current which comes prom pin 12 of U101. I have not yet worked out what purpose these two transistors serve. They are not mentioned in the manual.
Good work - this looks like progress!
It's quite likely that Q103 will have its gate shorted if it's been subject to more than about 20v gate-source - and that's quite likely to have happened if the power transistors have failed.
I wondered if these where supposed to implement a crude PFC by modulating the voltage feedback signal from U102 in time with the mains, but that can't be right... Then I wondered if they're a brown-out protection. That doesn't make much sense either.
In any case, get those devices replaced and see what happens. I take it that the numbers on that schematic are internal part numbers or something? I couldn't find a FET called a "252043", but I reckon anything should do just to prove the point.
I've found the description of the function of Q103 and Q104. It is at the top of page 11 of the document in the link in post #29 It is to provide a low decay of the voltage to the power amps when the unit is switched off. I missed this reading the document first time. The MMBT3906 is available from Farnell https://uk.farnell.com/fairchild-se...PMkA6A3fvrAEVD6uU1yOIn0nmy57jQHov5RoCEknw_wcB
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
