| Electronics > Repair |
| Power supply board damage - reversed polarity on two CONs |
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| vladc77:
According to the attached "PCB-Trace-View.png" diagram CN-5pin1 (+12v) and CN-5pin2 (+5.6v) are directly connected to one trace line that is from IC401p8 (+12v). There is no trace line from C411 to CN-5pin2 to deliver 5.6v as it can be seen in the electrical schema. CN-5pin1 and CN-5pin2 are directly connected. How CN-5pin2 can have (+5.6v) but CN-5pin1 (+12v)? I missing something here. The same for CN5pin3. How CN5pin3 can get -12v? CN5pin3 is not connected to IC401pin4 but to the ground accoring to "PCB-Trace-View.png". Where -12v are coming from to CN5pin3? Finally, CN5pin5 (+15v). Looking at the PCB Trace view, CN5pin5 (+15v) is connected to IC401pin4 (-12v) and has no connection to ZD403 (+15v). How CN5pin5 can get +15v? Currently, it receives -12v which is not as specified in the electrical schema. It should be receiving +15v from ZD403. I hope you can educate me on this. It may help to trace the current issue. I already removed all transistors for testing, most caps, and some diodes and they all seem to be OK. The caps are not 100% perfect but were working fine before. So, I do not know what else to check. Thank you again! |
| vladc77:
I still cannot resolve the issue with connector CN-5 pins 2 and 3 not getting the correct voltage. It seems like all components in the chain are good while testing. --------- CN-5 --------- 1. CN5p2 gets +12v but should have +5.6v 2. CN5p3 gets 0v but should have -12v 3. CN5p5 gets -12v from IC401p4 but should +15v 4. CN5p6 gets -12v but in the schema, it is marked as AC Power The voltage values are the same as for CN40 pins. Looking at PCB trace lines, CN40 pins are connected to the same trace lines as CN-5 pins. The most difficult is to understand how these values can be different. The way I test voltage is simply by connecting a black probe to the ground and a red probe to the pins. According to the electric schema diagram, CN-6 connector pins 2 and 3 are getting voltage from the same sources as CN-5 connector pins 2 and 3 and even directly connected. I cannot find trace lines on the PCB that connect CN-5 p2 and CN-5 p3 to the sources as CN-6 pins. CN-6 pins get voltages very close to the specified in the schema while CN-5 pins are not. --------- CN-6 --------- CN6p2 - (+15v) from ZD403 CN6p3 - (+5.6v) from C411 I did not find damaged components. I tested them on the PCB circuit board and removed the caps, diodes, and transistors. I am stuck here. Can anyone help to understand what is wrong? |
| MathWizard:
It's a lot to follow along with, but really I'd say forget the schematics, and make your own. I find it fun especially on big old stuff that's easy to trace out. I mapped out most all of a 1980's stereo this winter or spring, as part of a repair/modification. I also removed all the electrolytic caps, to test them, but also just to make it way easier to map and check the rest of the circuit unpowered with a DMM. At some point I started powering different sections of my bench PSU, making sure it all seemed good. But I keep forgetting to ask or say that I'd fully disconnect the PSU from the rest of the stereo. And then map it all out and make sure those voltages make sense. And if you have any power resistors, or dummy loads, measure the DC and AC ripple. And for the rest of the stereo, if you like soldering, and writing down which cap goes back where, I'd remove the electro's, and then fly through rest of the circuits with a DMM. Either way, I'd separate each board, and just go through unpowered with a DMM. Some people would just apply power, and just go check all over the places that way. I'd want a lot of wires soldered on for that, but I'd want to know where I was putting them. But sometimes too, with a current limited voltage source, it's ok to slowly up the voltage to a board, and see if it starts using way too much current. Depending on your external PSU, that can be pretty safe and quick way to narrow down a problem. Or inject voltages into just 1 section of a circuit. Sometimes I'll cut traces, and remove a few parts, just to power up 1 section to see what it does. And I'd make the circuit or parts of it in LTSpice, anything I can't calculate or don't know about, LTSpice usually does. |
| vladc77:
Thank you, MathWizard! To clarify, are you suggesting to try connecting the Power Supply Board to the rest of the boards and checking the voltage with loads along with other deeper investigation methods? I am afraid to damage anything else. When I installed connectors incorrectly, a 12V load could have been coming from the logic board to the CN-5 connector on the PSU which itself was supplying voltage. it is unlikely it damaged much. I think that because all fusible resistors are good, all fuses are also not damaged and I did not find any damaged parts yet. I removed almost all parts of the chain. All other connectors on the PSU have the correct voltage even IC401. The most confusing is understanding how CN5p2 and CN5p3 get their voltage. Looking online, I've read that is possible that pins connected to the same trace may have different voltages similarly as specified in the schema. I cannot find a good article explaining this. Regardless, these pins do not have the voltage as specified in the schema. Maybe, I need to measure the voltage somehow differently. |
| MathWizard:
No I'm saying to disconnect the PSU as much as possible, from everything else, and check all the PSU voltages. Yeah I'm forgetting this is an old tape deck, I'd say it could be a nightmare to disassemble, and another to get back together correctly. But that might be the only way to narrow it down, if it's that bad. But yeah unplug as much as you can, like the motors, see what they read with a DMM, maybe try powering them off some 5-12V supply, but yeah I'd put some resistor in series if I was just going apply power to those motors. If you hooked up some connector backwards at the start, then it would be pretty easy to pop polarized caps, like maybe around a motor, or base-emitter junctions, IC's. |
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