A quick update.
Found a video of some tech with the exact same TV and identical voltages on the PSU output.
He also noticed that the BGA chip on the main board had overheated so switched the main board and the TV came back to life. So, have ordered a new board and I'll confirm the result when it arrives.
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The exploded cap was C9877.
Looking over the main board, nothing 'looks' out of place and all the electrolytics have been checked for high ESR. The one thing that is clear is that the BGA chip has been running hot and clearly scorched the board through to the other side. Nothing extreme, but the heating is clear.
I'm still far from convinced that the PSU board is working properly....
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In the end, I put a 12v bulb onto the 12v output and sure enough, once mains connected, the lamp lit up.
However, the 24v output dropped to 18.1v.
Surely this can't be correct?
Also, when the PS/On pin is bridged to the Stand-by pin, then the 12v lamp goes off.
Surely this is wrong? It's like the stand-by circuit is connected in reverse. When you close the circuit, the TV goes off, open the circuit and it turns on.... ?????
As for when I said 'trusted replacement', that is only what the seller stated. This is Brazil....
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The voltage on the filter caps is 360vdc across the bridge rectifier is 140vdc - The mains AC supply here is 127v.
I'll stick a resistor across and see what happens...
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LCD TV Power Supply Board question
First off,
Happy New Year everyone!
Someone recently asked me to look at their Philips 42pfl4508g/78 TV which had stopped working.
A quick visual check of the PSU revealed a ceramic cap with a hole burnt through it, and that the output voltages were wonky. The 5.2v Standby was only 3.2v. The 24v - 19.1v and the 12v - 11.2v. We found a cheap 'guaranteed working' replacement PSU online, which they bought and installed but it still the TV didn't switch on, and when I went back to test the new PSU board, it showed the same kind of low output voltages....Last edited by Crystaleyes; 01-01-2025, 09:44 AM.
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Thank you for the suggestions. I'll give that output capacitor a go and let you know how it goes. If that works and the voltage stabilises, then I'll give the diode a go too.
Nice one!...
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Think I'm just gonna put in some new ones. I have plenty here......
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Giving the socket a quick clean and a close look, nothing jumps out at me.
One other thing, while I remember, is that when the voltage is set to zero, and then the amperage is also set to zero. the voltage goes up to around 5 -6 volts. Not sure if that might be a dodgy pot but a quick test of them shows that the voltage pot goes from 1K down to 1.8 ohms and the amperage pot goes from 10k down to just under 4 ohms....
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Hi all. Thank you for all the input. I'm learning how this thing works thanks to you.
So, re-sitting the filter cap worked. At least in the sense that it no longer blows the 3055, and turned out that only 3055 had blown.
One thing that still needs resolving is the output voltage is fluctuating. Quite savagely. The unit goes from zero up to around 42v but the digital display is constantly and rapidly, jumping around. On lower voltages such as 7v it might jump between 0.3v up and down, although up around 40v it is flying around, anywhere within a 2v difference....
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So do you think there are any minor modifications which could be made to improve this PSU?...
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I think I might have found the problem.
As mentioned, this is something I last messed with several years ago and only came off the shelf as I'm having a total revamp in my little overcrowded workspace. In fact, I posted here before even taking a renewed look, as I've just been putting up shelves and sorting boxes. I thought there might be a design flaw which someone might spot?
Anyway, just now I went to actually take some voltages and immediately saw that one of the two main filter caps which I installed maybe ten years ago, was ever so slightly touching the wire connecting...
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Here is the original list I made of the board components, before any changes.
R1 1R
R2 1R
R3 1k8
R4 4k7
R5 4k7
R6 1K5
R7 3k9
R8 4k7
R9 200R
R10 18k
Q1 BC 547
CI 1 LM 7805
CI 2 LM 7905
CI 3 CA 747CE
C1 1000µF 25V
C2 1000µF 25V
C3 2200µF 40V
C4 2200µF 40V
C5 Not used
C6 472 Ceramic
C7 22µF 25V
C8 22µF 25V
D1 1N4006
D3 1N4006
D4 1N4006
D5 1N4006
D6 1N4006
D7 1N4006
D8 1N4006
D9 1N4148...Last edited by Crystaleyes; 11-15-2024, 06:05 AM.
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This is absolutely possible.
In that photo from when it was bought, it certainly appears that the BC547 has been re-soldered, so it is also either not original or was pulled for some reason? Looking at the datasheet the base collector max is 50v... Perhaps I should try with a 546?
I also have some 3773's here....Last edited by Crystaleyes; 11-14-2024, 03:54 PM.
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Apologies for the delayed response, but thank you both for the input.
Camera is playing up but there's a couple or few images below..
As for he voltage, I thought I remembered it going up to 30v but perhaps it might be 50v as when the 3055's fail (as now) it shows 51v on the LED display (see photo). For the brief time that the PSU does work, the display and voltage output works fine. It never lasted any decent amount of time before killing the transistors.
The story with this thing is that I bought it back when I was first tinkering with electronics...Last edited by Crystaleyes; 11-14-2024, 01:26 PM.
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The 1N4007's are used for the 2N3055's. I know they take up to 15A but surely this PSU can't be expected to handle 15A - It must be 1.5A, no?
Gonna upgrade the diodes anyway but didn't get to the shops today... I'll see what I have in the parts boxes...Last edited by Crystaleyes; 11-12-2024, 01:28 PM.
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Hi fella.
Yeah. They're in parallel. Did think about upgrading them to the 2n3773's but the current ratings are almost the same, only the 3773 can take higher voltages.
I have wondered if the CA747/LM747 Op Amp is responsible and what might be a suitable replacement?
Seems to me that the main reasons that these transistors would blow might be; a) something shorting, b) too much Emitter-Base voltage, c) too much Base current.
But I don't immediately know the answer as I'm not too familiar with Variable power supplies.. yet....
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In fact, just looking at the PSU again (first time is maybe three years), it appears that the main rectifier diodes on the 35V input are 1N400x which being 1A diodes, clearly won't work in a 1.5A PSU.
Gonna stick in some 3A diodes and see what that does.
The photo below was taken on the day I bought the PSU and those diodes D1, D2, etc. are what it came with......
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Bench PSU question
Hi all.
Looking for some help regarding the workings of this unit.
Some time ago I picked up a digital bench power supply from the local flea market here in Rio. It's a 'Sincler SPS 3015/D' which I imagine is 30V, 1.5A 'Digital'.
Even though it would turn on and show a display, it wasn't working, and no matter how hard I have looked, I've found no reference let alone any kind of schematic. I imagine it is from the late 70's or early 80's? I can only assume that all the components are to the original specs although it had clearly had prior work done so it 'is' possible...
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I haven't measured any voltages but yes, it has a transformerless power supply.
The next thread was actually going to cover this
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