Whilst sat browsing Facebook one day, I saw a post from someone local asking if anyone had a CRT for sale as theirs had broken, and they included a photo of their lovely 28-inch Sony Trinitron TV. I messaged him and asked what the fault was, and apparently the picture went off and was now just blinking the power LED with 2 blinks, which according to the service manual translates as Over Current Protection (OCP) triggered.
I said that if he was going to bin it, then I would like to pick it up and attempt to repair it. After all, you can’t really learn to repair stuff without having some broken ones to fix.
A couple of weeks later, he messaged me to say if I wanted to pick it up, then I could have it. So I got in the car and went and collected it. With some help from the Mrs, we got it home and down into my workshop, where I could then start my testing.
The first thing I needed was lots of information about CRT TVs and exactly how they work. And for this, I highly recommend watching videos by Randy Fromm on YouTube (Start with this one) After watching those, I had a very good understanding of what I was looking for. Even so, this Trinitron turned out to be quite a puzzle.
The first thing I did was test the TV to see what happened when I powered it on. And just as the guy had said, upon powering on, all I got was a blinking power LED flashing twice, then pausing and repeating. The issue with this error code is that it isn’t really very descriptive. Basically something, somewhere in the TV was apparently pulling too much current.
The other thing I noticed was that I wasn’t hearing any high voltage at all. Normally, when you power on a CRT, you will hear a crackling sound from the high-voltage circuit. So I downloaded the service manual for the set and started to look at all of the voltages to start with. This wasn’t that easy, because as soon as the TV detected the overcurrent, it shut down the power supply! Luckily, I have an oscilloscope, and this meant I could visualise the voltage for the very brief moment it was there.
From what I could see, the main voltages were present, specifically the 135V b+ voltage, which would go on to generate the high voltage via the flyback transformer. Another common fault on CRTs is the Horizontal Output Transistor; without this part working, you won’t get any image on the screen, and that part could easily pull too much current if it were to be faulty. But after testing it, it looked fine to me, and on my scope, I could see the required 135V pulses being sent to the flyback. So I really expected to hear some high-voltage crackling when the set powered on.
After looking around, I found that two fusible resistors had blown, which were right next to the flyback. I was hoping that they may have been the cause of the issue. But I wasn’t going to be that lucky, and after replacing the resistors, there was still no life in the set.
At this point, I decided I needed to test or replace the flyback. Unfortunately, I didn’t have any way of testing it at the time. So I looked around and luckily I found a replacement flyback on eBay from Malaysia. The part was new old stock, so I snapped it up and waited for it to arrive.
Fast forward to delivery day, and I swapped the flyback over, held my breath and hit the power button. For a split second, I heard a nice static crackle from the CRT, and then it shut down and sat blinking its LED at me. At least this proved that the flyback was indeed faulty.
The next few weeks were slow and painful. My bedtime reading was the schematics for all the various boards inside the TV. Every now and again, I would have an idea of something to test, so I’d run down, test it and come back to bed depressed that it still wasn’t working.
I went through all of the test points on the CRT and compared the waveforms to what was in the service manual and I just couldn’t find anything wrong.
I then had an idea, a potentially dangerous idea which I don’t think I would recommend to anyone! But I had basically run out of things to try.
The way the Over Current Protection circuit worked, is that it measures the voltage drop across a resistor and if that voltage drop was too highm(two much current) it would trigger an OCP line that originates from the power supply board and gets sent to the microprocessor board which would then shut down the TV.
So what if I simply disconnected the OCP wire between the power supply and the processor? It wouldn’t get the OCP signal and would keep the TV running. Maybe then, whatever part was causing the issue would very quickly get hot, and using my thermal camera, I could see where the issue was.
Either that, or it would burst into flames and I wouldn’t need to worry about repairing it anymore 🙂
So, I pulled the OCP wire out of the connector block, pointed my thermal camera at the back of the TV and nervously hit the power button.
And……..
It came on and just worked. I carried on looking around with my camera, but nothing looked wrong. So I left it running for a while and carried on monitoring the situation. After about 20 minutes, everything was still fine, so I powered it off, reconnected the OCP wire and was straight back to a blinking light again.
After scratching my head until I had no hair left, my conclusion was that there wasn’t an overcurrent issue, but actually an issue with the overcurrent protection circuit itself.
I took a look at the schematics for this section of the board, didn’t understand how on earth it was supposed to work, so I went and watched some videos about over-protection circuits. The main thing seemed to be that for the OCP to trigger, there would need to be a large voltage drop across the current sensing resistor, but I was getting a 0.13V drop, which seemed perfectly fine to me. So the next step in the circuit was two transistors, which would monitor that voltage and turn on when overcurrent was detected. It looked like one of the transistors was outputting the OCP signal from its emitter, but with the voltage being in spec, there was no reason why it should. So, just for completeness, I swapped out both of these transistors as there wasn’t much else left that it could be.
After replacing these parts, I finally had a fully working CRT again and was able to reconnect the OCP wire.
So the moral of the story, although these newer CRT sets have the luxury of running their own diagnostics. Don’t just blindly trust what they are saying, and don’t rule out the fact that the diagnostic circuit itself could be the fault!
I would also like to point out that although this story seems quite short, these events were spread out over 6 weeks! But I do feel I have learnt a hell of a lot about CRTs on the journey so for that I am quite happy with myself.
Nothing left to do now but kick back and play a bit of Jumping Flash on the PlayStation to celebrate my victory 🙂
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