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Dead IBM 8513 repair, quiet ticking, no HV - I have no ideas left

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First post, by Alec M

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About a year ago I inherited a dead IBM 8513 monitor from my aunt.

The issue seemed like it would be a simple fix of resoldering all the commonly troublesome connections such as the AC jack, replace C120 along most of the other capacitors on the power supply, and a few potentially bad diodes.
Unsurprisingly as you may have guessed based on the length or title of this post, this didn't fix the issue I'd been having where when the monitor was turned on it would make a quiet ticking sound with no high voltage, picture, or tube heater.
After a few seconds of being powered off, the power LED would flash for a short time and there would be a squeal from the back. Adjusting the contrast/brightness knobs did nothing.

Today, I decided to pull it out again and work on it, checking more connections and individual components, all of which turned out to be working properly.
Yet again I powered it up just in case my fiddling had helped it. The ticking was still there, but it was more random, and the power LED would still flash and there was still a squeal when powered off.

This time around I narrowed down a hot smell I noticed last time I powered it on to the the posistor that turns off the degaussing circuit, and when I touched it, it was burning hot.
I believe this is how it's supposed to be, but I'm not entirely sure, if someone could back me up on this that would be great.
Just to rule it out as a potential short or power drain, I removed it, since it's only purpose is to eventually turn off the degausser. Doing this did change something, since the ticking was much slower, but there still was no HV, and the same thing occurred with the LED flash and the squeal after the monitor was turned off.

I fear the the only possible thing left that could cause HV not to turn on is the actual flyback transformer, since that provides power for the heater as well as the tube.
However I do think I've seen a very similar model/identical shape of flyback on eBay for around $15 USD.

TL;DR
I've tried a bunch of stuff, it still clicks when turned on, has no picture or high voltage, and the power LED will flash after being turned off for a few seconds accompanied with a short squeal. There aren't any components on either boards that look potentially blown either.

If you have any other suggestions, or know how to fix this specific issue, please let me know. I don't really have any ideas left.

Thanks,
Alec

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Reply 1 of 4, by 133MHz

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Did you test the horizontal output transistor? A shorted HOT is the most common cause for the tick-tick-tick sound.
And yes, it's normal for the degaussing PTC to get hot.

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Reply 2 of 4, by Alec M

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133MHz wrote:

Did you test the horizontal output transistor? A shorted HOT is the most common cause for the tick-tick-tick sound.
And yes, it's normal for the degaussing PTC to get hot.

I didn't test any transistors when I was checking components, mainly just passives.
Would that cause the entire HV circuit to come down?
Also, where is the horizontal output transistor?
Good to know that's normal for a PTC 😀

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Reply 3 of 4, by 133MHz

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Alec M wrote:

Would that cause the entire HV circuit to come down?

Most CRT displays derive the high voltage supply from the horizontal deflection stage. The horizontal waveform drives both the deflection coil and the flyback transformer, so most of the time when the horizontal deflection goes out you lose high voltage as well. The horizontal output transistor works very hard and when it fails, it shorts. So you basically have a dead short circuit from the main voltage rail (B+) of the power supply to ground when the HOT fails, what usually happens is:

  • 10 Power supply tries to start up
  • 20 Short circuit causes current to go through the roof
  • 30 Overload detected! Power supply goes into protection mode and shuts down
  • 40 Hey the overload has cleared now, let's try starting up again
  • 50 GOTO 20

This is why it makes that tick-tick sound, you're hearing it repeatedly trying to start, fail, retry.

Alec M wrote:

Also, where is the horizontal output transistor?

It's a big-ass power transistor that's usually located physically close to the flyback transformer, mounted to a large heatsink. I had a quick look around online and found the service manual for your monitor, from there on page 8 I can see that the horizontal output transistor is labeled Q403 and it's a 2SC3883, an 800V 5A NPN with integrated damper diode. You should desolder it and test it for shorts (don't test in circuit, you'll get false readings), then with the transistor removed hook up a regular 25-40W household incandescent bulb (you saved a couple of those, right?) across the collector and emitter connections on the board, turn on the display and measure the DC voltage across the lamp - it should be around 115V. If it's low, high or absent you have a power supply problem and should troubleshoot that first. The lamp is needed to test because the power supply could also malfunction if no load is placed on the main voltage rail.

Something's overloading for sure, it's either the horizontal or the power supply. Also you should check out this resource if you haven't already. 😀

http://133FSB.wordpress.com

Reply 4 of 4, by 133MHz

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I gave a more thorough read to that Sci.Electronics.Faq repair page and it mentions that the flybacks on these monitors are failure prone. 🙁
I'd still test the HOT and check the B+ under a dummy load though. If both the HOT and PSU test good by themselves then you can be sure it's the flyback.

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