IBM 5151 making a loud whining noise after warming up

[kdr]

I have an IBM 5151 display that is working great (stable raster, good brightness, decent focus) except that it starts making an awful whining noise after about 20-30 minutes of use. Turning the display off and letting it rest for an hour doesn't have any effect. Once it's started to make the whine, it will start the whining noise as soon as power and video signal are applied. (When powered on but without a video signal, there is no whine.) If I put the 5151 back on the shelf and return to it the next day, I get another 20-30 minutes of blissful silence before the whining begins anew.

The raster remains perfectly stable even when the whining noise is happening. The whining noise is constant and unchanging in pitch or amplitude.

I can't quite hear the normal 18432hz sync (it's about 750-1000hz too high) and the whine sounded around 9khz, so I used a basic audio spectrum analyzer app on my phone to confirm that before the whine begins, the only sound is an (inaudible to me) 18432hz tone; once the whine begins, there's an additional (audible to me) 9216hz tone...



The fact that the whine is precisely half of the horizontal sync frequency is quite interesting. What could be causing this? Is there some component that is slowly heating up and that needs a significant amount of time (multiple hours) to cool back down again?

I haven't opened up the 5151 yet. When I do pop it open (it won't be the first CRT that I've worked on) what should I be looking for?

 

[SomeGuy]

I seem to recall I had a cracked linearity coil that intermittently made a high pitch noise until I stabilized it by putting some glue on it.

 

[Chuck(G)]

Good observation. The linearity adjustment coil has a ferrite slug in it. If, for whatever reason, the slug gets a bit loose, it will vibrate. If you have a nylon/delrin tool, you can test this by just pressing against the slug inside the coil to see if the whine changes. Do not use a metallic probe--it will heat up in seconds.

 

[Hugo Holden]

Most likely it is coming from an inductor in the H output stage. Unlikely the LOPT (line output transformer) as these are potted, more likely as noted a Lin coil or width inductor. It can come directly from the wires on these coils or loose cores. The deflection yoke is another possibility. Even the H driver transformer could do it, but I have not seen one do it, yet. Other components can generate noise from the Piezo electric effect, but this is normally quieter and less bothersome. It might be a job for a small electret microphone insulated on a chopstick, to act as an "Audio Probe" to find out which component is doing it. Its hard to find the exact physical location of high pitched sounds by ear.

 

[Chuck(G)]

There's one other possibility--an arc sometimes makes noise, usually happens between windings in the flyback transformer. The only permanent solution there is to replace the transformer.

 

[kdr]

These are all excellent suggestions, thanks! I will take a close look at the various coils in the horizontal output stage, in particular the linearity coil, and also see if poking/prodding them with a long plastic rod has any effect on the sound.

 

[nevadaOZman]

Agreed, many good suggestions. I know of two instances where dust on/around the high voltage lead arched to near-by metal creating a whine, as well as one instance of a cracked flyback diode "squelching". All I have, but if it's non-dust-related the clue is some high voltage component is breaking down or losing it's integrity. Good luck.