MOSFET or IGBT in place of bipolar CRT horizontal output?

[james_s]

I occasionally find myself repairing vintage CRT displays, both monitors and TV sets. Unfortunately the high voltage NPN BJTs used for the horizontal output drive can be hard to find these days, these are typically rated 1500V and are quite low gain. I got to thinking recently, MOSFETs and IGBTs with sufficiently high voltage ratings are readily available so I started wondering how feasible it is to substitute one for the original transistor? Typically the HOT is driven by a transformer which is a technique that is also used with MOSFETs. Anyone ever try this? I'm tempted to get out the scope and take a look at the drive waveform on a TV or just try tossing a suitable IGBT into an already working TV in place of the HOT to see what happens.

[amyk]

Typically, the components in the horizontal circuit are selected carefully for their characteristics and don't have much operating margin. While a MOSFET or IGBT can act like a switching device too, I suspect you'll have to reengineer much of the other surrounding components to make something that works.

[james_s]

That was kind of my fear, the circuits are tuned resonant tanks and very clever, but rather fiddly hence the difficulty of repair when an exact replacement HOT can't be easily sourced. Still, as far as parts go, it seems like the HOT should be one of the easier ones to sub since it is only acting as a switch.

[fzabkar]

My memory is very hazy, but I believe I once worked on a Philips (?) TV that had a MOSFET or IGBT as a LOP transistor. Of course (?) there would be no need for a drive transformer.

[T3sl4co1l]

Yeah, too finely tuned, even just replacing the HOT with a bulk equivalent (similar V, I, hFE, t_s..) probably isn't great.  For example I tried a FJL6920TU in place of the 2SC5047CA in the Trinitron I had.  Worked for a little while, H linearity was definitely off, and it ran hotter.  Probably other things were already going bad (electrolytics?) that took it out not long after (couple weeks, whatever it was?).

Tim

[Kleinstein]

At least in Germany one can still get a few of the high voltage BJTs for cheap ( < 2$ range, down to the 50 cent range) as left overs. The exchange to a different type may be tricky, but likely less of an issue than going to a MOSFET or IGBT.

[hfleming]

At some stage the BU508 was the generic replacement for the HV-voltage transistors, with sometimes a bit of mechanical wizardry to make it fit. They are still available all over flea-bay.

[james_s]

Ebay brings its own issues. One of the reasons I got to thinking about this in the first place is that I bought a replacement HOT from an ebay seller and it popped the second time I powered up the TV, I suspect it to be a counterfeit part but who knows.

[fzabkar]

1500V IGBT for CRT Horizontal Deflection Circuit:

https://www.fujielectric.com/company/tech_archives/pdf/36-04/FER-36-04-162-1990.pdf

https://www.transistormosfet.com/transistor?search=1MB12-150



https://www.sciencedirect.com/topics/engineering/cathode-ray-tube

[Uunoctium]

long time ago a friend of mine had an repair workshop and contracts with some importers.
Repaired large quantities of VGA-CRT Monitors ( >1k/month).
He used and recommended BU2525(AF) for near all puposes incl TV-sets.

If it's not a fake, price for original NXP is 1,82€ (1,94$) @ Reichelt (Ger)
https://www.reichelt.de/hf-bipolartransistor-npn-800v-12a-45w-sot-199-bu-2525af-phi-p51648.html?

[CaptDon]

It could probably work. Many of the HOT transistors included a built in damper diode. A mosfet would probably have the same action with its body diode. If the original HOT transistor was transformer driven you would probably need a load resistor from gate to source to hold down the drive voltage and not destroy the gate. The turnoff time of the mosfet would be extremely critical since you would go from near zero to about 1000 peak volts in just 3 to 5 microseconds. The circuit may require some sort of Baker clamp to keep the mosfet out of hard saturation and the drain to gate miller capacitance could also be a real problem at the 15,734hz speed and high drain flyback voltage.

[james_s]

long time ago a friend of mine had an repair workshop and contracts with some importers.
Repaired large quantities of VGA-CRT Monitors ( >1k/month).
He used and recommended BU2525(AF) for near all puposes incl TV-sets.

If it's not a fake, price for original NXP is 1,82€ (1,94$) @ Reichelt (Ger)
https://www.reichelt.de/hf-bipolartransistor-npn-800v-12a-45w-sot-199-bu-2525af-phi-p51648.html?

That part looks promising if I can find a local source. That site is in German so I'm guessing if they will ship to the USA it will be expensive.

[T3sl4co1l]

From IRC:

Kuschelpony: just to add a note:
Kuschelpony: BU2525 is a totally fine transistor, but care about the letters
Kuschelpony: if the external circuit has no recovery diode, you MUST use an BU2525D (unisolated) or DF (full isolated)
Kuschelpony: the ...A and ...AF will fail after few seconds to minutes
Kuschelpony: also i strongly recommend to check the electrolytic capacitors in the base drive, when they age the transistor also overheats
Kuschelpony: was my former job, replacing thousands of them

FYI

Tim