MOSFET or IGBT in place of bipolar CRT horizontal output? - Page 1
Forum section: electronics / repair

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2022, 11:10:44 pm
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: June 01, 2022, 12:33:52 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: June 01, 2022, 12:47:30 am
content: 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.

author: fzabkar
author_rank: Super Contributor
timestamp: June 01, 2022, 05:20:03 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2022, 06:01:15 am
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2022, 07:37:37 am
content: 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.

author: hfleming
author_rank: Regular Contributor
timestamp: June 01, 2022, 05:35:13 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: June 01, 2022, 05:52:25 pm
content: 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.

author: fzabkar
author_rank: Super Contributor
timestamp: June 01, 2022, 05:59:29 pm
content: 1500V IGBT for CRT Horizontal Deflection Circuit:https://www.fujielectric.com/company/tech_archives/pdf/36-04/FER-36-04-162-1990.pdfhttps://www.transistormosfet.com/transistor?search=1MB12-150https://www.sciencedirect.com/topics/engineering/cathode-ray-tube

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 01, 2022, 06:03:17 pm
content: 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?

author: CaptDon
author_rank: Super Contributor
timestamp: June 02, 2022, 02:28:43 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: June 02, 2022, 04:52:10 pm
content: Quote from: Uunoctium on June 01, 2022, 06:03:17 pmThat 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 02, 2022, 05:39:49 pm
content: From IRC:QuoteFYITim