choosing a suitable replacement flyback transistor - Page 1
Forum section: electronics / repair

author: lilshawn
author_rank: Regular Contributor
timestamp: September 13, 2017, 03:15:01 pm
content: I have a CRT deflection chassis I am repairing and it's main flyback deflection transistor is no good. It is currently specified as a 2SC5387, but I don't immediately have an exact replacement. I do have a pile of 2SC5144 which seem to match pretty good or exceed the original specs. some of the gain specs are different, but I noticed the testing method is different due to the increased voltage capability (1500 vs 1700 volts CBO)the only physical difference is the potential replacement has a metal (collector connected) back, while the original is totally resin encapsulated... not really an issue i can mica/silicone pad insulate it if it would work electrically.http://dalincom.ru/datasheet/2SC5387.pdfhttp://www.trzrus.ru/sourse/analog/2sc5144.pdfsecond opinions?

author: Toasty
author_rank: Regular Contributor
timestamp: September 13, 2017, 03:48:18 pm
content: eBay2SC5387 - Encapsulated as original, pack of 5 for $12.50 US + s/h.T

author: lilshawn
author_rank: Regular Contributor
timestamp: September 13, 2017, 04:07:23 pm
content: I know I can buy online, it's not that they AREN'T available, it's that they aren't available NOW. no parts in stock at local shops.I would like to replace this now to see if I potentially need more parts. I mean, if this replacement is suitable, I'll leave it, but if not I guess I'll just have to order online and wait.

author: Armadillo
author_rank: Super Contributor
timestamp: September 13, 2017, 04:08:32 pm
content: IMHO, the spec is better and I would install it.

author: Ian.M
author_rank: Super Contributor
timestamp: September 13, 2017, 04:14:47 pm
content: If its not clip mounted don't bother trying - its *EXTREMELY* difficult to get adequate isolation from a silpad or mica washer with a mounting screw going through it without modding the heatsink. Even if you use a nylon screw, you wont get enough creepage distance between the tab and the heatsink round the edge of the silpad or washer at the screw hole.

author: lilshawn
author_rank: Regular Contributor
timestamp: September 13, 2017, 04:23:40 pm
content: it's all good, this particular transistor has an insulated mounting point as well as sufficient clearances on the backside around the screw.

author: Armadillo
author_rank: Super Contributor
timestamp: September 13, 2017, 04:28:50 pm
content: Quote from: Ian.M on September 13, 2017, 04:14:47 pmI thought the screw should comes with the insulating sleeve cap as a standard kit.?

author: Ian.M
author_rank: Super Contributor
timestamp: September 13, 2017, 04:42:46 pm
content: @Lilshawn,You are right, that type of package is generally OK screw mounted without extra precautions unless there are extreme high voltage spikes at the collector@Armadillo,Yes, but that doesn't work for TO-257, TO-220 and other metal tab packages unless you drill out the heatsink hole to take the sleeve of the top-hat insulating bush.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2017, 05:01:16 pm
content: It will work in most cases, as both do not have an integrated diode in them anyway ( datasheets do indicate this if present, and give specs on them, typical device is a BU508 and BUu508D, one without and one with the diode and otherwise nearly identical as example) and are otherwise going to be driven the same in both cases.Check before replacing why the old one died, they normally are the symptom, and typical causes are open circuit or high value resistors in the base drive ( yes that 10R or so resistor in the BE path is needed, along with the other resistor across BE junction), dry joints on the drive transformer, dry joints on the LOPT ( resolder all of the LOPT joints, they often hide the cracked solder joint under what seems to be fine looking ones), dry joints on the film capacitors ( and on a multisync monitor there will be a few of them and some high voltage MOSFETS switching them into circuit that can also have dry joints) and otherwise just dry joints in the area. This is the one area where a bipolar transistor is still king, other power devices ( other than Sony and those so disliked and now unobtanium GCS thyristors they designed) simply were never robust enough to survive.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 14, 2017, 04:17:26 am
content: Quote from: lilshawn on September 13, 2017, 04:07:23 pmHave a look at the schematics of other devices to see what they use.Back in the day, I often replaced the 2SC1413a transistors which Sony used in their KX27 " Profeel" monitors both as horizontal output devices, & in the SMPS with BU508D , which Philips used in the Horizontal output of many of their TVs.The transistors you have sound close---I had a similar problem with an odd model Sony mon which we didn't have any spares for.I ended up replacing the transistors with spares from a Barco one.This had the same problem of insulated -v-non insulated tab, but I found it was easy to devise a workaround which was reliable.Working in a TV Studio, the main requirement was to have the stuff up & working (& in "spec"),as soon as possible.That never worked well with waiting for the "correct" component to arrive.( although Sony were extremely fast in supplying our orders).

author: Toasty
author_rank: Regular Contributor
timestamp: September 14, 2017, 05:25:01 am
content: I would be confident using the 5144s for testing. The gain is off by 30%, but it would definitely work. Everything else is just a touch bigger, better, stronger.T

author: SeanB
author_rank: Super Contributor
timestamp: September 14, 2017, 07:22:56 pm
content: In this application gain is typically around 0.9, simply because of the very hard drive required to start conduction fast. that is why the capacitors and low value resistors are there on the base, to turn it on fast, and importantly turn off equally as fast even with Miller effect dumping charge into the base trying to keep it on.

author: Toasty
author_rank: Regular Contributor
timestamp: September 14, 2017, 10:53:09 pm
content: Thinking about what you saidSeanB, and pulling the book out to refresh my soggy brain, I should've mentioned to check the caps surrounding that transistor before attempting to run the circuit. Check the driver circuit too.T

author: lilshawn
author_rank: Regular Contributor
timestamp: September 15, 2017, 01:05:17 am
content: yes, i've already gone ahead and replaced all the capacitors. they are all still original from factory and it's usually about time. This monitor was brought to me dead. The SMPS is gone overcurrent protection so i'm trying to track down all the issues. this chassis is a little odd (Wells gardner D9410) in that it uses 2 separate HV voltages and switching circuits, one is used to drive the flyback, the other is used to drive the horizontal deflection. wherein usually it's a combined circuit.The one i found shorted out was the one doing the horizontal. the flyback "HOT" tests out okay.still not working yet, but still a work in progress.

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2017, 03:33:50 am
content: Quote from: lilshawn on September 15, 2017, 01:05:17 amActually that's pretty normal for multisync monitors as otherwise it is very difficult to maintain the same EHT voltage at different line scan rates.Check for bad tuning caps on the line scan circuit. They may be switched in by relays to accommodate different scan rates or there may be some other arrangement. In addition to checking them for shorts, or opens, as they are commonly wound foil capacitors you need to check for loss of capacitance, which can happen if they have arced internally and also, if possible Hi-Pot test them out of circuit as close to their rated working voltage as you can manage. A burnt or welded relay contact can also cause problems, as the wrong capacitance value for a horizontal frequency range can result in excessive current in the line output transistor or an excessive voltage on its collector during flyback.