Transistor Replacement Confirmation - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: July 02, 2022, 01:42:51 pm
content: I have an old mini TV that I obtained strictly for connecting my old C64 to it. It stopped working, so I found the horizontal transistor to have low resistance (about 50 ohms) from base to emitter; and a 10uF 160V bulged capacitor.The transistor is a TT2084 (I believe the datasheet in the attached is correct even though it includes LS) and is obsolete. After searching for a replacement, the only ones that seem to match this and are available are:NTE2640 and NTE2679.Can someone confirm that either (or both) are compatible to the TT2084?

author: nfb
author_rank: Contributor
timestamp: July 02, 2022, 08:37:04 pm
content: I would check your transistor once more - it has B-E resistor.

author: Benta
author_rank: Super Contributor
timestamp: July 02, 2022, 09:05:39 pm
content: CRT TVs in 90% of the cases I've seen fail due to aging and temperature stress in the solder joints around the high-voltage and horizontal deflection circuitry.Be certain to discharge the CRT and the capacitors in that area before starting work.Remove the old solder (pump, wick) and resolder with good eutectic.If that doesn't do it. you at least know that that area is OK.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 03, 2022, 01:22:34 am
content: I once repaired an old CRT TV.It did a "beam current measurement" when starting up and because of aging the beam current was too low due to aging.Adding a resistor to fool this beam current measurement worked, but it distorted the picture.I fixed it by adding a microswitch and a piece of string to the resistor, so you pull the cord just after turning it on to get beyond this beam current measurement, and this simple hack got a few more years of life out of that old CRT.But as nfb noted, that 50 ohm apparently is normal, so you have to look elsewhere for the real error.

author: bostonman
author_rank: Super Contributor
timestamp: July 03, 2022, 02:13:18 pm
content: I saw that resistor, but, since the datasheet didn't mention it, I thought it was part of the test circuit. How do you know the value is 50ohms since the datasheet doesn't make mention of it?Years ago I use to repair CRT monitors, so I'm quite familiar with issues of cold solder joints. The only reason I have/kept this TV is due to its small size and curved CRT which gives the C64 the more natural display.If I can't fix this easy enough, I'll just toss it. I dislike tinkering with high voltage stuff unless I have someone around just in case.

author: nfb
author_rank: Contributor
timestamp: July 03, 2022, 03:03:51 pm
content: Quote from: bostonman on July 03, 2022, 02:13:18 pmBecause it is very common value in this application.

author: bostonman
author_rank: Super Contributor
timestamp: July 04, 2022, 02:32:04 am
content: Is the resistor there for a purpose other than what I assume is to hold the base low when the transistor is off so any leakage current doesn't cause it to turn on?

author: nfb
author_rank: Contributor
timestamp: July 04, 2022, 09:15:54 pm
content: Quote from: bostonman on July 04, 2022, 02:32:04 amIt's the damping resistor. It damps unwanted oscillations from the output transformer of the horizontal driver.

author: bostonman
author_rank: Super Contributor
timestamp: July 05, 2022, 01:31:21 am
content: I remeasured the resistor and it's more like 57 ohms if that matters. Initially I stated 50ohms because I thought the transistor was bad and figured 50 or 57 ohms, at that point, it didn't matter because the transistor is shorted; obviously I know better now.Per my original question, ignoring the resistor, is the transistor I selected as a replacement a good replacement? As for a resistor being included in the transistor, apparently I'd need to either add one or buy a transistor that has one.What would I have to search for in order to find a resistor included with the transistor?I don't plan to repair this TV, it's just not worth my time at this point. I gave the boards a quick inspection and didn't see any cracks nor anything burnt. It apparently sat in a damp basement for some time because it has rust on various sections, but, I always take a moment to learn a little something which is why I'm inquiring about the transistor.