Vector Deflection Transistor Replacement

Update - 31 Dec 2017

I’ve encountered a similar issue with the screen after replacing the transistors. I’m pretty sure there’s a loose connection somewhere causing it, because the problem goes in and out. I may have just been unlucky that the screen issue first appeared using the NTE284 and then went away after replacing it with the NTE130. So, the NTE284 might work fine as a replacement for the 2N5878/2N3716. However, I still think a matched pair of NTE219 and NTE130 transistors makes sense as replacements to the original transistors and they seem to work well.

More on the screen issue and diagnosis soon... 

Original Post

Part of the restoration list is replacement of parts that are old and likely to fail. The high-power transistors than control vector beam deflection, which are mounted on large heat sinks on the side of the monitor assembly, do a lot of work and can generate a lot of heat so they can be sources of failure. Failure of these transistors can endanger the display board, so these were high on my list to examine.

IMG 4695 (1)

At any moment, one transistor handles vertical movement, another the horizontal movement. However, as the beam moves from one side of the screen to another, control changes from one transistor to a similiar but opposite transistor. So, a pair is required for vertical movement, and another pair for horizontal movement. I believe these are examples of what are called push-pull amplifiers. The table below lists the transistors and their function.

Vector Deflection Transistors
Schematic Type Original Deflection Side
Q110 NPN 2N5878 (2N3716) Vertical Left
Q111 PNP 2N5876 Vertical Right
Q210 NPN 2N5878 (2N3716) Horizontal Left
Q211 PNP 2N5876 Horizontal Right

The transistors in my Star Castle appear original and I decided to replace them. One transistor is my unit, however, was a 2N3716,in place of a 2N5878. I learned online that this was a less costly substitute, so I’ve listed it in the table as well.

These transistors aren’t made anymore, so suitable replacements need to be found. My local electronics shop stocks NTE transistors, so I looked up replacements at the NTE website. Here’s what I got:

NTE Suggested Replacements
Schematic Type Original NTE
Q110, Q210 NPN 2N5878 (2N3716) NTE284
Q111, 211 PNP 2N5876 NTE219

But here’s what happened when I replaced the transistors for the horizontal control, Q210 and Q211. Whoa! What’s going on with the left side of the screen?

IMG 6436

Maybe the NTE284 isn’t a suitable replament for the 2N5878. The right side of the screen looked okay, so the NTE219 is probably okay. Looking at the data sheet for the NTE219, I realized it also details it’s NPN counterpart, the NTE130. It started to look like this was probably the transistor I should be using instead of the NTE284.

It was about this time I noticed that you can also buy the NTE219 and NTE130 in “matched" pairs. Im not an expert on transistors, but it appears transistors of the same type can have quite a bit of variation in them. By testing them individually, NPN and PNP transistors of the same type can be matched, having very similar, if opposite specifications. Here, in their push-pull arrangement, it seems like having the transistors as matched as possible is a good idea, but I cant say they need to be. Getting matched pairs was pretty easy and cheap, so I got the matched set from NTE Parts Direct. They worked!

IMG 6584

I’ve tried studying the differences between the 2N5876/2N5878 and the NTE219/NTE130, but again, I’m no expert on transistors. The one thing that struck me is the NTE transistors are rated for 115 watts while the originals are rated for 150 watts. (It’s interesting to note that the NTE284 is rated 150 watts.) I’ve been checking the temperature of the new transistors and only one seems to even get warm, so the lower power rating seems acceptable. 

One more detail thats hard to quantify: the noise you here associated with moving the vector beam as it runs seems to have gotten quieter. Its not scientific, but it makes me think the system is happier moving the vector than it was. So Im going to stick with the matched NTE 219/130 pairs.

In summary, if you want to replace the power transistors, here are the replacements that works for me:

My Replacements
Schematic Type Original NTE
Q110, Q210 NPN 2N5878 (2N3716) NTE130
Q111, 211 PNP 2N5876 NTE219

IMG 6529