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475 repair question


jeff corey
 

My project as of late has been refurbing a 475 I picked up in non working condition. I recapped the power supply and got my -15 back up with no ripple, still had no trace at all. I have HV, but beamfinder shows nothing. I disconnected one end of cr1343 as per the troubleshooting Z axis steps and can get tp1364 to swing from +6 to +25 when pushing beam finder.

I started testing transistors and q1338 tests bad. it is listed as an ska6516, which i cant find anywhere. can i just sub in a 2n2222 npn transistor for it?

jeff corey
wb9kaz
microsystems development


Simon
 

One thing to try is to press the beam finder in a darkened room and see if there is a slight glow from the CRT. I had a bad Y output amplifier which put the spot way off the screen. It cannot be retrieved by the beam finder, which acts on earlier stages. You should also measure the voltage between the CRT grid and cathode using a plastic meter. There are a couple of test points for this under the metal HV section shield. It should be around -80V and vary with intensity and grid bias trimpot setting. You could also put a scope on the junction between CR1371, R1372 and CR1373 and see how it behaves with intensity and grid bias trimpot setting.
You might also put the scope on the X amplifier outputs (red and green wires going through a hole around position E8) to check that the horizontal section is OK.
You could also check the Y amplifier outputs by checking the voltages on the connections of the forward Y CRT pins at their junction with the resistive network R497. Treat R497 with care as it is not easy to replace ….
Simon


Tom Lee
 

Hi Jeff,

I took a quick look at the schematics. There's nothing in the circuit that demands extreme properties of Q1338, so a 2N2222 should function as a sub, although I'd probably use a 2N3904, myself. This particular circuit is relatively insensitive to Q1338's ft, but somewhat sensitive to output capacitance (unblanking speed will be slower with the 2222).  A 3904 looks a lot like a 2222 of one-third size, with correspondingly lower output capacitance. So if you happen to have a 3904 in your junk box, I'd choose it over the 2222 in this case. If you don't have one handy, no problem. Q1338 has to be replaced for you to proceed, and the 2222 will certainly function.

I think you're close to finding and fixing the problem. You've got working HV, you've found a bad transistor in a part of the circuit that could cause what you're seeing. Let's see where things stand after you pop in a new transistor.

Happy hunting!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/4/2020 10:25, jeff corey wrote:
My project as of late has been refurbing a 475 I picked up in non working condition. I recapped the power supply and got my -15 back up with no ripple, still had no trace at all. I have HV, but beamfinder shows nothing. I disconnected one end of cr1343 as per the troubleshooting Z axis steps and can get tp1364 to swing from +6 to +25 when pushing beam finder.

I started testing transistors and q1338 tests bad. it is listed as an ska6516, which i cant find anywhere. can i just sub in a 2n2222 npn transistor for it?

jeff corey
wb9kaz
microsystems development