Hello, I just bought this CRT with a smearing/streaking issue. Already recapped all of the caps and the issue is still there. Can you suggest anything to check? I believe this is a clone of Sanyo VM 4509, the schematic is here https://elektrotanya.com/sanyo_vm4509.pdf/download.html
What you're seeing is referred to as "flaring" or "bleed". First thing to do is turn down the brightness and/or contrast and see if the problem goes away then.
This problem can be caused by both a bad tube, or by a bad video amp transistor circuit in the electronic part of the set. The possible tube problem can be diagnosed and maybe repaired using a CRT analyzer device.
It can also be caused by a bad cable between the computer and the monitor, or by incorrect termination (should be 75 ohms) at the end of the video cable. This is a form of ringing where the signal is underdamped.
So in order of easiest first, check:
My money is on being switched to High Impedance instead of 75 Ohms.
This is possible - many of those metal cased industrial monitors had switches on the back panel for high and low impedance, as well as multiple video inputs for different types of cables.
And if this is a true clone of the Sanyo monitor, then from the manual you cited, here, on page 3 is the impedance switch:
monitor.jpg
Hello, thanks for replying. I already tested point 1, 2, 5, and it is still the same. Although it seems to be darker when using high-impedance mode.
For number 5, because I didn't have the new replacement, I replaced it with another transistor on a different module, but still the same.
Still ordering the new cable, hope it makes a difference.I already searched for a datasheet for the CRT tube but still cannot find one. The tube was from CPT (a Chinese company) with serial number CPJ310N1U. But the heater voltage is around 10v, seems normal to me. Btw, can you point out the Screen pot from the datasheet? I did not find one.
Hello thanks for replying, the output still the same, but now little bit darker using the high impedance mode
Yes, you are right.
Do you know of anyone who has a CRT analyzer, like a Sencore CR-70 or CR-31, or a BK 467? Testing the cathode's emission would be very useful as it isolates the problem to either the tube or the electronics.
To simplify the problem a bit, when flare happens, the emission of electrons from the cathode to the screen lags behind the video signal. The immediate cause is that the voltage on the cathode, which is low for bright dots and high for darkness, can't climb fast enough to cut-off the beam after the video signal goes from bright to dark.
This would happen if the transistor that amplifies the video signal is weak, or has insufficient bias voltage. But even a good transistor will struggle if the cathode of the CRT has low emission. To get any beam to flow at all, the transistor needs to bring the cathode voltage very low, which means that the transistor must "saturate" and conduct a huge current. Transistors in saturation have slow recovery, so it takes longer than normal for it to shut off again and raise the cathode voltage, cutting off the beam. The result is that the beam stays on longer than it should, visible as flaring.
I would also suggest measuring the impedance of the video in connector on the set, to ensure that the 75 Ohm switch isn't simply broken internally. The set doesn't need to be powered on for this measurement.
With the switch in the 75 Ω position, DC resistance from the center pin to shield of the video input should be close to 75 ohms. In the high impedance position it should be somewhere around 1M ohms. Keep measuring until the reading stabilizes, because there may be a coupling capacitor that will be charging at first.
Very good explanation, thank you. But currently, it's hard to find a CRT analyzer in my town. The CRT tube itself is physically good and I think there is no sign of heavy use. I already swap the cable using a good one, but still the same. The impedance is correct when I switch to 75 ohm and High Impedance (around 1M ohm).
The thing that I can do right now is try to replace the transistor, I hope there's a miracle here lol
From the analyzer setup book,
CPJ310N1U should use a filament voltage of 11.8 V, pinout: F1 (3) F2 (4) K (2) G1 (5) G2 (6) with bias 52 V. These values are used for cathode emission testing.
Looking at the schematic you linked to the VM4509, you will want to check the SUB BRIGHT pot (VR203) because it controls the range of the BRIGHT control VR202. There are electrolytic capacitors in the video output circuit: C201, C202,C204, C208, C209, C401, C622. The video amp is Q201, feeding Q202 and Q203. Were you able to see if your set uses the same schematic?
The schematic has 10.5 V for filament voltage and -62 to -13 V for bias.
It lists tubes 240XB4A and 240YB4, which the setup book has at 11.2 V filament, and 36V bias, so not identical, although the same pinout.