Help diagnosing my CRT for Apple II

11 posts / 0 new
Last post
Offline
Last seen: 3 weeks 2 days ago
Joined: Mar 2 2024 - 06:03
Posts: 5
Help diagnosing my CRT for Apple II

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

 r/crtgaming - Smearing CRT issue 

 

 

 

Offline
Last seen: 13 hours 54 min ago
Joined: Feb 27 2021 - 18:59
Posts: 498
flare

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:

  1. if turning down brightness and contrast solves the problem
  2. if the termination can be switched to 75 ohms (not "high impedance")
  3. if the problem goes away with a different video cable
  4. if the video amp bias voltage is too low, raise it using the SCREEN voltage pots
  5. test or replace the video amplifier transistor
  6. check the filament heater voltage, adjust it if it is too low based on the data book for the tube
  7. with a CRT analyzer, check emission and rejuvenate the tube if it is low
  8. try to boost the filament heater voltage slightly (devices called "brighteners" were sold for this purpose)
  9. replace the CRT tube.
CVT
CVT's picture
Offline
Last seen: 52 min 55 sec ago
Joined: Aug 9 2022 - 00:48
Posts: 1020
My money is on being switched

My money is on being switched to High Impedance instead of 75 Ohms.

Offline
Last seen: 3 days 1 hour ago
Joined: Apr 26 2016 - 08:36
Posts: 685
CVT wrote:My money is on
CVT wrote:

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:

 

Offline
Last seen: 3 weeks 2 days ago
Joined: Mar 2 2024 - 06:03
Posts: 5
robespierre wrote:What you're
robespierre wrote:

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

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. 

Offline
Last seen: 3 weeks 2 days ago
Joined: Mar 2 2024 - 06:03
Posts: 5
CVT wrote:My money is on
CVT wrote:

My money is on being switched to High Impedance instead of 75 Ohms.

Hello thanks for replying, the output still the same, but now little bit darker using the high impedance mode

Offline
Last seen: 3 weeks 2 days ago
Joined: Mar 2 2024 - 06:03
Posts: 5
baldrick wrote:CVT wrote:My
baldrick wrote:
CVT wrote:

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 ci

Yes, you are right. 

Offline
Last seen: 13 hours 54 min ago
Joined: Feb 27 2021 - 18:59
Posts: 498
Do you know of anyone who has

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.

Offline
Last seen: 13 hours 54 min ago
Joined: Feb 27 2021 - 18:59
Posts: 498
impedance

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.

Offline
Last seen: 3 weeks 2 days ago
Joined: Mar 2 2024 - 06:03
Posts: 5
robespierre wrote:Do you know
robespierre wrote:

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 th

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

Offline
Last seen: 13 hours 54 min ago
Joined: Feb 27 2021 - 18:59
Posts: 498
comments

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.

Log in or register to post comments