CRT Repair Help on ADDS 4000 Terminal, Hissing/Arcing Noise

[Edison94]

Hello All,

I am currently trying to repair an old ADDS 4000 CRT terminal. It powers on, but there is a consistent hissing sound coming from somewhere around the analog part of the board and no picture. So far I have removed, cleaned, and socketed one chip (MC74F38N) that had visible corrosion/residue surrounding many of the pins and cleaned the whole PCB (it was very dusty). I have also replaced all capacitors that were visibly leaking, and reflowed all the solder joints on the flyback. I do have all the necessary electrolytic caps in case I should need to replace all of them. The terminal showed no change after doing these things.

The board has several bodges that were already there when I got it. I am not sure if these were done at the factory or if someone did them later. It appears to have been powered on for a long time as there is some serious screen burn on the tube and there are several brown spots on the board around power resistors. The heater on the tube appears to glow, and the 5V logic seems to come up as well (it beeps after a few seconds). I also disconnected the deflection and focusing coils and ruled them out as the source of the hissing noise.

I suspect that the flyback transformer is bad, however it is hard to pinpoint where the sound is coming from and I cannot see any visible arcing, even in the dark. The flyback has no visible cracking or bulging. I have not done any live probing yet over saftey concerns, but I have discovered that the horizontal output transistor (Sanyo C3175 NPN) is shorted across pins 1 & 3 which I believe are the base and emitter, which could suggest a shorted primary in the flyback that took the transistor down with it. During a few powerons, however, I did notice that static was building up on the front of the screen, so the flyback could still be working to some extent. 

I have also noticed there is 15 across the 5V rail for the terminal logic, however as I said I have not done any measurements of this voltage while it was on.

There is very little information about this terminal online, and even less about this specific flyback transformer which says it's a 52500027 / 350-09800. I found one website which still sells them: https://www.donberg.ie/catalogue/line_output_transformers/hr_diemen_line_output_traformers/hr_42079.html however the minimum order amount is 50 euros so I would rather not buy one if I don't have to.

Any help on where to go from here or how to further debug this safely would be greatly appreciated.

[Technotronic]

If the HOT is shorted then it could be the FBT or the damper.
I have sold flybacks for decades and done a lot of CRT repairs,
this page explains a little why its always a good idea to replace
them at the same time:

https://technotronic-dimensions.3dcartstores.com/crt-repair.html

The hiss is probably HV escaping from the FBT somewhere. Remove it for inspection
for cracks, holes etc and under near the pins and the potting compound.

Also check the HV anode wire for cracks, remove the CRT anode cup for inspection
or improper fit....and discharge the CRT properly with a screwdriver and alligator clip
before you work that area because older monitors usually keep a charge of a few KV
like a capacitor. Bleeder resistors were later built into the flyback to mitigate this danger
but that happened sometime in the 1990's.

[p.larner]

can you smell ozone near it?,also power it up in a dark room with the covers off and look for any blue corona near the eht sections,anode cap etc+ the flyback.

[inse]

I think I can see cracks in the FBT on the photo.
Most probably the HOT is still OK, driving the FBT and releasing some high voltage through the broken isolation.
If you are brave, take a isolated test lead with tip, connect one side to chassis ground and carefully probe the surface of the FBT.
You will instantly notice any leaks.
Do this at your own risk!
I had to edit my post:
A less brutal approach would be to use an isolated (electrician style) screwdriver to probe the FBT, the tip will provoke corona discharges at isolation faults.

[inse]

No further findings on the hissing source, Edison94?

[R.J.]

At the last photo there are cracks in the high voltage transformer.
You have to replace it.

[CaptDon]

It is normal for the base to emitter junction to appear shorted in circuit. You are probably seeing the driver transformer winding. Test the transistor out of circuit. It is probably not bad since you hear the hiss of high voltage. If the filament is lit and you have high voltage you may want to look for and scope a video signal present at the CRT cathode or grid to be sure there is actually something to display. Have you sent data to the terminal so it actually has something to display? You should at least see the blinking cursor. The CRT could be so weak that it would need an extensive warm up time. Don't worry about the hiss or replacing the flyback until you verify the rest of the video circuitry is working.

[CaptDon]

IF you have a good 10M input resistance DVM or VTVM measure the cathode and grid voltage on the CRT. If the grid is close to the cathode voltage (cathode 100vdc, grid 95vdc) you should have screen illumination. If the grid is much lower like cathode =100vdc grid =70vdc the beam will be cut off. A video signal brings the grid to cathode voltage closer together and away from cutoff. Often there is a 'drive' or 'CRT Bias' control that can be adjusted to compensate a weak C.R.T. The C.R.T. filament voltage may possibly be derived from the flyback transformer. In any event the C.R.T. filament should be at an orange or even yellow/orange operating temperature and not a dull cherry red. It is possible the high voltage rectifier located within the flyback is failing which would give low or no high voltage. The only good way to test high voltage on the CRT anode cup is with a meter specifically designed for the job, typically about 100 megaohms in series with a microammeter. The resistance is spread out over about 6 inches to eliminate arcing within the resistor or across the body of the resistor. Every T.V. shop owned one of these.
 

[Paceguy]

I also notice some corrosion on the FBT itself. There are cracks visible and they might be fixable with some high voltage corona dope, or no arc spray. Since that board appears to have been in a damp environment, I would suggest removing the FBT and looking to see if something on the underside of the FBT could be the cause of arcing.