X-Ray protection, Commodore 1084S-D1 CRT

[Overkill]

Hello all, I'm hoping to find a CRT expert here

I got hold of a monitor that had a high-pitch whine and smocking, long-story-short I found and replaced some fault components in the Horizontal Output Transistor circuit and also replaced the flyback with what was listed as a compatible replacement.

The monitor then switched on and I got high-voltage for a second and then the monitor shut down.

After some thinking it looked like maybe the X-Ray protection circuit was cutting in, which I have proved by unsoldering the X-Ray pin on an IC.

I've since tested the monitor on a very crude current limiter that I put together with some stuff lying about the house , It now stays on and I get a nice white spot on the screen when I cut the power.

After doing some Googling, I found someone that had the same issue when using the same replacement flyback and they fixed theirs by realising that their original flyback wasn't faulty, and putting it back in the monitor fixed the issue.

So, from that information I assume the replacement flyback needs less voltage or the winding for the X-Ray protection circuit is different (creates a higher voltage).

I don't have a high-voltage probe (although I have plenty of 0.25W resistors).

I've stopped testing the monitor now until I get some advice.

The closest service manual I can find is here:
https://www.zimmers.net/anonftp/pub/..._400413-01.pdf

The original flyback is a DCF-1580, but I replaced it with what is supposed to be a compatible MSH1FCT31

Thanks for looking

[Maarten]

The X-ray protection seems to be derived from the heater supply. Does the tube glow too bright? Are the other line-derived voltages such as the 15V line correct?

[Overkill]

Thanks for the reply, I've not wanted to leave this on for long to test it, but the screen is dark until I cut the power and it collapses to a white spot.

Since disabling the X-Ray protection, I've not run the monitor on full power until I got some advice.

I'm assuming there is no danger from X-Rays because the screen isn't bright, but I'd like an expert to second that assumption I think I'm more worried about the HT being super high and blowing up

Would it be best to remove the HT lead from the tube and then test voltages?

This is my first CRT repair apart from replacing the odd ON/OFF switch.

[Maarten]

Is the picture size correct when turned on? If it's about right or too large, the EHT will be roughly correct or too low respectively. If it's small and bright, you should switch off before something flashes over.

Also you could measure the +15 voltage by first attaching your multimeter to it with wires soldered to the PCB. If it's correct, EHT will also be correct.

I think the most important criterion to switching it off at that point in time would be heater colour and brightness.

[Overkill]

The screen is dark, maybe I should adjust the 'Screen' trim on the flyback?

Originally Posted by Maarten

Also you could measure the +15 voltage by first attaching your multimeter to it with wires soldered to the PCB. If it's correct, EHT will also be correct.

Sorry, it took me awhile to realise what good advice this is I will try it.

[Overkill]

Just as I think I'm getting somewhere, confusion sets in

I've possible found the problem, but I don't understand it

The circuit diagram (which might be the wrong one) shows 105v to pin 7 of the flyback, BUT on the underside of the board it says 110v.

The circuit diagram shows a DCF 1580 flyback which is the same as the flyback I removed.

I tried seeing if I could test the 15v (DC right?) while it was connected to my current restrictor, but it just read millivolts (maybe because my current restrictor was messing it up).

I noticed that the screen was bright and not full, probably an inch to 2-inch boarder at the sides and it was concave.

I also noticed smoke, which looks like it was coming from the flyback

So, it looks like the flyback must be getting 110v instead of 105v, but it doesn't make sense that it's the same flyback

It's a shame I can't find datasheets for these flybacks.

Should I desolder something and test to see if it's 105v or 110v ?

[G6ONEDave]

If the loptx has screen (A1) and focus controls on it, try turning them to minimum and then power up, hopefully that will stop the set from tripping.

Dave

[Overkill]

Just to make sure, "down" is anti-clockwise?

[Welsh Anorak]

5v won't make much difference - I'd set it to 105v.
A concave screen suggest an E/W fault, and it's likely the smoke is coming from that area rather than the new LOPTx.
Yes - down is anticlockwise.

[Overkill]

Originally Posted by Welsh Anorak

5v won't make much difference

I've been reading elsewhere that every extra 1v in makes a big difference to the output.

Originally Posted by Welsh Anorak

I'd set it to 105v.

I would love to try that, but how would I adjust the voltage? I see nothing adjustable in the circuit diagram.

Originally Posted by Welsh Anorak

A concave screen suggest an E/W fault, and it's likely the smoke is coming from that area rather than the new LOPTx

an E/W fault?

Originally Posted by Welsh Anorak

Yes - down is anticlockwise.

Thanks

[G6ONEDave]

If the monitor powers up without tripping, after you've turned the A1 (screen) volts down, then with the set powered try increasing that control very slowly and see if anything appears on the display before the set trips again, such as all grey or white luminance or a centered horizontal or vertical white line. If the monitor is RGB, then the display might show a dim colour cast and or a coloured central line.

E/W being East/West will show as a possible lack of width and or possibly curved vertically at the display edges.

Ideally watch the display in a mirror so that you can remain behind the set. That way you can keep an eye on what is happening on the pcb, as well as the display and of course where your hand is! You might need to do this with the lights turned down.

Dave

[Overkill]

Thanks for the info Dave, although I'm still concerned about the input voltage possibly being too high, will an extra 5v hurt?

Is that voltage to the flyback called the "B+"? Because I've read in lots of places that the B+ must be checked and can only be 1% out, because every extra 1v can = 230v extra at the output. The trouble is, you get conflicting information from different places

I will try it again with the "Screen" turned right the way down.

[PaulM]

B+ is a hangover from ancient wireless days in the USA. It just means 'HT' (High Tension) supply. There were low voltage 'A' supplies (valve/tube heaters), 'B' supplies (valve/tube main HT) and 'C' supplies (valve/tube grid bias supplies).

At this stage of fault finding, 5V either way makes no odds at all.

It does sound like an E/W fault along with something else causing the trip to operate. CRT heater/filament supplies derived directly from the LOPT like this are very hard to measure. The pulsed nature of the power makes it very tricky to work out accurately what the real DC or sinusoidal AC equivalent would be. I do wonder if your flyback period is right? Can you check that? It has a big effect on things.

Best regards,

Paul M

[Overkill]

Hi Paul

flyback period? is that to do with what the HOT circuit creates? This is my first CRT repair attempt apart from power switch replacements and things like that.

[Philips210]

Hi.

It's not clear if the service manual is correct for the monitor so it's difficult to give advice if the circuit diagram is not necessarily correct.

You could check the power supply circuitry in your monitor making comparisons with the circuit to see if the component reference numbers/values agree. A list of components replaced so far could be helpful. Perhaps post a few pics of both sides of the chassis so that we can make a few suggestions?

What we used to do when faced with TV faults like this was to disconnect the HT feed to the line output stage and then connect a 60W mains filament lamp across that feed. That way the power supply can be load tested and the HT can be measured. This loading method can not be applied to all types of power supply but can in most cases.

Reduced width with a very bright display is often a sign of high EHT. That could be due to excessive HT to the line output stage. Another cause is the flyback tuning capacitor(s) either going open circuit or falling in value. The replacement LOPTx could possibly be another likely cause.

Often with reduced width there's a less serious cause. The pincushion distortion combined with reduced width will be associated with the EW modulator circuit and is often due to faulty diodes or capacitors. They're usually located around the line output transistor as is the tuning capacitor(s). Sometimes blue coloured ceramic capacitors are used here and they can and do fail. A good look at the physical state of these components is worthwhile but with the tuning caps it's best to check its capacitance to be sure. Their value is fairly critical and the voltage ratings for these is often 1.6kV to 2kV.

Re measuring the EHT. I wouldn't attempt this with a makeshift voltage divider with resistors unless you can be certain it's safely insulated. You would the need to verify its calibration. A commercially available EHT meter or adapter is a safer option.

Good luck with it!

Regards,
Symon

[PaulM]

Flyback time is at the end of the TV line and is when the spot rapidly goes back to the other side of the screen. To do that takes a lot of voltage and the resonant nature of the LOPT circuit does that for you. It's this huge voltage transition that makes the EHT and also drives things like the heater supply, the A1 (screen supply) and also the X Ray protection (in this case). I'm greatly simplifying things here!

As Symon states, this could be a consequence of changing the LOPT. If the flyback time is wrong, the EHT (and heater voltage in this case) can be horribly adrift too. The flyback time is governed mainly by the characteristics of the LOPT and the flyback tuning capacitor. A <<different>> LOPT may not be 'tuning up' correctly and could need the flyback capacitor changing slightly. It looks like C207 here. It may just be faulty . . .

If you have an oscilloscope, just waving a probe near the LOPT will induce a voltage large enough to see and measure the flyback pulse width. Don't go too close, you can zap your 'scope! Its shape is also important.

This is all separate to the E/W fault that seems to be present.

Making your own EHT probe is not a good idea!

Best regards,

Paul M

[McMurdo]

what was the fault with the original LOPTY, did you ascertain this was causing the squealing? And have you found out what is causing the smoke?

[Maarten]

The + 15 should be measured behind the rectifier, for example across the electrolytic. You didn't by any chance try to measure it directly at the flyback pin?

[Overkill]

Originally Posted by McMurdo

what was the fault with the original LOPTY, did you ascertain this was causing the squealing? And have you found out what is causing the smoke?

I'll write a little bit of the repair history below

[Overkill]

Originally Posted by Maarten

The + 15 should be measured behind the rectifier, for example across the electrolytic. You didn't by any chance try to measure it directly at the flyback pin?

Yes, from the flyback, I will try across the cap.