• Please review our updated Terms and Rules here

Strange horizontal interference in Mac SE screen

P

PgrAm

Experienced Member
Joined
Sep 28, 2011
Messages
276
Location
Toronto, Canada
Hi all,

A little while ago I dug out my Mac SE FDHD and replaced it's dying hard disk with a much newer model. Afterwards as I was playing around with it, I noticed some strange horizontal lines in the video image, that look to me like an messed up sync signal. I made a video below which demonstrates the issue (sorry for my somewhat loud breathing).


Does anybody know what's causing this or if this might have anything to do with the new HDD, maybe its drawing more current than the old one and causing noise in the PSU? I'm a bit hesitant to open this thing up again since now the capacitors are fully charged and I'm recovering from an unrelated electrical shock, but eventually I'll psych myself into it.
 
PgrAm said:
Hi all,

A little while ago I dug out my Mac SE FDHD and replaced it's dying hard disk with a much newer model. Afterwards as I was playing around with it, I noticed some strange horizontal lines in the video image, that look to me like an messed up sync signal. I made a video below which demonstrates the issue (sorry for my somewhat loud breathing).


Does anybody know what's causing this or if this might have anything to do with the new HDD, maybe its drawing more current than the old one and causing noise in the PSU? I'm a bit hesitant to open this thing up again since now the capacitors are fully charged and I'm recovering from an unrelated electrical shock, but eventually I'll psych myself into it.
Click to expand...

PgrAm, discharging a CRT is not as bad as you think. They just tell you about the voltage so you pay attention and dont make any dumb risks. All you need is a long think screw driver and one or two alligator clip leads. Connect one clip to the screw driver and one to the base metal on the chassis. I always put one hand behind my back (old school training I guess) and with the hand your working with(make sure not to touch any exposed metal on the screw driver, just the plastic handle) slowly press the screw driver under the anode cap until you see a small spark... sometimes there isn't a spark which isn't satisfying. at that point just poke around for a few seconds touching the metal contact. Then unclip the anode and all is well. Probably a bunch of you-tube videos out there. Once you get past this portion, there are soo many more things you can work on... Sorry for the Rant.

I would take off the Tube cap and put it on and off a couple time cleaning the contacts. Let us know.
 
VERAULT said:
All you need is a long think screw driver and one or two alligator clip leads. Connect one clip to the screw driver and one to the base metal on the chassis.
Click to expand...

Directly grounding a vacuum tube to discharge it is really hard on the tube, you should use something like a 1 mega ohm resistor in series to gradually discharge the tube instead.
 
GiGaBiTe said:
Directly grounding a vacuum tube to discharge it is really hard on the tube, you should use something like a 1 mega ohm resistor in series to gradually discharge the tube instead.
Click to expand...

Gigabite, what type of wear and tear does it actually do? This is just the way I have been taught and the way I have seen others do it. I think the emphasis has always been on personal safety rather than equipment longevity.
 
Yeah I know that its totally safe if I take the right precautions. I've even discharged CRTs using this method a few times before. I suppose its irrational but every time I think of the volts going through that thing it gives me the heebie-jeebies :crazy:, just takes me some time to overcome my fear.

Also from what I recall isn't it not the tube that the discharge is hard on but rather the capacitors?
 
VERAULT said:
Gigabite, what type of wear and tear does it actually do? This is just the way I have been taught and the way I have seen others do it.
Click to expand...

Because they have no experience with high voltage circuitry.

What happens when you put and instantaneous heavy current across a thin foil sheet? It starts to melt/burn/vaporize. If there are any defects in the foil, the current path tends to deviate towards them since they're more conductive than the rest of the foil.

The inside of a vacuum tube is coated in a thin conductive material and is subject to the same damage. The connection points themselves can get spot welds and be damaged as well.

But that's not the only subject of potential damage. Inside flyback transformers, some of the windings are thinner than a human hair, what happens if those suddenly experience heavy current? They will burn up.


VERAULT said:
I think the emphasis has always been on personal safety rather than equipment longevity.
Click to expand...

It seems like your and the people that you've learned from's emphasis on personal safety is to do it as fast and as reckless as possible. Using a resistor to properly bleed the charge from a vacuum tube does nothing to reduce personal safety, but it does everything to increase the longevity of the equipment.

You need a bit of a reality check here, this is 2018, not 1995. CRTs have not been in commercial production for over a decade and going the extra mile to preserve existing, working tubes for as long as possible is a must. It's stupid to think that taking your time with a repair reduces personal safety.

PgrAm said:
Yeah I know that its totally safe if I take the right precautions. I've even discharged CRTs using this method a few times before. I suppose its irrational but every time I think of the volts going through that thing it gives me the heebie-jeebies :crazy:, just takes me some time to overcome my fear.
Click to expand...

CRTs do store enough energy to kill you, but only under the right circumstances. It's why you only keep one hand in the chassis at a time, so it doesn't make a complete circuit through your heart, and any potential shock goes through your feet or other grounded appendage.

PgrAm said:
Also from what I recall isn't it not the tube that the discharge is hard on but rather the capacitors?
Click to expand...

No, it IS hard on both the tube and the flyback. Capacitors are designed to be charged/discharged at high frequencies. And technically the CRT is a capacitor itself.
 
Last edited:
Gigabite... Cocky much? Fast and reckless? I received my training in the Military, So if snail pace is fast to you I suppose. And Reckless? No I think not. Maybe just provide instruction and education and check the attitude at the door. I don't think berating anyone is helpful in this forum. If you want to teach a better, smarter method... Have at it. We are all here to learn on some level. Whats name calling going to do exactly?
 
So if I were to discharge the tube through a 1M resistor, how many watts would it need to be rated for? I have some spare 1Watt 1M resistors, would those work?
 
Last edited:
Rather than join the debate on how to discharge the tube, I'm going to be practical and ask why discharge it at all.
You are at greater risk of a shock discharging it than just leaving it alone.
Just reseat the board on the end of the neck and the pcb end of the cable running to the deflection yolk, there is no risk of shocking yourself from the energy stored in the tune.
 
PgrAm said:
So if I were to discharge the tube through a 1M resistor, how many watts would it need to be rated for? I have some spare 1Watt 1M resistors, would those work?
Click to expand...

A 1W resistor will work (and I've used them), but I'd recommend a 5 or 10W resistor that's specifically rated to handle high voltage. I personally have a fat 50W 1 mega ohm resistor for doing this, but I didn't buy it for the task, it came in a parts drawer I got at a garage sale a decade or so ago.

If we do the maths on an average flyback voltage of 10 kV, a 1 mega ohm resistor would pass about 100 watts, but this assumes continuous duty. The brief pulse from discharging a tube won't be enough to heat up a 1-5W resistor too much.

Smaller CRTs generally have a lower flyback voltage though, as do B&W CRTs because they don't have to drive three guns.
 
If you think you'll discharge a CRT (or any other high voltage component) multiple times in your life, it is well worth the price to buy a high voltage probe voltmeter. (I think I paid $10 for my Eico one) Get one with a d'Arsonval mouvement (not a digital one) because it has impedance low enough to discharge tens of kilovolts in a reasonable amount of time.

This is the safest and most convenient method. But there is one caveat. If you never see the meter move, you never know if it's accurate. But you can verify its operation simply by measuring the second anode voltage of your CRT (with the power on!).
 
KC9UDX said:
If you think you'll discharge a CRT (or any other high voltage component) multiple times in your life, it is well worth the price to buy a high voltage probe voltmeter. (I think I paid $10 for my Eico one) Get one with a d'Arsonval mouvement (not a digital one) because it has impedance low enough to discharge tens of kilovolts in a reasonable amount of time.

This is the safest and most convenient method. But there is one caveat. If you never see the meter move, you never know if it's accurate. But you can verify its operation simply by measuring the second anode voltage of your CRT (with the power on!).
Click to expand...

That does sound useful KC9UDX, can you provide an example online of one in particular or the one you use?
 
VERAULT said:
That does sound useful KC9UDX, can you provide an example online of one in particular or the one you use?
Click to expand...

I found mine either at some kind of flea market or something, don't really recall. It's an Eico brand, probably 40 years old or more by now.

You can buy new ones
https://m.ebay.com/itm/BK-Precision...840778&hash=item2ccc0a22da:g:QMYAAOSwiLdV8IH3
or used ones
https://m.ebay.com/itm/LEADER-HIGH-...581132?hash=item2136b262cc:g:9vUAAOSwtGlZLBKv

If you buy a used one, it's probably best to wear appropriate gloves the first time you use it. Also, before using a used one it might be a good idea to thoroughly clean it.
 
KC9UDX said:
I found mine either at some kind of flea market or something, don't really recall. It's an Eico brand, probably 40 years old or more by now.

You can buy new ones
https://m.ebay.com/itm/BK-Precision...840778&hash=item2ccc0a22da:g:QMYAAOSwiLdV8IH3
or used ones
https://m.ebay.com/itm/LEADER-HIGH-...581132?hash=item2136b262cc:g:9vUAAOSwtGlZLBKv

If you buy a used one, it's probably best to wear appropriate gloves the first time you use it. Also, before using a used one it might be a good idea to thoroughly clean it.
Click to expand...

Wow thats a big price difference. Definitely going to get the used one.. Thanks KC9UDX
 
All you need is a flat-head screwdriver with a rubber or plastic handle, and a wire with alligator clips. Clip one end to screwdriver (metal part) and the other to ground (on the device).

Put one hand behind your back and with the other hand, stick the screwdriver under the little cup on the monitor until it touches the metal piece. If there is a charge, it will make a little "tick" sound and you're done. That's how I was trained and I've never had a problem in 25 years (did it 100 times at least).

https://www.youtube.com/watch?v=-jbnFuVWTdk

The "one hand behind your back" is a "just in case" and supposed to keep any charge from going directly through your heart, somehow, which can kill you. I'm not sure why this would help, but that's how I was trained.
 
Last edited:
Don't hurt him GiGaBiTe! He must be one of those " your and the people that you've learned from.." types you lumped me into.
 
GiGaBiTe said:
Because they have no experience with high voltage circuitry.

What happens when you put and instantaneous heavy current across a thin foil sheet? It starts to melt/burn/vaporize. If there are any defects in the foil, the current path tends to deviate towards them since they're more conductive than the rest of the foil.

The inside of a vacuum tube is coated in a thin conductive material and is subject to the same damage. The connection points themselves can get spot welds and be damaged as well.

But that's not the only subject of potential damage. Inside flyback transformers, some of the windings are thinner than a human hair, what happens if those suddenly experience heavy current? They will burn up.
Click to expand...

Just a reminder, a CRT under OPERATING conditions only uses a few hundred miliamps on the anode. I can see your concern if this was a few amps where but the damage incurred by a direct grounding in the entire HET section while the machine is not powered is pretty much negligible. Chill out.
 
You must log in or register to reply here.
Back
Top