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Powering up a VT100 - now the EHT FLybyck xfmr died...

MattisLind

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Well. I have been hi jacking the VT180 thread for too long. And now when I powered up the VT100 for a second try the EHT flyback transformer gave up. There were some smoke coming out of it. The EHT winding is now open circuit :-(

A found a tip from Lou which was dated 2013 that a company called Dalbani could supply these. But that seems to be history, since no they are on back order. The chance that they are getting new ones are slim I guess.

Any other sources for the EHT transformer of VT100?
 
I don't know your experience, and don't mean to question your work, but are you sure the winding is open? I find this doesn't happen very often (but it does happen). It's difficult to determine, because you typically can't measure it with an ohmmeter. Have you verified that the other secondaries are working?
 
I don't know your experience, and don't mean to question your work, but are you sure the winding is open? I find this doesn't happen very often (but it does happen). It's difficult to determine, because you typically can't measure it with an ohmmeter. Have you verified that the other secondaries are working?

No problem. I have no deep knowledge of EHT flybacks. But measuring with ohmmeter both direction from ground to EHT out gives mega ohms. Why can't you measure it? It just the coil and a diode in series. All the other windings measure fine. In the few ohm range.

And there were smoke coming out.

So what is the best procedure to check out the EHT flyback anyway?
 
The diode rectifier to produce high voltage is actually a whole series of diodes and does measure in the megaohms. If you saw smoke from it and it probably has an odor now? Then it's likely toast. The only way to properly check a flyback is with a ring test which definitely requires a scope. Even then with the later integrated flybacks a ring test could pass but still have a short in the HV silicone diode / voltage divider area, at least in my experience. I asked a question in the other thread if the focus pot was integrated into the flyback? If cleaner was sprayed into it, a short can develop. I've seen the whole focus pot area actually catch on fire, not from cleaner, though but a lot of energy buried in all that silicone... Maybe KC9UDX has other ideas to try. I know one thing would be unsolder all the diode supplies generated from the flyback in case a short it there external.

Larry G
 
How close, mechanically, is the HOT to the flyback? I've been burned before by a smoking HOT that looked exactly like the smoke was coming from the flyback. In my experience, HOTs die more frequently than flybacks. But my experience isn't necessarily indicative of what's normal.

The easiest way I know to confirm an open secondary is to verify that first there's indeed no output at all on that secondary, but second that there is proper output on the other secondaries. That's provided there are other secondaries, which in most cases there are. I don't know anything about the VT-100 though.

To do that you don't even have to take anything out of circuit. Just be careful; keep the 2nd anode button connected to the CRT; don't assume it's really dead. If it is open, it could still arc internally and come back to life unexpectedly. My point is, you don't want to accidentally come in contact with it if it does.

If the other secondaries are outputting correct (or nearly correct) voltages, you've usually eliminated any kind of problem at either the primary, or another shorted secondary (including the suspect one).

Of course if one isn't working, that doesn't guarantee the flyback isn't at fault either. But if you do find that, at least you have a path to follow, and things that are easier to check than the EHT output.

Else I'll second everything Larry said.

Matt
 
I was able to read the other thread and don't have much to add but it occurred to me that I should add a warning to not measure anything with high voltage pulses lest you damage your meter/scope. Without studying the schematic I can't say from here what to watch out for. Generally avoid anything connected to the horizontal deflection coils. Maybe Larry can add something to that.
 
The diode rectifier to produce high voltage is actually a whole series of diodes and does measure in the megaohms. If you saw smoke from it and it probably has an odor now? Then it's likely toast. The only way to properly check a flyback is with a ring test which definitely requires a scope. Even then with the later integrated flybacks a ring test could pass but still have a short in the HV silicone diode / voltage divider area, at least in my experience. I asked a question in the other thread if the focus pot was integrated into the flyback? If cleaner was sprayed into it, a short can develop. I've seen the whole focus pot area actually catch on fire, not from cleaner, though but a lot of energy buried in all that silicone... Maybe KC9UDX has other ideas to try. I know one thing would be unsolder all the diode supplies generated from the flyback in case a short it there external.

Larry G

Yes. There is an odor now. According to schematic there is just one diode but it might lie or the diode is actually a number of diodes in series.

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The focus pot is not integrated in the flyback itself. It is on the board.

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What is the normal Vf of the HV diode in the flyback? If apply I enough voltage across the winding it just have to let through a few micro-amps, or am I wrong. If I apply lets say 150 V across it wound't it conduct then?
I was under the impression that ring test is to conclude that there aren't shorted turns in the winding.

In any case. Since there were smoke I am pretty convinced that the flyback is toast.

How close, mechanically, is the HOT to the flyback? I've been burned before by a smoking HOT that looked exactly like the smoke was coming from the flyback. In my experience, HOTs die more frequently than flybacks. But my experience isn't necessarily indicative of what's normal.

They are far away. The other side of the chassis. Connected using wires and a connector. At least 30 cm. So there is no question about it. The smoke was from the flyback.
The easiest way I know to confirm an open secondary is to verify that first there's indeed no output at all on that secondary, but second that there is proper output on the other secondaries. That's provided there are other secondaries, which in most cases there are. I don't know anything about the VT-100 though.

To do that you don't even have to take anything out of circuit. Just be careful; keep the 2nd anode button connected to the CRT; don't assume it's really dead. If it is open, it could still arc internally and come back to life unexpectedly. My point is, you don't want to accidentally come in contact with it if it does.

If the other secondaries are outputting correct (or nearly correct) voltages, you've usually eliminated any kind of problem at either the primary, or another shorted secondary (including the suspect one).

That is of course possible that there is the other secondary that is shorted and not the HV secondary. Didn't really think of that option. But in either case I think the flyback is gone.

Of course if one isn't working, that doesn't guarantee the flyback isn't at fault either. But if you do find that, at least you have a path to follow, and things that are easier to check than the EHT output.

Else I'll second everything Larry said.

Matt

Does anyone know of a replacement flyback transformer. In this thread Lou specify the DEC part number, 16-16463-00, and suggest that Dalbani.com carry it. But unfortunately they don't any longer. I need another source for this or other good replacement.
 
Unfortunately, the characteristics of that diode are usually a mystery. I've never succeeded in testing one, unless it's external (older ones were).

What I've found in recent years is that flybacks just aren't available at all anymore. I did find someone selling one with the correct part number that I needed, a couple years ago. It looked the same but had one more secondary winding than my original one. I couldn't get it to work at all, I tried. I've got another one that also is the correct number and looks the same, but doesn't produce enough high voltage.

I've got two TVs and a monitor right now that I can't fix due to two flybacks and one custom rectifier. It's really irritating. (Oh and one oscilloscope)

I think the only thing scarcer than flybacks at this point is replacement CRTs.

If you do have one shorted secondary, you may be able to burn that winding out and buy our build an external transformer just for that circuit. But you'd completely change the characteristics of that flyback and would have to do a lot of adjustment to get the right picture. I'm speaking theoretically here, what I would think of as a last resort. I've never attempted anything like that.

I don't think this helps you anyway. You've only got two secondaries; it's not like some where you have six.

Just in trying to exhaustively eliminate anything else: are there any other components in close proximity to the flyback? C440 or 439 perhaps?

Oh also, don't go applying power to the flyback to test the rectifiers or the windings. Flybacks are designed to operate at high frequency. If you use DC or line frequency, you can burn up windings real quick. This is a very hard lesson to learn and why I have a dead oscilloscope. The thing that stinks is I really should know better.

That flyback is so simple that you might be able to get someone to rewind it. I have no idea what you'd do for a solid-state rectifier though. For all I know there might be a transformer shop that can deal with that.
 
Are you positive that the flyback stinks from burning up? Flybacks can stink anyway because they collect smoke, dander, and whatever else is in the air.

Are you really sure the primary circuit is functioning?
 
You can measure the ohms on cathode of CR409 to ground which is a secondary supply. It should be high resistance of 5 mega ohms based on focus control.
If CR409 were shorted or a short on it's load it would cause EHT to run hot. I think you said the raster you saw was out of focus.

Larry G
 
What is the normal Vf of the HV diode in the flyback?
I do not know, but what I know is that you cannot check it with an ohmmeter .
You need more than 9 VDC to check it. What you could try, is using a regular ohmmeter ( with internal 9V battery, that is a must ) and ADD, in series ( outside of the ohmmeter of course ) another 9V battery in such way that both 9V ADD UP ( that you can check with ANOTHER multimeter ) .... Then you have a chance to measure something.
Of course do not short the two leads of the ohmmeter ( to zero it ) when the external 9V battery is in the circuit, Ohmmeter may not like it.
That is what I know from books .... I never try it for real, but I think this measurement should NOT be harmful, IF you do not short directly the two probes.

(PS : that is adding a 9V DC Bias to the Ohmmeter circuit )
 
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I have checked all semiconductors know. There are definitely no shorted semiconductors, CR409 and all others shows a OK Vf. Some is not possible to confirm in-circuit, like the HOT. The 2A (not 2.5 as in the schmatic) fuse is blown though.

Now if I are able to get a new flyback I am a bit reluctant to just push it in without checking that there is something else in the circuit that causes it to fail. Could a bad or marginal electrolytic cause something to behave so badly that the EHT flyback is killed? Maybe I should get a ESR meter that can do in-circuit testing. I have been looking at the Der EE DE-5000. We'll see if I can afford one.

Glowing cathode: The first time I switched it on it glowed. Then when the smoke appeared I switched everything off and then I now recognize that the fuse is blown so I expect nothing to happen if I connect it.

To measure the EHT winding: What if I measure the current through the winding when supplying current using an external supply setting it to lets say 25V. That should be about the same?
 
A shorted flyback will burn out the HOT. A shorted HOT will blow the fuse.

It doesn't happen instantly; usually the whole unit will function for a while (maybe with reduced brightness and incorrect image size) for long enough for you to notice, when it happens. At least, that's been my experience: Flybacks don't go dead short, they seem to have just a few shorted turns. They do go completely shorted if you keep running it like that.

That C439 (or is it 438?) could short and cause exactly the same symptom. Is it original? A capacitor like that is sort of a common failure. What bothers me is that if I read that schematic right, it says it's rated 6V. I think that's a misprint. If someone replaced it in the past, they may have put the wrong capacitor in after reading the schematic, and it would certainly fail early like that.

Here's what I'd do at this point: remove the flyback completely, remove that capacitor, replace the fuse, and power up. If the new fuse blows, either the HOT is fried, or something completely unrelated is a problem. I'd then first replace the HOT and check the old one, compare it to the new one. If then you power up and the fuse doesn't blow, I'd investigate the capacitor first and then the flyback. But if the fuse blew again, I'd completely forget about the horizontal section for now and try to locate that problem.
 
Looking at the picture of your flyback, is that a crack at the end where the anode lead is? It looks swelled too?
An internal short or even a crack in the HV diodes would arc and heat and swell. If you think that is so, I have
something to try that is dangerous and not for weak hearts :)

Larry G
 
Could a bad or marginal electrolytic cause something to behave so badly that the EHT flyback is killed? Maybe I should get a ESR meter that can do in-circuit testing.

That is what I thought at first ....

Going a little "backward" in the fixing process : The first thing I do when I deal with such equipment : I check ( with an ESR AND a capacitor meter ) the TWO "tank" capacitors ( C9, C14 ) .... and surprising enough, I often find ONE ( ?? Why only one ?? I do not know !! ) totally dry !!

EDIT : The more I think, the more I suspect C439 !!! I fully agree with KC9UDX, with a "small" detail : C439 COULD BE 6 V rating, as it is in a "pump" circuit .... But I agree, it is it working voltage, so I would strongly recommend replacing it by a 16 V rated one !!
( It is rated 6 VDC in DEC part list, see sheet A1 of A3 .... but this is "dangerously close" in my opinion!! )
 
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A shorted flyback will burn out the HOT. A shorted HOT will blow the fuse.

It doesn't happen instantly; usually the whole unit will function for a while (maybe with reduced brightness and incorrect image size) for long enough for you to notice, when it happens. At least, that's been my experience: Flybacks don't go dead short, they seem to have just a few shorted turns. They do go completely shorted if you keep running it like that.

That C439 (or is it 438?) could short and cause exactly the same symptom. Is it original? A capacitor like that is sort of a common failure. What bothers me is that if I read that schematic right, it says it's rated 6V. I think that's a misprint. If someone replaced it in the past, they may have put the wrong capacitor in after reading the schematic, and it would certainly fail early like that.

Here's what I'd do at this point: remove the flyback completely, remove that capacitor, replace the fuse, and power up. If the new fuse blows, either the HOT is fried, or something completely unrelated is a problem. I'd then first replace the HOT and check the old one, compare it to the new one. If then you power up and the fuse doesn't blow, I'd investigate the capacitor first and then the flyback. But if the fuse blew again, I'd completely forget about the horizontal section for now and try to locate that problem.

I FULLY agree, particularly about C439. That is why I asked about the cathode heater glowing.
 
A lot of good advice here. Thanks to all those that help out!

A shorted flyback will burn out the HOT. A shorted HOT will blow the fuse.

It doesn't happen instantly; usually the whole unit will function for a while (maybe with reduced brightness and incorrect image size) for long enough for you to notice, when it happens. At least, that's been my experience: Flybacks don't go dead short, they seem to have just a few shorted turns. They do go completely shorted if you keep running it like that.

That probably describe pretty well what has happened. First the display was very dim. The cursor was almost not visible. The next time there were smoke. In general terms what could cause the flyback to give up? Someone told me that moist can go into the mold and then make them break. Is this true? What is then the cure? Bake them in the oven at low temperature to remove any moisture?

Do they go short on the HV secondary or the low voltage secondary? I assumed the short was on the HV side.

That C439 (or is it 438?) could short and cause exactly the same symptom. Is it original? A capacitor like that is sort of a common failure. What bothers me is that if I read that schematic right, it says it's rated 6V. I think that's a misprint. If someone replaced it in the past, they may have put the wrong capacitor in after reading the schematic, and it would certainly fail early like that.

I will check C439. It has not been replaced that is for sure. The actual value is 16V not 6V.
The schematics provided earlier was a bit small. I provide a link to a bigger one here.

Here's what I'd do at this point: remove the flyback completely, remove that capacitor, replace the fuse, and power up. If the new fuse blows, either the HOT is fried, or something completely unrelated is a problem. I'd then first replace the HOT and check the old one, compare it to the new one. If then you power up and the fuse doesn't blow, I'd investigate the capacitor first and then the flyback. But if the fuse blew again, I'd completely forget about the horizontal section for now and try to locate that problem.

If I pull out connector P102 then the flyback transformer and L301 is out of the circuit. Is this a problem to remove L301 from the circuit? What about the deflection coils. Shall those be connected as well?

I also ordered an Der EE DE-5000 ESR meter to check the capacitors with. Hopefully it can do in-circuit checks.

I doubt that the EHT flyback transformer has a crack. I think it how it was molded.
 
Pull out P102 then measure with an ohmmeter the collector emitter resistance of Q414 HOT. If your meter has a diode check you should get the normal .6V E to C (for the internal damper diode) and infinity in the reverse.
Any low ohm reading then desolder Q414 and check again. You will have to desolder the base to check the B to E junction, same readings. With Q414 desoldered, there should be no low resistance C to E on the board traces, either.
Do you have a scope? Then you could solder Q414 back in and leave P102 unplugged and measure the drive on Q414. It should represent somewhat of a square wave, at least sharp vertical edges. It's penciled in on the schematic. Not long ago in this forum we had a bad hor out transistor that had a curved waveform on the B-E junction by trying this. Essentially removing P102 removes all power to the horizontal output section so none of the other coils matter. Like KC9UDX was saying, if the fuse blows with P102 removed then the problem is elsewhere. Another possibility is your flyback smoked after the transistor shorted and may still be ok. However, Q414 is a beast, rated at 60 watts, 10A peak collector current. It could probably drive a short in the load for awhile
and create smoke ...

PS - I see a note in schem that there may be an external damper diode CR406 which means a different part# for Q414 ...

Larry G
 
Pull out P102 then measure with an ohmmeter the collector emitter resistance of Q414 HOT. If your meter has a diode check you should get the normal .6V E to C (for the internal damper diode) and infinity in the reverse.
Any low ohm reading then desolder Q414 and check again. You will have to desolder the base to check the B to E junction, same readings. With Q414 desoldered, there should be no low resistance C to E on the board traces, either.
Do you have a scope? Then you could solder Q414 back in and leave P102 unplugged and measure the drive on Q414. It should represent somewhat of a square wave, at least sharp vertical edges. It's penciled in on the schematic. Not long ago in this forum we had a bad hor out transistor that had a curved waveform on the B-E junction by trying this. Essentially removing P102 removes all power to the horizontal output section so none of the other coils matter. Like KC9UDX was saying, if the fuse blows with P102 removed then the problem is elsewhere. Another possibility is your flyback smoked after the transistor shorted and may still be ok. However, Q414 is a beast, rated at 60 watts, 10A peak collector current. It could probably drive a short in the load for awhile
and create smoke ...

PS - I see a note in schem that there may be an external damper diode CR406 which means a different part# for Q414 ...

Larry G

I thought I checked Q414 already. It measure fine 1-2 and 2-3, but 1-3 is short circuit! As far I can see from the data sheet this is Base-Emitter. Unfortunately it is not very clear since I cannot find a pin vs function mapping in the data sheet. But normally the collector is the heat sink which is 2.

I didn't think of the internal diode before. When I got two PN junction testing fine I just stopped testing this...

Q414 dead. Need to get a new one. Mouser only carry the one without built in protection diode. Maybe I get get that one and install an external protection diode in CR406. What is a good choice for this diode? Vrmax at least 400V I guess, but what If value is a good choice? 1A fast recovery diode?

Now when the HOT is dead is this caused by a failing flyback or is the smoke from the flyback caused by the failing HOT and the flyback might still be good?

I really appreciate getting all this TV / monitor knowledge from all you guys! Thanks!

BTW. I ordered an ESR meter, Der EE DE-5000, so I can check the capacitors.
 
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