Quote:
Originally Posted by ohohyodafarted
Good discussion guys! Now lets have some input on your theories as to the causes of flyback overheating in sets that have HV that is within acceptable limits. It has been my experience that a transformer does not overheat unless it has an excessive load placed on it, or the transformer has shorted windings. What do you fellows think? Are you going to point more to a defective flyback with a short or did something place an excessive load and that caused overheating and then a resultant short. IF it is excessive loading, what was the most likely cause?
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So you're asking the chicken/egg question? lol
The simple answer is we're working with 60-70 year old electronics at this point and it's a miracle any of it works anymore, but beyond that these are the culprits I've personally run into.
1. Running sets with higher line input voltage than called for in the service information. All the sets I've ever worked on were specified to accept 115-117VAC input from the wall, and the reality is these days we can have anywhere from 115 to 130VAC at the wall plug. There's not much regulation going on with the lines coming from the utilities, so during summer when everyone is running the AC it's usually on the low side and right now when it's cold at night time and demand is low it can get pretty high. This has unintended consequences with the horizontal circuits in a set but people don't think about it, because it doesn't affect anything else in their home because we now have switching supplies that account for it in the rest of our appliances.
Because you have a linear power supply with step up transformer in old TV's, the line drifting high causes the B+ supply to drift up as well. Only a few volts difference on the primary can cause 50+ volts extra to show up at the secondary, which then gets passed directly to the horizontal section and flyback transformer. Those extra volts will also cause a rise in horizontal cathode current all else being equal (the primary resistance of the flyback hasn't changed, so check ohm's law!), which means the flyback is being strained and it's natural to expect it to run hotter as a result.
Because of this, I refuse to run any of my sets without an autotransformer or variac and verifying AC input voltage.
2. Not having the horizontal section aligned correctly. This could be anything from a wave shaping coil set wrong to the HV regulator tube drawing excessive amounts of current, it all falls under the same banner. Some are content to simply go through the procedure in Sams and assume it's good, meanwhile it's running away with excessive current draw and next thing you know it melts down for seemingly no reason. I don't stop till I can put a number on HOT current, which I then annotate in the schematic as an accepted norm. This way if I ever mess with it again, I know what it was last time and can watch for changes.
3. Moisture. This one we can't do anything about so far as I know, but when sets sit all the components will be absorbing moisture from the environment. If said environment happens to be a damp/wet basement, watch out. It can infiltrate the windings of the flyback and cause the copper wires to corrode leading to shorts, intermittant pops, overheating, incorrect resistance readings, nearly anything you can think of within reason. I know shango dunnked a fly in hot wax and boiled out the moisture, but for the vast majority of us this will not be an option. Best thing you can do is actually run the set, because the heat from the flyback will drive the moisture out naturally if given enough time.
4. Failed CRT. Sometimes despite your best efforts a fly gets taken out by an arcing CRT, and you can't do anything about it. Sometimes parts fail, just the way it is. Doesn't have to be a reason which always sucks, but like I said 70 year old TV's...