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Thread: Does anyone know old TV's?

  1. #31
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    I don't think I ever saw any in military surplus equipment. That should have been a hint.

  2. #32

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    Dwight is right about bumblebee caps, they are notoriously poor and nearly always electrically leaky.

    One good thing about video/TV scanning raster faults, it is like having an oscillograph showing you what is wrong. In this case the scanning raster lines are compressed together at the bottom. This means that the required sawtooth current in the yoke, in the vertical scan coils, is not climbing in a linear way. As the current increases toward the bottom of the scan , the rate of change of current with time is falling off. So this is the highest current part of the scan that is defective. There are multiple causes, will list here:

    As the vertical scan output tube ages, its emission drops, and it cannot support the peak current required for the scan. Other causes for this can include a cathode resistor gone high. It was also common for a linearity control to be included as a variable cathode resistor. There may or may not be an electrolytic bypass cap on the cathode resistor that has dried up.

    The drive voltage to the grid of the vertical output tube could be abnormal, due to leaking wax-paper capacitors, or resistors could be high in the linearity compensation networks in the vertical output stage.

    Depending on the design and the vertical output tube used, triode or pentode, the screen voltage & bypass cap needs checking. If you could post the schematic I could remark on where to look for this fault and what to do.

    The first initial move would be to check the HT supply to the vertical output stage and plug in a new vertical output valve to eliminate that, then move on to the capacitors & resistors.

    ..just had a look at the schematic, it uses a triode output stage and linearity control in the cathode and a 200uF cathode bypass cap, these are the three items to check first, then the coupling caps in the triode's grid circuit.
    Last edited by Hugo Holden; June 21st, 2019 at 10:19 PM.

  3. #33
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    Quote Originally Posted by Chuck(G) View Post
    Depends on the capacitors. Are multi-section high-voltage caps readily available, say, 40-40-20 @450VDC? I hadn't looked, but thought that they'd gone the way of the dodo.
    A quick google search turned up this:

    https://www.tubesandmore.com/product.../%20Can%20Type

    They have several multi-section caps, but not the values you specified.

    But that doesn't stop you from replacing the single multi section with several individual caps. Sure it will take up more space, but it'll work.

    Quote Originally Posted by Chuck(G) View Post
    But yeah, bumblebees were really terrible--they were supposed to be superior to the paper-wax variety, but that turned out to be a bad assumption.
    It's a bad assumption that any capacitor using a liquid electrolyte will survive more than a decade, with the poorer quality ones lasting even less. The capacitor plague ended years ago, but I still regularly service newer equipment with dodgy chineseium capacitors that have failed in pretty much every way it's possible for a capacitor to fail. Even dry or solid electrolyte caps like tantalum or ceramic are prone to failure, especially the SMD variety from mechanical stress.

    Those bumblebee caps still use paper, but replace the electrolyte with oil. The paper will still eventually break down and from the brief research I've done, it seems they use castor oil, which is definitely not stable over 70 years.

  4. #34
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    I'm of the "if it isn't broken, don't fix it" school. In particular, if a filter cap (one of the multi-section ones) goes bad, it'll manifest itself with AC hum in the video or audio--and can easily be tested for. If the DC supply voltage is within tolerance with regard to ripple, I'm inclined to leave the bloody things alone rather than "shotgun" replace them because they're old.

    As I've mentioned, I have gear with 50+ year old electrolytics that's still working just fine, thank you. OTOH, I have equipment that's 5 years old that's had to have been re-capped.

  5. #35
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    Just swapped out that vacuum tube and the issue is still the same. I'm betting one or more capacitors needs to be replaced. I'll bring it to that workshop in NJ on July 20th when I go. Hopefully someone there can help with that and a couple other CRT's that I have that need some work
    Last edited by RadRacer203; June 24th, 2019 at 11:23 AM.

  6. #36
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    Quote Originally Posted by Chuck(G) View Post
    I'm of the "if it isn't broken, don't fix it" school.
    I used to have that mentality until I started getting equipment with slightly failing capacitors that rapidly deteriorated suddenly and nuked entire power supplies and distribution boards, turning them into charcoal briquettes.

    One more recent failure was a power supply board which had one bad 2200uF 16v cap that went off, it caused a cascading failure from the secondary side of the supply that went up the chain to the primary side, nuking diodes, transistors, transformers and the push/pull mosfets all in succession. Of course the board was unobtanium, so I had to rig one of those generic 300W chineseium PSUs up and cascade two buck converters off of it to get the three voltages required (24v, 12v and 5v.)

    For me, if it's more than 5 years old, all of the caps get replaced if I plan on keeping it for any length of time. Capacitors are cheap, equipment is not. I got two parts bins half loaded with different capacitor values that cost me less than $50 to stock, so replacing caps on something only costs me a dollar or two.

  7. #37
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    If it helps to narrow down the problem, this is what the image looks like on the screen. The only thing I have right now that can connect to it is my Atari 2600, so this is what Frogs and Flies looks like. Loads of interference, which I think is probably from filter caps going bad. Hugo, I'll definitely have my friend check those 3 things out when we look it over.
    20190625_113649_HDR.jpg

  8. #38
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    Could be caps or resistors or both.

    If you do replace the caps, I'd recommend polypropylene dipped caps over electrolytic unless you're on a budget. Poly caps are more tolerant to heat and don't degrade as badly over time unless they're insulted by power spikes.

    The only issue with polypropylene caps is that you have to identify the outside foil end of the capacitor, which requires special equipment. You can put them in either way around, but ensuring the outside foil end is connected to the lower impedance side of the circuit will drastically reduce noise.

    Mr. Carlsons Lab on Youtube has lots of great info regarding old tube gear and his patreon page has schematics for all of the test gear he's designed, including a capacitor tester that's way better than the generic chineseium stuff on ebay.

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