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#31
09-29-2015, 10:28 PM
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C'mon...I have been knocked on the ground a FEW times...ALL by the HV of a set. I have been ALSO hit by a boost converter--(around 400 Vdc), H-out cap and focus voltages. It is NOT that bad really. NONE of those...have discouraged me from loving to work on this stuff...old and NEW.
Just an unfortunate aspect of this hobby--or profession... 
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#32
09-30-2015, 06:25 AM
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With rectifier diodes, you can go up in current and voltage handling capability. The diodes in your set are probably 400V, 1A diodes, maybe 600V, 1.5A diodes. As long as the voltage rating of the diode (PIV, or PRV) is higher, you can use the diode. Same goes for the max current rating or IF (I-forward). Higher is better, but please make sure there is a fuse around if on the secondary of a transformer - transformers are the one thing we don't get to upgrade easily, and they can be taken out by shorted rectifiers if not fused.
__________________
Brian USN RET 22YRS (Avionics/Cal) CET-Consumer Repair and Avionics ('88) "Capacitor Cosmetologist since '79" When fuses go to work, they quit!
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#33
09-30-2015, 06:39 AM
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I've seen guys get shocked, shake it off, and drop dead within hours - their hearts damaged by enzymes that elevate after a shock. Likewise, I've seen first hand the nerve damage done by boost voltages - it was my Dad, and he had nerve damage that lasted for years, with white tissue scarring all along his arm where the voltage traveled along. He couldn't pick up a coffee cup with that hand for 6 months, and the shooting pain (actually nerves repairing connections) went on for months. The scarring took years to disappear. So yes, let's be careful and mindful of safety at all times. And please fault no one that goes the extra mile to be safe. I'd rather read of their fortunes than misfortunes....
__________________
Brian USN RET 22YRS (Avionics/Cal) CET-Consumer Repair and Avionics ('88) "Capacitor Cosmetologist since '79" When fuses go to work, they quit!
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#34
09-30-2015, 09:37 AM
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I like to blow things up and set things on fire. Never had a fire get out of control, and never damaged anything I didn't intend to with an explosion. Same principle applies to doing my own electrical work. Again, never been shocked, never caused the house to burn down. Actually saved the house from burning down once because I kept getting a faint whiff of smoke and traced it to aluminum wiring for a light fixture twisted onto copper wiring. The connection had corroded and was close to starting a fire. So I rewired the fixture using proper copper wiring. It all comes down to giving dangerous things a healthy respect and not touching them unless I know what I am doing. And with discharging a CRT, I'd be too nervous to keep a steady hand and would jump at the slightest pop, possibly causing the screwdriver to go somewhere it shouldn't. So I'll work around the problem. Luckily these diodes are on the opposite side of the chassis, so I have room to get to them.
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#35
09-30-2015, 09:41 AM
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Well, BE SAFE guys !!,, don't be stupid when you repair your TVs.. No need to know if the current is okay by touching something around the HV cage or else with your hands ouff !!..The warnings are there to be respected when the TV set have need to be repaired...Sorry my English not good .
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#36
09-30-2015, 09:57 AM
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I assume you want to pull the chassis out ? Usually
you can flip it on edge & run it. Just dont put strain on the wires. Can usually put a box behind the set to rest it on. Alternitive is flip the whole set on its side HV area down & you can run it & do tests. Sets with bottom plates like Zeniths are almost always done that way. Just remember the purity will be way off, thats normal. One other important thing. Until you change the thermistor JUMP IT OUT or the VDR will burn. They stink too !!! On HV shocks be as careful as you wish. If you are healthy it should not harm you but....... I have got nailed many times by HV. Worse it did was give me A-fibs for maybe 20 sec. a few times. In fact if you already have A-fibs it may cure them if the timing is right ! When they fix them they put a ground on your back then on your chest. The machine has a sync curcuit & at the right point SNAP ! And you are back to normal rhythm often. You are asleep during it. I had it done 3 times. 73 Zeno 
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#37
09-30-2015, 09:58 AM
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#38
09-30-2015, 10:24 AM
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#39
09-30-2015, 05:33 PM
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syl p# was 38-17071-2 It will cross to a workman ?FR922? For now just short it out & repair the set. The degauss just wont work. 73 Zeno
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#40
09-30-2015, 05:51 PM
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Are you sure about shorting the leads together? I ask because the last thermistor I replaced, shorting the leads would have fried the degauss circuit. This was on a much newer RCA CTC... 187 I believe, but I figure a degauss circuit works pretty much the same?
If shorting the leads together is safe, I'll do that.
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#41
09-30-2015, 06:18 PM
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When the thermistor is in the set, it starts out at ~ 120 ohms, and DROPs in resistance as it warms up, to about 2 ohms or so hot. Shorting it out puts no voltage in the degaussing coils, so your degaussing is disabled until it is replaced.
From the RCA patent: The thermistor is a device which when cool has a relatively high resistance but when hot has a relatively low resistance. The degaussing magnet coils are energized by the voltage drop across the thermistor in a circuit which includes a varistor (voltage dependent resistor). The varistor is a device which has a relatively low resistance when a high voltage is impressed across it and a relatively high resistance when the impressed voltage is low. It will be observed that the resistive properties of the thermistor and the varistor are of a reciprocal nature. The energization of the degaussing coils is as follows, assuming that the thermistor is cold, i.e. at room temperature. immediately upon closure of the power switch the current flow through the thermistor to the power supply produces a relatively large voltage drop across the thermistor. At this time the resistance of the varistor is low so that alternating current flows through the degaussing coils at considerable amplitude. As a consequence, a relatively strong alternating field is established in the screen region of the picture tube. As the thermistor warms in response to current flow it's resistance decreases thereby decreasing the voltage drop across it. As a result of the decreased voltage the resistance of the varistor increases thereby decreasing the current flow through the degaussing magnet coils. As a consequence, the strength of the alternating field produced in the picture tube decreases. The thermistor ultimately heats sufficiently that its resistance and the voltage drop across it are very small so that the resultant voltage applied to the varistor is so small that the current through it and the degaussing coils is so small as to be effectively zero. For BPC sets, the thermistor is about 16-25 ohms cold, too low to energize the coils in your set effectively. Get an FR191, FR291, or FR922 thermistor for your set. All three have relevant RCA part numbers with the last digits of the RCA part number matching the 191, 291, and 922......betcha a lot of folks wondered how they got numbered.....now you know: 107191 = FR191 107291 = FR291 RCA drawing number 942922 and all suffixes = FR922, or FR291, both are heavy duty 120 ohm thermistors..... Cheers,
__________________
Brian USN RET 22YRS (Avionics/Cal) CET-Consumer Repair and Avionics ('88) "Capacitor Cosmetologist since '79" When fuses go to work, they quit!
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#43
10-02-2015, 10:10 AM
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__________________
"When resistors increase in value, they're worthless" -Dave G
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#45
10-02-2015, 12:18 PM
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You should get an HV probe to use it to discharge the CRT, and confirm HV reg is adjusted properly once you get it going. If you use an HV probe, the CRT second anode should not 'recover' any voltage, and you get to see that it has discharged to a safe voltage. Just clip the ground lead of the probe to the chassis, hold probe by user handle stick the tip on it's far end under the HV suction cup or to the plate at the base of the HV rect tube if more convenient, and watch the needle jump and fade back to zero.
Honestly, if you are wearing rubber soled shoes and not touching any metal that leads back to the chassis you could touch the HV lead with the set off and it would not be much worse than a static shock from shuffling your feet across the carpet and grabbing a door knob.....If you change that situation to holding the handle of a large insulated screw driver with the metal tip touching the HV you should feel nothing since there is far too much insulation for current to flow thru you even if the ground jumper were to fall off. If the CRT has not lit for you there probably is no or very little HV. I can understand being a bit squeamish after a jolt, I've had enough of them to know (99% were reconnecting the lead after the screw driver discharge and bounce back, which I learned to avoid by leaving the CRT's HV connector grounded until reconnect), but your irrational fear of the unknown is just being a big freaking chicken....Unless you have a bad heart condition or the like. If you get shocked from the yoke somehow (I say somehow cause the only time it happened to me I could not figure out how) with the set on, you will then not worry about small HV jolts.....I've never had my reflexes hard triggered by the HV (sure I'd consciously pull back in a controlled way), but the yoke....without trying everything in my way was smashed out of it including some wires. Yoke voltages and 120VAC are the two things I work with that I'm thoughtfully cautious around.....Now that I have an HV meter I just discharge and stop worrying. If you are truly dedicated to/focused on working on something (as any experienced auto mechanic will tell you) you will not notice or care as much about small pain. When I was starting out soldering I often would cause my self small burns and not notice until minutes or hours later. I've given my hands small cuts while working on some things and not noticed until long after. MAN UP!
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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