NotTaR of Television Sets : Interchangeability of components        
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Interchangeability of components

The question often arises: If I cannot obtain an exact replacement or if I have a monitor, TV, or other equipment carcass gathering dust, can I substitute a part that is not a precise match? Sometimes, this is simply desired to confirm a diagnosis and avoid the risk of ordering an expensive replacement and/or having to wait until it arrives.

For safety related items, the answer is generally NO - an exact replacement part is needed to maintain the specifications within acceptable limits with respect to line isolation, X-ray protection and to minimize fire hazards. Typical parts of this type include flameproof resistors, some types of capacitors, and specific parts dealing with CRT high voltage regulation. However, during testing, it is usually acceptable to substitute electrically equivalent parts on a temporary basis. For example, an ordinary 1 ohm resistor can be substituted for an open 1 ohm flameproof resistor to determine if there are other problems in the horizontal deflection circuits before placing an order - as long as you don't get lazy and neglect to install the proper type before buttoning up the monitor or TV.

For other components, whether a not quite identical substitute will work reliably or at all depends on many factors. Some deflection circuits are so carefully matched to a specific horizontal output transistor that no substitute will be reliable.

Here are some guidelines:

  1. Fuses - exact same current rating and at least equal voltage rating. I have often soldered a normal 3AG size fuse onto a smaller blown 20 mm long fuse as a substitute.

  2. Resistors, capacitors, inductors, diodes, switches, potentiometers, LEDs, and other common parts - except for those specifically marked as safety-critical - substitution as long as the replacement part fits and specifications should be fine. It is best to use the same type - metal film resistor, for example. But for testing, even this is not a hard and fast rule and a carbon resistor should work just fine.

  3. Rectifiers - many are of these are high efficiency and/or fast recovery types. Replacements should have at equal or better PRV, Imax, and Tr specifications.

  4. Posistors - many of these are similar. Unfortunately, the markings on the devices are generally pretty useless in determining their ratings. Note, however, that the prices for replacement posistors may be quite reasonable from the original manufacturer so it may not make sense to take the risk of using an unknown part.

    (From: Stefan Huebner (Stefan.Huebner@rookie.antar.com).)

    In most cases you can use a standard 3-terminal-device, the resistance of the temperature dependent resistors in it are nearly identical. Here is a list of possible replacement devices:

    380000-01, 24340521, 2199-603-1201, 163-024A, 163-035A, CO2200-N66, C8ROH, QX265P05503, 32112026, 4822-A1-11240148, 02199-003-120, 15-08-001A, 5391560067, F400001.

  5. Transistors and thyristors (except HOTs and SMPS choppers) - substitutes will generally work as long as their specifications meet or exceed those of the original. For testing, it is usually OK to use types that do not quite meet all of these as long as the breakdown voltage and maximum current specifications are not exceeded. However, performance may not be quite as good. For power types, make sure to use a heatsink.

    Also see the section: Replacement power transistors while testing.

  6. Horizontal output (or SMPS) transistors - exact replacement is generally best but except for very high performance monitors, generic HOTs that have specifications that are at least as good will work in many cases. Make sure the replacement transistor has an internal damper diode if the original had one. For testing with a series light bulb, even a transistor that doesn't quite meet specifications should work well enough (and not blow up) to enable you to determine what else may be faulty. The most critical parameters are Vceo/Vcbo, IC, and Hfe which should all be at least equal to the original transistor. I have often used by favorite BU208D as a temporary substitute for other HOTs and SMPS (chopper) transistors. Make sure you use a heatsink and thermal grease in any case - even if you have to hang the assembly by a cable tie to make it fit.

    For that matter, you can usually substitute a similar HOT with the D suffix instead of the A (or no) suffix. These have a built-in damper diode and two in parallel (the external one) will not hurt (or remove it). Naturally, the reverse is not true since a damper diode IS essential and the HOT will probably not last beyond the click of the power relay without one!

    On SVGA monitors, there will likely be additional circuitry between the HOT and the damper so this trick doesn't work for them.

    However, using a HOT with much better specs may actually result in early failure due to excessive heating from insufficient and/or suboptimal base drive. See the document: "TV and Monitor Deflections Systems" for more info.

    For more information, see the document: TV and Monitor Deflection Systems.

  7. Deflection yokes - in the old days, particularly for B/W TVs, all of these were quite similar. It was common to just swap with one that fit physically and at most need to adjust or change a width coil. With color TVs and high performance multiscan monitors, this is no longer the case. Sometimes it will work but other times the power supply won't even be able to come up as a result of the impedance mismatch due to different coils and pole piece configurations. In addition, there may be other geometry correction coils associated with the yoke that could differ substantially.

    However, if you are really determined, see the section: Swapping of deflection yokes.

    Also see the section: Replacement power transistors while testing.

  8. Standby power transformer - this most likely only has a single secondary so locating a standard UL approved (for safety reasons) power transformer with the same output voltage should not be difficult.

    Check the service manual or the Sams' Photofact for the set to determine the required output voltage and if a centertap is needed. Current should be quite low.

  9. CRTs - aside from the issues of physical size and mounting, many factors need to be considered. These include deflection angle, neck diameter, base pinout, focus and screen voltage requirements, purity and convergence magnets, etc. Color CRT replacement is rarely worth the effort in any case but trying to substitute a different CRT is asking for frustration. For monochrome CRTs, there is less variation and this may be worth a try.

  10. The following are usually custom parts and substitution of something from your junk box is unlikely to be successful even for testing: flyback (LOPT) and SMPS transformers, interstage coils or transformers, microcontrollers, and other custom programmed chips.

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