Lighting hit at phone ports- resistors now crispy

latelle4 said:

Hello, I am currently recapping some old Audio equiptment and thought I'd try to resurrect 2 Tivos with lifetime service. The issue that took the 2 Tivos out of service was loss of audio on both units after lighting took out our phone service. Picture & functions seemed intact, just no audio. After cracking open the case, I have found the same resistors fried just off the phone input jack. (see photo). I don't see any other signs of failure & don't know if this area would even effect the audio. I haven't found any schematics to give me resistor values, so I'm stuck. I'm no electronic engineer, but if this can be fixed, I'd like to give it a shot.

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If you can do surface mount device (SMD) de-soldering and resoldering, the fastest way to fix that S2 DT is to pick one up cheap off of Craigslist and transfer the Atmel crypto chip to it. That'll move the TiVo Service Number to which the lifetime sub is tied.

It's not a job for a beginner, and you need to practice on something expendable first, but with a magnifier, some chipqwik, braid, and a small point iron, it's do-able if you're patient and very careful.

The damage to the internal modem section shouldn't have done anything to the audio section, but that doesn't mean that the spike(s) that took out the modem might not have found its/their way over to that section as well.

One thing you can try is tracing the phone jack's signal path back through the damaged components (so that you know you're only dealing with stuff that's part of the internal modem circuitry) and unsolder the damaged stuff in case it's shorted or semi-shorted and pulling down the supply voltage to both itself and the analog audio output section.

Have you tried recording anything and transferring it to computer via TiVo Desktop and playing it there, or transferring to working TiVo, to be sure it's only an analog playback problem?

Also, take a very good look around for capacitor disease, especially on the output side of the power supply.

switch998 said:

Those aren't fuses, marking FB*** indicates a fuse or 0 ohm resistor in most cases, try removing the bad components and jumpering it with a wire.

Make sure all FETS are good, use a resistance meter to make sure that they're not shorted or open. FETs should only let electricity through in one direction, your resistance meter shows it's open (infinite resistance), or shorted in both directions, then remove the FET and test it on the bench.

Same thing for capacitors. They're designed to let current through one way, so use the same method to test.

Here's a better explanation on testing fets: mayohardware.blogspot.com/2011/02/tips-on-testing-mosfets.html

I have a bad S2 board that I can send out to you for parts (missing PROM pads), if you're willing to pay shipping. Other than that, the tivo was fully functioning, until I attempted to socket the PROM.

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What you have said here could be misleading to some.

Diodes and triodes (which includes transistors, including Field-Effect Transistors, or FETs) are designed to allow current flow in one direction and to oppose it when it tries to flow in the opposite direction, and in the case of triodes there is, in addition to the "in" and "out" terminals. a control element that determines how much current is allowed through in that one direction.

Capacitors, however, do not block current in one direction and pass it in another. Technically, they block it in both directions, but allow changes in that current to be communicated through them.

A capacitor is a conductor separated from another conductor by an insulator. When a voltage is connected across a capacitor, electrons leave one of the conductors and head for the more positive of the two ends of the voltage source, and electrons leave the more negative of the two ends of the voltage source and head for the other conductor in the capacitor, and that goes on until the one conductor is just as positive as the positive end of the voltage source and the other conductor is just as negative as the negative end of the voltage source, at which point the capacitor is fully charged and there is no longer a difference in voltage to cause those electrons to move. Those electrons want to jump from the negatively charged conductor in the capacitor to the positively charged one, which is why current flows long enough to fully charge the capacitor, but the insulator that separates them prevents it.

Some capacitors, referred to as electrolytic capacitors, use an insulator whose design depends on the capacitor only having voltage applied across the capacitor so as to cause current flow in one direction until the fully charged state is acheived. These are called polarized capacitors, and can be damaged if voltage is applied in the wrong polarity.

Once you have established beyond doubt which of your ohmeters leads is negative and which is positive, which is to say which is which when you switch your multimeter to the resistance or continuity measuring function, then you can connect the positive lead to the positive lead of an electrolytic capacitor and the negative lead to the negative, and watch the resistance climb until the capacitor is fully charged, at which point the resistance appears infinite, like an open circuit.

Connecting your meter to a capacitor the wrong way round may damage the capacitor and cannot be counted on to provide any useful information about the capacitor even if it doesn't damage it.