First off. Hello, my name is Todd. I am new here. I just bit the bullet and bought a working used TiVo HD Lifetime off Ebay yesterday.
I am somewhat of an expert at repairing SMPS (Switching Mode Power Supplies). I taught myself about 15 years ago when I was finding dead VCR's at garage sales and reviving them. Now I use this trick on HD TV's that my neighbors are throwing out.
Everything I have read so far on this forum on this subject has failed to mention the temperature rating of the replacement caps. Most electrolytic caps are rated at 85°C. This is not sufficient for the cycling duty that these caps endure and failure may quickly reoccur. Spend a little more and get 105°C rated caps. These are available from the NTE rack in the blister pack. uF must be the same. Voltage rating can be higher as long as it fits on the board and in the case.
It is good practice that ALL caps should be replaced, not just bulged ones. In fact a cap can be bad without bulging.
The surest way to determine if a cap is bad is to test it for ESR (Equivalent Series Resistance). In order to do this, you will need an oscilloscope and a square wave generator. I am fortunate that my brother lets me use his old oscilloscope. There are applications that allow a PC sound card to stand in for a low frequency oscilloscope. The wave generator can easily be bread boarded from a 555 timer chip. Frequency should be ~100 kHz. Peak to Peak voltage around 100 mV, although this not critical. You need enough to inject a good signal, but not so much that you blow the cap. Shorting your test leads together gives you the reference waveform...very little change in the square wave amplitude on the O-Scope screen (no ESR). Now place the test leads on the leads of the cap. A GOOD cap will show very little decrease in amplitude. On a BAD cap the amplitude will completely collapse. A cap that collapses about 50% should also be considered suspect and replaced. The uF rating of a GOOD cap will effect the ESR. There are charts available that show the expected % amplitude of a GOOD cap.
I am not an Electrical Engineer, but I play one on TV.
Todd K.
I am somewhat of an expert at repairing SMPS (Switching Mode Power Supplies). I taught myself about 15 years ago when I was finding dead VCR's at garage sales and reviving them. Now I use this trick on HD TV's that my neighbors are throwing out.
Everything I have read so far on this forum on this subject has failed to mention the temperature rating of the replacement caps. Most electrolytic caps are rated at 85°C. This is not sufficient for the cycling duty that these caps endure and failure may quickly reoccur. Spend a little more and get 105°C rated caps. These are available from the NTE rack in the blister pack. uF must be the same. Voltage rating can be higher as long as it fits on the board and in the case.
It is good practice that ALL caps should be replaced, not just bulged ones. In fact a cap can be bad without bulging.
The surest way to determine if a cap is bad is to test it for ESR (Equivalent Series Resistance). In order to do this, you will need an oscilloscope and a square wave generator. I am fortunate that my brother lets me use his old oscilloscope. There are applications that allow a PC sound card to stand in for a low frequency oscilloscope. The wave generator can easily be bread boarded from a 555 timer chip. Frequency should be ~100 kHz. Peak to Peak voltage around 100 mV, although this not critical. You need enough to inject a good signal, but not so much that you blow the cap. Shorting your test leads together gives you the reference waveform...very little change in the square wave amplitude on the O-Scope screen (no ESR). Now place the test leads on the leads of the cap. A GOOD cap will show very little decrease in amplitude. On a BAD cap the amplitude will completely collapse. A cap that collapses about 50% should also be considered suspect and replaced. The uF rating of a GOOD cap will effect the ESR. There are charts available that show the expected % amplitude of a GOOD cap.
I am not an Electrical Engineer, but I play one on TV.
Todd K.
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