Motorola VT-71 modification question

[Telecruiser]

Back in the 1960's when many of these sets came in for service, we just lifted one lead of the selenium and placed a diode in the circuit. The voltage would go up a little, but nothing alarming. It worked then, I don't see why it wouldn't work OK now. Remember the tolerances on most consumer TV's is about 20%.

[Kamakiri]

So to resurrect this discussion, if you use a ballast like this one AND have diodes in place of the seleniums, is a dropping resistor required?



The set's running fine, but that 30 ohm resistor starts to smoke after about a minute.....

[timmy]

I have diodes in place in 3 of these sets I have and I have a 50 ohm 10 watt resistor in place of the 37 ohm and it's good.

[Kamakiri]

Aha! Thanks! I'm a bust out the soldering iron

[timmy]

The 10 watt does get hot but it is a ballast and this resistor does hold up.

[Kamakiri]

I was gonna say.....that resistor is getting mighty toasty, but it is holding.

[timmy]

Well we really don't know what the actual wattage is for each resistor within the ballast itself so maybe a 15 watt would run Alittle cooler. I have had my 3 sets with that resistor in there since they were done and not one ever opened, lol , not yet.

[bandersen]

Measure the voltage drop across it. Watts = (V * V) / R. A good rule of thumb is to use a resistor rated for at least twice the power it dissipates.

[Kevin Kuehn]

Keep in mind that heat dissipation is all about surface area and air flow over that surface. Some modern day components are rated for higher wattage simply because the materials used can handle the heat, but that alone will not keep them cooler. Resistors for example have gotten smaller for the same wattage rating.