What frequency limit I should plan for when choosing oscilloscope for TV repair?

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Here's what I see on the scope from the output of the voltage sense circuit. This is not the one in the variac , but the identical circuit. The input voltage is 100V and Ohms law says the output should be 33mv, so it's pretty close, but that 1.3mv that it's off amounts to the meter being over 3v off. The one in the variac is probably more accurate, because I've upgraded to all 0.1% resistors.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

So I gutted everything from the sense board which was related to the power supply circuit. The first waveform is with the bridge rectifier in place, and it's equivalent to a single diode at this point. That is the waveform I expected before I altered anything. Obviously, I forgot about what happens when the current changes polarity.

Second waveform is with the bridge rectifier removed, and a jumper to complete the circuit between neutral and R6. That waveform matches the original one. I'd call that a success. I'll do that to the one in the variac and adjust my voltage divider so it's correct, then it's on to the control board. I think I just need to decouple the AC sense line from the power supply circuit, and maybe add a pot to the voltage sense input on the micro.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Solved! I removed everything which wasn't part of the voltage sense circuit, and now its within 3 to 4 hunredths of what my Fluke reads at any voltage. Some of that could be from the single digit resolution on the meter. I could probably add range adjustments and more resolution in the software if I could ever identify the micro, or at least figure out how to dump the firmware. The lowest it goes is 0.7v, but that's the variac itself.

[Agent24]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Nice!

Even if you could identify the micro-controller, they probably protected the code so you can't dump it (with software) anyway, though.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Would have been a cool project to try and make it auto ranging. Still though, I'm happy with how it turned out. I also took a file to the back stop and got it down to 0.2v. If I had a meter which could cancel out it's lead resistance, I have no doubt I could make this as accurate as my Fluke. It's all about the voltage divider. The voltage is sampled between a 1k resistor and 3 1m resistors. I think they're all 1%, maybe .1%. It's been 2 years since I did the rest of the mod. If I put a pot on each side of the divider and dialed them both in perfectly, I think I could close that .3v difference. But really, I'm not lugging this thing around to use as a multimeter, I just want to be confident that if it says 120v, it's close enough to not damage anything. That, and if it's close, the wattage displayed will be close to accurate.

[awyatt2]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

I use a tek 2430a scope, 150 MHz dso. sometimes you can pick them up reasonable, they have a dallas sram and time is at hand for the srams to be replaced on these. I know because I just did mine and have had it since new. if the internal batteries go it requires a complete calibration or it becomes a brick. remember these are internal batteries, (inside the chip) you cannot simply replace the batteries. the chips aren't expensive and thanks to a friend I was able to get the program from both srams to save the calibration. find one that's failing the self test with a low battery and replace the dallas chips and its good for another 25 yrs or so. a tl866 or other programmer will do these easily. it is around a three hour job to remove, put in a pair of zif sockets, read and reprogram the memory and you are good to go.