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

[stj]

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

you can shield the coil with mu-metal if it comes to it.

[lookimback]

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

That's good to know. In the LTspice simulation I did, I found there to be 33mV at J1 with a 100v supply. Of course, if I did the math, I'm sure I'd come up with the same. I'm wondering if there's any correlation to the one resistor which is 33Ω. I'd expect their programming to be based on a perfect model, but in actual practice, there will be variances. So, if those three 1MΩ resistors aren't exactly 1MΩ, or if that 33Ω resistor isn't exactly 33Ω, then the calculations will be off. Measuring them, I get about .966MΩ. What's interesting is that they are supposed to be 1% tolerance, but that reading makes them way out of specs. Maybe I'll just replace all of them with some 0.1% ones and see what effect it has. This is, of course, no longer just about adding a digital display to a variac. Now it's an opportunity to experiment and learn.

[Uniballer]

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

...Measuring them, I get about .966MΩ. What's interesting is that they are supposed to be 1% tolerance, but that reading makes them way out of specs.

Don't forget the tolerance of the meter that is measuring them. Especially if it has not been calibrated recently.

[lookimback]

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

Don't forget the tolerance of the meter that is measuring them. Especially if it has not been calibrated recently.

I hadn't thought about that.

[stj]

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

dont forget any replacements must be rated for a couple of 100v running.

[lookimback]

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

dont forget any replacements must be rated for a couple of 100v running.

Everything I got was rated for 250v.

[lookimback]

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

nice, the small board has 2 pins on the ribbon for current, the other 3 are the supply and sense-voltage.
so just need to draw a schem of the small board.

btw, if you can use a meter to see how the 4pin header on the edge and the crystal connect to the 24pin chip around the ribbon we can id the microcontroller.

The 4 pin header only connects to U3, 5v, and Gnd. Does it sound like U3 is a flash IC? I marked Gnd and 5V on all the ICs.

[stj]

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

u3 may be a microcontroller,
but with that power layout it's not a pic or an avr

[lookimback]

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

u3 may be a microcontroller,
but with that power layout it's not a pic or an avr

I really want to decap them just to satisfy my curiosity, but I don't want to play with nitric acid without the proper safety equipment. I'm just kind of prefer my fingers, lungs, and eyes the way they are.

[stj]

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

just download datasheets for mcu's with that many pins from lots of company's until you find one with the power on those pins.

[Alastair E]

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

Hmm.............

Is it just me, or do the last 6 Pages of stuff have little--or no-- relevance to the thread title??

It turns from Scopes to Variacs at about page 4, then onward to some mad attempt at reverse-engineering cheap Chinese testers....

I can tell ya--Most use Arduino chips....

Even if you find what those chips actually Are--What does it achieve? What relevance does it have to Scope functions and bandwidth, and what help to the world does it offer!

Looks to me this abortion of a thread needs splitting into around three different subjects!

Hmm--Not impressed.

[stj]

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

scopes? we are on to putting digital readouts on a reostat now.
it's relevent because the reostat is doubling as an isolation transformer.

[lookimback]

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

There were very smooth transitions from scopes to isolation transformers, then from isolation transformers to an isolated variac I purchased, and then from the variac to the current subject which is modifying the variac to add a digital display. Where it goes from here, I just don't know. Maybe a servo to change the voltage remotely? Actually, it would be neat to set a desired voltage, then have a servo adjust the variac.

[lookimback]

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

Well here's something I didn't think about in my plan is to use a pot to adjust the value of the 33Ω resistor. I'm replacing the resistor with a 27Ω one to give me a bit of adjustment room, and prevent any damage to the circuit by having too little resistance if the pot was turned down too far. The resistor appears to be a 3 watt, so I replaced it with a 3 watt 27Ω resistor in series with a 3 watt 50Ω pot. Now, what just dawned on me is that I didn't need a 3 watt pot because most of the heat will already have been dissipated by the resistor. Would that be correct?

[stj]

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

maybe, but not so much that it should be less than a 1w cermet pot
and remember the voltage rating needed
as for the resistor, avoid wirewound because they are inductive - stick to metal-oxide types.

[lookimback]

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

The 27Ω is MOX. The rest are metal film.

[lookimback]

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

I guess I'll go with this, but look at the difference between Digikey and Allied. Allied is about an hour from me and way cheaper for a lot of stuff. I just don't like parametric search.

https://www.alliedelec.com/bourns-30...01lf/70154527/

http://www.digikey.com/product-searc...=3059Y-1-101LF

[lookimback]

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

Done for now. I ended up replacing R6 with a 866Ω resistor in series with a 200Ω, 1W, 25 turn cermet pot. It isn't quite right though. I can dial it in perfect, but it doesn't stay dialed in when I change the voltage. I'm going to buy another meter to play with, but for now I need the variac usable. I'm thinking it's a software problem. I think it looks really good though.

[lookimback]

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

Just the before and after pics.

[lookimback]

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

So, I've learned a lot in the 2 years since I modified the variac. Still never got it perfectly accurate, and I'm ready to open it back up. I found a different revision of the meter I used, and found that the big 28 pin IC is a TM1640, which is an LED Drive Controller. Still no idea what the other two ICs are. I suspect one is an eeprom. I want to completely eliminate everything which isn't part of the voltage sense circuit with the hopes of finally making it accurate.

The first attachment is the power supply circuit.
The second is how I think current flows through the voltage sense circuit (ignoring the power supply portion of the circuit).
If I'm correct, then would the third attachment be right or would there be some parallel equivalent resistance from the power supply portion of the circuit which would affect the sense portion?