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

[tom66]

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

I picked one up from a local test equipment surplus store for about £25 which is about 40 dollars. Mine is 500VA continuous, 750VA for 30min max. If you look around you'll be able to find one. Perhaps enquire with power tool rental companies if they're getting rid of any of them. Most of these transformers have to be derated by half for continuous loads but I've seen 750VA (1500VA "power-tool rating") ones for not too much money.

[lookimback]

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

So 1KVA should be good for 500VA continuous?

[tom66]

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

So 1KVA should be good for 500VA continuous?

If that's a 1KVA power-tool rating yes...but most isolating transformers are rated for their continuous load. The workman's transformer (as used on construction sites) being the exception where generally they are rated for 15min load with 15min cooldown at 2x continuous load.

[lookimback]

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

Gotcha. Thanks!

[lookimback]

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

So, I decided on this 10A isolated variac which will probably be more useful. I got a scratch and dent one for $150 with free shipping.


http://www.amazon.com/gp/product/B00...ef_=sr_ph&sr=1

[stj]

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

shipping on variacs is usually a killer - lucky it's free!!

[lookimback]

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

shipping on variacs is usually a killer - lucky it's free!!

That's the main reason I went with this one. It's like 21 pounds, so I can only imagine the shipping cost.

[Agent24]

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

Looks good but be careful with it when driving a SMPS. Some can't handle low voltages and will blow up! - or so I read on the RepairFAQ anyway...

[lookimback]

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

Glad you told me that. Any idea how low is too low?

[stj]

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

it's called brownout, it's the startup threshold where the psu just pulses and damages itself.

the exact voltage varies depending on the psu design.

[lookimback]

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

Maybe I'll try to make a variable constant current source. Right now, I just use a light bulb limiter.

[tom66]

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

I have used a variac with many power supplies and never had any blow up on me.

If it starts drawing a lot of current and the backlight pings repeatedly, obviously, that's bad and can wear stuff out, like the tubes and inverter transformers. But a power supply won't just blow up immediately if you try and operate it on a lower voltage. After all, they do this anyway when the power is disconnected, as the main caps discharge and the TV turns off.

Good that you got one with isolation. I found two for sale locally for £40.00. Bargain of the century because I shipped one for £120 to an eBay buyer! Always pays to check local classifieds etc.

[lookimback]

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

Good that you got one with isolation.

Yes, I lucked out on even finding it because I wasn't really looking for it. I was looking for an isolation transformer that was at least 500 Va and this popped up because of the keyword "isolation." I checked online and confirmed it is really isolated, and the price, with free shipping, was actually less than what I could find any isolation transformers for. I think where this is going to get it's most use for me is as a makeshift bench power supply, until I get ready to buy a real one. I have tons of bridge rectifiers and voltage regulators from scrap boards which I can use.

[lookimback]

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

Variac arrived. It functions properly and is fully isolated. You can see from the pictures the little dent that saved me $30. I'll have to open it up and fix it so the front panel will stay tight, but that's no big deal. Talk about heavy though. This thing weighs 22 pounds. I need to find a permanent spot for this and never move it. Now it's time to learn how to use the scope and have some fun.

[Agent24]

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

Nice one. Don't forget the isolation only protects the circuitry\scope to some extent (don't forget probe\scope voltage limits etc) - the transformer does not protect you.

[lookimback]

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

I've watched a couple YouTube videos where people say things like, "This will keep me safe," referring to an isolation transformer. Obviously, nothing could be further from the truth. Electrocution is still always a hazard. But, eliminating the risk of your probe blowing up in your hand certainly makes you a bit safer.

[tom66]

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

Isolation makes it harder to get electrocuted with just one hand. But it's still possible, so you need to remain vigilant at all times. It's primarily for the safety of the equipment.

[lookimback]

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

Isolation makes it harder to get electrocuted with just one hand. But it's still possible, so you need to remain vigilant at all times. It's primarily for the safety of the equipment.

Right, because (without isolation) if you are grounded and you touch a hot wire, there is a complete circuit and you just became a big resistor.

[lookimback]

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

Couple issues I've noticed right away with this. The maximum voltage is supposed to be 130vac but it's 145vac. Is that because there's no load, or is there some way to adjust it?

Also, the volt meter is wrong. It reads 110vac when my Fluke reads 145vac. I'm sure there's a way to adjust this. Not really a big deal because the gauge is so small I can't see it anyway. I think I'm going to modify it and add a digital display. I'll have to rearrange the panel to do it though, because I can't find a display small enough. I've found one that's 79mm x 39mm. I think it will fit if I get rid of the power switch. I'm thinking I'll move the fuse to the side, and put a round push button switch in it's place. Then it should fit and all I'll have to do is figure out how to power it. Or, I may just get some sheet metal and make a new panel. I haven't opened this up yet, but I'm figuring there should be a way to power it from the transformer, and of course, a rectifier circuit.

[budm]

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

Did you open it up to verify if the LINE, Neutral, and safety GND are wired properly?
Verify if the the Neutral goes straight through from the AC plug to the outlet Neutral, and the fuse in on the line side not on Neutral.

http://instrumentation-central.com/p...r_drawings.htm