Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[bsfeechannel]

Would a new 122VAC primary winding need to have fewer "turns" than say the 100V or 115V primary windings that are in it now? If so, is it possible to maybe just shorten one of the existing primary winding without disturbing the secondary windings?

In your case the primary would have to have MORE turns. What you could do is to reduce turns of the secondary windings. But I wouldn't recommend you to do it if you're not familiar with the "useless theory" mentioned by the guy in the video you posted.

[Smoky]

I did smile when I heard him say that

We'll see how this transformer comes apart tomorrow, or if it even can.

I'm not sure what other options that there are, but we can surely say that we gave it a good run!

[bsfeechannel]

You can try to install a modern 5W resistor instead of the 3W you purchased. Then you can try to reduce the HV from 1500 to 1425 V, i. e., less 5% to put less strain on the oscillator.  You can try to install additional power resistors in series with the D301 and D302 to bring TP301 and TP302 to specs and consequently the voltage drop on R316. There are plenty of options.

I think tinkering with a power transformer, which is a critical and safety component, is not the best of strategies for the moment. But that's just what I see from my point of view. If you're having fun, that's what counts.

[Smoky]

Thank you BSFeeChannel.

The original 3 Watt R316 resistor is installed. FloobyDust ruled against using any of the new resistors.

The HV has been resting at -1400VDC for the past few start-ups.

I like the idea of adding the power resistors in line with the diodes.

Tomorrow I will  disconnect the output leads from the power transformer to the oscilloscope. I will then measure exactly what the output voltage is at each Secondary winding.

[Smoky]

Alright, I put a new fuse in the scope and hooked-up my DMM to the 140V output taps. Each measured 155.1V. The 35V output taps measured 38V.

 When calculating a dropping resistor, I wasn't sure what current value should be used in the equation, so I just used 400mA (the fuse size). At that amperage, and the needed drop being 15.1 volts, an online calculator came up with a 38 Ohm 6 Watt resistor.

 I have no idea what the current draw is of this oscilloscope when it's working properly, so would I need to experiment with different values until we find the happy medium? We know what the voltage is now and we know where it needs to be, so here are other values based on different currents:

At 150mA = 101 Ohm 2.26 Watt
At 200mA = 75 Ohm 3.05 Watt
At 250mA = 60 Ohm 3.77 Watt
At 300mA = 50 Ohm 4.53 Watt
At 350mA = 43 Ohm 5.28 Watt
At 400mA = 38 Ohm 6 Watt

Surprisingly, I see plenty of space to bolt a heatsink to the aluminum panel of the chassis.

We need to dump 11% of the voltage somewhere. Do we need a chimney

[floobydust]

I would add a small power transformer (in auto-transformer connection) to lower mains voltage. I think I see room in the chassis. A 120-12.6VAC transformer maybe 1A could shed 12.6VAC or about 10% with very little heat.
See: ]https://www.eevblog.com/forum/projects/hp-3410aac-microvoltmeter-overheats-power-supply-mods/msg2818584/#msg2818584]
Just ensure the little transformer is phased properly so it does not end up boosting +10% (instead of buck). Try it on a light bulb.

[Smoky]

Thank you FloobyDust!

So I get up this morning and I'm thinking, why not find out exactly what the input supply voltage should be that brings the transformer outputs to the correct voltages as written on its label?



So I did, I brought out the Variac and plugged in the oscilloscope. Any guesses?

...with the Variac set to 110VAC the outputs of the transformer fall in line!

   

Thank you for the "auto-transformer" idea too. I never heard of such a thing. And look how it is wired. I never imagined something like that could be done. So I type that topic in on Google, and it looks like many folks are in the same boat as I am. Old equipment/screwy voltages.

Now, do you think that there is a small 1 amp 120VAC to maybe 18-22VAC transformer available? And could the transformer be rated as low as 700mA? And maybe a product is out there that I can cannibalize its transformer from, a transformer that is sleek/small or custom-shaped so it fits better?

And the secondary winding needs to have a "center-tap" for this to work.

We need to shave 22 volts!

...now everybody get looking

[floobydust]

You could try a 24VAC transformer from a home alarm panel, a home doorbell transformer is 16VAC. But little transformers have poor regulation even 20% so they will have higher output at light loads, especially if they too are rated "115VAC" but fed the high line. Digikey has lots of different ones. It's just they might not give the exact expected results, and the power factor is not 1 for the scope I imagine.