Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[Smoky]

Thank you FloobyDust!

When the new film capacitors were installed and the scope was powered up and found to show good HV improvement in the Flyback, I stopped using the Variac to power-up the scope. I use the on/off switch.

It's only when I use VR401 to raise TP402 to -1500 VDC. About 15 seconds later, R316 starts to get screaming hot.

As for measuring the DC resistance of the Primary and Secondary windings in the Flyback and power transformer, I never did such a test before. All I can say is that my Kepco power supply can maintain very stable current from a few mA up to 3 amps. It also can produce up to 30 volts DC that remains rock steady.

Is this sufficent voltage and current to do the resistance tests?

Thanks again guys!

[floobydust]

I would just use a multimeter for the winding resistances in-circuit. Side-effects are OK just need ballpark numbers. Don't need a PSU or to take apart anything for this.

[Smoky]

There were a few comments about capacitor C402 on the HV Oscillator board.

It is a new Panasonic .01uf capacitor measuring 2% tolerance, 10,200pf.

Here's a shot of the HV Oscillator as it sits right now:



Next, I'm going to take the resistance measurements from the Flyback transformer's windings and the 128 volt supply winding from the power supply transformer.

[Smoky]

Here are the Flyback windings resistance measurements

My DMM was set on the 200 Ohm setting (the lowest range).



Yellow and Green wire winding (A & B) = 1.5 Ohms

Red and Orange wire winding (C & D) = .3 Ohms

Secondary winding (E & F) = 20.2 Ohms

[Smoky]

I am now working on the power supply transformer. On the schematic, this would be the 128 volt winding going to diodes D301 and D302. They are two Orange wires:



You guys aren't going to believe this! The Orange wires go to taps 8 and 9 on the power transformer:



No wonder why I read higher voltage at those diodes. They're attached to 140 volt taps.

There are no 128 volt taps on this transformer. Now we know my oscilloscope is built slightly different than the way it appears in the schematics.

I'm going to detach the orange wires from the diodes and read the 140 volt winding's resistance.

The resistance on the 140 volt Secondary winding is 281 Ohms.

[Smoky]

...and the test on the newest neons. A smaller package size but the current draw is the same. They fire at 52.5 Volts AC:



So the 70 VAC neons are installed, should I try a pair of the 52.5 VAC neons?

[David Hess]

Now make your neon bulb strike voltage measurements with the bulbs alternately in the light and in the darkness.

I forget what they are called but typically the bulbs they use in circuits like these have a little bit of radioactivity so that they have a consistent strike voltage whether in the darkness or the light.

[Smoky]

Thank you Dave! That is a very cool test.

Starting with the first neon in the picture above (52.5 VAC), it fired at the same voltage in light and in total darkness (these are installed in the scope).

The second neon (original 50.0 VAC) fired also at the same voltage in both light and total darkness.

The third neon (70 VAC) fired at 67.2 VAC in the light and 68.8 VAC in total darkness.

[floobydust]

I noticed the neon lamps end up in-series with the CRT (cathode), so they could be for start-up. The flyback oscillator would see little or no load until they are ionized. My opinion is it would not make a difference in changing the firing voltage between parts. edit: note the originals have crooked electrodes, close spacing could also be the reason for the lower firing voltage.

Sometime measure the power transformer's 6.3VAC filament winding so we can settle if the transformer is putting out way too much or not. Leader upped the secondary voltage from 128VAC to 140VAC it seems, as well as changing the base drive from 820R to 470R.

When you increase the HV trimpot, there is a point where the oscillator is pushed too hard and efficiency drops and Q401/R316 are just gonna cook and get real hot. It may be the HV section is fine at say -1300V but can't really pull off -1500V. So this might be the problem, you're trying to get more that it can do right now.
You can look at TP401 with different HV output settings and see when current shoots up.

[Smoky]

Sounds good FloobyDust.

It is written in the Service Manual that TP402 is to be -1500 Volts +/- 100 Volts. So I will bring it to -1410 the next time and see if the beam comes up on the CRT.

Should I slowly back down VR401 until it diminishes?  Then raise it back up until the beam reappears?

Ultimately, I should be able to see the CRT function without R316 overheating.

Anyway, I installed the original 3 watt resistor in the R316 location. It is huge and it should be a good indicator for temperature.

...and if the CRT works at a voltage below -1500 VDC, I'll be able to use my Tektronix P6009 HV probe (1500VDC + Peak AC) with the TDS420 to get good measurements at all test points and components.

[Smoky]

Here's the reason for the waiting for the new .68uf capacitor for the back of the trigger level knob pot.

This is a picture of the original .68uf film capacitor. The sides delaminated and I can catch the edge of the foil with my finger. The new Panasonic PP cap measures .678uf and is now installed:

               

[Smoky]

Here is the only mention of test point TP402 (-1500v) regarding the CRT circuit after reading the entire service manual:

"If the spot cannot be extinguished or made to appear with the Intensity control knob, check the following voltages and adjust as necessary. Check TP305 for +27, TP306 for -27, and TP402 for -1500v +/- 100v."

[Smoky]

FloobyDust,

The first test I did tonight was to use my DMM and measure across the CRT heater pins #1 and #14. They measure 6.58 VAC.

[Smoky]

I turned the Intensity control knob fully clockwise for max brightness.

I turned VR401 until the dot appeared on the CRT.

The voltage read -1385 VDC at test point TP402.

The voltage incoming on R316 is +170 and outbound +150.

I can hold my finger on R316 for an extended period of time but it is hot but not melting hot.

Next, I turned the Trigger Level knob to "Auto" mode and that's when R316 goes into meltdown!

So the dot appears on the CRT at -1385 VDC and the Intensity control knob can make it appear and disappear, just like it is stated in the service manual.

The question now is, why does R316 go into meltdown when the Trigger Level knob is turned to "Auto" mode?

BsFeeChannel, I will scope TP401 while TP402 is at -1390 VDC without the Trigger Level knob in the "Auto" mode.

Here it is:



*After five minutes of it running on its own, R316 hot but not burning, the main fuse blew   It is a 0.4A "slow-blow" type.

New fuses are on order!

[bsfeechannel]

The question now is, why does R316 go into meltdown when the Trigger Level knob is turned to "Auto" mode?

C410 is there both to convey the blanking pulse and to enhance the brightness when the beam crosses the screen.

It pulls the average grid voltage up when there are triggering pulses and the CRT conducts more, loading the HV supply.

BsFeeChannel, I will scope TP401 while TP402 is at -1390 VDC without the Trigger Level knob in the "Auto" mode.

Here it is:

Nice. Try to hook the scope probe on both leads of R316 (each one at a time) so that we can add the current that goes through C402.

You can measure the current through the CRT by measuring the voltage across R418. With and without auto trigger.

It'd also be a good idea if you could publish, if not the whole manual, at least the whole schematic. The more information you give us, the more we can help you.

[Smoky]

Will do BsFeeChannel.

I placed an order for the 400mA 250v 2x50mm slow-blow fuses that the scope requires.

This weekend I will scan the schematics and post them to either my website or here. One way or the other, you will have a link to them soon.

[nigelwright7557]

I have designed a few SMPS and made my own transformers.
Once I have made the transformer I need to find out primary inductance and leakage inductance.
Rather than use an LC meter, I put a capacitor in parallel with primary of transformer and put a resistor in series with both.
I then use a sig gen to apply a signal.
I move signal up and down until I find resonant frequency.
Then work out from 1/2*pi* square root (LC) the inductor inductance.

I then short out secondary and do the same again and work out leakage inductance.

Its long winded but gives accurate "real" results.

[Smoky]

I've been calling R316 a 3 watt resistor because that's what the service manual lists it as in the parts legend.

I just physically measured it with my calipers and it is .95" in length by .31" in diameter. It's huge!

Could this resistor be a 5 watt after all? ...and yet again, another factory revision? I just found this chart on the internet too, .945" = 24mm and .31" = 7.874mm. So after all of this time, it could be a 5 watt resistor that's getting blazing hot!

         

* I went back and reread some of the posts just in case I missed a request by someone for something. Floobydust, I did check the 1.8MOhm resistors (R410,R411, and R412). They are spot-on.

I also want to point out that the supply voltage at the wall outlets in my house average 122VAC. This Leader oscilloscope was originally intended to run on 115VAC. I just recently repaired a Tektronix 310 oscilloscope and I can attest to the fact that the 7VAC increase at the plug made the unregulated supplies within the Tek scope rise ~27VDC higher. Could this supply voltage increase cause an "imbalance" within this Leader oscilloscope

I'll be posting schematics tomorrow

[Smoky]

Hey guys and gals,

 I’m bringing back up a repair thread that I started a few months ago regarding this Leader oscilloscope.

 I finally was able to take apart the service manual and scan the schematics for everyone to view.

 I also have the new fuses to get her back under power.

 Each page of the schematics is ~1.8MB so it may take a few replies to upload them all (8 pages).

 BTW, I also uploaded this group of LBO-302 schematics by .ZIP file to Ebaman.com.

Smoky

[Smoky]

Power Supply        Vertical Input

[Smoky]

Vertical Amplifier      Calibrator

[Smoky]

CRT Circuit      Trigger Sweep

[Smoky]

Timing Switch           Horizontal Amplifier



The oscilloscope will be back on my bench this weekend and I'll start by testing both leads of R316 and posting the results. I really appreciate all of the guidance I've received so far regarding this restoration/repair!

Also, I was recently given this Huntron Tracker but I'm not familiar with it. Maybe it's not too useful in helping to find the fault in this repair unless you have an idea?



Here's what I've decided to do. Having had such a good time restoring that Tektronix 310 oscilloscope, this Leader is going to get the same treatment. I have already replaced all of the electrolytic and HV ceramic capacitors, rebuilt the Flyback transformer, and installed new neon lamps.  Starting with the CRT circuit, I'm going to pull every single resistor, transistor, and small capacitor. I'm going to replace every single part. There aren't that many. I wish I had a curve tracer to really get down and dirty with testing transistors in the event that the current "gain" is too high, but then again, new transistors of this era are cheap too, and some of the bolted-in ones I already have. Since some of the transistors that are installed have part numbers that differ from the schematics, I'll pull all of the data sheets and compare hFE specifications.

If you look at the parts legend, you'd see that the majority of resistors are labelled as 5%, others are 1%, and some are 0.5%. It appears that this scope was assembled with very tight tolerance parts. So, I imagine, it wouldn't take too many parts to wander and throw this machine out of whack. I really like those new Vishay metal film resistors that I used in the Tek scope, small and spot-on. So I'll invest in the Dale 0.1% and 0.5% for the entirety. And I wonder what those small low-voltage brown Mica capacitors have to say too? I'll measure each one to see if I find a fault.

There is something drawing too much current in this machine, and I'm going to find it!

Again, it's when I turn the Trigger Level knob to "Auto" mode that makes resistor R316 go into meltdown.

Time to dig into my 2019/2020 Winter project 


*Something just popped in my mind, what effect would Silicon diodes have in a Flyback transformer circuit when they are used to replace Selenium diodes? Isn't there a "voltage drop" factor to take into account?


Another observation, the power supply transformer is wired for 115VAC but I have 122VAC at the outlets in my house. That's an increase of about 5%.

Floobydust asked me to measure the filament voltage at the CRT heater pins. I read 6.58 volts, it should be 6.3. That's an increase of ~5%.

We also discovered that the transformer taps supplying R316 are 140 volts and not the 128 volts as outlined in the schematics. If this is an unregulated supply rail, I believe the voltage is higher than it should be, but I don't see a voltage reference at this point (R316) in the schematics to verify this.

I think there are errors now that are "component value" related due to the change of the power transformer's output voltages. I have used "dropping" resistors in the past to adjust filament voltages and panel lamps in my mono tube amplifiers with excellent results. But, calculating and making adjustments to higher voltage outputs and making component changes within this scope would be well above my pay-grade!

I know very little about circuits, but I can fix things mechanically pretty well. If you feed a transformer more voltage at its inputs it only seems logical that you will over-drive the regulated supplies (additional voltage dissipation required through heat along with increased stress) and you will raise the voltage of the unregulated supply rails. Maybe, a custom transformer can be made? Or can the original transformer be modified? Or a voltage regulator be added to the outputs?

Fun stuff and just thinking out loud

[bsfeechannel]

Welcome back.

Floobydust's hunch is that the 3-W resistors of yore are advertised as 5-W resistors today. My hunch is that R316 was dimensioned to operate on the limits of its power dissipation. The PCB cut-out above it is a hint. That, coupled with the fact that your line voltage is greater than the rated 115 V, makes me suspect that a modern 3-W resistor will turn your scope into a barbecue grill.

It is, however, a good idea to fully inspect your circuit to discard any other causes.

[Smoky]

So last night I stayed up to watch three videos on the winding of transformers. This was the first one, and I like the music too



Instead of messing with component testing tomorrow, I'm taking out the power supply transformer. We're going to re-wind it!

There are so many pluses here, one, I won't need all of the primary windings re-installed. I only need one, and that one will be custom-sized to 122VAC.

The secondary windings needed are four, 6.3, 10, 35, and 140V. Where the 10V winding goes is a mystery to me, I see the letter "E" below it (I'll trace it and let you know where it goes). The 6.3V wires are large green ones going straight to the CRT heater pins. Since this scope came with this transformer, those output voltages need to remain the same.

So, yes, this transformer is slightly different than the one in the schematics. But based on what I watched in the videos, the iron core size and gauge of wire are the most important factors to know when building a transformer. And if the winding has a "center tap" you divide the number of turns in half for each voltage.



Thank you for the help, I'll need it!

Whoah, what if it's possible to access just one of the primary windings? 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? And if that's the case, it looks like the primary windings have a center tap, which means that I would need to shorten the wires a little on both ends.

And the Mailman just dropped off a Digi-Key box on my porch! The parts inside are for the repair of the HP 35660A Floppy drive I have going on in another thread.

It feels like Christmas already