Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[Smoky]

Ok, I have the Variac supplying the full 120 VAC to the oscilloscope.

VR401 is sending +27.0 volts to the base of Q406, supposedly, cutting off voltage to the bases of Q402 and Q403.

The high voltage test point TP402 is reading +1.646 volts DC (-1500 VDC test point).

So far, this is the first time resistor R316 does not burn while in this state. R316 has +185.1 volts DC on its incoming lead and +184.9 on its outgoing lead.

Here's where I see an issue before I go any further. On the base of Q402 I measure +.996 VDC, and on the base of Q403 I measure +1.451 VDC.

Shouldn't both of these base measurements be zero since they should've been cut off by Q406?

The Orange and Red Primary wires entering and exiting the flyback transformer read +.539.

The base of Q401 reads +.536.

Shouldn't these also be zero?

[bsfeechannel]

Now turn VR401 to reduce its voltage to around 14 volts. Do that slowly and carefully, always monitoring TP-402 for any strange voltage and R316 for overheating.

[bsfeechannel]

Here's where I see an issue before I go any further. On the base of Q402 I measure +.996 VDC, and on the base of Q403, I measure +1.451 VDC.

Shouldn't both of these base measurements be zero?

They're consistent with what you're measuring at TP402. Q406 is providing Q402's and Q403 bias.

[Smoky]

Good to see you back BSFeeChannel!

I slowly lowered the voltage to 13.9 on the base of Q406.

R316 started to get hot again but only as I scrolled downward past 16-17 volts.

TP402 measured -1204 volts while 13.9 volts was measured on the base of Q406.

[Smoky]

As I scrolled down from +27 volts on the base of Q406, the voltage on TP402 changed smoothly to -1204.

I must say, it takes a little time for R316 to get hot once I get to +14 on the base of Q406.

**Also, if I continue to turn VR401 to +10.6 on the base of Q406, TP402 easily reaches -1500 volts DC.

So at least we know the rectifier circuit we made is doing its job!

...and seeing the heater at the base of the CRT glow is a good sign too

[Smoky]

BSFeeChannel,

 I didn't see this because the scope was on its side while I was adjusting VR401.

 But when I start to turn the variable resistor, and before it even reaches +26 volts on the base of Q406 (-110 VDC on TP402), both neons illuminate.

 I carefully watched a second time and the neons fire at or around -95 volts DC.

[Smoky]

While adjusting the variable resistor, VR401, I reached over and pressed the "identify" button on the top of the scope.

I seen a green glow in the lower left corner of the CRT.

I made a quick adjustment of the "centering" and the "focus" knobs.

I couldn't believe what I seen when I ramped the VR back up the second time!

[Smoky]

So we know now the rectifier circuit and CRT function.

R316 still gets hot now, but after a much longer delay.

So there is something still that's causing R316 to react this way.

I replaced all of the electrolytic and film capacitors.

I believe only original small ceramics remain.

Anyway, what a great night

[bsfeechannel]

Good to see you back BSFeeChannel!

Thank you.

I slowly lowered the voltage to 13.9 on the base of Q406.

R316 started to get hot again but only as I scrolled downward past 16-17 volts.

TP402 measured -1204 volts while 13.9 volts was measured on the base of Q406.

As I scrolled down from +27 volts on the base of Q406, the voltage on TP402 changed smoothly to -1204.

I must say, it takes a little time for R316 to get hot once I get to +14 on the base of Q406.

**Also, if I continue to turn VR401 to +10.6 on the base of Q406, TP402 easily reaches -1500 volts DC.

So at least we know the rectifier circuit we made is doing its job!

...and seeing the heater at the base of the CRT glow is a good sign too

BSFeeChannel,

 I didn't see this because the scope was on its side while I was adjusting VR401.

 But when I start to turn the variable resistor, and before it even reaches +26 volts on the base of Q406 (-110 VDC on TP402), both neons illuminate.

 I carefully watched a second time and the neons fire at or around -95 volts DC.

I've redrawn the HV section of the LBO-302.



V302 and V402 apparently do more than just offer protection for the CRT. They act as voltage regulators to maintain the filament and the cathode with a 100V potential difference, being the filament more negative than the cathode. Normally, you'd find the filament and the cathode at the same potential, but not in this case. It seems that the cathode helps to accelerate the electrons emitted by the filament, or something like this.

So every time TP-402 goes past -110V, the neons will glow.

D405 and D409 form a voltage reference for Q405, which is a current source for the intensity circuit.

The voltage doubler output, therefore, has to deliver at least -1610V, so that, after the voltage drops across the neon lamps and the zeners we have -1500V at TP402.

Of crucial importance is C418 and C408. If they haven't been tested for leakage, they should. I'd recommend Xavier60's method: using a 10k resistor in series with the capacitor and a high voltage source (anywhere between 100 to 1000V would do, I guess).



Any leakage in those capacitors would make the HV supply to deliver an output voltage more positive than -1500V and will load the voltage doubler which will make R316 overheat.

[floobydust]

When the HV is up, measure CRT pin 2 (control-grid) and pin 3 (cathode), that will tell us a bit. Both are at high voltage, close to TP402. I think it's trouble if the neons stay lit.
Q405 is a constant-current source for controlling beam current (intensity) and that circuit might not be working. The (4V7?) zeners D405, D409 and diode D406 should be conducting through R416 and turning Q405 on to back off CRT current.  If not, then you have (too) high CRT current which would explain R316 getting hot.

I would look for open INTEN trimpot/potentiometer (does it do anything?), open Q405, open R416, open D405, D406, D409.
You said C410 470pF 3kV tests good... the 1.8MEG's I would also take a peek at.

You don't want the CRT control grid trying to go +ve or zero wrt the cathode, then it would be full on too much current.

[Smoky]

Thank you BSFeeChannel.

C408 and C418 are new and both were tested before the first rectifier module was installed, and again, after the second rectifier module was installed (I desoldered the caps and tested them for leakage at 600v out of circuit on the Z-meter).

I will remove the caps and test them again, or even replace them with another pair of new 470pf and 1000pf caps.

When you mentioned at first to turn the VR401 down to 14 volts at the base of Q406, are you saying that -1500 should have been read at TP402 (14v @ Q406= TP402 -1500)?

It wasn't until I turned VR401 down to 10.6 volts on the base of Q406 before TP402 reached -1500 VDC.

I also turned VR401 down even further and exceeded -1650 VDC on TP402.

When I stop turning VR401 and remove my plastic screwdriver, the voltage on TP402 remains rock steady.

FloobyDust:

When TP402 is at -1510 VDC, Pin #2 on the CRT measures -1190 VDC and Pin #3 measures -1500 VDC.

The variable resistor for intensity (VR403) works great.

The "intensity" pot on the front panel (J403) works great.

And BTW,  if the neons are in series on the -1500v rail and remain lit, what are they sensing?

...and I think she's smiling

[floobydust]

Ahhh cathode rays 

If you have intensity control, then the CRT bias is reasonable and all looks good so far 
Focus seems to be OK, I don't see retrace lines either.

I redrew the circuit, stared at it and realized the cathode current path is either through R416 or the neons.
My guess is leaky C417 or C408 could load down the HV and light the neons without upsetting the CRT's operation, up to a point.
Note the LBO-301 schematic, mostly similar- except there are no neon lamps.

Otherwise, you would have to scope TP401 to see what primary current looks like now, with the new caps and other parts.
The factory had some issue if they lowered R404 from 820R to 470R.

[Smoky]

This is Test Point TP401.

The LBO-302 has -1505 VDC on TP402 while TP401 is being scoped.

On the Tektronix 222A's screen, you'll see that every vertical division represents .5 volts.

In the lower left hand corner you'll see "1.70v" which represents the location of the (+) sign in the waveform. I set that because that is where the top of the waveform should be as outlined in the service manual. As you can see, the waveform reaches ~2.25 volts at its peak.

I imagine this shows the irregularity of the HV regulation. The waveform should be lower by .5 volts yet still produce -1500 volts at TP402.





I will also hook up TP401 to my TDS420 oscilloscope tonight.

BsFeeChannel, I will also get the voltage of the HV Red wire where it enters the main board of the scope so you can have the differential between it and TP402.

[floobydust]

The peak current you measure of 225mA verses service manual 170mA I would say is due to the high raw DC rail at C302.
LTSpice R316 avg. power dissipation is 5W at 165VDC and 225mA peak for Q401 (assuming flyback primary resistance is 140 ohms). R316 sees peak power dissipation over 20W.
I think current at TP401 looks reasonable.
It's a delicate circuit, even 10% change in C403 value or raw DC causes R316 to go from say 3W to 5W.

[Smoky]

Thank you FloobyDust.

I was thinking at work today about the difference between the voltage at the cathode and the control-grid of the CRT.

It reminded me of the tube amplifier repair I went through last Fall. The screen-grid to plate resistors. I recall adding a 1K Ohm resistor to keep the screen grid at a lower potential than the plate. Parasitic oscillation also comes to mind.

Should the voltage spread between the CRT's control-grid (-1190 VDC) and the plate (-1500 VDC) be so huge? I imagine that's a lot of heat to dissipate.

In those tube amplifiers the voltage differential was at most 200 volts.

So less voltage differential equaled less heat dissipation by the resistor.

And C402 and C403, those are the new .01uf 3% film caps we just installed, I don't have to think, they measured less than 1%. Almost all of them measured exactly 10200pf.

I want to adjust the components to bring the scope under less stress. Would we be looking at a larger cap for C402 and C403?

And do you recall when I mentioned that resistor R404 on the HV Oscillator board measured 470 Ohm and not the 820 Ohm listed in the service manual? Should that resistor be adjusted too?

I apologize if I'm not making any sense, I'm just so happy to see that green dot!

I'll have more voltage measurements for BsFeeChannel in a little bit too.

[bsfeechannel]

The peak current you measure of 225mA verses service manual 170mA I would say is due to the high raw DC rail at C302.
LTSpice R316 avg. power dissipation is 5W at 165VDC and 225mA peak for Q401 (assuming flyback primary resistance is 140 ohms). R316 sees peak power dissipation over 20W.
I think current at TP401 looks reasonable.
It's a delicate circuit, even 10% change in C403 value or raw DC causes R316 to go from say 3W to 5W.

I calculated the average power R316 dissipates if the voltage at TP-401 were at 1.7V peak. Its over 2W. For 3W carbon resistor, the surface temperature rise is expected to be in the range of 100°C above ambient.

You could barely touch it.

However with TP-401 at 2.25 V peak, R316 dissipates 3.6W, which exceeds its rated power. I'd spend a little more time trying to hunt down the cause of this discrepancy.

[bsfeechannel]

I redrew the circuit, stared at it and realized the cathode current path is either through R416 or the neons.

The neons are there to provide Q405 plenty of leeway for the voltage at its collector to be up to 100V more negative than the cathode. The heater is also 100V more negative to repel any emissions in its direction.

So the voltage at the control grid should not be more positive than -1500V with the maximum intensity, and not more negative than around -1600V with zero intensity.

[Smoky]

BsFeeChannel, I just measured TP402, it is right on at -1500 VDC.

The HV Red wire from the Flyback transformer where it attaches to the main board reads -1640 VDC on the nose.

Your calculations are very close.

* I also forgot to mention last night, during one of the start-ups I had turned the "Trigger Level" knob all of the way to "Auto."

I noticed that the neons would flicker randomly, but not constantly, even when Q406 was getting +27 volts at its base.

[bsfeechannel]

So, from the voltage point of view, things seem not to be out of the norm.

Check, please, R403 to see if it is not out of tolerance. Also measure the voltage across R316 now that we managed to get -1500V at TP402.

[Smoky]

BsFeeChannel,

Resistor R403 measures 10-10.1 Ohms in circuit (it is listed at 10 Ohms 5% in the parts legend too).

For resistor R316, the incoming lead measures 170 volts and the out-going lead to the HV Oscillator board measures 149 volts.

That's quite a few volts being dumped.

I'm going to measure the to legs coming from the power transformer, they should be at 128 volts.

BsFeeChannel, I'm measuring 154.1 and 154.4 volts AC going to Diodes D301 and D302. I believe it should be 128 volts AC on the schematic. I'll check again. Yes, with TP402 at -1510, Diodes D301 and D302 are receiving ~149 volts AC from the power transformer.

[bsfeechannel]

P_R316 = (170V - 149V)²/470Ω = 0.94 W.

That's even below my calculations for 1.7V peak at TP-401.

What we can do now is to measure the temperature at the surface of R316. It should be around 70 to 75°C above ambient.

[Smoky]

So, we're talking 167 degrees Fahrenheit plus another 70 degrees Fahrenheit of room temperature.

237 degrees Fahrenheit.

I'm going to check the voltages again when R316's temperature starts to rise quickly.

I don't own an IR thermometer.

[Smoky]

BsFeeChannel,

R316 remains at 170 VDC (incoming) when it starts to get hot. So the voltage doesn't change.

Now get this, R306, a 47 Ohm 1/2 watt that FloobyDust noticed that was discolored from heat a couple of weeks ago. I replaced it with another new 1/2 watt resistor two days ago for this new start-up (It's listed as a 1/2 watt in the parts legend too). It's the only other "hot" resistor.

I just measured it. There is 44.9 volts going in, 39 volts coming out. 44.8-39=5.8, 5.8 x 5.8=33.64, 33.64/47 ohms = 0.7157 watt. Wouldn't this justify a 1 watt resistor if the voltages are proper?

R306 and R316 "Feel" the same on my finger

I need to beg, borrow, or steal an IR temperature gun!

R316 does feel like piercing heat   ...and maybe 237 degrees Fahrenheit should?

As you can tell, I'm mumbling to myself!

Could this scope be that close to Ok voltage wise?

I think we're doing very well 

[bsfeechannel]

So, we're talking 167 degrees Fahrenheit plus another 70 degrees Fahrenheit of room temperature.

237 degrees Fahrenheit.

Well, I said Celsius because specs for resistors are given in that unit.

I'm going to check the voltages again when R316's temperature starts to rise quickly.

I don't own an IR thermometer.

You can use a thermocouple.

[Smoky]

Wait a minute, water boils at 212 degrees Fahrenheit!

Of course my finger will find it piercing

Another great night, and more to follow!