Leader LBO-522 repair - CRT too bright

[Ortzinator]

Hey all! I’ve got another oscilloscope I’m trying to repair.

The power supply voltages all look good, as well as their ripple.

Symptoms:

* CRT appears to be on full brightness.
* Intensity control has no effect on trace brightness but does affect the unblanking pulse.
* Unblanking pulse looks as described in the schematic
* Disconnecting the unblanking pulse also has no effect
* The intensity trimmer has no effect

I saw in another thread that the high value resistors in the CRT circuit often increase in resistance with age, but as you can see, they look ok (readings are out of circuit). The 1.5M resistor reads 1.55M. Is that enough to make a difference?

[Andy Watson]

Is C44 good?
Also, double check the circuit against what you actually have - there's at least one drawing error on the tube base - they have the X deflection connected to a grid whilst the X-plate goes nowhere!

[Ortzinator]

Is C44 good?
Also, double check the circuit against what you actually have - there's at least one drawing error on the tube base - they have the X deflection connected to a grid whilst the X-plate goes nowhere!

C44 is good.

I couldn't find the datasheet for the tube but the Iwatsu SS-5702 uses the same tube, and assuming their schematic is not also incorrect then it looks like the Leader is actually hooked up correctly.

[floobydust]

Are the three neon lamps V1-V3 lit?
I would you say you lost grid bias, so G1 is not as -ve potential as the cathode. But the CRT schematic symbol looks wrong, cathode is pin 3 "K" but drawn as control grid.
There are Toshiba CRT 150BTB31 datasheets out there.

You can make a poor man's HV probe with a bunch of 10MEG resistors in series and measure cathode voltage at TP6 and G1, P1 to see what is going on.
1S2463 diode is DO-35 package 300PIV 300mA and I wonder if they are OK. I would measure across C43 and see what the intensity control does there.
Careful to discharge the CRT and HV multiplier before going in, I use a 10's kohm power resistor to GND to make sure there is no stray charge that can zap a multimeter and kill it.

[Ortzinator]

Are the three neon lamps V1-V3 lit?
I would you say you lost grid bias, so G1 is not as -ve potential as the cathode. But the CRT schematic symbol looks wrong, cathode is pin 3 "K" but drawn as control grid.
There are Toshiba CRT 150BTB31 datasheets out there.

You can make a poor man's HV probe with a bunch of 10MEG resistors in series and measure cathode voltage at TP6 and G1, P1 to see what is going on.
1S2463 diode is DO-35 package 300PIV 300mA and I wonder if they are OK. I would measure across C43 and see what the intensity control does there.
Careful to discharge the CRT and HV multiplier before going in, I use a 10's kohm power resistor to GND to make sure there is no stray charge that can zap a multimeter and kill it.

I have a HV probe actually.  K and G1 are both -1910V and P1 is around -1500V. The neons are not lit.

C43 goes from 47V to 138V when adjusting the intensity trimmer.

I tested the 1S2463 diodes in circuit and they seem ok. Diode mode on my dmm shows a 0.6v forward drop for all of them.

My Google Fu is still failing me but here’s the CRT circuit from the BK model 2120 which uses the same tube.

[George Edmonds]

Hi

G1 shold be around 50V negative compared to the Cathode

G Edmonds

[George Edmonds]

Hi

Manual with schematics here

https://elektrotanya.com/leader_lbo522_lbo523_instruction_manual.pdf/download.html

G Edmonds

[George Edmonds]

Hi

The CRT on the LBO-522 schematic has G1 and the Cathode mixed up.

On the LBO-523 schematic it is correct.

G Edmonds

[floobydust]

G1 looks is too +ve, it should be around -2,000V, certainly more -ve than cathode K TP6 to bias the CRT dim/off.
I would check the extra HV multiplier stage for that. D16, D17, D18 (you already tested OK) they are LV diodes on a HV rail. C40, C41 not as likely to fail. C42 maybe.
Or something is loading that down.

The blanking circuit here is a bit hairy. Hitachi V212/V211 also uses the Toshiba 150BTB31 CRT, and very similar circuit. That schematic is drawn better with some waveforms.
Basically a HV multiplier making -1,900VDC to the cathode, and I think extra multiplier stage to make additional 100V or so but modulated by the blanking transistors and the intensity control setting the overall  DC level.

[Ortzinator]

I’ve had some time to do some more component testing. I’ve attached an updated schematic showing my testing. Green means it was removed and tested out of circuit. Yellow was tested in circuit. Not sure where to look next. Maybe the unblanking circuit?

[edavid]

I tested the 1S2463 diodes in circuit and they seem ok. Diode mode on my dmm shows a 0.6v forward drop for all of them.

They usually fail by becoming leaky.  You need to test them for reverse leakage out of circuit, with reasonable voltage applied, maybe 50V.

[George Edmonds]

Hi

Your current method of fault finding usually ends in tears as multiple other faults are normally introduced by your method.

You need to understand just how this circuitry works, which you clearly do not, and then apply logic coupled with a few checks using another oscilloscope to narrow down the fault and rectify it.

The basics of how this circuitry works are not rocket science and were virtually standard with all oscilloscope manufactures using CRT’s

G Edmonds

[floobydust]

I'm posting the Tektronix explanation for the same basic circuit used in the 2225. It's not easy to understand because it is modulating the CRT grid sitting on -1,900V yet using low voltage transistors to do that. This is why it is complicated yet everybody uses the same approach.
The lower tap on the transformer is for ~100VAC, I wonder if you have anything from that. I think the cathode voltage looks fine, it's just the extra -ve voltage to blank/dim the CRT is not happening.
OP have patience, it's supposed to be fun figuring it all out

[Ortzinator]

Thank you for the help and encouragement.

I decided to go back to basics and probe voltages. I found a broken trace! I had previously looked over the board several times. But the broken trace was underneath the felt pad separating the board from the CRT shroud. Lesson learned to inspect the ENTIRE board.

The oscilloscope is now working perfectly. Thanks everyone!