Predicta Predicament - Videokarma.org TV - Video

Down Under · #1 07-31-2014, 07:42 AM

Hi guys,

I'm currently working on a Preficta Holiday. Although I now have an image of sorts on the screen, it's far from perfect. The really strange thing is that the brightness and associated voltages around the chassis seem to be directly related to the horizontal oscillator frequency.

For example, if I turn both the horizontal hold and the horizontal range controls fully clockwise, I get a reasonably bright display. Not as bright as it could be, but ok. Of course, the sync is way off, as the oscillator is now running at around 40kHz. With the tv running like this, B+ is at 260V, and boost is way up at 620V.

As I adjust the controls back to where they should be, the width of the pic shrinks, the brightness almost fades to nothing and B+ is now down to 230V, boost at 310V. Overall though, the raster is still too small, not at full height or width, even with both of the associated controls set at maximum, and this applies regardless of what frequency the horizontal osc is running at.

I've rebuilt all the K networks except the vertical integrator and checked all the tubes. Replacing the networks has made virtually no difference. Of course, I've also replaced all resistors and caps on the board and most of the resistors on the chassis as well. I've replaced all chassis caps except the ceramics.

Currently, I'm running the board on jumper leads, so I can test my networks properly.

Anything I should be looking for? There's not much left to replace! The resistances on the flyback all read ok as well.

Cheers,

Adam

Fairlane500skyliner · #2 07-31-2014, 08:55 AM

G'day Adam, didn't think I'd see you on here! How many of these danged Predictas do you own now?

Chris

init4fun · #3 08-01-2014, 06:20 PM

Quote:

Originally Posted by Down Under

Hi guys,

I'm currently working on a Preficta Holiday. Although I now have an image of sorts on the screen, it's far from perfect. The really strange thing is that the brightness and associated voltages around the chassis seem to be directly related to the horizontal oscillator frequency.

For example, if I turn both the horizontal hold and the horizontal range controls fully clockwise, I get a reasonably bright display. Not as bright as it could be, but ok. Of course, the sync is way off, as the oscillator is now running at around 40kHz. With the tv running like this, B+ is at 260V, and boost is way up at 620V.

As I adjust the controls back to where they should be, the width of the pic shrinks, the brightness almost fades to nothing and B+ is now down to 230V, boost at 310V. Overall though, the raster is still too small, not at full height or width, even with both of the associated controls set at maximum, and this applies regardless of what frequency the horizontal osc is running at.

I've rebuilt all the K networks except the vertical integrator and checked all the tubes. Replacing the networks has made virtually no difference. Of course, I've also replaced all resistors and caps on the board and most of the resistors on the chassis as well. I've replaced all chassis caps except the ceramics.

Currently, I'm running the board on jumper leads, so I can test my networks properly.

Anything I should be looking for? There's not much left to replace! The resistances on the flyback all read ok as well.

Cheers,

Adam

How is your picture tube ?

I ask this because raising the voltages to get "normal" brightness on the screen could indicate a CRT with weak emission . Course , it could be other things as well , but since I didn't notice mention of the CRT's condition I figured I'd ask .

bandersen · #4 08-01-2014, 07:07 PM

Does adjusting the width control have much effect ? I've noticed with my Holiday the B+ varies with the width control.

Down Under · #5 08-01-2014, 10:52 PM

Hi guys,

Thanks for the replies. The CRT tests quite good, with strong emission. I count myself very lucky considering it's the low voltage heater version.

The width does have an effect... it's currently set fully clockwise. As I turn it the other way, the picture loses width and brightness, but blooming also has an effect, starting to increase picture size, as the high voltage starts to drop away.

Cheers,

Adam

bandersen · #6 08-01-2014, 10:54 PM

Did you replace the germanium power rectifiers with silicon diodes ?

Don Lindsly · #7 08-01-2014, 11:44 PM

Based on your trouble description and 260 B+, the rectifier diodes are fine.

I would start by replacing the horizontal output and damper tubes, followed by testing or bridging the electrolytics. The one tying the AC input to the rectifier diodes is fine.

Check for shorted/open hor out screen resistor and B+ divider, audio output tube and stacking shunt resistor. Something should point in the right direction.

Align the hor osc circuit per instructions.

Down Under · #8 08-02-2014, 03:29 AM

Hi guys,

Yep, I've replaced the diodes as per one of your videos, Bob. I liked your trick of installing them on the existing framework to keep it neat.

I've subbed in new damper, horizontal, vertical out and video amp tubes, no change. All electros have been re stuffed, but I may have done something wrong in the mufti sections. Although they do seem to test OK.

I'm not sure about which resistor is the hor out screen resistor, but I've probably replaced it. There are a couple of wire wond resistors I've left alone, since they test ok. Also not sure which part of the circuit form the B+ divider or the stacking shunt resistor! but they do sound like viable options to check.

Cheers,

Adam

old_coot88 · #9 08-02-2014, 12:11 PM

What voltage(s) are you getting on G2 (the screen grid) of the 12DQ6 when you turn the width control stop-to-stop?

Down Under · #10 08-02-2014, 11:48 PM

Hi,

The voltage on the grid varies between -42.9V with the control fully counter CW and nothing on the CRT. When fully CW, the reading is -40.5V, and an image of sorts, albeit quite dim. Both voltages taken with the hor osc running at the correct frequency.

Cheers,

Adam

old_coot88 · #11 08-03-2014, 11:07 AM

Quote:

Originally Posted by Down Under

The voltage on the grid varies between -42.9V with the control fully counter CW.... When fully CW, the reading is -40.5V,.....

That's the control grid, G1. The negative voltage you're seeing there is good.
What are you getting on G2, the screen grid? That's pin 4.
It should be some positive value, and vary with the setting of the width control.

Quote:

...and nothing on the CRT.

Um.. the term "screen grid" doesn't refer to the CRT or CRT screen. It refers to the second grid in a tetrode or pentode tube.

Down Under · #12 08-03-2014, 07:58 PM

Sorry, I had my grids mixed up! I knew about the names though, just thought I'd mention that the CRT goes black (as B+ and boost decrease) when the width control is fully CCW. As to pin 4, the voltages vary between 190V with the width fully CW, and 151V with it fully CCW, again, no image on CRT. In the state with no image, B+ increases to 248V, compared to 236V with an image...I can 'turn the image on and off' with the width control for these tests.

These voltages on the HOT seem ok, certainly within range of the diagram, it's a weird fault. I'm not completely sure that my K networks are fully correct, but it would be strange to have my new ones and the original networks displaying the same fault, so I think it's something else.

Cheers,

Adam

Down Under · #13 08-06-2014, 11:00 PM

I think I'm getting there. And I suspect I have a partially shorted flyback

I have read that a lower frequency horz signal being fed into the flyback drives it harder, due to longer on times from the plate of the HOT. So that may explain why I get reasonable B+ and boost voltages when I run the horz frequency too high.

As I lower the frequency to the correct value, things go haywire as voltages start to drop. If I disconnect the yoke and the 15 ohm resistor that connects between pins 4 and 5 of the flyback, B+ jumps to 280V. I'd expect an increase because there's no load. But the damper and deflection systems seem OK with this resistor in circuit, they're just being underpowered. So I've either got a shorted flyback (my guess) or a poorly regulated power supply. Given the simplicity of this supply, and the fact that the diodes and all electros have been replaced, I doubt this is the culprit.

Thoughts?

Don Lindsly · #14 08-06-2014, 11:21 PM

Usually if the boost increases when the yoke is disconnected, it is a bad yoke. First remove the damper and note if there is any picture at all. If a distorted picture shows briefly, (it is a series set) then look for a shorted cap from boost to B+. If not, i suspect the yoke. If it is OEM code 274, it's definitely bad. Other brands need verifying before replacing.

Good luck.

Don Lindsly · #15 08-06-2014, 11:28 PM

If disconnecting the yoke increases boost, that usually indicates a bad yoke.

With the TV on, remove the damper and check for voltage on the damper cathode. If there is any, look for a bad cap from boost to B+. If none, I would suspect the yoke.

If the yoke OEM code is 274, it's bad. If not, verify before buying a replacement.