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  #16  
Old 12-09-2018, 12:19 PM
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Quote:
Originally Posted by AlanInSitges View Post
I mailed B&K about this thing asking if they had any documentation, and they told me they never made a PAL version.
That's funny. I hope you emailed them a picture of the impostor piece of equipment. Possibly there was another B&K division over near your country that made them.
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  #17  
Old 12-10-2018, 03:11 PM
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Nah, I didn't bother, the lady mailed me the same schematic for the 1077 that is floating around the web.

OK, another day, more head-scratching. I probably picked a tough place to start this hobby, but I'm in it now and hopefully I'll see it through to the end. I really don't understand the architecture of this thing. On a high level yes, but getting down into detail I'm confused. This goes to more general TV circuits and not just the 1077. Here's my understanding, which is clearly wrong, no need to be gentle:



The horizontal and vertical oscillators depend on boost voltage (vs B+) for their plates. The boost generally comes from discharging a capacitor (which I don't see in the 1077) off the damper tube, which conducts as the magnetic field in the flyback collapses after the horizontal output tube stops conducting at the end of each scan line. So...if the horizontal oscillator is not running, there is no pulse on the grid of the horizontal output tube, which does not conduct, which does not cause the magnetic field in the flyback to collapse, which does not cause the damper to charge the capacitor, which means there is no boost so how does the horizontal oscillator start with nothing on its plate? I can see the chicken but there seems to be no egg.

Someone please straighten me out!
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  #18  
Old 12-10-2018, 04:17 PM
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The set starts on the 225v B+ supply which has a DC path from the boost supply back through the flyback then through the damper to the 225 rail.... the 225 starts it then once the osc starts switching the output generating boost the boost adds to the 225 it started on.

Voltage checks against the schematic are a good starting point in major failures like this.
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  #19  
Old 12-12-2018, 05:02 PM
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Thank you for that. Now I get it.

I was able to track the problem to a shorted filter capacitor (10uf/450V) in the decoupled boost supply that took out a 4.7k resistor. I replaced those pieces and now could hear the horizontal oscillator running (the frequency sounds low to me, especially for PAL).

But I still had no high voltage, and no screen illumination, so I started poking around. I didn't measure the plate of the horizontal output tube since Sams always says DNM on those, but checked the cathode and had about 10V, so presumably there was current flowing through the tube.

I decided to look at the grid with a scope to see if I have any drive, and got a waveform that resembles those I see in tube TV schematics, sort of a cross between a sinewave and a sawtooth. I saw pretty much the same thing on the HORIZ GRID DRIVE jack, and the grid of the top half of the 6FQ7 horiz osc; the moment I touched the probe to the grid of the bottom half of this tube, the horizontal oscillator got much, much louder (still at a frequency that I don't think seems high enough but not really sure), and checking again I have about 4KV of high voltage - no idea what the number should be, there are no voltages on the schematic. And still no raster.



I have double-checked the PCB and don't think it was a mechanical change I made to the circuit when I touched that grid, and poking around further didn't cause any other change. It would really be helpful to have some voltage measurements. Tomorrow I'll get back to it, but for today I'm pretty excited about the progress. Thanks everyone for the help and suggestions.
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  #20  
Old 12-13-2018, 12:51 PM
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The schematic shows that C30 is 470 pf. It is part of the frequency determining components. It may be a different value for PAL. The stray capacitance of the probe changed the total on the grid and so probably changed the frequency.
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  #21  
Old 12-14-2018, 09:37 AM
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I should have been more clear. The oscillator changed when I touched the probe to the grid, but stayed that way once I removed the probe. It's like I started it, or something.
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  #22  
Old 12-14-2018, 10:42 AM
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My manual says approximately 12.5kv for the CRT. Theory of operation starts on page 104 of the manual.
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  #23  
Old 12-14-2018, 10:46 AM
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That's interesting. It could be that touching it brought the frequency more in line with what it should be. The flyback circuit performance is frequency dependent so if it is far enough off the generated voltages will be low. This includes the boost which is used to power up the horizontal oscillator. So maybe touching the grid caused the frequency change that increased the boost that got the oscillator running better. This may indicate the oscillator tube or other part that changes with voltage.
BTW, I've got a schematic for the 1076 and a voltage chart for the 1077B that both show the CRT anode voltage to be 11.5 KV.
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  #24  
Old 12-16-2018, 11:41 AM
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We. Have. High voltage! There were a few dodgy-looking solder joints around the horizontal oscillator tube socket; I repaired all of them, powered back on, and:



So, it's a looong way from working, but every bit of progress is encouraging.
  • There's about 300V of boost, and HV of about 9.5kV.
  • The H frequency is way off (way too low), and you can't see very clearly in the photo but the horizontal has a big bright line down the middle. The raster is only about 1cm high, and awfully jiggly.
  • Cranking on the CALIBRATE control varies the pitch and intensity of the horiz oscillator, with a tiny bit of change on screen but not much.

At this point it all seems solvable, and it's probably time to start replacing those capacitors and checking resistors. On a whim I checked C30 (it's 390pf in my model) and it's right on the money. I connected a frequency counter to the horiz drive socket and it says 12.925khz. If that is correct, it's not a small amount.
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  #25  
Old 12-16-2018, 12:09 PM
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That's great. I don't know what the PAL frequencies are, but you shouldn't have too much trouble finding out.
I suppose you know you should turn the intensity down so you don't damage the CRT phosphor down the middle of the screen.
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  #26  
Old 12-16-2018, 12:31 PM
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Yes, I'm pretty stoked! PAL horiz frequency is pretty close to NTSC 15,625Hz, and just found what I guess is the intensity control on the cathode of the CRT - I'll lower it, good suggestion.

According to the service manual:

Quote:
The horizontal oscillator provides a 15,750Hz sawtooth waveform from a multi-vibrator type circuit. An internal adjustment is provided for frequency adjustment. The output of this stage is available at the HORIZ GRID DRIVE jack for injection into television receivers. The horizontal oscillator is synchronized with a sub-multiple of the 189 KHz color bar generator.
I can't find anything resembling a frequency adjustment in the horizontal oscillator circuit - could it be something besides a potentiometer?

It seems to me that R52 and R53 are the most likely causes of the off frequency, along with C101. That's what I'll check next.
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  #27  
Old 12-16-2018, 05:02 PM
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Quote:
Originally Posted by AlanInSitges View Post
Yes, I'm pretty stoked! PAL horiz frequency is pretty close to NTSC 15,625Hz, and just found what I guess is the intensity control on the cathode of the CRT - I'll lower it, good suggestion.

According to the service manual:



I can't find anything resembling a frequency adjustment in the horizontal oscillator circuit - could it be something besides a potentiometer?

It seems to me that R52 and R53 are the most likely causes of the off frequency, along with C101. That's what I'll check next.
The horizontal freq is derived by a divide by counter(CD4013) driven by the color burst oscillator. There's a trimmer cap(C100) in series with the color crystal in the grid circuit of V6A that should allow some adjustment.

Before I'd worry much about the horz frequency I'd be looking for the cause of your vertical deflection collapse. Possibly a bad cap, connection, or tube in the vertical section. Possibly even the vertical height or linearity controls.

Last edited by Kevin Kuehn; 12-16-2018 at 05:07 PM.
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  #28  
Old 12-16-2018, 11:17 PM
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You'll probably have more correct boost and HV when you get the horizontal frequency on spec. I haven't seen the h osc circuit schematic of one of those in years, but the osc is probably adjusted with a slug tuned coil or possibly a trimmer cap...Dirty secret of most TVs with a hold pot: the pot is the fine frequency and often varies the effect of the AFC tube that pulls the frequency of the oscillator...The course hold is usually slug(s) in the H osc transformer (some sets especially Zenith and portables did away with the pot and made the transformer slug the user control).

On some sets, the boost also feeds the vertical osc/output, and powers other stages...If the vert stage is defective in such a way that it is loading down the boost rail it could be causing the H osc to not get enough boost to run on frequency...May want to prioritize vertical and troubleshooting of boost powered stages first.

BTW may want to pull the photomultiplier tube if you are operating with the lid removed and fluorescent shop lighting. The flying spot CRT produces ultraviolet light as does some non-incandescent lighting (especially fluorescent) and if you overload the photomultiplier with more ultraviolet light than the CRT makes you could damage it.
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  #29  
Old 12-17-2018, 12:58 PM
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One thing to keep in mind with this series of instruments is that unlike a TV set where the deflection circuits are intended to sync to an external signal, these instruments originate the sync. Of course the intent is to generate these signals to the relevant standard.
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