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Old 2nd Oct 2017, 8:59 pm   #1
Mazda Man
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Default Frame cramp Baird M676 help

Evening all.
Ok . Im struggling with this Baird. What should have been an easy problem is proving elusive.
Fault is very poor frame lin and lack of height and frame lock up one end of the control with frame bounce.
So far I have fitted a brand new Pcl805 replaced c163 c168 c172 c176 c169 c164 c175. Checked r131 r132 r155 r144 replaced interlace diode x11
Height . Lin and form control all operate and but are miles away from correcting the scan.
voltages as follows
p1 99v
p2 -21v
P3 0v
P6 245 v
P7 262 v
P8 23.5v
P9 0.9v
Heater 18.5v
I must be missing something. Any suggestions welcome as I really want to fix this set but getting a bit frustrated as I don't normally get stuck with frame faults.
Thanks
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Last edited by Mazda Man; 2nd Oct 2017 at 9:02 pm. Reason: Missed out something
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Old 2nd Oct 2017, 9:08 pm   #2
Laurdys
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Default Re: Frame cramp Baird M676 help

Have you checked the cathode bypass capacitor on the frame output valve?
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Old 2nd Oct 2017, 9:16 pm   #3
flyingtech55
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Default Re: Frame cramp Baird M676 help

That used to be caused by the cathode resistor in the output stage of the frame stage...in this case the PCL805. Renew the cathode resistor and the cathode bypass capacitor in the frame output stage.

If that doesn't fix it.......you've quoted the voltages you've got but what are they supposed to be according to the service manual?

HTH

TimR
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Old 2nd Oct 2017, 9:28 pm   #4
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Default Re: Frame cramp Baird M676 help

I haven't got a circuit of this set but I would expect a cathode resistor of about 330 ohms in the pentode of a PCL85. The cathode voltage you give suggests the cathode current of the pentode section of the PCL85 is over 70mA (far too high) or the cathode resistor has gone high in value. So, as above, check the cathode resistor (and the capacitor).
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Old 2nd Oct 2017, 10:10 pm   #5
flyingtech55
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Default Re: Frame cramp Baird M676 help

There's also probably some form of coupling capacitor to the control grid of the pentode section of the PCL805. If renewing the resistor and capacitor in the cathode cct. doesn't correct the problem look at renewing the control grid coupling capacitor. It doesn't take much leakage to upset things.

HTH

TimR
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Old 2nd Oct 2017, 10:11 pm   #6
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Default Re: Frame cramp Baird M676 help

Thanks Ukol and flyingtech 55. The cathode resister is made up of R131 100r and r132 330r. Both test ok and the 330 r had been replace as a precaution along with the cathode caps c164 100uf and c169 2.5uf
The voltages the manual quotes are as follows
P1 115v
P2 -45v
P3 to chassis
P4 p5 heater
P6 235v
P7 245 v
P8 not quoted In manual
The most abnormal reading I'm getting is on p2 at -21v not -45 as per the manual.
Bit of an odd one.
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Old 2nd Oct 2017, 10:19 pm   #7
Mazda Man
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Default Re: Frame cramp Baird M676 help

Here is the frame circuit
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Old 2nd Oct 2017, 10:29 pm   #8
flyingtech55
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Default Re: Frame cramp Baird M676 help

Try renewing C172. You don't need to fit the new one properly at this stage, just remove the existing one and tack the new one in its place and see what happens.

What voltages are you getting on pins 8 and 9?

TimR

PS. Sorry I see you've renewed C172.
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Last edited by flyingtech55; 2nd Oct 2017 at 10:35 pm.
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Old 2nd Oct 2017, 10:32 pm   #9
Mazda Man
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Default Re: Frame cramp Baird M676 help

C172 replaced allready in my first attempt at resolving the fault. No change I'm afraid Tim!
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Old 2nd Oct 2017, 10:44 pm   #10
Boater Sam
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Default Re: Frame cramp Baird M676 help

Daft question but are the replacements serviceable, its not unknown for new to be faulty.
Pin 2 is triode anode? If that voltage is wrong, it needs to be corrected. Is the valve definately OK? Grid cap?
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Old 2nd Oct 2017, 10:47 pm   #11
flyingtech55
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Default Re: Frame cramp Baird M676 help

What's the height control setting. I'm wondering about the 820K (can't make out the cct reference) in the boost cct.
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Old 3rd Oct 2017, 12:09 am   #12
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Default Re: Frame cramp Baird M676 help

The appearance of the scan itself is analogous to an oscilloscope recording of the output valve's plate current, which shows there is a failure to produce adequate current as the scan progresses. As noted this could be due to the valve itself, cathode resistor & bypass cap etc.

However, if it is true that all these parts are good, there other ways it can still happen. For example if the power supply to the output stage is starting to collapse as the scan progresses (bad power supply electros) it could do it.

The way to sort it out is to put a scope on the control grid of the output valve and look at the drive voltage wave shape, does it correspond to a decent trapezoidal or basically sawtooth wave? Is it a failure of the output tube and or power supply to produce adequate scan current, or is the drive voltage corrupt ? This is the first question that needs to be answered looking for the cause of the fault.

Other tests can include seeing if the tube's anode or cathode current profile matches what you see on the screen by putting a small resistor like 10R in series with both the cathode resistor and its bypass cap to ground, to look at the cathode current with the scope across that resistor.

Its always worth putting the scope directly in the screen grid to make sure the voltage is stable there and not collapsing toward the end of scan, so also scope the HT rail.
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Old 3rd Oct 2017, 4:07 am   #13
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Default Re: Frame cramp Baird M676 help

I could add to the post above that in this particular circuit, the triode acts as the discharge tube, conducting during vertical flyback and the sawtooth forming capacitor is C176 0.047uF that is charged by the 820k resistor from a high voltage source (that supplies the focus pot) this is so the charging section of the waveform is as linear as possible.

Unfortunately, in frame deflection circuits like this one, where the oscillations are generated by the feedback loop with the output pentode and the triode, it can sometimes be difficult to "untangle" where the fault is, because wherever it is in the circuit, it upsets all the wave forms in the feedback path, so all over the circuit. There is one way to get around this if you get really stuck:

If you disconnect the center of the height control (this will collapse the scan so make sure brightness is at 0) you can then patch in a normal sawtooth drive voltage with a 0.1uF coupling cap and a 1meg resistor to ground, from either a generator or another tv set's normal grid drive voltage ( with the two chassis connected) to the connection that you have disconnected that feeds the pentode control grid and check if at least the output pentode & yoke circuit is ok. Then you can scope the remaining circuit to find the fault. This is called breaking the feedback loop.

In the meantime check that the high voltage feeding the 820K charging resistor is normal and double check that the total resistance of the resistor chain with the height and form controls is correct.
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Old 3rd Oct 2017, 8:28 am   #14
Mazda Man
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Default Re: Frame cramp Baird M676 help

Hi Boater Sam. Pin 2 is the the triode section grid and the voltage is at -21v against a quote voltage of -40v . the anode is pin 1 which is at 99v against a quoted voltage of 115v. The 820k is good and has been replaced and the boost voltage is at around 800v which looks about right.
All caps are brand new or new old stock and tested at greater than 100 meg leakage at 250v before fitting.
Hi Argus25.
Thanks for some ideas to explore. I hadn't considered the ht rail folding during the sweep period. Something to look at. Also interesting idea to substitute a seperate ramp wave form from another source. I will also double check the resistance network in the vertical form and height network as they haven't all been checked. Will report back soon
Dave
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Old 3rd Oct 2017, 9:45 am   #15
Peter.N.
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Default Re: Frame cramp Baird M676 help

I remember having a similar problem on a Bush many years ago and it was due the the oscillator anode resistor having gone high thus reducing the drive. I only recall having it once though - that's why I remember it.

Peter
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Old 3rd Oct 2017, 11:01 am   #16
flyingtech55
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Default Re: Frame cramp Baird M676 help

Unfortunately it can also be shorted turns in the output transformer. Hope it's not that!

TimR
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Old 3rd Oct 2017, 11:47 am   #17
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Default Re: Frame cramp Baird M676 help

Hi Mazda Man, try C153 and C158 both are known to present the problem you describe, particularly C153 in the anode of the pentode, might also be worthwhile checking the VDR2.
Good luck.
Victor.
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Old 3rd Oct 2017, 12:19 pm   #18
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Default Re: Frame cramp Baird M676 help

Is there a thermistor in the scan coil assembly? I can't tell from your circuit snip.
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Old 3rd Oct 2017, 1:28 pm   #19
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Default Re: Frame cramp Baird M676 help

Is this any clearer.
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Old 3rd Oct 2017, 1:59 pm   #20
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Default Re: Frame cramp Baird M676 help

Thanks Bill.

Mazda Man, check the thermistor in the field scan coils is may be open circuit with all the scan current going though the 15 ohm resistor.
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