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Old 4th Aug 2017, 6:15 pm   #61
peter_sol
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Default Re: Pye VT2 EHT problems

Can you make the image roll up and down using the hold control?
I can't see the blanking bar which should be at the top and bottom of each frame so maybe it is still at the wrong speed. You could juggle the frame hold value to increase the range either way to see if it will lock. (put a pot in series first at one and then the other) Is the height too high?
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Old 4th Aug 2017, 8:13 pm   #62
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Default Re: Pye VT2 EHT problems

Hi Peter,
Yes I can make the picture roll when operating the frame hold. The frame will lock about halfway through its travel so would say this operation is normal. I'm thinking its possibly the frame oscillator area?

Height pot also working fine, advanced just enough to fill the shadow mask
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Old 4th Aug 2017, 8:20 pm   #63
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Default Re: Pye VT2 EHT problems

Could you post a picture displaying test card C, its a better diagnostic.
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Old 6th Aug 2017, 11:47 am   #64
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Default Re: Pye VT2 EHT problems

The attachment shows the sync separator and frame timebase circuit of the Pye V2 and VT2. A negative going sawtooth waveform is developed across C45(0.25mfd). Timebase frequency is determined by C45 and 390Kohm resistor R60. Bottom end of the frame hold control is returned to the cathode of the video amplifier valve. The 12mfd electrolytic capacitor C44 decouples the HT supply to the frame TB and sync separator and also the screen grid supply to the screen grid of the video amplifier valve V4. Low capacity in this capacitor will cause all sorts of funny sync faults.
It's possible that C45 is connected to the cathode of V14 to improve the linearity of the sawtooth developed across the capacitor, the parabolic waveform across the cathode bypass capacitor C48 is used for this purpose.
Interlace diode V8B will only conduct on the negative going integrated frame pulses. A leak in frame sync coupling capacitor will C43 cause weak frame hold.
A leak in coupling capacitor C46 can cause cramping at the top of the picture, the negative going waveform biases the output valve to near cut-off.

DFWB.
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Old 6th Aug 2017, 8:31 pm   #65
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Default Re: Pye VT2 EHT problems

Thankyou David that has certainly helped me understand the circuit a bit more. Today I have been playing around with component values, in particular R60 390k. This had risen slightly (up to around 420k) so corrected this and got a slightly more recognisable test card (1st two pics) it's still too fast so I dropped it down again to 300k and the image is better but still misplaced. Anything lower than this causes more disruption and anything higher and it won't lock. Is it worth experimenting with the value of C45 (0.25uf) and maybe R61 (82k)
All the caps are new polyester types and all values are correct to circuit.

Here's some pics
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Old 6th Aug 2017, 8:38 pm   #66
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Default Re: Pye VT2 EHT problems

Oh and this pretty little 9" GEC followed me home today! Needs the full treatment but I'd really like to get the Pye finished off before I start this one. It's full of rust and muck but complete. I dabbed the meter across the CRT heater pins and it has continuity, but that's all I know so far!!
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Old 6th Aug 2017, 8:38 pm   #67
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Default Re: Pye VT2 EHT problems

Looks like something else could be wrong as well as the line lock, i bow to David's knowledge on this one.
Nice GEC BT2147, i have one. The 6505A tube has bad habits of going open circuit on the cathode as well as the heater going open.
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Old 7th Aug 2017, 11:00 am   #68
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Default Re: Pye VT2 EHT problems

Looking at those pictures I get the impression that the frame timebase is running too fast, is there any evidence of flicker? That will prove that the FTB is running too slow, that is <20mS. Reducing the value of R60 (390Kohms) will increase the speed of the timebase. It's assumed that capacitors C46 (0.5mfd) and C49 (0.25mfd) have been replaced. Has the decoupling electrolytic capacitor C44 (12mfd) been checked or replaced?

DFWB.
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Old 7th Aug 2017, 12:15 pm   #69
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Default Re: Pye VT2 EHT problems

Hi David yes all the capacitors C46/49 and electrolytic C44 have all been replaced. I've substituted the EB91 and ECC81, the voltages are all very close to the circuit and the HT is all right.
I've experimented with the values of C45 and R61, I can make the test card bend and stretch but I can't correct the speed problem. I do agree that the frame timebase is running too fast, doesn't seem to be any flickering. I though I was reducing the speed by reducing the value of R60. So is it worth trying some higher values here then? I temporarily hung a 500k pot in place of R60 and adjusted it very slowly to see what effect it had but it was much the same as yesterday. With the pot reduced the picture stabilises but the bottom of test card c is cut off and the 'C' appears at the top of the screen. All of the picture information is there, its just wrong.
I do think it's something really silly causing this but I just can't locate it. It's so frustrating!

Matt
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Old 7th Aug 2017, 1:37 pm   #70
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Default Re: Pye VT2 EHT problems

Can you reduce the height and position the image so the top & bottom edges are visible on screen. There appears to be problems with the video which might be causing erratic line and frame triggering.
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Old 7th Aug 2017, 2:13 pm   #71
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Default Re: Pye VT2 EHT problems

Do you have an oscilloscope ?, it would make finding the issue much easier.
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Old 7th Aug 2017, 3:05 pm   #72
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Default Re: Pye VT2 EHT problems

I don't have any experience of oscilloscopes unfortunately Stephen, however I've just made a bit of progress. I've increased the value of R60 to 560k and this has corrected the test card. The frame hold is poor now however and the image is flickery. anything below 550k and I lose the picture. This is about the best compromise I can get. The Do you think I could correct this elsewhere or is it totally dependant on this resistor?
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Old 7th Aug 2017, 3:37 pm   #73
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Default Re: Pye VT2 EHT problems

I still say there is something else wrong, however my experience is less than David or John so best let them guide you; I have marked what I think is wrong but its unclear as to what is generating these, I don't believe they are flyback lines
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Old 7th Aug 2017, 4:09 pm   #74
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Default Re: Pye VT2 EHT problems

We have got so used to the Aurora converter we now assume the source is 100% reliable. How does the image look on other sets?
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Old 7th Aug 2017, 4:45 pm   #75
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Default Re: Pye VT2 EHT problems

The EB91 and ECC81 won't have any effect on the frame TB frequency but a faulty V14 (ECL80) will. You mentioned that the bottom of the test card is cut-off. We know that the sawtooth drive is a negative going waveform so if the cathode voltage of V14 is too high the valve will driven into cut-off at the end of the frame scan. The cathode bias resistor is 820ohms, this resistor could have gone high in value. Check or replace the cathode bypass capacitor.
It might be worth checking the value of R58 (22Kohms) the resistor connected to the "bottom end" of the frame hold control. The voltages developed across R57 and R58 should be roughly equal.

DFWB.
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Old 8th Aug 2017, 1:54 pm   #76
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Default Re: Pye VT2 EHT problems

Thanks again David and all who have offered advice. R64 820R had risen to 870R so I replaced this, R58 reads fine, all voltages are correct. I've tried 3 different ECL80's, been through all the capacitors and resistors in the frame circuit and everything is correct. I think the fault must lie elsewhere with this one.

To Summarise
Fault - no frame hold and picture information wrong. C45 390K controls the frame frequency. Reducing this value will lock the picture but the bottom half of test card is cut off and appears at the top of the screen. Picture is quite stable and doesn't want to roll.

Increasing the value of C45 (anything over 550K) will correct the picture information but it won't lock properly and becomes very flickery. There is what looks like flyback lines but they have breaks in them. These disappear when a lower value resistor is fitted.

The aurora is working ok, have tried another set to rule this out. I hate to admit it but I just don't have the knowledge or expertise to get this one finished sadly. If John W is reading would you be interested in casting your expert eyes over it?!
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Old 8th Aug 2017, 2:02 pm   #77
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Default Re: Pye VT2 EHT problems

What's the voltage on the bottom end of R58?

Lawrence.
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Old 8th Aug 2017, 2:16 pm   #78
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Default Re: Pye VT2 EHT problems

Hi Lawrence 92v on the Pot tag, end of R58 2v
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Old 8th Aug 2017, 2:50 pm   #79
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Default Re: Pye VT2 EHT problems

Hi Stephen,

The contrast is at minimum, advancing it does affect the hold and eventually causes the the picture to break up. The contrast does 'work' however
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Old 8th Aug 2017, 2:57 pm   #80
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Default Re: Pye VT2 EHT problems

According to the schematic the voltage on the wiper of the frame hold control should have a range of 68 volts to 150 volts from what I can make out.

Lawrence.
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