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Old 22nd Feb 2018, 4:43 pm   #21
FERNSEH
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Default Re: Marconiphone VT161 17" TV

Cover of the Electrical and Radio Trading magazine, 26th January 1960.

The VT161 was in still production in 1960. 62guineas = 63 2s 0d. = 63 10p.

Top quality set and good value for money.

DFWB.
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Old 22nd Feb 2018, 11:42 pm   #22
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Default Re: Marconiphone VT161 17" TV

Thanks for posting that picture: its always nice to see adverts from the time. Also the reference to the VT161 in the British relay thread was interesting.
There seems to have been a variety of similar Marconiphone models available in the 59/60 period. 17 & 21", 90 & 110 degree tubes and at least three different chassis- some with full pcb & the VT161 still hand wired.
All the best
Nick
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Old 23rd Feb 2018, 11:18 am   #23
FERNSEH
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Default Re: Marconiphone VT161 17" TV

Hi Nick,
The Marconi VT163 and VT164 are fitted with the BRC 500 series chassis, electrically similar to the Ferguson 516T and 508T respectively.
Info in the 1960/61 Radio and Television servicing book.

DFWB.
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Old 28th Feb 2018, 11:53 pm   #24
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Default Re: Marconiphone VT161 17" TV

During the last week, I've been gradually sorting the remaining issues such as getting the tuner to work on all channels, attempting to improve the line linearity and dismantling the concentric contrast/ hold controls so that both spindles don't turn together!

It now produces a very good, watchable picture. However, one niggly fault remains which I have not been able to get to the bottom of:- the top 1/2" of the scan is only composed of about 10 lines, widely spaced.
By careful adjustment of the linearity controls, the effect can be reduced but is still just visible on a picture and it annoys me!
Connecting a 'scope to the grid of the PCL82 output valve, clearly shows the problem. Immediately after the flyback, the waveform rises rapidly before the linear part of the scan starts, hence, the first 1/2" of scan happens much too fast so the lines are spaced too far apart.
The top linearity control affects this bit of the waveform.

The frame oscillator is a multivibrator comprising 1/2 of an ECC82 and 1/2 of a PCL82. What is most significant is that the anode of the ECC82 is 75V which is correct, but the anode of the PCL82 (triode section) is only 20V! It should be 55V. It's fed from the height control so is obviously variable. If the height is increased to 55V, the waveform on the 'scope looses that initial peak and the scan looks linear (on the scope).
The picture is then extremely tall!! So I suspect some form of feedback issue but I have not been able to find the cause. All the caps have been replaced in the frame stage, the actual circuit has been traced out on the chassis and agrees with the book. All resistors are in spec and apart from the PCL triode anode, all other voltages are OK. A different PCL82 has been tried.
So I'm rather stuck for ideas
I'll try and post some pictures of the problem a bit later on tonight.
All the best
Nick
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Old 1st Mar 2018, 12:37 am   #25
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Default Re: Marconiphone VT161 17" TV

Here are some pictures showing the fault. Also the scope trace from the grid of the PCL82 output valve (pentode section) and the circuit of the frame timebase.
The height control was at minimum for these pictures and the picture still nearly fills the screen!
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Old 1st Mar 2018, 10:04 am   #26
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Default Re: Marconiphone VT161 17" TV

Just a couple of thoughts. Make sure the CRT coating is earthed to chassis. I checked for balancing resistors across the two frame scan coils typically 2k. The circuit does not show them and it would be interesting to see what happens if you introduce them.

Check the components in the feedback circuit for correct value.[C94 C95] then suspect the frame output transformer or possibly the scan coils. Try connecting a green spot VDR similar to the type used in the later 850 chassis from point D to Point C on the transformer. John
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Old 1st Mar 2018, 10:48 am   #27
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Default Re: Marconiphone VT161 17" TV

Hi Nick,
have you scoped the waveform present on the anode of the triode section of the PCL82? Notice that the bottom end of the ramp forming capacitor C91 is returned to the cathode of the pentode section. A parabolic waveform is developed across the cathode bypass capacitor C92 100uF and this is added to the sawtooth waveform developed across C91. The coupling capacitor C93 has an unusually low value, 0.02uF.
That strange pulse at the beginning of the forward scanning stroke could introduce ringing in the scanning coils. The frame blanking period is 1.4milliseconds so in fact that pulse will introduce the disturbance outside of the picture part of the frame scan.

DFWB.

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Old 2nd Mar 2018, 12:02 am   #28
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Default Re: Marconiphone VT161 17" TV

Hi David & John,
Thanks for your suggestions and information on circuit operation. I did look at the waveform on the PCL82 triode anode but was not entirely sure what I was expecting to see! As the height control is increased to allow the PCL to work with 55V, the sharp bit at the start of the forward scan disappears. With correct picture height and only 20V, the rest of the scan is remarkably linear!
Could a faulty output TX or coils cause this excess height? Assuming the component values printed in the R&TV book are correct, mine correspond to them.

Presumably, resistors across the scan coils might damp out that initial ring- if that is what it is?

Out of interest, some circuits use a two valve multivibrator, and some can manage with only a single triode (my Pilot where they have done everything to reduce the valve count). Are there advantages with the two valve arrangement?
The set has now been run for many hours over the last few weeks and I've actually watched it for an evening's entertainment. That's when I decided that I needed to fix the frame fault- even though it is pretty minor!
The repaired LOPT is working extremely well and runs nice and cool even after several hours of use so that was a worthwhile exercise.
I won't have a chance to look at it again until the weekend.
Many thanks
Nick
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Old 5th Mar 2018, 12:40 am   #29
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Default Re: Marconiphone VT161 17" TV

Quote:
Originally Posted by FERNSEH View Post
have you scoped the waveform present on the anode of the triode section of the PCL82?
DFWB.
The waveform on the PCL triode anode is a perfectly linear sawtooth with no trace of the sharp pulse present.
I re- checked the circuit and component values to make sure I had not made any errors.
I'm still not sure why correct height is obtained with only 20V on the PCL triode anode?
I will try John's suggestions of resistors across the coils to see what effect that has.
Thanks for your help
Nick
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