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B&W TV Marconiphone VT161

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Nuvistor
(@nuvistor)
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@pye625 
Probably find C68 is as leaky as a sieve, did you try a resistance reading? Those capacitor meters seem to read high with a leaky capacitor.

Presume they use a known AC frequency and use the reactance measured to determine the capacity. A DC leak through the capacitor will lower its impedance and fool the meter  to read high. I could be talking rubbish, anyone know how capacitor meters work?

 

 

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Posted : 30/06/2021 10:58 pm
PYE625
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Posted by: @nuvistor

@pye625 
Probably find C68 is as leaky as a sieve, did you try a resistance reading?

Just have and it is around 500k on a Fluke DVM. I won't even bother with a voltage test. It will be in the bin all in good time.

Judging by the fact that several have already been replaced a long time ago, any remaining ones will likely be useless too. Not to mention a couple of the red and black Plessey electrolytics on the signal's chassis that look baked to a crisp. There is one directly above the mains dropper resistor and what was left of the label fell away like a burnt leaf.

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Topic starter Posted : 01/07/2021 5:14 pm
PYE625
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Last night I re-installed the CRT after the cleaning session. I replaced the boost HT capacitor along with the line drive capacitor to the grid of the PL81. I also reformed the main HT electrolytic can and thought why not try a first power up as there appeared to be no real reason not to.

Well for a crt that measures low emission, the raster is not bad. In fact, it can be turned up pretty bright.

IMG 5930 50
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A raster is all that can be achieved though as there is no signal getting through whatsoever. No switch noise upon channel change either. She would appear to be stone deaf. Oh, the sound output is working, but no detected audio of course.

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Topic starter Posted : 03/07/2021 8:27 am
Lloyd and Nuvistor liked
PYE625
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After some fault-finding fun, I have eventually traced it down to no IF coming from the tuner. The reason for this is that there can be no HT getting to V2B anode because there is an open circuit between L8 and pin 6. Typically, pin 6 can only just be seen under a rats nest of other components.

Here below is the circuit, L8 next and finally pin 6 where there is an open circuit. Keyhole surgery anyone?

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InkedIMG 5952 50 LI
InkedIMG 5953 50 LI
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Topic starter Posted : 03/07/2021 10:16 am
Cathovisor
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Posted by: @pye625

Well for a crt that measures low emission, the raster is not bad. In fact, it can be turned up pretty bright.

But isn't this the old story of "mono tubes always read low on a BK 467"?? If that upper scale is the beam current in milliamps, 200µA was a pretty respectable figure for an old mono tube - most contemporary testers used 250µA movements then and 80% of FSD would be a "good" tube.

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Posted : 03/07/2021 10:52 am
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PYE625
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After I repair the broken (at pin 6) and corroded wire from L8 to pin 6 of the PCF80 in the tuner, hopefully we can get a better idea of the CRT capability. Until then, it will be fun trying to get a soldering iron down to the pin connection on the valve base.

Here you can see the effect of corrosion that caused a break in the wire. Green spot is the culprit, and I think a reaction has occured with the PVC insulation that covered the small portion of wire at the valve base. Heat from the valve had made the insulation brown and brittle and it simply disintegrated upon removal.

IMG 5954 50
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Topic starter Posted : 03/07/2021 11:44 am
PYE625
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Having mastered the art of keyhole surgery, with fine tweezers and a needle, I have managed to thread and solder a new piece of fine wire between pin 6 PCF80 anode and L8. Keeping the same length and position of where it was to start with too. I used new sleeving to insulate the wire.

The result is that the tuner is operating and there are very good results on screen, after some adjusting of the various preset controls. There is nothing wrong with the CRT emission either. I thought it was going to be poor. Obviously not.

I am rather glad that I did not immediately reach for the "rejuvenate" button upon initial testing on the B&K !!

IMG 5959 50
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Topic starter Posted : 03/07/2021 1:09 pm
PYE625
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The frame linearity is very good and I think this is due to the RS capacitors that long ago replaced many of the Hunt's type in the frame stage. There are still several Hunt's dotted about and really ought to be replaced, especially where any appreciable voltage across them is concerned. This includes the audio stage too. Thus far though, the results are rather pleasing.

So, if your Fireball has gone out, re-ignite by banishing the dreaded green-spot !

I wonder if that broken wire was the reason for the set to be finally retired after it's long service life.

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Topic starter Posted : 03/07/2021 1:26 pm
PYE625
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Earlier today the replacement mains dropper I bought from ebay arrived. I have fitted it into the set and will now have a go at rewinding the old one. I shall hopefully then have a good spare. The two uppermost sections, 97 and 100 ohms are for the heater circuit. It was these sections that were open circuit, not the 40 and 25 ohm sections I thought were open to begin with.

Here below is the chassis all cleaned up and I have replaced several dodgy Hunt's capacitors on the signals chassis. Next the newly installed dropper and then the old one scraped off and ready to receive new resistance wire.

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Topic starter Posted : 03/07/2021 8:04 pm
PYE625
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Although some of the timbase valves had been replaced, it would seem like all of the signal chassis one's are original. As the sound was rather lacking in "oomph", I tried the PCL82 in the frame ouput position. The valve is pretty well worn out as only a raster about an inch high could be produced. Replacing the valve in the audio position then brought about a much improved sound output power.

The video output, a PCL84, was replaced and then it became apparent that the contrast range was improved. Previously, you would reach a point where the peak whites would flatten out, rather similar to the effect of the spot-limiter control being used.

Back to the timebase, and here I decided to replace the rather sad looking PY81 as the base of the valve was full of white flaked-off heater material. This made no difference to the picture, but I feel happier knowing that a flashover in this valve is now much less likely.

Incidentally, after an hour or so operation, the LOPT is still running cool. Quite unlike say an RBM lopt that would be pretty warm. It looks to me that Thorn knew how to make a reliable and well designed transformer.

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Topic starter Posted : 04/07/2021 10:34 am
PYE625
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Posted by: @cathovisor

But isn't this the old story of "mono tubes always read low on a BK 467"?

It most certainly would appear so.

After a few hours worth of use, the measured emission has increased. But in reality, the quality of the picture is clear to see with plenty of contrast and good focus.

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Topic starter Posted : 04/07/2021 11:07 am
PYE625
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This evening I had a go at rewinding the dropper. After the RBM one I did last month, this is much easier. Probably because I had previous practise, plus this one has two sections each of 100 ohms. (Well, one is 97 to be precise). It is good because I can take a break half way through for a cup of tea. 😋 

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Oh dear !  It's half an ohm out. Whatever shall I do? 😀 

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Topic starter Posted : 04/07/2021 7:10 pm
PYE625
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With the other 100 ohm section wound and tested, the first application of heat resistant paint is applied.

IMG 5973 50
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Topic starter Posted : 04/07/2021 7:47 pm
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colourstar
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That's a really zingy-looking picture Andrew and interesting to read about the significant improvements from a fresh PCL84. Just goes to show what these models are capable of when working properly. The crts in sets of this era seem to last exceptionally well and it's rare to find a duff one in my experience. Well done on the detective work and subsequent keyhole surgery on that broken wire.  I restored a VT161 some years ago and was very impressed with the results. It was a real joy to watch vintage programmes on!

Steve

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Posted : 04/07/2021 9:26 pm
PYE625
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Thanks Steve, yes it really is a very good performer and I was rather surprised at the results to be honest. The video stage is DC coupled so no varying black level either.

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Topic starter Posted : 04/07/2021 9:55 pm
PYE625
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It could be that the fault of an open circuit corroded lead to pin 6 in the Fireball tuner is not unheard of.

Here, Heatercathodeshort mentioned a while back the exact same problem over on UKVRRR....

https://www.vintage-radio.net/forum/showthread.php?t=16554&page=2

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Topic starter Posted : 04/07/2021 10:15 pm
Lloyd
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Sounds a bit like the plastic sleeving breaking down and some sort of acid coming out of it, eating the wire. I’m sure I‘ve seen it happening on other wires too before now.

 Regards 

Lloyd 

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Posted : 05/07/2021 1:29 pm
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PYE625
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The old mains dropper is all finished now, just going to leave it another 24 hours for the paint to fully harden and then pass some current through it to simulate a load.

When I first tested the set I had fitted a new PL81. This was temporarily replaced by the old one, a Marconiphone branded type, and it worked ok for a while but with slightly reduced width. I then begun to have trouble with drifting line lock until it eventually stopped oscillating altogether causing line collapse, whilst adjusting the hold control. Back in went the new Mullard and no more trouble. I then decided to test a couple of used PL81's and similar issues occured with varying degrees of severity. It would seem to me that the PL81 has to be in tip-top condition for the line stage to operate properly.

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Topic starter Posted : 05/07/2021 10:29 pm
Katie Bush
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Posted by: @pye625

It would seem to me that the PL81 has to be in tip-top condition for the line stage to operate properly.

That reminds me of my Murphy V350. It has a self oscillating line output stage, and as it would seem, failure to oscillate was a common issue in these sets, so much so that there was a modification which incorporated a PCF80 and a handful of discreet components to construct a separate line oscillator to drive the output stage.

I do believe there was a "special" version of the line output valve, with an "MR" suffix to identify it as being specifically made for self oscillating Murphys. So, 30P4MR in lieu of the more common 30P4, though equally it seems, a brand new 30P4 would self oscillate - But for how long would it last is anybody's guess. I'd say your surmise is pretty much going to be spot on.

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Posted : 06/07/2021 3:28 pm
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Cathovisor
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Posted by: @katie-bush

That reminds me of my Murphy V350. It has a self oscillating line output stage, and as it would seem, failure to oscillate was a common issue in these sets, so much so that there was a modification which incorporated a PCF80 and a handful of discreet components to construct a separate line oscillator to drive the output stage.

ITYM 'discrete', unless they were well hidden...! 😉 

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Posted : 06/07/2021 4:39 pm
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