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Old 1st Aug 2016, 6:03 pm   #1
davidh1041
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Default Bush/Rediffusion CTV 25

I recently acquired this set from Neil, my first colour tv i owned was a Bush baged one of these. It has the later timebase unit with the quadrupler. Unfortunately as is usually my luck, the LOPT was knackered. With some help from Keith who has a Mullard application book, we decided that a Decca Bradford LOPT could be substituted. I removed the original LOPT Saturation choke and the Quadrupler and fitted the Decca LOPT in place, just had to drill one new hole. Then wired it in and fitted a tripler with the extra diode from a KT3. My plan is to use the diode output to control the PD500 later. After a bit of fetterling the set is now working, the EHT is currently a bit low at 20kv, but it's producing a reasonable picture. I have washed the timebase and decoder chassis to remove the 40 years of grime. I am now working through other faults, the two transistors in the PL802 output stage were both short causing flyback lines, the 2k7 anode load was O/C. One of the BF115's legs had rotted off, the poor thing only had two left so was a bit unstable. I have replaced all of the electrolytic's and high value resistors that i found were well out of tolerance. The next job is realignment of the decoder. When its all working properly, i will then concentrate on reconnecting the PD500 EHT regulator(Shunt stabilizer).
Thank you Neil for agreeing to sell me the set, and Keith for your help and moral support!
More to follow. David
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Old 1st Aug 2016, 7:09 pm   #2
line sync
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Default Re: Bush/Rediffusion CTV 25

Hi David
Well done on getting this set to work using a decca Bradford LOPT and sorting out the other faults.
I`ve got a liking for these sets and restored a 19" version on this forum 2 or 3 years ago.
Its interesting looking at the pictures of your set to see how different the chassis is laid out compared to the 19" model.
I`m looking forward to reading more about your CTV25.

Robin
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Old 1st Aug 2016, 9:48 pm   #3
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Default Re: Bush/Rediffusion CTV 25

The main set in my mate's house in the 70's was one of these. I always thought it was a bit odd because it was difficult to tell its age. It kind of looked modern and old-ish to my eyes at the time. All the same, I thought it had the look of a quality item.

Regards,
Paul
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Old 3rd Aug 2016, 4:39 pm   #4
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Default Re: Bush/Rediffusion CTV 25

Perhaps you can post a picture of the front? The only Rediffusion version of this set I've seen was a wired cable set with no tuner and a big red light on the front in a rather plain cabinet. Is yours dual standard or has it been modified?
The Bush CTV25 and Murphy CTV2510 were identical with a pale wood cabinet with doors. The slightly more ordinary 25" was the CTV167 in a darker cabinet without doors. The 19" was the CTV162 which looked quite pretty but was a feat of engineering to get the boards all in a small space! The decoder and CDA boards were mounted vertically.
The last appearance of this chassis in the Mk3 form was in the CTV174D which was a 22" using 'push through' tube presentation. It had a redesigned decoder using an SL901 IC on the print side and solid-state RGB tube drive. I don't think there are any of these still around.
Don't forget the leaded glass cylinder and full screening can if you're going to re-instate the PD500. To be honest I'd be tempted to leave it out if you have stable EHT as it'll draw a lot of current from the poor old Bradford LOPT, and was probably what led to the demise of all the CTV25 LOPTs. Besides, all other contemporary sets that used triplers seemed to manage very well without one.
Glyn
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Old 3rd Aug 2016, 6:00 pm   #5
dazzlevision
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Default Re: Bush/Rediffusion CTV 25

Quote:
I don't think there are any of these still around.
Hello Glyn,

Yes there are (well, at least one). I have a Murphy CV2210D, which is a fascinating "half-way house" design between the original Mk1 and 2 hybrid chassis and the solid-state A823A chassis.

Regards,

Dazzlevision
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Old 3rd Aug 2016, 8:21 pm   #6
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Default Re: Bush/Rediffusion CTV 25

Hi
That's nice to hear - along with the Philips G22K503 and the Decca CS2220 I agree these were interesting sets. I doubt many were sold.
However the Bush/Murphy one is unique in that it wasn't simply an evolution of the original design but, as you say, a half way house.
Does it work? I know Mikey 405 is after one of these so guard it with your life!
Glyn
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Old 4th Aug 2016, 7:30 am   #7
dazzlevision
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Default Re: Bush/Rediffusion CTV 25

Hello Glyn,

When I first acquired it in the 1980s, I overhauled it and stripped out the 405-625 system switching. It then produced a very good picture on UHF/625 with its original Mullard CRT. I did keep all the 405-625 switching components, so it can be reversed. It hasn't been switched on for some years now, but I will get around to fully restoring it soon.

Regards,

Dazzlevision
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Old 4th Aug 2016, 5:18 pm   #8
davidh1041
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Default Re: Bush/Rediffusion CTV 25

Hello Glyn, i will post a picture of the front next time i am in Norwich with the set. With reference to the PD500, the set seems stable enough with out it, but its the challenge you see, i like to play! Don't worry i will respect the Xrays. i did own a 22" version many years ago with the SL901 hanging off the back of the board. It was very reliable and produced a very good picture. I remember that the sound was good quality as well and a good looking set. Keith has one but unfortunately the timebase tower is missing. We are trying to source enough bits to build one up for it.
Looking forward to having some time to work on the set soon.
David
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Old 4th Aug 2016, 5:21 pm   #9
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Default Re: Bush/Rediffusion CTV 25

Hello Dave, this set still has the system switches in place, and the Decca LOPT has a 405 tapping on it. (Pin 6). I have not tried it on 405 yet, will concentrate on sorting out on 625 first.
Regards David
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Old 8th Aug 2016, 6:17 pm   #10
Vintage Engr
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Default Re: Bush/Rediffusion CTV 25

Hello David,

Great to see one of the CTV25's again, after so long.
I worked for Rediffusion (Redihire), which was the division that rented off-air sets rather than the wired types used in the wired-only regions.

The CTV25 was one of the first CTV's we rented out, and I was lucky enough to go on the early training courses. I still have just a few bits & pieces lurking somewhere around, but nothing major. I can't remember how many of these I've set up. Rediffusion had their own manufactured degaussing coil, green, wooden, & looked like an electric toilet seat. Customers used to look at me suspiciously when I turned up to install their new CTV, brandishing this green monster!
I seem to recall a fair number of PAL switch faults, as well as line timebase faults. There were some large Suflex capacitors in the LTB that used to fail.

Good luck with it, I will enviously follow your progress.

David.
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Old 9th Aug 2016, 9:37 am   #11
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Default Re: Bush/Rediffusion CTV 25

Quote:
Originally Posted by davidh1041 View Post
Hello Dave, this set still has the system switches in place, and the Decca LOPT has a 405 tapping on it. (Pin 6). I have not tried it on 405 yet, will concentrate on sorting out on 625 first.
The Decca Bradford LOPT? Most interesting!
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Old 15th Aug 2016, 6:07 pm   #12
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Default Re: Bush/Rediffusion CTV 25

Hello Focus Diode, yes, in the Mullard application book, pin 6 of the Decca LOPT is listed as 405 line output. In the Bradford it's used to drive the convergence circuits. Here is a picture of the set running on 405, still needs some work on the convergence, there are 2 coils missing from the board.
David
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Old 15th Aug 2016, 6:41 pm   #13
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Default Re: Bush/Rediffusion CTV 25

Have made progress on the decoder now, have washed the panel, then replaced all the electrolytic's and any suspect looking resisters. Then out with the scope and carefully set it up. Another BF115 croma amp 5vt1 was leaky and was replaced, setting the Ref Oscillator frequency to just running through with its trimmer is quite tricky, its very critical. Measuring the ACC voltage and tuning the burst coils for maximum negative voltage was a easy way of setting them.
I still have a problem with the bistable, its a bit lazy to lock. After the set has run for 2 minutes it slowly wakes up, it will then run all day no problem. I have replaced both transistors, and the 2200pf line pulse coupling cap. But still its stubborn. I have a healthy 100 volt line pulse and 4 volts peak to peak on the emitters. Just attaching the scope probe is enough to trigger the thing. Any ideas welcome!
Other than that i need to sort out the convergence and the pincushion circuit is not working. Unfortunately i have run out of time again and it will have to wait two more weeks.
Here are some pictures of it running in glorious technicolor
David
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