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Old 11th Apr 2019, 12:57 pm   #1
davidh1041
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Default Decca CTV 25 (Battersea Chassis)

Help!

I have been trying to improve the picture on this set, the video looks crushed, it looks like a very poor freeview picture with shiny plastic looking faces etc. It's the same if you back off the contrast and brightness. New PL802 fitted and votages correct around it. The CRT reads well on my BK.

I know these sets were never very good performers, but should be better than this. Please can anyone please give some more pointers, i am desperate to get this monster off my bench.
Thanks David
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Old 11th Apr 2019, 1:22 pm   #2
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Default Re: Decca CTV 25 (Battersea Chassis)

Hi,
this is only a bad educated guess !
But I wonder if the signal from the modulator you are using is a bit large for the set ? Have you seen if using an aerial attenuator makes any difference ?
Of course you may have tried this already.
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Old 12th Apr 2019, 9:35 am   #3
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Default Re: Decca CTV 25 (Battersea Chassis)

This isn't a chassis that I ever had much to do with but it looks like too much voltage on the A1s to me, try a different balance of the brightness control setting and the A1 volts. Sometimes it is flattering to try and get a bright picture without having the brightness control set too high by winding the A1s up, but this is the sort of picture can sometimes result. We noticed a similar thing with the BRC 9000 chassis when we were selling a lot of those, the answer was always to set the A1 control to minimum and make up the difference with the RGB background settings. Testing the voltage is difficult because of the high impedance of the feed, but it is worthwhile checking that it is approximately right.

If its not that, the cathode circuit of the PL802 is worth a look. Do these sets have a blanking transistor there? If so that's worth a check, also the more obvious resistors. If the PL802's screen grid has a decoupling capacitor that could also give effects like this (even if the voltages in the stage are roughly correct).
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Old 12th Apr 2019, 2:31 pm   #4
davidh1041
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Default Re: Decca CTV 25 (Battersea Chassis)

Thank you Peter and Tim for your replies. Peter the picture is poor on different signal sources and also no signs of sync crushing issues, it's just shiny plastic looking picture like a heavily compressed freeview channel. Tim, Have kept the A1 as low as possible, bridged all the electrolytic in the video channel and decouplers around the PL802. Yes there is a blanking transistor in the cathode circuit; I have confirmed that it is working, (shorting it out produces flyback lines as you would expect) all very strange. I have traced the video signal through from the IF strip to the anode of the PL802, a monochrome step pattern, and can't really see any signs of distortion. I think my next step is to temporarily hang a NOS 22" CRT in it and see if this improves things. Cheers David
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Old 12th Apr 2019, 3:22 pm   #5
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Default Re: Decca CTV 25 (Battersea Chassis)

Well if the response of the vision receiver is linear and the tube tester says that the tube is OK that doesn't leave a lot! Is the tube heater supply in the set correct? I'm guessing that the EHT must be about right as the picture looks the right size and is well focussed, however it is still a good idea to check any resistors in series with the focus supply just in case.

Back-to-backing two sets to swap the connections to the tubes over - never an easy operation but sometimes needs must. You could try instead running the tube heater from an external supply and raising the voltage a little - about +20% perhaps. If the picture then gets better the tube is definitely suspect.
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Old 13th Apr 2019, 12:54 am   #6
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Default Re: Decca CTV 25 (Battersea Chassis)

I have a set of panels for the singkle standard "tripler" version of this set that I have recapped, you are welcome to borrow any of them if you wish.

My only thought with this (and I am no expert here) is possibly some sort of "agc" fault? I also vaguely remember reading that the tuner in these sets was not very good with excessively strong signals, but again I may be barking up the wrong tree...
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Old 14th Apr 2019, 8:05 pm   #7
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Default Re: Decca CTV 25 (Battersea Chassis)

Hi.

There is a useful servicing article on the Decca CTV25 in TELEVISION magazine entitled "Renovating the Rentals" by Caleb Bradley. The relevant issues can be found here https://www.americanradiohistory.com...UK-1972-07.pdf and here https://www.americanradiohistory.com...UK-1972-08.pdf

My thoughts were also AGC related having had a similar problem on a GEC hybrid colour set. In the servicing article, mention is made regarding setting up the video level.

Regards,
Symon
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Old 14th Apr 2019, 8:18 pm   #8
its ur aerial
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Default Re: Decca CTV 25 (Battersea Chassis)

Back in the day, the quality of ANY PL802`s that was not a Mullard valve was questionable,fitting another brand resulted in poor HF response.
And as for the solid state replacement....who remembers them ? they were awful.

Ken, G6HZG
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Old 15th Apr 2019, 10:43 am   #9
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Default Re: Decca CTV 25 (Battersea Chassis)

I only ever saw PL802 from 1 or 2 Philips factories (not certain which, might or might not include Mullard), never knew anyone else also produced it.
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Old 15th Apr 2019, 10:50 am   #10
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Default Re: Decca CTV 25 (Battersea Chassis)

Quote:
Originally Posted by Maarten View Post
I only ever saw PL802 from 1 or 2 Philips factories (not certain which, might or might not include Mullard), never knew anyone else also produced it.
I also have a Tungsram and a Toran sample of the PL802.
Not at hand to check though, so they may have been manufactured in a Philips factory as well.

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Old 15th Apr 2019, 11:31 am   #11
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Default Re: Decca CTV 25 (Battersea Chassis)

The Decca CTV25 should always display an excellent colour picture, after all the circuits are based on a Mullard design.
Read about my travails with a CTV25 dual standard CTV: https://www.vintage-radio.net/forum/...ad.php?t=27126

And here: https://www.vintage-radio.net/forum/...ad.php?t=77419

https://www.vintage-radio.net/forum/...ad.php?t=10269

DFWB.
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Old 18th Apr 2019, 8:16 pm   #12
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Default Re: Decca CTV 25 (Battersea Chassis)

Hi

A close up pic of the set displaying a test card would be interesting to see. I wonder if there's some impairment in the video signal or in the luminance and CDA output stages, poor HF response?

I think a heater cathode leak in the CRT may cause a smeary display but that would affect only one colour unless all three guns have a hk leak but that seems unlikely.
Could there be a problem with the G2 (A1) supply? I was thinking about the smoothing of that supply, possible o/c 10nF 1kV capacitor.

Regards
Symon.
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Old 19th Apr 2019, 8:33 am   #13
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Default Re: Decca CTV 25 (Battersea Chassis)

Yes a very good colour picture. An absolute horror to service due to the tuner and control panel remaining in the huge cabinet when the chassis was removed, connected with short B9A/B7G plugs and sockets on short leads.

I had four customers with these, Overall reliability was very good other than the EHT overwind. That was another story and I took a few excursions to Ingate Place to obtain replacements. They stunk the house out when they failed! John.
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Old 19th Apr 2019, 12:14 pm   #14
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Default Re: Decca CTV 25 (Battersea Chassis)

HKS wrote: "Yes a very good colour picture. An absolute horror to service due to the tuner and control panel remaining in the huge cabinet when the chassis was removed, connected with short B9A/B7G plugs and sockets on short leads."

And didn't those B9A plugs often track between the pins as they did in the DR31 series mono TVs?

Battersea CTVs fitted with the EHT voltage tripler were more reliable.

DFWB.
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Old 19th Apr 2019, 12:29 pm   #15
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Default Re: Decca CTV 25 (Battersea Chassis)

Yes! Reminds me of the GEC BT302 I.F. connection B7G plug..How can something so small make so much noise when it fails? J.
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Old 19th Apr 2019, 2:24 pm   #16
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Default Re: Decca CTV 25 (Battersea Chassis)

Just a thought, what is the video response like on a 405 line picture, assuming the set works on 405. A bit of a long shot but I wonder if there could be a problem with the 625 video detector diode if it uses 2 separate diodes for each system. Not that familiar with this chassis, the Bradford was more my era.

Alan.
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Old 19th Apr 2019, 3:26 pm   #17
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Default Re: Decca CTV 25 (Battersea Chassis)

Circuit diagram of the IF amplifier and vision demodulator. Noteworthy is the separate detector for the 6mc/s intercarrier sound.

DFWB.
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Old 19th Apr 2019, 5:08 pm   #18
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Default Re: Decca CTV 25 (Battersea Chassis)

Looks like it uses the same video detector diode for both systems followed by a video phase splitter transistor (like the RBM A640 chassis), so may be a problem around here.

Alan
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Old 19th Apr 2019, 6:47 pm   #19
davidh1041
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Default Re: Decca CTV 25 (Battersea Chassis)

Managed to spend a little more time on this set today, i have been checking around the PL802 checking the value of the anode and screen resistors thinking it might have possibly had the wrong value fitted in a previous repair, but they were all okay. I did find that the track on the blue back ground control was burnt out, after replacing this 50k pot and resetting both back ground pots the picture has improved greatly. Also replaced the AGC pot as it was hot happy being set to the 2v P-P video across R203 as specified in the manual.

This set must be one of the last 25", it uses the tripler line output stage, no system switches ever fitted, but does have two aerial sockets. I agree about the difficulty servicing these things, with wires and plugs dropping off all over the place as soon as you tip the chassis up. However the layout meant that it does have plenty of air circulating, which is why they reliable back in the day.

I will take some better pictures and leave it running for a while time permitting, since retiring at Christmas i seem to be working harder and have less time for hobbies, but hopefully work load will settle down soon! Thank you all for your interest in the post and will report back soon.

David
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Old 19th Apr 2019, 10:04 pm   #20
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Default Re: Decca CTV 25 (Battersea Chassis)

Davidh1041 wrote: "This set must be one of the last 25", it uses the tripler line output stage, no system switches ever fitted, but does have two aerial sockets"
Then it must be a model CS2500 or CS2520 single standard receiver. These receivers were in terms of reliability a great improvement over the original dual standard models. The Philips multiband tuner as used in the dual standard models was retained some single standard models. Later sets use an IC as the audio amplifier.

DFWB.
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