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Old 26th Jul 2019, 2:45 pm   #1
regentone001
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Default KB WV20 Warwick

Hi
I picked up this tv, at least my mate did for me, at a local auction for the princely sum of 15.When it was home I gave it a cursory look over and brushed out some of the dust. It had been played with but didn't look too bad so took a chance and plugged it in. No bangs or flashes but after a short time a blank raster and loud buzz, adjustable with the vol control on both systems, a very faint click was heard on turning the 405 line tuner.
The next day I took a better look and found that a number of switches on the system switch had wires soldered over them including the I.F from the tuner which meant that the I.F on 405 was shorted to chassis and that the sound for both systems were linked together. The I.F can from one set of coils is missing, this is the can with the video detector diode in. I removed the wire links and replaced several RS replacement caps and tried again.
On 405 full scan and with the aurora connected I got sound and very very faint signal on the tube. On 625 I got the loud vol adjustable buzz and no visible signal.
Anybody ideas where to go from here? Could it be the GD12 detector diode? I would ideally like to get 405 working correctly before trying anything with UHF. Is it possible anyone has a suitable I.F can for the one that is missing. I will have to put a request in too for a 405 channel change knob as that is missing.
Thanks for any input
Steve
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Old 26th Jul 2019, 3:16 pm   #2
dazzlevision
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Default Re: KB WV20 Warwick

STC “GD” series Germanium diodes from this era are very suspect. Do a forward and reverse resistance test, with any parallel circuit path disconnected.
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Old 26th Jul 2019, 8:13 pm   #3
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Default Re: KB WV20 Warwick

the diode appears to be ok, I did try disconnecting it and subing another just in case but still the same results. Another thing I found was that the EF183 and EF184 had been swapped over so neither was in the correct position but changing them back still made no difference. I have noticed that changing the channel causes a nice flash on the screen which appears to mean the signal is getting through but still no picture. Could the I.Fs have been got at? It wouldn't surprise me at all with what I have found on this set.
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Old 26th Jul 2019, 8:54 pm   #4
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Default Re: KB WV20 Warwick

Another possible cause of virtually no signals on UHF or VHF might be the screen feed (g2) resistor of the mixer valve in the VHF tuner - usually around 22kOhm, half watt carbon composition type. Worth checking, I’d say. The mixer is also employed as an additional IF amp stage on 625 operation, so it would affect 405 & 625.

I’d also check the screen feed resistors of the EF183 and EF184 stages.
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Old 28th Jul 2019, 11:38 am   #5
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Default Re: KB WV20 Warwick

Hi dazzlevision I have found the reason for no picture on this chassis. The 2nd vision I.F coil has been wrecked, the one with the GD12 diode in. It Looks as though the core had been screwed in until in jammed and then it broke the former off at the base and broke the wires to the coils. It was had to see when the coil was on the chassis but now I have taken it off it is easy to see. I found out because I was only getting 33 volts on the anode of the EF183 instead of an expected 215 volts as stated in the makers manual.
I have put in a request in the forlorn hope that some one has a scrap chassis with a good coil
Steve
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Old 28th Jul 2019, 12:00 pm   #6
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Default Re: KB WV20 Warwick

I have replied to your "wanted" post.
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Old 28th Jul 2019, 4:21 pm   #7
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Default Re: KB WV20 Warwick

Hi replied to your post on my wanted ad
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Old 4th Aug 2019, 3:23 pm   #8
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Default Re: KB WV20 Warwick

While waiting for the replacement I>F coil to arrive from dazzlevision I decided I had nothing to lose by having a look at the broken original. Apart from the broken former, snapped off at the base, there were 3 snapped wires 2 from L41 and one from L42. I managed to find the ends of the 2 from L41 but the one from L41 was buried under some sort of very hard gloop that had been put on to hold the coil in place, L42 consists of 6 turns of fine wire. I put a ferrite core into the base of the former to hold it together and then re-soldered the 2 wires from L41 shorting out L42. The original core I screwed up to the very top of the former. I then soldered the whole thing back into circuit. and switched on. On 405 I got a very poor picture and good sound. After adjusting the original core the picture became very good. I switched to 625 and was rewarded with good picture and sound. This proves that the set is a good un and when I have the coil from dazzle it should work extremely well.
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Old 4th Aug 2019, 4:43 pm   #9
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Default Re: KB WV20 Warwick

When my IFT arrives, check the detector diode is OK. These STC white bodied “GD” types do often fail.
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Old 5th Aug 2019, 12:51 pm   #10
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Default Re: KB WV20 Warwick

Any chance of a picture of your set, Steve?

Steve
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Old 9th Aug 2019, 10:14 am   #11
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Default Re: KB WV20 Warwick

Well the I.F transformer from dazzlevision duly arrived,many thanks Dave, and I fitted it with a bit of jickery pockery as the connections on it are different to the original and it works. As can be seen from the photos though it has what I can only refer to as streaking as can be clearly seen especially in the photo of the forces network image. I should know what causes this but for the life of me can't remember can anyone help please?

Steve
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Old 9th Aug 2019, 10:18 am   #12
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Default Re: KB WV20 Warwick

Hi Steve
please see attached photo.
Ideally I need to get an original channel change knob as the one I have on it is from a Regentone Ten-17

Steve
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Old 10th Aug 2019, 3:13 pm   #13
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Default Re: KB WV20 Warwick

Very nice too, Steve. I think that was the model Which magazine featured in its group test of BBC2 sets in July 1964. Maybe someone has the article they can scan see we can see how well it did...

Steve
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Old 10th Aug 2019, 5:00 pm   #14
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Default Re: KB WV20 Warwick

Quote:
Originally Posted by regentone001 View Post
Well the I.F transformer from dazzlevision duly arrived,many thanks Dave, and I fitted it with a bit of jickery pockery as the connections on it are different to the original and it works. As can be seen from the photos though it has what I can only refer to as streaking as can be clearly seen especially in the photo of the forces network image. I should know what causes this but for the life of me can't remember can anyone help please?
Steve
Hello Steve,

Of course, it could be that the IFT you fitted will require realignment, as it is in a different chassis to that in which it was aligned at the factory.

However, I would first check for problems in the video amplifier, as the symptoms appear to be of a poor low frequency response. There's usually a decoupling electrolytic in the cathode and possibly in the screen grid as well (I don't have the circuit to hand as I type this). Are any carbon resistors well out of tolerance? Have you replaced the paper dielectric capacitors in this area (e.g. yellow TCC types, Hunts black or brown Moldseals, etc.), as they will probably be very leaky?

Are the symptoms the same on 405 and 625 operation?

Did you check the vision detector diode in the IFT I sent you (as I advised)?

Dave
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Old 10th Aug 2019, 6:57 pm   #15
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Default Re: KB WV20 Warwick

Hi Dave
yes it could be that it needs realigning so I will try that and I have been looking accusingly at a 50uf cap c62 which can be seen in the clip of the circuit on post 8.
The symptoms are the same on both 405 and 625 and yes the diode checked out ok.

Steve
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Old 19th Aug 2019, 8:11 am   #16
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Default Re: KB WV20 Warwick

I have tried replacing the electrolytic cap d62 and c59 as well as a couple of out of spec resistors. C59 is named as a possible cause of poor hf response in the makers manual. But the picture is still much the same. I have noticed if I off tune the picture the streaking does disappear so I am wondering if it could be an agc problem. I have to replace the frame output valve holder as it is so loose that even walking across the floor can cause the picture to collapse.
Any Hunts and TCC caps have been replaced the problem I have is that some work was done on the set before I got it so its possibly something that was done before and its a matter of tracing it.
Steve
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Old 19th Aug 2019, 10:08 am   #17
dazzlevision
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Default Re: KB WV20 Warwick

Have you replaced the (I think 0.22uF @ 250Vdc working) capacitor that couples the video from the PCL84 pentode section's anode to the CRT cathode? If it has been left out, it could well cause poor LF response on both line standards.

In production, they usually fitted either a TCC (Supamold or Duomold) yellow plastic cased or Hunts red dipped, resin cased type, with axial leads.
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