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Forum 141

B&W TV Ultra 6641 Cub

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Jayceebee
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Mention of the Thorn 1590 chassis a few days ago in another thread prompted me to do something with a Thorn portable I picked up some while back, a 12" Ultra 6845 Cub fitted with the 980 chassis. This was I believe the last ever 405 line only set ever produced, there was also a 16" version with small changes known as the 981. This was a fairly simple all valve design but punched well above it's weight, the well known tuner with PCF805 and PC97 valves used with slight changes could been seen in many Thorn sets and is very sensitive. I believe these sets were given away free with 2000 chassis CTVs in the early days of colour to try to boost sales. 

Back to set in question, this was collected some years ago and the cabinet has had some major damage probably having been dropped sometime in it's life. The set came from @Alastair and he told me there was some damage to the PCB also. After getting it home and inspecting the internals the damage appeared to be confined to the line and frame hold pots which are accessible from the the rear and a broken fixed resistor. New Piher PT15 pots were fitted and with their extensions fitted they looked the part. First light revealed no signals, a lack of width and frame linearity faults. The set was put to one side until today. The CRT emission is very good so hopefully this set can be brought back to full working order but I'm not sure what can be done about the cabinet shell.

IMG 0092
IMG 0093
980 3
980

John.

 
Posted : 09/11/2023 10:00 pm
WayneD, crustytv, ntscuser and 3 people reacted
slidertogrid
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I suppose working on the principal that it won't look worse a repair with fibreglass car body repair filler may be an option? It is going to be difficult to make any repair invisible due to the textured surface but a patch flicked in with a suitable colour aerosol paint may be acceptable... I don't suppose you have the missing bits? 

I have a 405 Thorn portable in my collection, looks like it may be a similar chassis but my set is larger with the controls on the top either side of the carrying handle. Maybe it is the 981 version that you mentioned? 

 
Posted : 10/11/2023 11:07 pm
Jayceebee
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@slidertogrid Yes, sounds like the 981 which was a 16” with the controls on the top. The cabinet is going to be tricky, I had thought of fibreglass matting and a skim of filler, never used the matting before. There is another issue also, the seems to be some breakdown of the plastic due possibly to UV exposure. The smooth parts at the edges scratch very easily leaving a powdery residue, not the case with the inside. Not too bothered really as being at the back it’s not noticeable, just really need to do something to prevent fingers being poked inside.

The no signals issue was simple servicing of the tuner, cleaning the biscuits and stator contacts, readjusting the tension of the springs of the latter. I’m in the process of checking the usual suspect caps in the frame stage for the linearity issue, access is not easy as the frame stage is tucked under the CRT bowl. Another fault has shown up in that touching the PCB anywhere near the line osc stage provokes tearing or loss of drive. No print damage that I can see so more investigation required.

John.

 
Posted : 11/11/2023 12:30 pm
slidertogrid reacted
Jayceebee
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A roundup of the usual suspects in the frame stage proved fruitful and removing several of the Dubilier capacitors all showed some leak or a rise in value particularly C61 which gave a reading of over 0.7uF instead of it's nominal 0.3. The two chicklet 0.047uf types were removed for testing but fell apart, I suspect they were probably OK but are quite fragile. For those not familiar with this chassis and the 1400 series if you look at the cathode of the frame output pentode (actually a beam tetrode) you will see it's returned directly to chassis instead of the usual bias resistor and bypass electrolytic. In this case you will see the grid bias voltage is obtained from the heater chain, the heater are fed with only the negative half cycle of mains by W9. If it was to fail s/c then the heater will be overrun leading to short life of the valves and CRT. I'm not sure if the designers actually intended this as an indicator but should W9 fail then the correct bias will be lost and results in a rolling mess on the screen. Unfortunately Joe public was not deterred by this and continued to listen to the sound of his/her favourite soap leading to the inevitable consequences.

The lack of width was traced to those awful cream PCB type Egen carbon composition resistors in the width circuit being high in value, they are absolutely notorious for this. I've found the later green coloured metal film ones are much better unfortunately none fitted here. Replacing the 6/30L2 line oscillator and PY801 boost diode valvebases has stopped the intermittent loss of line but there is still a very occasional line ragging/twitch to sort. Another capacitor which I regard as another change on sight component are those orange TCC electrolytics, the one here is C36 1uF in the line oscillator stage, they can also be encountered in the 1400 and 3000/3500 chassis.

As I said earlier these are super little sets and this one is coming along nicely. The photo really doesn't do it justice, looks like the interlace is a little poor but I assure you that it is a camera effect.

980 frame
IMG 0098

John.

 
Posted : 11/11/2023 10:02 pm
Tazman1966, Cathovisor, LSmith and 1 people reacted
slidertogrid
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Looking good! 😎 

 
Posted : 11/11/2023 11:35 pm
irob2345
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Thorn made a very similar looking TV in Oz. well prior to the AWA joint venture.

Here are the circuits of the two versions.

Thorn P1
Thorn Atlas P2
 
Posted : 12/11/2023 8:19 am
Jayceebee reacted
Jayceebee
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Very interesting, unlike the R2M chassis (AWA 58 series) that was for the antipodes, apart from the tuner I don't see any Thorn brushstrokes there at all. Coupled with that excellent tuner and four VIF stages it must have had very good gain, an elaborate design including a sine wave line oscillator. Something for use in fringe areas, outback maybe?

John.

 
Posted : 12/11/2023 7:13 pm
irob2345
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No, it was a low-end market product. One of the cheapest TVs you could buy.

Aussies expected portables to work just as well with the rabbit ears as their "big" set with an outdoor antenna. I used to do warranty service on TVs in those days and the sales blurb "Inbuilt antenna gives perfect reception anywhere" made for a few arguments with disgruntled customers. Therein lies a tale! Which I'll relate if anyone's interested...

Thorn were based in Melbourne and, unlike most of Sydney where the proverbial piece of wet string would get you a picture, average signal levels were much lower there because the transmitters were well east of the suburban area on Mt. Dandenong. That might explain the necessary sensitivity.

 
Posted : 13/11/2023 8:43 am
Nuvistor and Jayceebee reacted
Nuvistor
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Posted by: @irob2345

I used to do warranty service on TVs in those days and the sales blurb "Inbuilt antenna gives perfect reception anywhere" made for a few arguments with disgruntled customers. Therein lies a tale! Which I'll relate if anyone's interested.

It appears “The Advertising industry” in Australia stretched things a little, like most places. So what’s the tale? 😀 

 

Frank

 
Posted : 13/11/2023 10:40 am
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