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Old 1st Oct 2020, 8:35 am   #61
Heatercathodeshort
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Default Re: My first foray into Old model TV repairs.

Looks good. I suspect the broken iron dust tuning cores especially as the picture is grainy. If they are damaged I can offer a solution. The aerial connector is a battery type AD4/35 type. John.
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Old 1st Oct 2020, 8:51 am   #62
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Default Re: My first foray into Old model TV repairs.

OK John thanks for the information, presently using bits of twisted wire into the sockets.

I will need to get a trim tool for the cores, I used to have one years ago as they would have been in some very old Pye radios.

One question, when I switch the set off it get a bright dot in the middle of the screen. Now I know that my original viewing age with 405 TV's would have been early 60's onwards but I can not remember that, do you get this on your 991T?

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Old 1st Oct 2020, 10:43 am   #63
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Default Re: My first foray into Old model TV repairs.

The switch off spot is quite normal, nothing to worry about.
The tuning cores break away from the threaded brass rods connected to the tuning plate that carries the tuning control. All visually looks OK but the cores are usually stuck in the coils and will require pushing out through the bottom of the chassis with a thin tool once you have eased out the spring metal clips that have the brass adjusters threaded through.

I have built some new tuning cores with some radio types but the threaded rods are a bit short. I have cut one of the brass threads off one core and super glued it to another threaded core. They are spot on. I can let you have some spares if you need them FOC.

A tuning tool can be made from a cheap screwdriver and a hacksaw.

The 991T is very high gain and when working correctly it should be possible to obtain complete white out overload with the gain and contrast controls advanced.

The pictures show: Attenuator in aerial input 12DB. Attenuator 30db and aerial direct with gain and contrast advanced. Complete overload with direct aerial connection with controls advanced. Remade tuning cores and modified cheap screwdriver for tuning and finally the switch off spot.

All the caps in the frame stage will be shot. The linearity should be 100% with plenty of free adjustment.

Incidentally I had to realign my I.F. strip to the makers data. It had been aligned for maximum gain. It must have struggled with a very weak signal.

John.
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Old 1st Oct 2020, 3:29 pm   #64
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Default Re: My first foray into Old model TV repairs.

Hi John,
that chassis needs a cabinet and I'm gonna make you one, sent me a picture of the table version.

DFWB.
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Old 1st Oct 2020, 4:43 pm   #65
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Default Re: My first foray into Old model TV repairs.

It's a table model David with the usual Ferguson 99/204 shape. Thanks very much for the offer. I was going to make a very plain cabinet and call it 'The Goggle Box'. I'll sort out some dimensions. John.
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Old 1st Oct 2020, 5:35 pm   #66
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Default Re: My first foray into Old model TV repairs.

I forgot to add. My example burst into oscillation after a few minutes use, blanking out the picture and sound. The I.F. decoupling capacitors used by Thorn are high quality .002uf ceramic types, that is all but one. The screen grid decoupler of the EF80 R.F. amplifier was the familiar light brown crunchy type and yes it was O/C. Replacement cleared the instability and gave a 100% improvement in gain. J.
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Old 1st Oct 2020, 5:56 pm   #67
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Default Re: My first foray into Old model TV repairs.

I replaced the 4 electrolitics today and have more range on the linearity controls, I did spend time emptying out the cardboard encased capacitors, was about to figure a way of getting new ones in side, then had a closer look under the chassis.

I thought about the 2 and 3 inch pieces of wire from the caps to where they are connected, and decided it was better to do it another way, so now new caps are just soldered to the valve pins directly and down to a nearby earth point all with standard wires on the caps, it goes with some of the previous repairs.

I have also been checking out the cabinet on mine, one side as mentioned has started to separate the plywood layers, but I have been managing to use wood glue and a series of clamps to get the top of the cabinet to stick it self back together. I have knocked out the side that has suffered the most and plan to use this as a template for a new piece of ply and then get the router out and see what I can do.

I do not believe the cores are broken, I have not removed them but will take the knob off and wind the control to the top to take a picture.

Here are some pictures of the original listing, I have cropped and resized them as I have no other pictures of the cabinet at present. Not sure if they will help and I have no problems providing measurements if required.

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Old 2nd Oct 2020, 12:06 am   #68
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Default Re: My first foray into Old model TV repairs.

Hi Adrian,
Good work so far.

Regarding changing exactly where new components are terminated, that's usually OK in the power supply and time base areas. However if you need to replace components in the IF or tuner circuits it is very important that the new components connect in exactly the same way as the old ones. This is even more true of the connections to 0v/chassis. I find it often helps to take a number of HD photos of the circuitry before replacing parts as it helps to remind me how everything was laid out originally and it can be a great help if I accidentally connect a component to the wrong place.

About IF alignment. Usually in a TV set from the 50s onwards you don't need to touch it unless someone has messed about with it in the past. Replacing valves in the IF section will not put the set out of alignment unless you are dealing with earlier valves such as the EF50 and SP41 etc. Generally the close tolerances of EF80s and EF91s mean that even using a different brand of valve (I would usually stick to the original brand if possible) won't normally cause a problem.
It is good practice however to only ever remove one IF valve at a time for cleaning etc. This of course will stand you in good stead if you end restoring earlier sets with EF50s etc.
To achieve the bandwidth required for television IF circuits were often staggered tuned, with each stage peaked to a slightly different frequency. If one of the IF valves goes low emission then it can give the appearance of being a misaligned stage. This is where the phantom twiddler often goes in and messes things up when all that was needed was to locate the faulty IF valve.

I hope this advice helps.

Cheers
Andy
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Old 2nd Oct 2020, 7:25 am   #69
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Default Re: My first foray into Old model TV repairs.

Quote:
Originally Posted by beery View Post
This is where the phantom twiddler often goes in and messes things up when all that was needed was to locate the faulty IF valve.
Andy
This was a very common disease in the 1960's. John.
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Old 2nd Oct 2020, 7:51 am   #70
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Default Re: My first foray into Old model TV repairs.

Hello Andy and John

This twiddling I think has happened on this TV, I followed the tuning instructions for just the tuner section and ended up with better results, did it on channel 3, but it certainly drops off at 5.

I need to set up my home made valve tester for the 7 x EF80's and check them, then move on to the 5 x ECL80's, one of which I have already replaced with a boxed one I had for the frame output. Then EBF80 and ECC81. The others, the LOPT valves, rectifiers and EB91's I will leave unless I want to change my tester. Not sure if my PSU will go to the PL83 heater Volts will need to check.

Have an order in for some 100uF caps at 400V will see if I can re-stuff them into the old can, if not, figure an alternative.

But I am going to start concentrating more on the cabinet to get that repaired and cleaned up, the sooner I can do that the sooner the TV may be accepted. It whistles was the comment I got last night, my answer was 'be grateful you can still hear it'.

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Old 2nd Oct 2020, 8:00 am   #71
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Default Re: My first foray into Old model TV repairs.

Quote:
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It whistles was the comment I got last night, my answer was 'be grateful you can still hear it'.

Adrian
Odd how no-one seemed to notice back in the day. I used to have an Ekco T283 and that had a very loud line whistle. Didn't matter though and no-one complained. Maybe we just got used to it and 'tuned' it out?
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Old 2nd Oct 2020, 10:46 am   #72
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Default Re: My first foray into Old model TV repairs.

Apparently the whistle is audible on youtube videos of working 405 sets, but not by me.
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Old 2nd Oct 2020, 2:24 pm   #73
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Default Re: My first foray into Old model TV repairs.

Quote:
Originally Posted by Heatercathodeshort View Post
I forgot to add. My example burst into oscillation after a few minutes use, blanking out the picture and sound. The I.F. decoupling capacitors used by Thorn are high quality .002uf ceramic types, that is all but one. The screen grid decoupler of the EF80 R.F. amplifier was the familiar light brown crunchy type and yes it was O/C. Replacement cleared the instability and gave a 100% improvement in gain. J.
The choice of screen grid decoupling capacitor might be thought to be rather strange considering all the others are high quality components. The paper dielectric Hunts capacitor will have been employed to to introduce a certain amount Cag feedback to increase gain and peak up the Q in the associated tuned circuits.
I met something similar in a Murphy V410. Weird patterning turned out to be a bumble-bee capacitor in the sound IF amplifier having gone faulty. All the other decoupling capacitors were of a different type and like the Ferguson being discussed here the designer had chosen the offending capacitor for it's characteristics which was a tiny degree of self inductance due the rolled foil method of manufacture.

DFWB.
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Old 2nd Oct 2020, 7:20 pm   #74
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Default Re: My first foray into Old model TV repairs.

The sound I.F. mentioned by David was a known trouble spot. I changed loads of them as a young lad in the 60's.

Murphy Radio recommended that the original capacitor is replaced by a Hunts W99 type, the original crunchy brown one! I always fitted ceramics and had zero problems. Faulty decoupling can cause no end of odd symptoms. John.
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Old 2nd Oct 2020, 7:25 pm   #75
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Default Re: My first foray into Old model TV repairs.

Yesterday's work on the cabinet carried on this afternoon. Getting glue between the separated layers of plywood and then clamping for an hour or two, have done about four areas all on the same side.
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This is the side piece I have given up on you can see how it must have been wet at some point.
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So I bought some plywood and got out the router and had a go at a copy, it is not perfect, but should look OK, this is a dry-fit just to see if all fits together.
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And then on to gluing and clamping. I will check in the morning that all has gone well and start adding the corner brace pieces.
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I am not putting in the hole for the Band3 tuner, it does not have one with it but it is a simple matter to make the hole at some point if required.

One last image for tonight, I noticed the quality assurance paper in the cabinet and a date which I guess is as close as I will ever get for the TV at 15tth April 1954
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Anyone have the component number for this cap and I will take a look at the one on this TV?

Adrian
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Old 3rd Oct 2020, 8:35 am   #76
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Default Re: My first foray into Old model TV repairs.

If it is the decoupling cap you are referring to it is C3 .002uf from pin 8 of V1 the R.F. amplifier. [Circuit reference Thorn manual.] John.
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Old 3rd Oct 2020, 12:45 pm   #77
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Default Re: My first foray into Old model TV repairs.

Thanks John, that seems to be a small banded capacitor and not the brown Hunts type that crumbles, so unless you suggest otherwise I will leave alone.
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The set has be plagued with a frame fault that I could not fathom for a while, the bottom of the raster would not form correctly and would distort badly, gradually going away after about 10 minutes, found it this morning C89 a 2nF cap, again a Brown Hunts cylindrical cap. I had already replace C88 which was a waxy cap anyway. The cap was leaky, so replaced with two 1nF 400 Volt caps in parallel.

With the rain I have not felt like going in the garage to the cabinet so I have been testing valves, replaced two EF80, one being the 1st sound IF amp and the other the tuner amp. Also replaced a few ECL80's with some pulls I have which were better but not perfect, raster is now looking better.

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Old 3rd Oct 2020, 1:19 pm   #78
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Default Re: My first foray into Old model TV repairs.

No....Change that cap! Nasty things...you'll find references to those in other threads. It'll either be O/C, leaky or short. They are sometimes referred to as 'bumblebee' capacitors.
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Old 3rd Oct 2020, 1:57 pm   #79
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Default Re: My first foray into Old model TV repairs.

Well there are another two in the audio section all caps 0.002uF or 2nF. Think I need to order some more caps.

It is a bit like the old Solartron scope I did, every cap tested was leaky so 99% got replaced, looks like it will be the same with this 66 year old TV.

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Old 3rd Oct 2020, 2:25 pm   #80
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Default Re: My first foray into Old model TV repairs.

I call those horrible little caps ‘firecrackers’, I have an Ekco TV that had loads of them in it, some decoupling valve heater’s, and when powered up with them in it they started blowing up like little firecrackers! They are as bad as, if not worse than the horrible Hunts crumblecaps!

Good work on the cabinet, I’ve got a similar looking Ferguson 204T in good original condition, they are nice looking telly’s.

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