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Cathode Ray Tubes CME2306 & CME1906 : Thorn 800 series Chassis

 
crustytv
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I was reading an interesting article from an internal 'Contact' D|E|R magazine. It was in reference to a considerable number of the above tubes having been incorrectly returned by branches, to their main stores. The tubes were in fact OK, the article covers why the incorrect diagnosis of low emission. I thought members might find this of interest and worthy of note.


Replacement Of Cathode Ray Tubes

Tests carried out on faulty tubes returned by branches to the Main Stores have revealed that a considerable number are serviceable and should not have been replaced. All types of tubes are involved, but the majority are CME.1906 and CME.2306 removed from the 850 series receivers.

In order to avoid this unnecessary and large addition to our service costs, it is essential that adequate and proper steps be taken to confirm that a tube has ended its useful life before any replacement action be taken. The problem is almost completely associated with apparent low emission and the following instructions apply only to this type of failure.

In the case of receivers other than those in the 850 series the checks required are simple. Symptoms suggesting low emission could result from low EH T, incorrect tube electrode potentials, lack of drive and low heater current, all of which can be assessed or measured fairly easily. Special attention must be given to receivers in the 850 series and, although the same checks are required, specific action must be taken regarding heater current.

The models concerned are 049, 198, 199, 231, 232 and also model 197 which was the last model in the 800 series. Whenever a tube fitted to one of these receivers exhibits signs of low emission, the heater current must be checked. If the current is not between 295 and 300 m/a the mains dropper should be examined and if the heater section (R118 in 850 series) measures 160 or 173 ohms the dropper should be replaced by a new dropper, part number OYO-063, which has a 150 ohm heater section. If this does not sufficiently increase the heater current to bring it up to 295 m/a, then the tube heater modification should be fitted. This modification is fully described in Contact, issue 5, pages 7-11, and in H.O. Directive number 66/22 dated 29th June, 1966.

The above instructions regarding mains dropper and tube heater modification must also be applied whenever one of these receivers is serviced in a workshop, for this will greatly reduce the possibility of the tube suffering from premature loss of emission. It should be noted that when the heater current has been low and is increased to 295/300 m/a the tube emission may not recover immediately, but an improvement should be noticed if the receiver is left running for an hour or so. Fault labels are supplied and must be affixed to all faulty cathode ray tubes. Each label must be fully completed with details of branch, engineer, fault, set model and serial number and date of replacement.

The responsibility for confirming that a tube is faulty rests with the Supervisor, Foreman or Senior Engineer in the branch concerned.

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Topic starter Posted : 17/02/2019 12:39 pm
crustytv
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As mentioned above there was a detailed modification procedure to address the false low emission problem. For completeness I have included the various procedures, covering the various models, below. Hopefully it may be of interest and perhaps a member might find it of use.


 

LOW EMISSION TUBES
There is little doubt that many Of the 19" and 23" tubes which are being replaced because of low emission could have their life usefully extended if the heater modification is properly used. Full details of this modification were contained in H.O. Directive number 66/22 issued in June, 1966, but for the benefit of engineers who may not have this information, full details are given below.

It must be stressed that the fitting of the resistors is unlikely to produce immediately noticeable improvement, but after the receiver has been operating for some time a gradual improvement in emission will result. It is obviously important that the correct value of mains dropper be fitted, and if shunt resistors have been added to the mains
dropper, a new mains dropper should certainly be fitted.

The Television Receivers concerned are:—

  1. Models 192, Phase 2, 194, 195, 019, 230.
  2. Model 196.
  3. Models 198, 199, 049, 231, 232.
  4. Model 197.

 

The reason for the four classifications is that while, electrically, the modifications have the same effect of increasing CRT emission, the physical connections are different, owing to Chassis layout variations. The current through the CRT heater is increased using the modification shown below:

dercrt 1

R1 and R2 are added to the existing circuit, the instructions on pages 8-10 showing details of mounting and location to be adopted for each model. Supplies of these special resistors and sleeving should be requisitioned from the Main Stores at Twickenham. The part number for the complete kit required for each receiver is OYO—300.

In future all engineers must carry these kits and, in the case of service calls involving these models, should modify the receiver whenever there is any doubt regarding the CRT emission. We do not expect any of these receivers to be returned to the workshop because of CRT low emission, unless this modification has already been carried out and sufficient time allowed for the emission to recover. Workshop engineers may only replace low emission tubes if they are completely satisfied that the modification has been tried and that improvement in the emission cannot be achieved. Once the modification has been fitted it should be left permanently in the receiver.


Instructions For CRT Heater Circuit Modification
Models : 019, 192 (Ph. 2), 194, 195, 230

dercrt 2

 

  1. connect 27k 3W resistor between tag 3 and tag8 on eleven way mains dropper tag strip. Adequate clearance must be ensured.
  2. Connect a wire from tag 8 on eleven way strip to pin 1 on the CRT base
  3. Connect a 2.7k ½W resistor from wire warp tag 2 to wire warp tag 4 on IF printed board.

 

dercrt 3

Model 196 Only

dercrt 4

 

  1. connect 27k 3w resistor between tag 1 and tag 3 on eleven way mains dropper tag strip. Adequate clearance must be ensured.
  2. Connect a wire from tag 1 on eleven way tag strip to pin 1 on the CRT base.
  3. Connect 2.7k ½W resistor between wire wrap tag 69 and wire wrap tag E1 on IF printed board. Postion resistor to clear wire link. Use sleeving to provide insulation.
dercrt 5

Instructions For CRT Heater Modification
Models 049, 198, 199, 231, 232

dercrt 6

 

  1. Connect 27K 3W resistor between tag 4 and tag 6 on the eleven way mains dropper tag strip. Adequate clearance must be ensured.
  2. connect a wire from tag 4 on eleven way tag strip to pin 1 on the crt base.
  3. connect a 2.7k ½W resistor between wire wrap tag 37 and wire wrap 72 on the timebase printed board.
dercrt 7

Model 197 Only

  1. Connect 27k 3W resistor as detailed in [1] & [2] above.
  2. Connect 2.7k ½W resistor between wire wrap tag 70 and wire wrap tage E2 on IF printed board. Sleeving must be used to provide insulation.
dercrt 8
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Topic starter Posted : 17/02/2019 1:50 pm
Nuvistor
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I saw few Thorn sets but this mod poses a few questions in my mind.

 

The mod indicates that the CRT was passing less than 300ma with this problem, noticeably they increased the current through the CRT but bypassed the other valves in the set. This was obviously to stop those valves being overrun.

 

I believe these CRT’s did have a faulty heater, perhaps it had gone higher in its resistance, this mod would compensate for this problem without overrunning the other valves. It would certainly save the company a lot of money but I question leaving it position if a new CRT was fitted.

 

So why did the CRT’s test good in a tester, probably do to the heater  being powered from a low impedance transformer winding instead of a series string of valve heaters. A CRT boost transformer on the non boosted winding would probably have worked but would have cost much more than a couple of resistors.

 

Just my thoughts.

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Posted : 17/02/2019 2:48 pm
Jayceebee
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I'm afraid although being ex D|E|R I'm unable to shed any light on this as the 850 models had almost all been exited out of the system when I joined the company in 1974, even 950s were pretty rare. I've been looking through back issues of PT mag online but haven't been able to find a full 850 circuit but it's seems odd that this hasn't cropped in bulletins from Ferguson and also wasn't it usual practice to have the CRT heater last in the chain though could the last two be the UHF tuner? I had been wondering if there was also some switching done in the heater chain for sets with VHF radio operation to cut off the supply to CRT and timebase valves.

From the mid 70s every field engineer carried a CRT tester/reactivator as several faults on a 3000/3500 could give the impression of it being low emission, they were taken out of the field for an afternoon and built their own in the workshop from a supplied kit. I remember seeing one turn up on Ebay a couple of years ago.

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Posted : 17/02/2019 10:35 pm
crustytv
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Posted by: Jayceebee

 I've been looking through back issues of PT mag online but haven't been able to find a full 850 circuit

Hi John, as the advert once said "Should have gone to Radio Rentals " or in this case Radios-TV. ? 

If you had looked under the "Service Dept" section of the blog, there's the section "black and white TV" which leads to a section called "Servicing B&White TV's". I gathered all the popular B&W servicing articles in one easy place. See here

The actual LLJ paper for the 850 is here

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Topic starter Posted : 17/02/2019 11:04 pm
Nuvistor
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I wonder how long the CRT lasted after doing the mod and getting emission back to acceptable, I suppose if they got 12 months it would be a big saving.

 

Ekco used that range of CRT’s, the biggest problem I had was an O/C cathode, very common problem, annoying because the emission was still excellent on most of them. Only permanent cure was a replacement CRT, seem to remember we used Mullard replacements.

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Posted : 17/02/2019 11:05 pm
MurphyV310
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Hi.

Sounds totally illogical to me. The correct heater current for the whole chain should simply be 300ma. Adding in a resistor to the valves above the CRT and those below the CRT will in effect reduce the current in the valves heaters but obviously increase the current in the CRT heater. Why did Murphy and other manufacturers that used the the very same CRT not have this issue? The current through the whole chain should be checked and if 300ma the voltage across the CRT heater should be checked, if it is below 12.6v then as far as I would be concerned I'd suggest the heater is out of spec. So in my opinion returning the CRT's for replacement would be the correct way to go. I also find the suggestion of fitting a different Dropper with a lower resistance odd as surely the set designer can understand ohms law?

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Posted : 17/02/2019 11:06 pm
Jayceebee
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Posted by: crustytv
Posted by: Jayceebee

 I've been looking through back issues of PT mag online but haven't been able to find a full 850 circuit

Hi John, as the advert once said "Should have gone to Radio Rentals " or in this case Radios-TV.  

If you had looked under the "Service Dept" section of the blog, there's the section "black and white TV" which leads to a section called "Servicing B&White TV's". I gathered all the popular B&W servicing articles in one easy place. See here

The actual LLJ paper for the 850 is here

I did try that Chris but I think I may have a browser issue as it takes may back to the initial page of the technical library. Your link works fine.

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Posted : 17/02/2019 11:19 pm
crustytv
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Posts: 10843

Next time try the download button rather than preview, I think some browsers have trouble with the preview function.

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Topic starter Posted : 17/02/2019 11:27 pm
Nuvistor
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Hi Trevor,

Nice to see you posting.

I have no doubt the heater was out of spec in some way,  it seems they tested CRT’s on return to stores and it must have initiated some further investigation. They found a problem, perhaps in contact with Mazda, then came up with a plan to save replacement and save some money. I don’t know how many CRT’s had the problem but it must have been worthwhile to modify the circuit. 

I find in interesting and would really like to know the story behind it all,  the service bulletin give the remedy, unfortunately not the background into DER’s thought process.

 

 

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Posted : 17/02/2019 11:52 pm