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Old 27th Jan 2018, 5:22 pm   #1
bionicmerlin
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Default GEC BT336 possible tube change

I'm looking to replace the original warn out picture tube in this set.
I believe has has a Gec 7601A. That infact is a MAZDA CME 1901
Looking at a 1970 MAZDA data book the 1901 has been superceaded to a CME1903s That cross references to a Mullard AW47-91 .
To cut a long story short i believe I have the Mullard version in a non working set that I would like to use assuming that one is not warn out..
Only other thing in the Mazda book it says the upgraded CME190 is for 300ma heater chains only .
Talking to a well known forum member the other day he tells me most tubes can by used as long as they will fit but I'm just checking before taking the plunge .
Thanks Andy
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Old 27th Jan 2018, 10:32 pm   #2
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Default Re: GEC BT336 possible tube change

According to my 1968 Mullard data book the AW47-91 is a suitable replacement for the 7601A and CME1901, it has a 1" shorter neck and the only circuit change required is the addition of a 21Ω 2W resistor in series with the heater as it's a 6.3V type as opposed to 12.6V for the GEC/Mazda. The heater current for all three types remains the same at 300mA.

The AW47-91 was superseded by the A47-14W, the latter having a tinted faceplate.

John.

Last edited by jayceebee; 27th Jan 2018 at 10:42 pm. Reason: more info
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Old 27th Jan 2018, 11:21 pm   #3
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Default Re: GEC BT336 possible tube change

Worth bearing in mind that the CME1901 has a 114 degree deflection angle whereas the CME1903/AW47-91 it is 110 degrees. For replacement purposes this shouldn't matter. CME1901 has a 12.6volt 0.3amp heater and the CME1903 is 6.3V. Full agreement with JCB, to do the tube substitution job properly a 21ohm 3watt resistor should be inserted into the heater chain. The AW47-90 has a long neck and if one can found it is a good replacement for the CME1901.

DFWB.
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Old 28th Jan 2018, 2:25 am   #4
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Default Re: GEC BT336 possible tube change

Thanks for your help guys. That explains why on testing the filament on my potential replacement it was different from the existing tube.
I will proceed when I can and report back Andy
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Old 5th Feb 2018, 8:01 pm   #5
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Default Re: GEC BT336 possible tube change

Update on the tube change. So far so good but i do have a couple issues hopefully someone can help me with .
after changing the tube i now have a black line less that a 1/8 of a inch going across the screen near the top this wasnt present before the change.
also i have a slight snaking picture and a slight lack of width. Both of these i had before the tube change . The HT is more or less correct .
Thanks Andy
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Old 5th Feb 2018, 8:42 pm   #6
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Default Re: GEC BT336 possible tube change

Change the frame output valve. These lines were known as 'telegraph wires, and were very common with the N379/PL84 valve. It is called bachxxxxxxI can't spell it oscillation!

The main smoothing block has probably deteriorated causing the snaking.

John.
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Old 5th Feb 2018, 9:22 pm   #7
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Default Re: GEC BT336 possible tube change

Quote:
Originally Posted by Heatercathodeshort View Post
Change the frame output valve. These lines were known as 'telegraph wires, and were very common with the N379/PL84 valve. It is called bachxxxxxxI can't spell it oscillation!
If you don't have a spare valve then you could also try putting a small magnet near the valve. I had the same problem with a GEC set and a magnet cured it for me.
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Old 5th Feb 2018, 9:29 pm   #8
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Default Re: GEC BT336 possible tube change

Barkhausen Effect: https://sciencedemonstrations.fas.ha...khausen-effect
And: https://tesla3.com/heinrich-georg-barkhausen/

DFWB.
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Old 5th Feb 2018, 11:04 pm   #9
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Default Re: GEC BT336 possible tube change

. I don't have the correct valve as a replacement it's got a N389 so I will give the magnet a try just to see for now. Then get a replacement .
I will change the main cap as John suggested. It's a 100mf x 300mf
It does test ok but about 50 % higher in value .
The only new ones I have are 150 mf and 330 mf I assume that's ok
Thanks everyone Andy
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Old 6th Feb 2018, 3:00 am   #10
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Default Re: GEC BT336 possible tube change

I was having having a lack of knowledge moment but I'm back on track now I think. I was getting the valves muddled up.
I don't have a replacement PL84 so I did try the magnet . What I do appear to have done is magnetise the valve and the line is still there but pulled right up to the top of the screen hardly showing .dont know if it will stay in that position. Thanks again Andy
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Old 6th Feb 2018, 10:30 am   #11
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Default Re: GEC BT336 possible tube change

There may be a leak between sections of the smoothing cap. The value you have will be fine. Testing these large capacitors can be misleading.

The PL81 line output valve [not used in the GEC] used to exhibit a similar line but this time it was vertical to the left of the raster. They produced what appeared to be a mini ion trap magnet that fitted snugly around the valve envelope.

OK David, yes I know. I should have paid more attention to spelling at school.

Regards, John.
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Old 8th Feb 2018, 3:13 am   #12
bionicmerlin
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Default Re: GEC BT336 possible tube change

I assume it's ok to carry on this thread with the mods as it's a little more than a tube change now.
John . I have replaced the caps as you suggested they are newish stock but I still have the picture snaking . It gets better the longer the set is on. Other problems I have is I still can't get full width. Also worse than that after the sets Been on for about 30 mins the focus starts to slowly deteriorate and after 45 mins it's terrible.
I really want to get this set going as my nan and grandad have the same model so happy memories.
Any more help would be appreciated Andy
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Old 8th Feb 2018, 8:21 am   #13
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Default Re: GEC BT336 possible tube change

Just a thought have you got a good connection to the outer coating of the replacement tube? I had a similar effect on a set.The dag earth spring was loose. worth a check..

Rich.
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Old 8th Feb 2018, 8:35 am   #14
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Default Re: GEC BT336 possible tube change

Couple of things come to mind, check the HT when the picture is deteriorated, if it is low and the set is still using the finned selenium rectifier it may pay to change to a diode and 47ohm resistor.
Also monitor the CRT heater voltage to see if that drops, you may have a partial heater short.
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Old 8th Feb 2018, 10:30 am   #15
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Default Re: GEC BT336 possible tube change

The clue here is that the width is short and I guess it gets worse with time? Can you also measure the EHT as well as the H.T.?
As Stephen has posted, the old finned rectifier will definitely be U/S that is if it has not been replaced by a silicon diode and 25 ohm 10 watt resistor.

Did you reposition the lin sleeve between the neck and the scan coils when you changed the tube? It may be jammed inside the coils. If in doubt, as a test, simply remove it completely.

It is possible that the snake effect is due to a slight heater cathode leak in the line oscillator or output valve. I will have a look at the circuit later.

The deterioration over time points at the LOPT overwind, the old problem encountered many times on this forum. I have had a similar problem with the earlier but very similar BT302 series but they have always responded, over a couple of days, to the pass a low current through the pitch insulated overwind in an attempt to drive out the moisture. John.
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Old 8th Feb 2018, 11:30 am   #16
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Default Re: GEC BT336 possible tube change

Quote:
Originally Posted by Heatercathodeshort View Post
The deterioration over time points at the LOPT overwind, the old problem encountered many times on this forum. I have had a similar problem with the earlier but very similar BT302 series but they have always responded, over a couple of days, to the pass a low current through the pitch insulated overwind in an attempt to drive out the moisture. John.
Hi Andy,
In addition to John's point above, have you checked the temperature of the LOPT after it's been on for 1/2 hour? I've had transformers that get way too hot in that time- usually the primary part of it. I'm not sure why some seem to get so hot when there are no other apparent faults. Turn it off before you stick your finger on it though- it will bite!
Can you post some pictures of the chassis & LOPT?
Good luck
Nick
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Old 9th Feb 2018, 2:55 am   #17
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Default Re: GEC BT336 possible tube change

Thanks all for taking the time to respond.
I haven't had much time today Nick I will put some pics on tomorrow if I can .
Maybe I should of said I have replaced the rectifier infact.
John I'm not sure if you have worked out I'm the guy who spoke to you on the phone asking advice and your going to look at my other set for me.
I have checked the dag strip and it wasn't making good connection so that's done ..
I have checked the ht voltage it's almost spot on the tube volts are 13.1 and stays that voltage over an hour. .
I also find the bottom of the picture is about 2 inch short but comes correct after a few mins. The LOPT is warm after about a hour but not hot. I got nosafe way to check the EHT volts.and the scan coil sleeve looks ok but will double check. I did replace The U26 valve as when I first got the set the picture would balloon very bad. This cured it by about 90% now it's almost cured its self with the LOPT slowly drying out I would guess. Hope to get some more advise . Thanks Andy
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Old 9th Feb 2018, 9:40 am   #18
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Default Re: GEC BT336 possible tube change

Replacement of the frame output valve will cure the slow warm up/telegraph lines. It sounds very tired. These output pentodes were very prone to this.

Providing you have replaced the boost cap it should get better as you say, as the overwind dries out.

I have a PL84 going spare. I have put it on one side for you when you visit. John.
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Old 10th Feb 2018, 1:47 am   #19
bionicmerlin
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Default Re: GEC BT336 possible tube change

John . Thanks for the advise and the valve.
For the last few evenings i have had the set powered up for about a hour each night.
no improvements. in fact tonight after about 45 mins the picture at the top randumly come unstable i could correct it with the horizontal hold but its then on border line to the whole picture breaking up.
One other thing i did notice tonight is when the focus goes the brightness and contrast goes very poor also on a very light background it appears almost like a stain over a large part of the picture tube .Up until this point the picture is very good. Could this tube be faulty as well. I would put a picture on but i dont think it will show.
Nick heres a couple pics of things you asked for . Thanks Andy
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Old 10th Feb 2018, 9:51 am   #20
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Default Re: GEC BT336 possible tube change

It sounds like you have an issue with the line oscillator, as well as an EHT fault.
The previous suggestion of passing a voltage through the overwind for a time may be enough to drive out the residual moisture that causing the overwind breakdown, its certainly worked for me on numerous occasions.
Out of interest, in your initial post you said the CRT was worn out, have you checked all the other valves conditions. If the CRT was worn out maybe some of the valves are tired too.
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Last edited by Freya; 10th Feb 2018 at 9:58 am.
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