Bush TV62 The restoration starts here...

[G3JZF_John]

John and others

Thanks for all your comments which are well noted.

I found out an old test meter (my AVO 8 is not working on AC) and measured the heater chain current. This wais OK at around 300 mA. The tube heater PD was about 3.8 volts and this clearly confirms the partial short circuit.
Although I served my time in physics and electronics, I have not come across this before-TV is more of a retirement hobby. Unless I can magic a cure with a rubber hammer, I will have to put up with a dim picture! I will pop in a transformer and run the set with a little more heater voltage until such time as I can get a replacement.

I would like to take this opportunity to thank all forum members who have come forward with helpful comments and suggestions- I would't have got very far on my own.

I now have to put the TV on the back burner for a while because I play in two brass bands and have four Christmas concerts and other carolling sessions. I have also promised to make my grandson a toy fire engine!!

Regards to all and a Merry Christmas.

John S

[PJL]

Hi John, when you get back to it I would measure the current by breaking the connection at the tube base as it is odd it was showing the correct 300mA with 6.3V applied. It's possible the short is intermittent but it could also be a faulty capacitor in the heater chain or some other odd fault.

[Heatercathodeshort]

Originally Posted by Refugee

If there is a partial short would it not reduce the currant to all the valves downstream of the tube heater as well?
With an isolated supply to the tube heater it would clear both the fault and the symptom.

We are talking a .3amp heater chain here. When the crt goes partial S/C it turns into a .6amp heater and will only drop 3-4v in a .3amp chain. This will only ADD around 3v to the heater chain, nothing to worry about.

I think John may have been mistaken when he took his readings but of course the heater chain would still measure 300m/a as it is a series chain. Remember the short can be intermittent and give confusing results.

The confirmation is the voltage drop when measured in the chain. If the heater decouplers were leaky and they would have to be virtually S/C to create the fault symptom robbing other valves and making the remainder glow brightly. John.

[Refugee]

The heater/cathode voltage will be more in the chain than it would with a transformer.
This could take half the heater currant out of all the valves down stream from the tube.

[Heatercathodeshort]

The CRT is last in the .3amp chain. If any of the heater decouplers are leaky V6 V7 and V4 would also be robbed of heater current and the remaining valves would be over run. John has stated that the receiver is operating OK other than the voltage drop across the tube heater. V4 is an ECC82, the line oscillator. All must be well as this valve will not function with only 50% of it's heater supply. The last heater decoupler is C67 from pin 4 of V10. V6, V7, V4 all follow with no decoupling followed by the CRT to chassis.
The effect is similar to placing a 21 ohm resistor across the tube heater pins. Half the current will pass through the resistor, the remainder will pass through the tube heater. The rest of the receiver will perform normally. As a test we are talking here of supplying the tube heater via a transformer divorced from the chain. A 6.3v transformer will light the tube at twice it's brilliance rather like passing 6v through a 3v lamp. The increased temperature of the over run cathode will increase emission but not for long. J.

[Heatercathodeshort]

Mullard tubes are very reliable and had only two problems.
The first was interelectrode leaks between grid/cathode and anode 1/grid in the early rectangular MW36-24/44 14" and MW43-64 17" series and this could usually be overcome.
The second was the partial short circuit heater aired here.
The tubes that tended to suffer from this heater problem were the 38mm neck 70 and 90 degree types. The 110 degree were completely free from this defect and last for ever.
[9"] MW22-16. [12"] MW31-16, MW 31-74. [14"] AW36-20, AW36-21, MW36-24, MW36-44, AW36-80. [16"] MW41-1. [17"] MW43-64, MW43-69. MW43-80, AW43-80. [21"] MW53-20, MW53-80, AW53-80.
Any of the above can suffer this fault but by no means to a high degree. It is just something that needs to be remembered when you have a Mullard tube with a weak picture of low brilliance that also tends to go negative when you turn the brightness up. Take a very close examination of the heater cathode assembly. All the Bush receivers used these tubes from the TV22 to the TV85. J.

[Refugee]

I did not realise that the tube heater was at the end of the chain.
It may last a bit longer on 4.5 or 5 volts.

[FERNSEH]

Originally Posted by Heatercathodeshort

[9"] MW22-16. [12"] MW31-16, MW 31-74. [14"] AW36-20, AW36-21, MW36-24, MW36-44, AW36-80. [16"] MW41-1. [17"] MW43-64, MW43-69. MW43-80, AW43-80. [21"] MW53-20, MW53-80, AW53-80.

All of these CRTs had the "M" heater. That's right. Ive never experienced any problems with the heaters of the Mullard 110 degree tubes. AW43-88 on.

DFWB.

[terrykc]

Originally Posted by Refugee

I did not realise that the tube heater was at the end of the chain.

This is the standard position in any set with a series heater chain - just look at a few schematics.

We still have a problem, though.

G3JZF_John said a while back that the CRT drew 300mA when fed by an external 6.3V external DC supply.

If this information was correct - perhaps John might care to recheck this and confirm the readings - the CRT heater must be ok.

However, we now have a reading of 3.8V in a 300mA heater chain.

These readings do not agree!

[Heatercathodeshort]

I think the reason for this is that John may have tipped the receiver on it's side to make his checks thus reducing the sag on the heater assembly. These shorts tend to be intermittent and might not have been present when the tests were made. It may be worth testing the receiver 'upside down' for a few minutes to see if the heater responds to the 'sag' releasing the short. If it works the tube could be reversed in the cabinet
The crucial factor here is the voltage dropped when connected in the .3amp chain under the fault condition. An AVO 8 on the 10v AC range should confirm this as John has done in his recent post.
The amazing thing is, another Forum member has had exactly the same symptoms only this week with a MW36-24. The voltage reading under the fault condition is identical to your receiver John, i.e. half the 'M' shorted out dropping approximately 3-4v.
I would totally agree that a transformer providing 4-5v would be a better bet 'force' feeding the heater for demonstration purposes. A resistor in series with a standard heater transformer could be experimented with to give the best overall result.

Sorry to make such a meal of this common fault and apologies to John for side tracking your interesting thread. Regards, John.

[Andy Doz]

If it is sagging causing the issue, would turning the tube upside down and refitting help?

Had a Decca 90 like that once

[FERNSEH]

Ref to #70. Turning the CRT upside down was one of the dodges we used to do in the 1960s to save the expense of fitting a replacement tube. It worked 30% of the time, but in reality it the tube had to replaced in due course, or in most cases the set was scrapped and replaced with one of the latest slimline models.

DFWB.

[G3JZF_John]

I have not had a lot of time over the last day or so and no time for experimenting.

I have decided to run the heater from a 12volt ransformer with a series resistor to aim at 0.3 amps times root 2- say around 420mA which should result in the same power in the heater. I thoght that it was better to do this to begin with because it would limit the start-up current and be kinder to the heater which did show a flare up in the original heater chain.
If the picture is still too dim I guess I willl just have to boost the heater currenr until it is reasonably useable. In the days of B/W TV I used to fit booster transformers to freinds' sets- I think these were around 9volts. I also used to fit re-gunned tubes, but not always satisfactorally.

I have noted the comments about upside tubes turned upside down etc -certainly worth trying.

John S

[terrykc]

Originally Posted by G3JZF_John

... I have decided to run the heater from a 12volt ransformer with a series resistor to aim at 0.3 amps times root 2- say around 420mA which should result in the same power in the heater ...

No, John! No, no, no!

The RMS value of an AC waveform is the value that gives the equivalent power of a DC supply of the same value!

6.3V @ 300mA AC is exactly equivalent to 6.3V @ 300mA DC!

Your 420mA will drop 8.82V across a 6.3V heater!

Actually, in your post you say you intend using a 12V Transformer - from the rest of the sentence I assume you meant DC Power supply, because there is no difference between using a 12V AC supply and 230V AC mains (except, of course, the series resistor will need to be a tad higher for the mains supply!).

[G3JZF_John]

Terry

Here is my reasoning:

The original heater is rated at 6,3 volts at 300mA
This is a resistance of 6.3volts/0.3amps=21ohms
The power consumed = 6.3volts*0.3amps=1..8watts

Assuming half the heater is shorted out, it will be 10.5ohms

To get the cathode up to temperature we still need to develop the same power in a heater with half the resistance. Both the current and voltage need to be multipled by the square root of 2 to achieve this. This means that the (half) heater will run at 4.45vollts,424mA

How long the tube will last is debateable, but I will see what the picture looks like before boosting it any further.

John S

[Heatercathodeshort]

Keep it simple. The easiest way is to use a 6v transformer and play around with a few low value resistors until you get a good picture with the lowest voltage. [CRT heaters do not like DC. For some unexplainable reason they tend to go low resistance but I feel this may be suitable material for another thread.] If you only have a 12v one then the values will have to be higher. It's not rocket science and you can get too deep with this ending up with slide rules, Bruno Tobacco and old tatty blazers with leather patches on the elbows.
Keep it fun and informative. That is what it's all about. Regards as always, John.

[FERNSEH]

Bow to the inevitable. The CRT will need replacing. I remember dealing with this problem 45 years ago and in almost all cases the tube had to be replaced.

DFWB.

[G3JZF_John]

As I predicted, I have not had much time to devote to my TV project.
I have quickly tried running the tube at a modest boost and get an excellent picture. I need to make some more accurate measurements before I can fathom out what is going on.
It may be after Christmas before I do much more work. I have been dropping hints about test meters!
Post again when I can.
Seasonal Greetings to All

John S

[G3JZF_John]

Bush TV 62 Restooration update.
Having had a new digitlal voltmeter for Christmas, I have been able to get abetter idea of the heater problem with my set.
I have fitted a 9 volt transformer for the heater supply and I am feeding the heater through a 6ohm resistor. It is running at 6.5 volts and taking around 375mA.
I can only conclude that the tube has self healed but it still does not like being run in the proper heater chain of the set. The picture is somewhat grey under these conditions
and I could boost it a little more. The set has been running for a couple of hours and is completely stable.
The resolution seems a little down and I wonder what it should be (certainly, I'm getting no more than 1.5 megs)
I've now put the back on the and will leave it at that because I want to return to the two Vidors sitting on the shelf!
Thanks for everyone's help; I will write up the project and put together a gallery of photos.
The cabinet has polished up spectacularly and is virtuallly in showroom condition. All I want now is a new tube.

73 de John G3JZF