Parts & Advice Request For Ekco TSC48 and CRM121 substitution

[DoctorWho]

Well I've finally had an opportunity to look at the Ekco TSC48 properly.

Overall it looks in pretty good shape, inasmuch as there is no damp damage, no rust and the like.

One big problem is the CRT, it's a CRM121 12" CRT, the original has gone white in the neck and has had the octal base glued back on, suggesting to me that the vacuum has been lost, I'd guess that the base was broken off at some time resulting in the loss of vacuum, it was probably then glued back.

There is also a lead on the chassis which is loose, from the circuit diagram it looks to be the "CRT Grid Lead", but can anyone point me as to where this should be connected please, as there is no obvious place on the CRT connector to show where it has come off, and I do not want to connect it back in the wrong place.

Finally there are two smaller items which are going to need replacing, the first is a choke underneath the chassis, L23, which should be 50 Ohms, this is open circuit, and secondly the focus control, a 2K Ohm wire-wound pot is also open circuit and will need replacing.

If anyone can help me with any of these parts, and/or advice me on the CRT Grid Lead I would be very grateful. If you happen to have any of the parts I need available for sale please drop me a PM.

Thanks in advance for any help.

Peter.

[ppppenguin]

Your diagnosis of the CRT is correct. The CRT has definitely "had it".

[tvden]

Hi
If your tube had been untouched and a bit soft it may have still worked to a degree but it looks like it has been bodged up at sometime.
Mazda tubes are not good at the best of times,
I am still looking for a Mazda CRM 121 tube myself.
If your 2k pot is a Colvern you can get a new one from RS.
Den

[DoctorWho]

Thanks for the input, and also for the advice on where to get a pot.

Can anyone suggest where I could find a replacement CRT?

Also, any ideas where the "CRT Grid Lead" is supposed to go?

Thanks again,

Peter.

[Panrock]

Peter the 'grid' connection to this tube goes to pin 5.

I'm afraid CRM121/123's are as rare as hens' teeth. I did find a good one once for an Ambassador 'corner' console but I had to modify the circuit in my own Baird Townsman to take another 12-inch tube.

Steve

[DoctorWho]

Thanks for the info Steve, at least the stray wire can be put back.

If necessary I'd buy a set with a CRM121 in it, just to get the CRT.

Peter.

[DoctorWho]

Out of interest would a CRM121B work as a direct replacement if one could be tracked down, or would modifications have to be made?

Peter.

[Panrock]

AFAIK it's very similar. The 'B' is rated for 10Kv EHT though instead of 7.

A CRM123 might be more desirable though, because it's aluminised. All should be plug-in replacements

Steve

PS. I posted a list of likely sets holding this tube some time ago and you should be able to search for it. However I got into hot water about this because the question of whether sets should be cannibalised in this way is a source of some disagreement.

[DoctorWho]

Many thanks for the info Steve, am I right to presume then that the CRM121B would plug in as a direct replacement, but it would perhaps be a little dim due to the reduced EHT?

I don't see a problem with posting details of other sets which used this CRT, I would be very grateful of such a list. Obviously none of us want to go around wrecking old sets, but if it comes down to having to choose which set you would most like to get going again then you simply fit your good CRT in to the best set and keep the other one until a CRT turns up.

Thanks also for the tip regarding the CRM123, that's worth looking out for too.

Peter.

[Panrock]

Originally Posted by DoctorWho

Many thanks for the info Steve, am I right to presume then that the CRM121B would plug in as a direct replacement, but it would perhaps be a little dim due to the reduced EHT?

Plug-in replacement, yes. As for the CRM121B being dimmer, not absolutely sure but don't think so. I would simply think it was a uprated version with the potential to display a brighter picture.

There are tube experts on this board who can probably tell us what would have been changed (ie. phosphor, cathode composition?) to permit the higher eht.

Steve

[DoctorWho]

Thanks Steve, that sounds worth following up. I'd be interested to know the difference just out of curiosity.

Peter.

[tvden]

Steve
I think it was me that started the other thread on the Mazda 121, I have just searched for it , but it must have been deleted as its not archived.
Peter you are not going to find one of these tubes ( I have been looking for a year ) very easily unless it's in another set and even then it will prob be soft or have shorts, and you may in the end have to modify your set to take another type of 12" tube.
I agree with Steve that the CRM123 is a better bet as long as you have at least 7.5 KV plus to run it.
TVDEN

[Heatercathodeshort]

Hello Peter,
The Mazda CRM121 is a pre war tube launched in 1938. It was designed to run from mains eht around 5kv. The higher the eht potential, the 'stiffer' the electron beam becomes therefore requiring stronger scanning currents to deflect the spot. The pre war receivers lacked scanning power with the valves and transformer cores that were available. Post war sets using RF as in your case or flyback eht were easily capable of higher voltages and were designed on their limits to supply enough scan current to provide just about sufficient width! The CRM121B was just an uprated version of this original tube using a slightly different internal construction within the bulb and neck. In practise tubes can be run at much higher eht potentials than the published ratings without any noticeable reduction in life and many were operated under these conditions for years. Aluminised tubes such as the CRM123 require a slightly higher eht potential to enable the elctron beam to penetrate the micro fine aluminium layer backing the phospher. In practise they will work fine in sets not designed for them. Its a matter of trial and error. Another Mazda tube with the same bulb dimensions is the CRM124. This is a later tetrode tube with ion trap. It is designed to run at a higher eht but will give a good picture when it is used to replace the CRM121. It will require a change of base and it has a 12.5v heater I believe. A second anode potential of around 300v will have to be found in the receiver but this might be slightly difficult with the TSC48. The HT alone may be high enough. As we have all said the Mazda tubes were very short lived in the early days suffering from heater/cathode shorts and low emission. Very few will have survived in good condition but I do have around 5 sets in my collection that use this tube and they are all ok. So there is hope. Regards John.

[DoctorWho]

Hello John,

Many thanks for the information, much appreciated. I didn't realise it was a pre-war design CRT, I just hope that I can find something suitable to replace it with.

Thanks also for the information on the CRM121B etc, at least I have a few I can look out for, if necessary I'll buy another set with a suitable CRT and use that in the Ekco, keeping the donor set safe until something suitable comes along.

From what you've said the CRM121B is obviously slightly different internally, but am I right to presume it could just be put in the place of the original and used if one should turn up?

Thanks again,

All the best,

Peter.

[Heatercathodeshort]

Hello Peter!
The full list of Mazda 12" tubes are as follows:
CRM121 [ Cathode Ray Magnetic twelve inch series 1]
CRM121B [Higher EHT version]
CRM122 [ .3amp version used in AC/DC sets]
CRM123 [Aluminised version of CRM121B]
CRM124 [ 12" version of 9" CRM93 used in the TMB272 Ekco]
These are TETRODE tubes and will require some minor mods to work in the
TSC48.
Unfortunately they are all rare crt's.
Any 12" tube that the scan coils will fit over can be made to give a picture in your EKCO with a bit of modification. Regards John.

[DoctorWho]

Thanks for the info John, much appreciated. At least I have a few different CRT's to chase up, hopefully I'll be successful in finding one of them.

I may well come back for some advice about using a different CRT if I don't come up with a CRM121.

All the best,

Peter.

[beery]

Hi Dr Who,
how did you get on with your TSC48?
I have one of these, still in bits awaiting delivery of the finnished cabinet. I finnished the electronic restoration some years ago.
The best way to get a CRM124 is to get one from a barrel murphy portable I think. They are realy excellent tubes, indeed the 121B fitted in my TSC48 is not particularly special.
Other than that, you could try a Mullard 12" tube. I have loads of the things in my parents loft.

Cheers

Andy

[DoctorWho]

Hello Andy,

Well it's getting there thanks, I did finally track down a CRT with the kind help for a forum member, and there is now a good healthy raster. More work is needed on the RF stage though as two of the little tuning slugs had fallen out, they were thankfully found but it has to be re-aligned now. Hopefully it won't take too much longer.

How come your cabinet has taken so long? It would be nice for you if you could get the set up and running again.

Peter.

[beery]

Hi Dr Who,
the cabinet on my TSC48 ended up with Fernesh after spending some time in bits in my parents' garage.
The cabinet is now complete and will be delivered soon. However after all this time I can't seem to locate the spring balancing mechanisms for the lid.
I will also have to re-do the felt and for this I will need an animal glue pot.

Here is a tip for when you get yours working. Poor EHT regulation is partly due to the EHT rectifier having some very odd characteristics at HF. I suggest a mod using a base adaptor and something like a DY86, R19 or U26. I have yet to try this in my set, but I will as soon as I get it back together.

Cheers
Andy

[DoctorWho]

Thanks for the advice Andy, I hope you are soon able to get your set back together and up and running.

Mine is now coming along well and gives a good picture and sound, the RF slugs are a bit lose though, as they do not seem to have any wax on them to keep them in place, nonetheless this should be easy to add and then the set will be up and running and back in use again. The two biggest obstacles were the CRT and audio transformer, both of which needed replacing, but with spares sourced it's now working a treat.

Peter.