Metrosil Replacement?

[Kat Manton]

Hi,

I've made a start on the Dynatron TV30CK (Ekco chassis, similar to Ekco TGC316 without the autochanger.)

So far it's not looking too bad, but on withdrawing the chassis for further cleaning I've found the 'Metrosil' varistor is broken off at one end.

I suppose "has anyone got one?" is a long shot (but you never know...), so I'm wondering what to do about it. It's R79 on the TGC316 schematic, and is connected between EHT and chassis for EHT regulation.

Regards, Kat

[Kat Manton]

I don't know if this is significant, but what looks like yellow paint at the EHT end is actually gold - I presume this may be some sort of colour code denoting its type.

Cheers, Kat

[BassoonBloke]

Hi Kat,

Have a look on Beery's website, he has devised a semiconductor replacement for metrosils (his is to replace a TV22's i believe). These are probably modifiable to suit your set's version.

Hope this is of some help,

Alan.

[Kat Manton]

Hi Alan,

I found that, it appears to be a circuit to replace a different type of Metrosil ("it usually comes in the form of a green or red painted ceramic disc which is about the size of a 10p piece") used in a different application, deriving the A1 voltage.

The one used in the Dynatron/Ekco circuit is an approximately 4" long black rod and is connected straight across the EHT. In the photo there are two leads visible on the left, one goes to the EHT rectifier, the other to the CRT EHT connector. It's a shunt stabiliser for the EHT supply.

I doubt that circuit could be modified for this application, failing a replacement about the only thing I can think of is adding a shunt stabiliser circuit similar to that used in first-generation colour sets (Decca CTV25 etc.) That would do the same job as the Metrosil VDR in much the same way, just using a triode and some other bits.

Regards, Kat

[Sean Williams]

Hi Kat,

I think this is used to regulate the EHT - So I wouldnt loose sleep over it - the set will work without it.

Cheers
Sean

[Mike Phelan]

Kat
We just used to leave them off altogther - the EHT regulation on a monochrome CRT with its lower beam current does not warrant a PD500 as in a colour set, and it is unlikely that any other triode with xx Kv on its anode could survive for very long!

[oldeurope]

Originally Posted by BassoonBloke

Have a look on Beery's website, he has devised a semiconductor replacement for metrosils (his is to replace a TV22's i believe). These are probably modifiable to suit your set's version.

Hi Alan, for g2 metrosil replacemts look here:

https://www.vintage-radio.net/forum/...light=Metrosil

Kind regards,
Darius

[Duke_Nukem]

Initially I'd try doing without the EHT metrosil and see how acceptable the picture is once everything else is sorted - without it I think the worst that'll happen is you get slight blooming depending on picture content (i.e just like cheap colour portable tellies down the supermarket!!)(and some not so cheap ).

I'm sure someone will correct me here if I'm wrong but I think most sets did without them. Thinking about it the only sets I've seen with them have been the more upmarket versions ?

TTFN,
Jon

[FERNSEH]

Just leave the metrosil out of the circuit. That's what I used to do in the sixties, the part was often found broken. You'll hardly notice any difference between a set fitted with the metrosil and one that isn't.

DFWB.

[Kat Manton]

Hi,

Thanks for the replies, for the time being I'll remove the remains and insulate the connector.

It looks as though it's already broken once before then the shorter remains have been refitted in the clamp on the right, then looks to have broken again and has probably been arcing away merrily judging by the state of the broken ends and all the black dust in the area.

If a replacement does turn up I think I'll modify the mounting/connection arrangements, it seems a bit odd to have a long brittle thing clamped solidly at one end, with the other end loose. Hardly surprising that it broke. I think connecting the chassis end with wire or braid might be a better idea.

Regards, Kat

[oldeurope]

Hi Kat, I am sure it can be repaired with conductive adhesive.
Note that the thing gets a bit warm during operation.

My EKCO TX295 has one and it is told that these things are
typical for British sets.
If you don't want the blooming effect Jon described, use a
HR1686. It is 167Meg ohms.

Kind regards,
Darius

[Station X]

It's sometimes a good idea only to support a component at one end. If it heats up and expands it's less likely to crack or break.

[Kat Manton]

Originally Posted by oldeurope

I am sure it can be repaired with conductive adhesive.

Unfortunately not; the ends don't match up at all, probably eroded due to arcing.

Originally Posted by G4ILN

It's sometimes a good idea only to support a component at one end. If it heats up and expands it's less likely to crack or break.

Noted - my idea was to support it at neither end, just leave it loose in its insulating tube, but prevent lateral movement by anchoring the connections.

Regards, Kat

[Mike Phelan]

I'd go for that, Kat. They sometimes just crumbled.

[dazzlevision]

Hi,

The single standard Pye hybrid colour sets used a long Mullard/Philips VDR, which might work in your Ekco set - maybe using a shorter section of it, as the Pye chassis worked with 25kV EHT.

It was connected between EHT and chassis, with a slider at the low voltage end tapping off the focus potential. The whole thing was housed in a two part plastic moulding. There should be some of these kicking around in junk boxes.

IIRC, it was mainly Ekco and Ferguson that used Metrosils in the 1950s. They did tend to disintegrate at the point(s) where they were clamped.

I think Metrosil was a trade mark of Metropolitan Vickers Ltd (later AEI), in Manchester. There is still a UK based firm called Metrosil Ltd, making VDRs and they have a website.

Regards,

Dazzlevision

[Kat Manton]

Hi,

Originally Posted by oldeurope

If you don't want the blooming effect Jon described, use a HR1686. It is 167Meg ohms.

The only reference I can find for this is an EHT cable for a Nordmende 1320, is that what you mean?

...meanwhile...

I've been studying the circuit for this set (well, I've been studying the circuit for the Ekco TGC316 and Ferranti TC1004/TC1005 as that seems as close as I can get.)

This chassis has gated vision AGC and the vision signal is DC coupled to the CRT.

It seems to me that this means when set up correctly there can be situations where the beam current is low (night scenes) or even zero (black). In this situation the EHT will rise, possibly over and above the 16kV specified as the maximum for the CRM172. I'm not sure what this would do, but arcing in various places and possible destruction of the LOPT are certainly things I'd be concerned about. (The U25 EHT rectifier fitted was gassy and was found to have a small crack, possibly due to overheating?)

However, the above design aspect of the set would explain the need for EHT regulation, hence the presence of the Metrosil.

Also, on this set with no EHT regulation, wouldn't the picture content dependent blooming be somewhat worse than that on a set with AC-coupled vision?

On a set with AC-coupled vision, black comes out as mid-grey and 'typical' scenes contain a mix of light and dark. It all averages out vaguely around the same beam current; hence EHT regulation is less important, and changes in picture size relating to scene content wouldn't be as severe.

OK, I was born ten years after this set was introduced so I could be spouting rubbish, but the above is my understanding after studying the circuitry.

Also I'd like to get the set working well, not just working. I'd like to be able to show that television in the fifties was pretty good. There seems to be an idea among some of a younger generation that back in the fifties people were huddled in the dark watching tiny flickering low-quality pictures. Given the vision bandwidth of the computer system I should be able to prove otherwise... )

So, if someone can offer me some means of restoring the EHT regulation the set was designed to have I'd appreciate it. Reasonable costs paid, all additional expenses reimbursed - original type of Metrosil VDR, Pye hybrid colour set VDR as mentioned above, any other suggestions?

Cheers, Kat

[oldeurope]

Originally Posted by Kat Manton

The only reference I can find for this is an EHT cable for a Nordmende 1320, is that what you mean?

Hi Kat, yes.

The voltage change depends in the load current.
The voltage change between no current and low current is high.
The voltage change between low current and high current is low.
A bleeder resistor permanently generates a low current.
The result is a low plate voltage change between dark and bright pictures.

The disadvantage of ac coubling is that dark pics get grey and bright
pictures lack in grey steps at the dark end. In dark pictures flyback
lines become visible. (except you have special stages to make them dark.)

I am still sure your metrosil can be repaired. Sent it to me and I'll do what
I can.

Kind regards,
Darius

[Kat Manton]

Hi,

Originally Posted by oldeurope

I am still sure your metrosil can be repaired. Sent it to me and I'll do what I can.

Thinking about this a bit I think I could repair it myself, if I could get another broken one of the same type to make up for the missing bit.

I've attached a better closeup photo; it looks to me like the original broke and the set was used in this state, eroding the broken end. Then someone refitted it in the clamp, discarding the broken-off end. Then it broke again but the set continued to be used, causing further erosion. What I've got left is too short, some of it having turned to dust.

If someone has a longish (maybe 2" or more) bit of broken late-fifties Ekco/Ferranti/Dynatron Metrosil I could either extend the remains of mine using conductive epoxy, or connect the two bits in series. Maybe coating the ends in conductive paint and then making a small connecting tube from thin brass sheet might work, along with suitable insulation to prevent corona.

So, has anyone kept a broken one they wouldn't mind parting with?

Regards, Kat.