Is this tube beyond redemption?

[jelida]

Having successfully restored my TV22, working beautifully with an Aurora (thanks Gerry at Crowthorne) I turned my attention to my first vintage TV attempt, a HMV 2805 that I picked up for £30 in my local auction a few years ago. All capacitors have been reformed or replaced -mostly the latter- and all resistors and valves checked and replaced if necessary. A very faint raster was obtained, and now a very faint picture from the Aurora. At the brightest I can get it, it is only visible in a darkened room - see pics. The CRT heaters are mormal at 4.4v, and the (mains derived) EHT measures 6.5Kv using an EHT probe and test meter. The tube is an Emiscope TA-10, so that should be adequate. Am I right therefore to assume that the CRT is low in emission and beyond saving? Or are there any tricks of the trade that might make this set more than an interesting ornament?

Jerry

[Anthony Thomas]

What a wonderfull television set you are to be congratulated.

Low emiision on the more common tubes was often cured with a CRT boost transformer, might this be the way to go increasing the heater voltage a little?

[peter_scott]

Hi Jerry,

See: http://vintagetvandradio.myfreeforum...php?f=5&t=4171

Peter

[jelida]

Hi both, and thanks for your comments.

Thanks for the link, Peter, I will try this.

regards,

Jerry

[Heatercathodeshort]

I was recently involved with the restoration of the TV/radiogram version of the 1805, the massive 1902 with the TA15. The chassis is identical to yours but is fitted with the 15" version of the TA10.
The tube had zero emission. Absolutely nothing even with grid and cathode shorted out and the room in complete darkness.
The TA10/15 when in good condition will produce one of the best pictures you will ever see on a receiver of this vintage [1948/9]
I connected my tube tester/reactivator [Bertha..] and at 4v the meter remained at zero. Now this is the point that a public warning must be given! Do not go beyond this stage unless you are willing to sacrifice the tube completely.

I increased the tube heater volts in steps to 8v, still zero even after a reasonable period of time had elapsed. Nothing to lose so I snapped it up to 12v. The meter kicked and a very low reading was registered. I quickly reduced the heater voltage and to my delight as the heater dimmed a flash over between grid and cathode commenced for just one second. That was enough and increasing the heater volts to the normal 4v resulted in about a 60% reading, more than enough for a reasonable picture. The gentle flash over must have removed a small amount of crud from the cathode exposing a fresh layer for service. It's a VERY delicate procedure.
This proved to be correct and even after long periods of inactivity the receiver still produces a good picture.
A dangerous procedure yes but there was nothing to lose so worth a bash.
I would add that Bertha is very gentle with the tubes and does not blast the hell out of them like many so called boosters and testers tend to do. Just a tickle is all that is required, not a shot gun.. Hope this helps, John.

[line sync]

Thats a very good picture for a tube that was scrap but you must admit john you were lucky .
I remember seing a large HMV similar to that one in davids shop over 12 years ago and was amazed at the size of it.
I,ve aiways found the CRM92,s and CRM121,s with the torpedo type electron gun can never be bought back to life once they,ve garn low .
The ones with the later gun assembly reactivate ok .

[Heatercathodeshort]

Yes I was lucky but those strange EMI Pyrex tubes do tend to go very stale. The vacuum is usually good. I would agree about the Mazda tubes. I have seen CRM121B's running at 50% boost for years!
A lot of success depends on the amount of reactivation administered. A blast will give an impressive picture for five minutes and then drop to zero. A soft tickle with a feather will last for years. J.

[mark pirate]

I wonder if I can improve my Mazda CRM92 in my Baird T164?
It is possible to see the picture, but it goes negative with a bit of brightness or contrast, I have tried the advise in the link above, but it has not really improved much, maybe I should try winding the heater volts up a bit more, I have only taken it up to 3v for a short period. if I have no luck with it, is there any other tube that could be fitted to my set?

Mark

[Heatercathodeshort]

Any 9" tube can be fitted and persuaded to work. Modifications may involve base changes, provision for differing heater volts, Adjustment to focus fields [generally triodes require a higher field than a tetrode] Provision for 1st anode supply [easy]. Sounds a lot but it takes longer to write it than carry it out. Some tubes have flatter screens than others and there may be slight differences in neck lengths. All these things are small when the alternative is a picture you can only see with the light out!
If your CRM92 is dim at 3v and it has so little emission that it won't 'kick' i.e. spark between cathode and grid when given mild reactivation, I think it is safe to say that parrot has died. Regards, John.

[mark pirate]

Hi John, it is brighter on 3v, almost watchable, but I will give it a bit of a tickle.
How many volts on the heater is safe(ish) though?

Mark

[Billy T]

Filaments can stand a lot more voltage/current then you'd think.

The attached file may help explain some of those 'miraculous' recoveries or inexplicable improvements.

The key to running heaters past their rated usage voltage and current is to avoid surges i.e. take the voltage/current up in a series of small steps. Rarely does a cathode lose its emissive coating, more likely it is poisoned.

I stumbled upon this 45+ years ago when my boss was too mean to sell me new valves at cost. Instead I recovered them from the rubbish bin, put them into the AVO valve checker and cooked them until I had usable emission. It was not in any way scientific, but I never blew a filament.

It wan't until I found this article a couple of years back that I understood the process. However, I suspect that battery valves may be somewhat more sensitive!!

Cheers

Billy

PS The original has been sympathetically edited to improve clarity and to expand some explanations that were a little brief. If you can't see where those edits are, I have succeeded in my objectives, but you can't count the italicised comment!

[Heatercathodeshort]

I would guess that 50% would be about the limit for the CRM92 series. I did have a zero emission CRM in an Ekco TS88 that refused to budge at 6v, then went O/C. It had obviously had enough of the Flower Pot Men..
The tube must be stimulated to get a small current to flow between grid and cathode to spark or boil off the dead crud, exposing a minute area of decent emissive surface. The cathode is tiny and it's delicate coating is easily destroyed beyond recovery. John.

[Heatercathodeshort]

Just to put things in perspective, when good old Mazda got it right they did a good job.
I have been using this Ekco TSC114 in my living room since November. It has the original CRM121 and has never been boosted. I have run the receiver for over 15 hours without the slightest drift. The CRM series were noted for their gun problems but if they had got it right we would probably have ended up with one of the most reliable CRT's of all time. Maybe just a quick round of applause to EDISWAN. John. [Pic taken a few moments ago in bright room]

[Billy T]

Originally Posted by Heatercathodeshort

The tube must be stimulated to get a small current to flow between grid and cathode to spark or boil off the dead crud, exposing a minute area of decent emissive surface.

That process does seem a little inconsistent with the opinions and advice of the author of the document I attached, as he proposed cathode currents of up to 50mA (or much more for line output valves) which might only be achievable by using positive G2 and anode voltages plus an appropriate G1 voltage, which could be negative or positive as the circumstances demanded.

That said, the original article lacked some specifics, so there is ample wriggle-room for other opinions and practices, and interpretation of his writings is open to the view that the grid was used as an anode in the early stages of the process.

When rejuvenating on the AVO, I used high heater volts, HT on the anode, and biased the grid to get a mid-range reading so that I could see any increase in emission. It was a long time ago now, but I know that it worked for a good number of valves, and personally I see little difference between drawing cathode current (which is the essence of the process) via the control grid, or drawing it via the anode and using G1 to control the current level.

Seems to me it is probably six of one and half dozen of the other, but personally speaking, I'd prefer not to abuse a G1, and on the AVO I could use the grid voltage to reduce the current as the emission came up.

Cheers

Billy

[Heatercathodeshort]

Hello Billy,
We are talking mostly triode CRT's here and I can only say that the method I have used for decades has proved very successful as a few Forum visitors can testify.
The tube tester/rejuvenator I use is very gentle and does not blast the cathode and destroy the CRT.
I have seen guys apply a blast from the line output valve anode and get splendid results for just 15 minutes...It also performs well at normal heater voltages and rarely needs to over run the heater unless the tube has virtually no emission.
Over the years I have used it with adaptors to clean up colour CRT's with almost complete success. The results very often last for years, in some cases longer than the original tube did from new.
We all have our own ideas and if it works and you feel comfortable with the results then what more can you ask. Pictures attached of my 'Bertha' found in a pile of rubbish at the rear of a closed down television repair shop in 1963. I think the original design was published in the 'Practical Television' for December 1957
She's done me well. Regards, John

[newlite4]

John, whats the TV pictured behind Bertha, is it one of those 'landscape' EKCOs?
Neil

[Heatercathodeshort]

No Neil. That was a deliberate plant...I knew someone would ask that question. I don't want to go astray on Mark's excellent thread but they were better known for good quality dry cells.. The tube in this one is U/S, well beyond any hope of even Bertha's gentle touch but it will make an interesting restoration for later. PM me if you want to know what it is. John.

[petertheorgan]

More likely to be a misspelling of The Tocatta that us organists have to dish up at weddings and which is normally described In the order of service as having been composed by J S Bach.

[ianm]

Originally Posted by newlite4

John, whats the TV pictured behind Bertha, is it one of those 'landscape' EKCOs?
Neil

Isn't that a Vidor CN369 or CN377?

[Heatercathodeshort]

Correct Ian. Now let's get back to tube reactivation. Regards, John.