Ekco TS46 1948

[matspar]

Hi all,

I'm slowly working my way through this lovely little Ekco TS46. Quite an early one dated January 1948. Progress has been slow but I've reached the point now where I've got a little stuck and need some advice.

I'm trying to keep this quite original. Its never been 'got at' or molested in its life. So far I'm happy with the power supply, the rf eht generator is alive (after a new P61) and there's been no disasters of any sort. So far I've replaced (restuffed) several duff caps, the pair of visconols and a few o/c and out of spec resistors. I've struggled trying to get any first light and was for a while suspecting the CRM92. To prove a point I briefly shorted the grid to cathode and was rewarded with quite a nice bright and well focused raster, which I guess proves the health of the eht supply and the tube! Try as I might, I can't get a raster to appear without introducing the short.

I've delved into the video output, cathode, grid, and brightness circuit, but so far haven't discovered anything amiss. Voltages all appear to be close to circuit. The trader sheet differs in terms of the brightness and video amp circuit which was revised on the later models.

With the aurora connected I'm getting nice strong audio, but nothing on screen.
Can anyone confirm I am looking in the right place, can this fault condition be caused by anything else that I've overlooked, could the tube still be duff?

Any pointers greatly appreciated -
Best regards
Matt

[Heatercathodeshort]

The tube looks OK. Check the cathode and grid voltages and check that the slider of the brightness control varies the voltage on the cathode. The tube is grid modulated. Check C22 .5uf decoupler from slider of brilliance pot.Check anode voltage of video amp V5 6F13. Should be an easy one. Don't forget any possible man made faults.
My guess is something is amiss around the brightness control. Regards, John.

[matspar]

Hi John!

Very many thanks for your advice. Well there's been some funny goings-on tonight which I can't quite fathom. First off, the crt grid is reading -173v the cathode 264v with the brightness fully advanced. With the pot turned down low this voltage is reaching 384v

As I was probing about tonight the raster appeared all on its own, but with loss of all signals from the aurora but responsive with the brightness pot. Bit strange. I left it running while I investigated this. After about 10 mins there was a pop and both 500ma fuses blew (Probably an intermittent short in the UU8 which I know these sets are prone to) In went a new pair of fuses and we were soon back with the blank raster. with no signals. After about another 10 mins the brightness pot had a little burn up and went o/c, but a raster was still visible at the high end of its track.
I found a suitable replacement pot and wired it in, and am now right back where I started with no raster but with plenty of audio and test card tone available.

The brightness circuit on my set is different to the trader sheet I'm working from, which only gives the circuit for the late models. On my ts46 one side of the brightness pot is returned to deck via a 56k dog bone resistor. I've already replaced the .5uf cap and the anode of V5 is around -220v. Certainly a bit odd!

[Freya]

Can you check the voltages on the crt cap with it disconnected from the tube. I suspect an internal short in the tube.
The P61 can draw excessive current and blow both fuses, as well as the UU8

[matspar]

Thanks Stephen I will take some readings and report back. Would be a real shame if it is the tube as the emission looks very good..

[Freya]

Try tapping the neck gently to see if it brings it back.

[FERNSEH]

"Try tapping the neck gently to see if it brings it back."

And rotating the tube might help.

DFWB.

[matspar]

With the crt base out of circuit the grid tag reads -216v and the cathode 308v.
On the circuit diagram the cathode volts are shown as 250v but no grid volts are quoted.

[FERNSEH]

"With the crt base out of circuit the grid tag reads -216v and the cathode 308v."

Is that 216Volts negative?

DFWB.

[matspar]

I've just tried rotating it 180 degrees, and some gentle tapping but with no improvements. Interestingly in its new position the frame scan is now central and fills the entire faceplate. (raster with G&C shorted) Previously the raster was too high and wouldn't fill the lower area of the screen even with the height fully advanced.
David yes the 216v is negative

Matt

[FERNSEH]

The cathode ray tube receives it's video signal on the grid, thus the video waveform is positive going. A negative voltage on the grid could be due to oscillation in the vision RF amplifier stages. Try removing the video amplifier valve (V6) and check the CRT grid voltage.

DFWB.

[Freya]

Just checked mine, the grid is 217volt to 300volt with the adjustment of the brightness, cathode is 229v with no signal input

[matspar]

Many thanks David and Stephen, I had assumed the grid bias was negative so this is very interesting. I won't condemn the tube just yet then! will let you know how I get on when I next have a session.

Best regards Matt

[Freya]

Very curious myself so its an interesting fault.

[matspar]

Couldn't resist it. With V6 (6F18) removed I still have a negative voltage (-256v) on the grid. Bit late now so I'll save it for another day. Any thoughts on diagnosis or how to proceed are more than welcome! Goodnight all

[FERNSEH]

According to the 1960/61 Ediswan-Mazda valve and cathode ray tube data book the cut-off voltage for the CRM92A CRT is -48 volts. So assuming the cathode voltage is say, 300, then the grid voltage will be 252 for the CRT to be cut-off.
Removing the video amplifier valve will cause the CRT grid volts to attain full HT potential. That being so it's best check the CRT voltages with the base connector disconnected.
Any evidence of instability in the vision RF stages will result in a negative voltage across the video detector load resistor (R18) when the TV signal input is disconnected.

DFWB.

[PJL]

Don't go bashing the tube as there is no evidence to support it being faulty.

As David says, instability is the only explanation for the -220V anode voltage on V5 and that would be a very large RF signal.

The inductor in the video output anode load might explain how the RF is able to break through from the grid to the anode via capacitive coupling.

Have you replaced all the caps in the video amplifier?

[Freya]

C19 is a good bet as its hidden between the chassis and the RF deck.

[FERNSEH]

Matspar wrote: "Couldn't resist it. With V6 (6F18) removed I still have a negative voltage (-256v) on the grid. Bit late now so I'll save it for another day. Any thoughts on diagnosis or how to proceed are more than welcome! Goodnight all"

Hi Matt,
So where's that negative grid voltage coming from? Here's an idea. There's some AC ripple or HF on the HT line that supplies the video amplifier. The interference limiter diode V7 is rectifying the AC signal. C23 is likely to be leaky. The negative CRT voltage is a result of the action of the diode D7. Remove D7 and see what happens.

DFWB.

[Heatercathodeshort]

Instability in the RF unit is indeed a possibility. My example uses silver mica caps for decoupling and didn't require any attention in this area. The CRM 92/121 is a rotten tube when it comes to low emission and H/K shorts but I have never experienced a grid/cathode short. John.