Kolster Brandes EV40L

[Radio Tech]

Hi David,

I am impressed on your achievement of a LOP transformer substitution, I just wondered if I could find a jelly pot LOP transformer to substitute the LOP in my KB EV40L, I am still looking for a 6BG6G LOP valve for it, got one to spare?

Regards

Ken

[FERNSEH]

Hi Ken,
Many years ago I fitted a BRC 1500 line output transformer into a Bush TV22 and recently there was a discussion about the the same subject in the VRAT Forum. A 6BG6G shouldn't be a problem I'll have look upstairs. It worth noting that the earlier KB CV40 employed an EL38 as the line output valve. Obviously KB prefer to use American valves and in their later sets the the 6BG6G was the choice in the DV40 and the EV40.

www.radiomuseum.org/tubes/tube_6bg6g.html
www.radiomuseum.org/tubes/tube_el38.html

DFWB.

[Radio Tech]

Hello folks again,

I know John has bee feeding me info on this KB EV40L and now it has got to the stage where I have managed to get hold of another 6BG6G line output valve and another U25 EHT rectifier but sadly the brilliance still fades after around ten or so minutes after it has warmed up and the heater on the U25 fades at the same time. Has anyone any other ideas what this problem could be, is there something I have missed around the line output stage.

Best wishes

Ken

[Freya]

What happened when you tried the warming on the lopt Ken ?

[Heatercathodeshort]

Hello again Ken,
It's so difficult attempting to diagnose problems at a distance. So many factors determine the outcome and it takes years and almost a sixth sense to solve some of the odd ones. I'm going to put aside what has been done so far and go back to basics.
First check is the coupling capacitor to the control grid of the 6BG6G. Is it definitely .1uf? It is very easy to fit an incorrect value ie .001uf. I still do it after more than 50 years! Next is the boost capacitor C71 .25 [.22] uf. The reclaim rectifer in your chassis is a nasty metal component. Have you substituted this with a silicon diode as test? I know I go on about it but have you very carefully checked the slider presets? They have a very important part to play in the operation of the line timebase.
I guess you have checked all the resistors around the oscillator/output stage for excessive out of tolerance. Anything within 20% is fine. Does the H.T. still drop? Does the line output transformer become warm [windings] after a short period? Ten minutes is a very short time for a transformer to play up. OK it can happen but a shorting turn will show up immediately, damping the stage usually to the point of total failure. Have you actually got a test card on the screen? Maybe a photo of the degrading scan may give us a clue. It's a stinker!
Failing everything you are more than willing to come over with the chassis and spend a few hours with me here in West Sussex playing with my junk. Nothing guaranteed but sometimes a second guy sees something that has presented a mind block to the original restorer. I might be able to eliminate or condemn the LOPT one way or the other.
Regards, John. [Junc 9 M25]
PS Remember it's only an old telly and we should be enjoying this..

[FERNSEH]

The circuit diagram of the KB EV30/40 line output stage. It would appear that the line output valve receives it's grid bias from the blocking oscillator via R69 which is in parallel with the coupling capacitor C84. R68 and R69 form a potental divider in order the correct negative bias is supplied to the 6BG6G. C83 is the sawtooth forming capacitor. It follows that if C84 becomes leaky the line output valve will be biased to cut-off, replace that capacitor and check the values of R68 and R69.
A metal reclaim diode is employed in this receiver. the rectified line flyback pulse is smoothed by C71 and the resultant voltage is supplied to the brightness control. Check or replace C71.

The circuit diagram was drawn in a "upside down" manner which means that the ground line is at the top. KB presented their circuit diagrams like this for many years.

I'm not absolutely sure but I believe some later sets employed a 6U4GT valve as the reclaim diode replacing the metal rectifier.

DFWB.

[Heatercathodeshort]

Hello David,
The reclaim rectifier used in the later models is a 5R4G. Very over the top for what it does. The upside down circuit diagram is very frustrating and can lead to confusion. John.

[Radio Tech]

Hello John, David, and Stephen

I haven't tried warming the lop transformer yet stephen, I will re-check all the other components John and see if I have made a bloomer, and also check what David has suggest, I am so near yet so far from fixing but will win in the end.

Regards

Ken

[Radio Tech]

Hello John, David, and Stephen

I have replaced the reclaim rectifier with a silicon diode but this only made a marginal increase on the performance, as for the lop transformer this doesn't get at all warm even after half hour or so. I took a HT voltage reading and after warm up the voltage sits at 238 Volts (see picture) I then put a scope probe on the grid of the 6BG6 line o/p valve and there is a sawtooth waveform at this point but the grid volts at initial switch on start at plus 1V then drift down to minus 23V and the grid volts and waveform stay but the brightness degrades the longer the set is on. I did wonder, as I have replaced the mains rectifier with silicon diodes and series resistors, should I have taken out the 10 Ohm w/w resistor that was originally in series with the metal rectifier, the pots that control the horizontal are or seem ok.

[PJL]

The drive is constant, the HT is pretty constant, the line output valve, EHT rectifier and efficiency diode have been replaced, the visconol is detached, but the EHT rectifier heater and CRT are still dimming.

You could monitor the boost volts but I am sure it will be falling away with the brightness. It does sound like a damp LOPT?

[Freya]

It really does sound the same problem as my ekco t221 set which was my first 405 television restoration. It was only when I tried a second lopt from another ekco (not the right one) that it was an instant cure. By heating the original it made the "dimming" fault change from a raster for maybe 2 mins to one that lasted 10-20 seconds.
At that stage in my learning it didn't dawn on me to bake it off and retry, although I still have it so it could be done.

[Heatercathodeshort]

A new line of thought. Disconnect the tube final anode connector and run the chassis. My original CRT [C12B] was slightly gassy. It would start up for a few moments then fade off with the usual argon coloured glow from the cathode. The EHT would fade away due to the heavy current demand. This will at least eliminate the CRT. John.[See thread]

[Radio Tech]

Hello John,

I first of all tried the hair dryer method that Stephen suggested but that didn't show anything, I tried running the chassis without the final anode connected but this didn't do much either but I did measure the voltage across the cathode and grid of the tube and with the brilliance turned completely down measures minus 193V and with it full up measures plus 37V, these voltages don't seem to alter much when the brilliance fades off, I have not been able to measure the EHT as nothing suitable to measure it with but I did notice that by playing with the noise interference control this only temporally and very briefly got the brilliance visible. As you know I put a silicon diode in with series resistance, should I have by-passed the 10 Ohm resistor that was originally in series with the old metal mains rectifier. I have yet to replace the very small brown Hunts capacitors in the RF vision and sound section and I might test all the valves in these sections.

Regards

Ken

Ps: Thanks everyone including you John for the help so far, this telly is one of the most intriguing instruments I have ever come across

[PJL]

Ken, what John was looking for was to determine if the tube was overloading the LOPT through excessive EHT current.

To do this, you should disconnect the final anode to the CRT and watch the EHT rectifier heater to see if it still fades away. As an alternative to the heater brightness, you could monitor the boost voltage but you would need to get some reference values with the anode connected first.

[Radio Tech]

Thanks PJL

I will try this again and make notes on voltages on where you suggest, it was my fault, I misunderstood what John meant.

Regards

Ken

[Radio Tech]

Hello PJL and John,

I have just tried what you suggested and with the final anode disconnected the EHT heater glows then fades right away, the boost voltage with or without the final anode connected remains at 274V at the cathode of the boost diode. So what do you think the problem is?

[FERNSEH]

Have you disconnected the EHT smoothing capacitor? C72 500pF 12KV.
The set will work without this component.
If C72 has been disconnected and there is no improvement then there is a likelyhood that the insulation of the EHT rectifier mounting board has failed. Disconnect the EHT rectifier V19 and also the heater wires from the transformer, reconnect the valve with the wires "in air" and see what happens.

DFWB.

[Freya]

Might it be the insulation of the heater winding itself that's breaking down ? I remember powering a u25 from a couple of batteries to prove this fault once. Very tricky as the whole lot was at EHT potential.

[FERNSEH]

Originally Posted by Freya

Might it be the insulation of the heater winding itself that's breaking down.

A very good suggestion.

DFWB.

[Radio Tech]

Hello David and PJL

I did disconnect the visconol but that didn't make any difference, but I haven't tried lifting the u25 away from the board, so I will do that next. It really does intrigue me what is the real fault but I will get there eventually.

Thank you for help so far.

regards

Ken