Kolster Brandes EV40L

[Radio Tech]

Hello Folks

I have acquired a Kolster Brands BW 405 line TV which has been modified with a tuner for other channels. So far I have started to change the paper caps and most of the electrolytic caps all bar two of them. The dc voltage for the ht is 250v but when the set warms up this sinks to 190v where or what can I try to find out where this is leaking off to, also the sound is almost non existent all but for a slight pu hum. any thoughts

best wishes

Ken

[Heatercathodeshort]

I have recently restored a EV30L which is very similar.
https://www.vintage-radio.net/forum/...ad.php?t=99998
Did you apply mains to it before you started to recap it and if so did you get any results?
It's very easy to make a wiring mistake when blanket replacing components that can be frustrating and time consuming to trace.
It is possible you have a partial H.T. short or the metal rectifier is weak causing the voltage to collapse as soon as current is drawn. There are two versions of the LOPT using different EHT rectifiers that are not interchangeable. Use the type fitted to your transformer, not what the service manual states. The EHT is high for a 12" model at 10KV. John.

[Radio Tech]

Hi John

I did plug it in via a vari transformer and brought mains up gradually but at the time it seemed the main caps had a problem as the smoothing was bad

Ken

[Heatercathodeshort]

Hello Ken,
Did you just replace the main electrolytic can then test it or replace the lot? It would be interesting to know what results were obtained after replacing the main can only. John.

[Radio Tech]

Hello John

I replaced main electrolytic can and tried it, that got rid of the hum but audio weak and just got full frame with weak brilliance, on saying that, the ht dropped from 240 to 178v even after replacement of most of the waxies and other electrolytic caps, trouble is now the frame has collapsed but I wondered if it was the rectifier failing when the current is being drawn after the valves warm up, I thought of replacing the rectifier with a modern type diode maybe a bridge wired as a single diode rec, mind you I would have to put it on a insulated heat sink.

Regards

Ken

[Heatercathodeshort]

So many possibilities. I think the best line of attack is to get a reasonable raster and forget about the signal circuits for the time being.
The drop in H.T. if caused by a partial short should result in some local overheating of components revealing the problem area. A failing rectifier would not give a total collapse. The rectifier can be replaced with a silicon diode [BY127 or similar] and a 25 ohm surge limiting resistor] Obviously the line timebase is working to a high degree to give you the visible frame fault. I would check around the frame circuit taking voltage readings with a view to getting a scan.
The frame oscillator is half of the 6SN7 valve and the output is a 6V6GT. Check the anode and screen grid voltage on the 6V6GT pin 3 227v and 4 227v. Pin 8 cathode 25v. Make sure both filaments in the 6SN7 are lighting.. If you have a scope, check for the frame waveform at the anode of the 6SN7. If a scope is not to hand, feed the signal from the anode via a .01uf [any small cap will do] to the input of a battery transistor amplifier/radio etc and you should here a loud 50c/s note that can be varied with the frame hold control. you can trace it to the control grid pin 5 of the 6V6GT. Remember the chassis can be live if the mains input is incorrectly wired. Check this carefully or use an isolating transformer.
The blocking oscillator transformer if generally reliable so my guess is that the preset control sliders are U/S. I made up a new panel as can be seen from my thread. You can always bodge a few test pots into circuit until you get it running.
Work on the frame timebase and you may discover that a couple of other faults may be cured if the frame fault has a common cause. It has to be traced logically or you will end up with steak and kidney pie and custard..
I have the original manual I can scan and forward via Email [massive file size] to maintain quality if this will help. There are a lot of circuit differences that are not mentioned in the manual so take care. Regards, John.

[Radio Tech]

Hello John

Many thanks for the info I will let you know in due course how I get on, yes I would like a copy of the manual, I will pm you my e-mail

Regards

Ken

[Heatercathodeshort]

OK Ken, let us have a look at what is happening here.
First of all check and make sure you have not swopped the 6SN7 and 6V6GT valves around. Very easy to do when fault finding.
With that out of the way I would suggest feeding the 6.3v heater line via a .01uf capacitor to the grid of the 6V6GT. This should produce some sort of frame scan if the output stage is working.
You said the oscillator starts, then fades away. Very odd. Have you very carefully checked the presets? They have all been useless on the two receivers of this series I restored. The vertical hold and height controls are highly suspect. Next to recheck are you component changes. It is very easy to connect a capacitor to a wrong tag.
The oscillator is a blocking type with a transformer with coupling between the anode and control grid of the 6SN7. There is very little to stop it working as there are so few components. R74/R77/The height pot/R78C74/C73/C76/R82/R31. Check anode voltage on pin.2. There should be a small negative voltage on the control grid pin 1 when the valve is oscillating.
If you have a silicon diode it might be worth wiring it into circuit as I suggested. It will give the HT a lot more 'woof' and any short will probably come to light

[Radio Tech]

Hello John

After applying a voltage to the grid of the frame output valve via a 0.1 cap I did get a frame of sorts which proved that part was ok, I then checked around the resistors on the frame oscillator valve found one or two not really on their proper value but did find the 390K was not what it should be, I got hold of a spare 6SN7 and away the frame went but brilliance not good but saying this I had set the mains input to 240 as our voltage here is 237V but not yet put the silicon rectifier in yet as I am a bit suspicious of the reclaim rectifier and this is a selenium type, I am thinking is this the culprit for dragging the HT down, I only think this as I checked the HT with all valves removed except the line oscillator and with all the other valves back in and the 6BG6G valve out the voltage on the HT comes up to around 200V and the reclaim rectifier is in the same area as the line oscillator valve, is my theory heading in the right direction.

I forgot to mention that I put the silicon rectifier in with a 66 ohm resistor to bring the HT down to 250V and can not only get a full screen but also the sound is there as well, the only thing is after a little while the brilliance drops so I am still bothered about the reclaim rectifier and maybe the EY51 not being good

Cheers

Ken

[Heatercathodeshort]

The reclaim rectifier in mine was high resistance giving lack of width and very poor line linearity. I replaced it as a temporary measure with a BY127, simply bridged it across the original and it works fine remaining cool. This will probably perk up the line timebase and give you some extra EHT. As I mentioned before the EHT is 10kv on this range of KB receivers employing the Brimar/Cintel C12B flat faced tube. They are usually good emission wise but occasionally suffer slight loss of vacuum.
It's probably time to check the vision circuits and see if you can resolve anything and if so get the timebases fully into shape. These are very high gain receivers and should produce a picture with 18DB's of attenuation from an AURORA. Lack of vision should first lead to a check on the video output valve 6AM6/EF91. They lead a hard life and I've had them die completely. Failing that you will have to check the vision IF amplifiers. If you have a signal generator the IF's are around 10mc/s and swinging the generator [modulated] around this frequency should produce a very large signal at the output valve giving black bars on the screen. Work back from the detector until the signal vanishes or becomes very weak. Complete lack or very weak signals should lead to a check on the anode and screen grid voltages of the 6AM6 valves. Shorted decoupling capacitors and O/C feed resistors are not unknown.
Good progress so far Ken and I have no doubt you will sort it. Regards, John.

[Radio Tech]

Thanks John for the extra bit of info, I am in the process of tidying the makeshift silicon diode arrangement for the HT then will work on the reclaim rectifier, then the vision circuits, I have got some spare valves coming for the set. I will also check the values of the other components and let you know

Regards

Ken

[Radio Tech]

Hi John

The set is still in trouble with the brightness / raster fading off after the set has been on for about ten minutes or so, at this stage I am not quite sure if it is the tube it's self or the U25 EHT rectifier, when this starts to happen, advancing the brightness control seems to make this happen more quickly but on saying that the phosphor on the screen dead in the centre has what looks like a 3/4 of an inch burn mark, this can be seen in a darkened room when the screen does have some raster on it, I hope it is not the tube because I don't think I stand much chance of obtaining on that is known to be ok, any idea where I can get one John.

regards

Ken

[Freya]

Hi Ken, it sounds like the line output transformer could be damp, to prove it, warm the transformer with a hairdryer.
As soon as the raster is up from a cold start, apply the heat and if its damp it should make the raster disappear much quicker.

[Heatercathodeshort]

I doubt if the Brimar/Cintel C12B is faulty. The dark patch in the centre is probably a slight ion burn but as this is almost impossible due to the screen aluminising it is probably visible due to low EHT. It will probably vanish when it is operating correctly. The fact that you are displaying a viewable raster with such low EHT tends to prove the CRT has good emission. It is a triode without an ion trap and being aluminised will require at least 7kv to give a reasonably bright raster. These receivers display a VERY bright picture when working well at 10kv.
When the picture fades out does the U25 heater fade also? Does it have a faint blue glow within the anode 'bell' with the room lights out? Have you disconnected the EHT smoothing cap? On mine it was a Visconol with a heavy leak. You can run the chassis without it with no visible defects. If your LOPT is designed for the 6.3v heater EY51, the 2v U25 will be incredibly over run and will lose emission after a few moments. The speed of the deterioration appears to direct attention away from the LOPT but of course it could be faulty.
Appologies if I have missed anything in previous posts. It's got a bit spread out but I'm sure we will get there. Regards, John.

[Radio Tech]

Thank you John and Stephen, all this feed back I am getting from you is extremely helpful. I will try applying some heat from a hairdryer as Stephen suggests and make notes on the outcome. Yes John, I did disconnect the visconol but that didn't make any difference but I think the lop has the correct rectifier, I will have a look in a dark room to see there is a blue glow, I did not check to see if the heater on the u25 is fading but what I did notice with regard to the HT, I took the 6BG6G valve out and HT rises to 271V

Regards

Ken

[Radio Tech]

Hello John

I did look at the way the heater on the U25 EHT rectifier looks and yes it does fade as the brilliance fades, does this mean the line o/p valve is not doing its job as it should or is this something more drastic like the line o/p transformer?

Regards

Ken