Hello everyone.
I have an AWA RADIOLA 520MY. The screen grid volts drop to 13 Volts . High tension voltage good. If I remove the front end valve 6A8 the screen grid voltage returns to normal. I have since replaced all the old perished rubber wiring all the coupling capacitors and the electro's. The 6A8 tests good and no interelectrode shorts. I have tried another in it's place also.
Does this mean the fault lies in the first IF transformer ? With the 6A8 removed if I place my finger on the top cap of the 6AR7 I do not get a hum. However if I remove the connecting lead to the top cap I now get an audible hum which to me suggests the first IF transformer again.I am trying not to take the IF transformer out and look inside if there is another possibility I have overlooked.
Any thoughts on this.
Regards.
Jimb.

Most radios have a resistor to develop the screen-grid voltage. Have you checked it?

I would check the resistor first, all of them should be checked when replacing waxed paper caps. Check for a wiring error.

What spare pentagrids do you have in a working set. That is a way of testing. I am preferably looking for another 6A8, 6J8, ECH35, 6K8.

The later may not work as desired but the first two will definitely work in the others socket & prove if the 6A8 has an internal short (which would not be unusual).

Marc.

If I'm looking at the correct schematic for that set, the three screen grids are supplied via R9 (12K5 2 watt).

Thank you for your replies.
The screen grid resistor is R 9 ,and I had already replaced it because it was a bit high, all other resistors were checked before the set was rewired. The radio developed this fault BEFORE it was rewired, it was working fine and suddenly went dead.
I had already checked the valves for internal shorts on valve tester and replaced the front end valve. However I should try the valve in another set , as another check,as you have suggested.
Jimb.

There are valve testers & then there are emission testers. The most reliable method is to steal another valve, as above,or put that in a known set.

The fact that the voltage collapses with it in, and not out, can only mean a faulty valve, or something is being shorted when it is put in the socket. That could be a solder blob, or a pin hitting a non insulated component wire.

The previous model used a 10K resistor. for the screens 12.5K is an odd ball number these days. They probably used 2W as the draw is around 0.9Watt on the screens.

Marc

Hello Marc.
I will heed your advice, and do what you suggest , I am stumped and that is my I posted this.
I agree with your comments on valve testers.I have one crappy one the other two are very good one his a Hickock the other is a Taylor 45C.
I will let you know how I got on.
Jimb.

I have a 1938 VCT valve & circuit tester. That has some extremely useful features. It is basically an emission tester: It will not test miniatures.
The other one I have that will, is transformer-less and whoever let it into the country has a lot to answer for. It is a death trap.

Despite the warning on it (put there by me) to use it with the isolation transformer; A guy that should have known better plugged it into the mains. That did not last long as the RCD killed the circuit.

Marc

I have tried the valve in an STC radio I have that uses a 6k8 and the valve worked fine, I also tried the 6k8 in the AWA and still had same fault. I then disconnected the oscillator grid pin 5 and the SG volts came up to the 100 V as specified the 50 k bias resistor is correct this fault is now heading into the osc coil. Keeping in mind this fault existed before anything was touched, and I since have changed the caps around the coil. This is where I am up to.
Jimb

FIXED !!!!!?
I always suspected spaghetti was not good for you. There was a short across the oscillator coil due to the spaghetti decomposing resulting in an oscillator grid to oscillator plate short.
A simple enough fault , however I did not find it simple to fix.
Thank you all for your input.
Kind regards.
Jimb

Those faults are specifically designed to save money, by not needing barbers.

Marc

I was so happy to have tracked it down, the radio had not been turned on for a few years it belongs to my brother and it played fine for 10 mins. , before developing that weird fault.
This radio for it's antenna ,uses the primary side of the power transformer via a 500 pf. 2000 volt mica cap. I did not have a suitable replacement , I should have checked but did not,to see if when I replaced the power cord ,if it is still wired to the neutral side if it ever was I left the old mica in place this idea is pretty smart , just not sure if it is safe .
What are the thoughts on this , it works well and is very tidy not having an antenna wire lying around.
Regards Jim.

for it's antenna ,uses the primary side of the power transformer via a 500 pf. 2000 volt mica cap

Don't like that idea one bit.

Me either. If that condenser shorts out then its time to start planning funerals. Condensers are used to isolate 240V in some cases but they are specially designed for the purpose and not always easy to obtain. One example is touch switches. Quite often, even then, the designer of the circuit will specify two such condensers in series to provide some protection for the user against shorts.

Anything on the mains should be a line cap. Normally they are decorated with marks of the "standards" of many countries. Eg "UL".

Voltage is marked 275V~ or 275VAC although the last lot I got were 300VAC.

Eliminate it and ground the chassis instead & use a real antenna.

Marc