I checked the 6X4 with a meter, only short I can see is there is a short between pin 6 (plate 1) and pin 7 (cathode)

Just checked the 6X4, pin 6 and 7 are shorted. (Plate 1 to cathode)
I guess that’s the problem. What usually causes it to short out?

Ah well that would do it every time! that would instantly short half of every AC wave from the transformer straight to ground.
no wonder the trannie was getting hot.

Well the valve could have distorted inside from over current due to a leaky filter cap. I've never come across a plate to cathode short but going off how 6X4's are constructed id say that's what's happened.

Thanks for your help on this. I haven’t got a spare 6X4 with me at present. I’m wondering if I just replace it with a bridge rectifier instead. Though I don’t have one of those with me either lol. Probably best to put another 6X4 in.

Time to replace it. Although I've never come across a 6X4 that spontaneously shorted in that way, I've heard of it happening.

The cause might have been a short circuit on the HT rail somewhere, either now or at some time in the past.

One thing I've noticed with radios that haven't been used for a while is to keep a close eye on the 6X4, if it starts emitting white sparks in and around the cathode, or strange light effects in that area, it means there's trouble. There's part of the cathode you can see between the 2 anodes, if it isn't smooth and glowing uniformly red then it's probably had it.

In my very early days I accidently included a short on the HT line of my radio, the transformer started humming followed by a fireworks display in the EZ80. The cathode became severed between the anodes and the heater could be seen. After that it was only a half-wave rectifier. It was a while before I found out that a 6V4 could be used as a replacement.

If you were so inclined to use a silicon rectifier, you would use two discrete diodes and either one or two power resistors to get the correct voltage drop to simulate a valve rather than a modern silicons greater efficiency. you could use just one diode and a dropping resistor for tests although you'll need a much larger filter cap because it would only be half wave.

Better off getting a replacement 6X4 but a little silicon won't hurt to help you fix up the rest of the radio.

Better to use a new valve as Silicon will substantially increase the voltage & all Electrolytics on "B" will have to be uprated to 500V minimum.

Thanks for that
I’ll just replace the Valve - I have replaced the electro caps and they are rated at 600v but I agree, prob better to just replace the rectifier valve rather than open myself up for more problems

Hi guys
An update on the Royaltone, I replaced the 6X4 and all voltages came back. The audio output stage is working fine though the radio side isn’t at this point. Also, there is a squeal when the tone control is turned up likewise at higher volumes too.
I’m not convinced the 7C7s are working properly as I have no way of testing them.
Any advice on why the squealing?
Cheers
Scott

Sounds like a dried out B+ bypass cap. The squeal when you turn up the volume means the 7C7 is working!

Did you change the other electrolytic?

The way you've described it, there might be some kind of feedback network centered around the tone control, which may be connected wrongly, or perhaps there's a faulty component in the area.

Without a schematic it's difficult to point out exactly which parts to look at.

If the speaker transformer has been replaced, perhaps the output wires got swapped when the new transformer was connected up.

Other things to suggest are to make sure the blue/red wires to the speaker transformer are kept well away from the other audio-signal wires, since feedback can occur.

EDIT: Try Ian's idea too.

I replaced the other electrolytic yes, it’s a 16mfd.
I haven’t replaced the old wax caps yet but will tomorrow and then update you.
Unfortunately I can’t locate a schematic as we still don’t actually know who built the chassis.

I consider it an exercise in futility fault finding a set with old waxed paper caps in it. Time is better spent consigning them to the bin.

It is best to replace all electrolytic caps, where fitted, not just the filters. and do check resistors as best as: they tend to go high, or occasionally open.

Marc

Well I replaced all the caps today. Radio still not working though I suspect the 6BE6 is the problem as moving the valve in the socket results is noise but no radio as such. I’ll replace it when I get one in stock and provide an update. As for the squeal, it’s still happening but does reduce significantly when I move the B+ lead to the OP transformer. The OP transformer isn’t original. I’m wondering if I re rout this wire if it will help, or would I be better to add a cap in the circuit somewhere that will eliminate the squeal.
Thoughts….