HI folks

I have an old Philips valve radio thats just been restored. But it has this problem where it goes completely dead after a while then comes back. I understand that it could be many things doing this but what are the most common problems making this radio do this?

Thanks guys.

Steven

A hairline crack in a resistor would be what I would look for. Use some cooling sprayb ( sparingly ) or a plastic toothbrush to tap components.

At least one knows the length of a piece of string.

You will really need to monitor voltage with preferably an analogue meter, as they won't shut off. Going open will tend to cause voltage to rise, & shorts brings it down.

It could be thermal, or one of those sneaky grid resistor fails. Do check for debris, arcing and bad jointing. A Chopstick for probing is always useful.

Bad caps & resistors are common, especially underrated filter caps & old type resistors with caps on them, are a horror.

Thanks alot for your help Marcc, one day id like to know as much as you do

Firstly, it would be good to know if the fault is before or after the volume control.

Suggest do a "burp" test using your finger on a screwdriver on the volume control pot, before the fault appears so you know what to expect, and after. The result of that simple test immediately rules out half the radio.

Could possibly be your mixer valve as well. 6J8 comes to mind . They can develop a fault as such. If so it's replacement time.

I used to train TV service techs.
One of the things I taught them was, particularly when looking for intermittent faults, was the importance of collecting evidence before you dive in and replace stuff. You will generally save time in the end and be more likely to find the "real" fault, rather than just disturb it so it goes away and comes back later.
Measure first. Disturb as little as possible until you nail the culprit.
That is the key to fixing intermittent faults on the first attempt!

Example: Let's say the "burp" test rules out the audio stages. You are then left with a fault in the mixer-oscillator (6J8?) or the IF amp.

What I would do is clip a lead onto the screen of the mixer to monitor its voltage before and after the fault appears. You will then know if it's in that stage.

Conversely, you might replace the valve and find that it fixes the fault when in reality the fault was a bad socket connection. Back in the servicing days that would have resulted in an angry customer and lost time and money.

you might replace the valve and find that it fixes the fault when in reality the fault was [...]

There were plenty of valve jockeys back in the day. Our family's Kriesler TV experienced a few of them.

Where a set has been sitting a long time, it is never unusual to have corrosion on the pins and in the sockets. It does pay to take the tunes out & put them back. With me that's usually via the tester. Gently wobbling the tube can be fruitful.

Try to avoid pulling the tube out by the envelope as that can break the bond between socket and envelope. In the case of Metallised Philips tubes, that can be catastrophic as it breaks the connection to ground.