Sneaky Valve/Tube Socket Problem

Aarghh!! Trying to work out why my valve radio stopped working was driving me crazy.

Sit back and enjoy the detective story. 

The Story So Far...

I've completed the restoration of the Murphy UL198 valve radio that I mentioned earlier.  I'm still working on a write up on that, it'll appear at some point.

It's been fun finding out what's actually left on MW and LW in the UK, and I'm surprised at the number of Irish and Isle of Man channels I've been picking up from West Scotland.

Anyway, one morning I turned the radio on and there was total silence.

Initial Investigation

Some quick voltage checks showed that there was no voltage on the cathode of the UY41 rectifier valve, everything up to that point looked fine.

So, the initial assumption was that the UY41 had failed. I ordered a replacement off the web and waited...

"New" Valve Arrives

The New Old Stock UY41 arrived a few days later. Replacing the original one with it was very... pointless. Exactly the same symptoms.

Second Investigation

The Murphy UL198 is a transformerless radio, all the valve heaters are wired in series, like the old Christmas tree lights.

I put the original UY41 back and measured all the voltages across the valves' heaters. The interesting values were the UY41 read 10V rather than 31V and the output UL41 read 56V instead of 45V. The other three valves were a few volts high.

So, both rectifiers showed the same symptoms; the common advice is "valves rarely fail", so I assumed that as my new one must be working then my old one probably was too. Was there a fault with the UL41's heater that was causing it to read too high a voltage and, because this dropped the rectifier's heater voltage, that the rectifier wasn't in emission?

At this point I decided to reach out to the really helpful vintage-radio.net people before I simply went ahead and ordered an expensive UL41. I also thought about bodging up a solid state replacement to test things further, but wanted to know if this might cause more problems.

Vintage-Radio Investigation

Summarising the Vintage Radio discussions and the actions taken on the various suggestions therein:

* The valve socket was tested for continuity; it was pointed out that sometimes the connections can break inside the sockets. All was ok.

* Just in case there was a socket issue I used jumpers to wire up the original UY41 by clipping its pins to the underside of the socket. It didn't work.

* At this point I wired up a solid state replacement using the jumpers; a 330R resistor in place of the heater and a 1N4007 in series with a 100R inrush limiting resistor between the anode and cathode. The radio sprang back into life and all the other heater voltages were spot on.

* Someone asked what current the UY41 was drawing. Testing the two UY41s out of circuit showed that the original one's heater was drawing 140mA while the new one was at 100mA. So it appeared reasonable that the original tube was indeed dead. So why wasn't the new one working?

* A few people were suspecting that the new UY41 was dead on arrival. I used the jumpers to wire it into the circuit and, indeed, it didn't work. An email has gone off to the supplier, but no response yet.

* Between the morning of running the UY41 current test and that evening the getter of the original valve had turned white, so air had somehow got in.

The Upshot

I've bought a tested UY41 from one of the forum members and am currently waiting for it to arrive.

So that I can listen to the radio in the interim, I dismantled the old UY41 and put the solid state components into it to make a solid state UY41. I'll do a separate post on that.

Root Cause

It was now clear that I had faulty valves, but why had the original UY41 leaked now, after all these years?  

I hate leaving things half answered...

While taking the old UY41 apart I noticed that pin 3 was at a different angle to the other pins, this struck me as odd. Taking a really close, magnified, look at the tube socket from the top showed that pin 3 was different to the others. On all the other pin sockets it was quite clear that the tines were open far enough to admit a pin. However on pin three the tines were tight together and at a slight angle to the centre of the pin hole. It really took a very close look to spot this. The tines were carefully teased apart and no longer rest at an angle after a valve is removed.

I'm now pretty sure that pin three was being pushed aside every time the valve was inserted and because I'd taken it in and out a few times while I restored the radio, that this had caused a leak around the pin on the base of the valve.

This doesn't explain why my new UY41 was faulty, it didn't work first time and its getter is still pristine, but at least I have a reasonable theory for the failure of the original.