Issues with Philips B2F70U.

[Montythegreat10]

Thanks for replying. What I meant was that when the set is turned off, the noises seem to die away with the light of the heater. Sorry it wasn't very clear. The output transformer appears to be fine, because when I put the meter across it on resistance or diode check, the speaker makes noise, even when the set is off. I will likely get an opportunity to test the voltages tomorrow evening, as this is when my dad gets back from work, as he seems to insist to be around whenever I turn the set on.

[Montythegreat10]

Hello, these are the voltages I got. They seem rather strange to me!

[ms660]

For the heater voltages was your meter set to measure AC ?

Lawrence.

[Sideband]

yes AC for the valve heaters. Those voltages all seem to be very high....as if none of the valves are drawing any current. 0v on the cathode of the output valve?? Only if the cathode is shorted to chassis in which case the anode volts would be very low and the valve would be red hot! The circuit says 6.5V which would be reasonable. There is no cathode capacitor to go short so the only way you would get 0V is if the cathode was open circuit at the valveholder....in other words, if the valve holder pin is broken (not the pin on the valve base). You are also showing 5 volts on the grid of the UBC81. No idea where that would be coming from unless the valve has developed an internal leak. there should never be any positive voltage on the grid.

Please check those voltages again, particularly around the UL84. If the cathode pin within the valveholder is broken, that will certainly account for why all your voltages are so high.

[Montythegreat10]

Oh, sorry, that was really stupid of me not to remember that the heaters are AC! as for the cathode voltage being low, on my particular set, the cathode appears to be connected to the chassis through a resistor, which makes me wonder where the 6.5v could ever come from. I measured everything from the bottom of the PCB, so it seems unlikely that the valve holder could cause this problem. Could a broken weld inside a valve cause this problem? I am only saying because at one point after the initial tests where the radio worked, I had to extract a stripped grub screw in a control to get the radio apart, and in the process of this I was lightly tapping a screwdriver into the screw with a hammer, and due to the way the wiring is, I have to balance the chassis on top of the cabinet, which is on its front, and it has slid down into the cabinet a few times. I am wondering whether all of these vibrations have damaged an internal weld in a valve, which is then causing these problems. Occasionally I get a loud crackle through the speaker when operating the radio, and when this happens, the current draw rises, and the anode voltage spikes, making it seem like internal arcing, which could be caused by something broken in a valve. I did research online, and it appears like these high voltages I am getting wont cause any damage to the valves if run for a short period of time, so I am feeling a bit more relaxed carrying out tests on this set. Thanks for the advice, and I will check that cathode connection

[ms660]

Note again that in the schematic the 6.5 Volts on the cathode of the output valve is the voltage when operating from 127 VAC mains, at 220 VAC mains it should be a lot higher (approx. 11 Volts) on 240 VAC mains maybe a bit higher still, the cathode voltage is developed across the cathode resistor due to the current flowing through it which is the current flowing through the valve....Ohms Law.

Lawrence.

[Montythegreat10]

Ok, thanks for the clarification. I will have another look at that cathode, as it seems important

[Sideband]

It's all important! You should read up on some valve theory! When the valve draws current the current flows through the cathode resistor. A proportional voltage will develop across the resistor (as said above Ohms law.....read up on it)! If the valve is not drawing any current, there will be no voltage across the resistor.

[PJL]

You also have 5V on the UBC81 triode grid, theoretically impossible when the cathode is 0V...

[Montythegreat10]

Have checked the circuit connecting the cathode of the output valve, and the holder is fine, and there is 150 ohms to ground from it, meaning that all is good. I am beginning to lean towards the idea that the valve has a broken internal connection to the cathode, which seems like it would generate the results that are happening here. Is this likely, given what has happened to the set between it working and now (detailed in my previous comment)

[Sideband]

It could still be the connector in the valveholder not making contact with the valve pin...it is far more common with some valveholders than you might think. Yes it could be an open circuit cathode within the valve but I have personally never come across that except in rectifiers.....always a first though. Try removing the valve and checking the connection fron the pin in the valveholder to chassis. The only way to prove if its the valve is to replace it! Fortunately there are plenty of UL84's available.

[Montythegreat10]

I have done that and it is fine. What I might try is tapping or wiggling the valve in the holder while measuring the cathode voltage to see if anything comes up.

[Sideband]

What type of valveholder is it? Can you take a good focused close-up of the of the contacts in the valveholder (with the valve removed of course). Some are fitted with contacts like tuning forks and one side breaks so it doesn't make contact with the valve pin or is intermittent....not so obvious if you don't know what you are looking for.

If that all appears OK, the only other thing you can do is try another valve. I might be able to find you a good working UL84 or if not you can ask in the Sets and Parts Required section and someone will probably be able to help.

[Sideband]

I have found a UL84 that I can send, looks new but has been used. Tests fine on the Avo valve tester. Yours for postage if you need it.

[Montythegreat10]

That is extremely kind of you to offer, thank you! I will send you a picture of the holder and do the test described. I will mention, as I have previously, that this radio was working before the whole business with the filter cap shorting, so it seems unlikely that the holder will have broken between now and then, although I have been pulling the valve a lot.

[Montythegreat10]

Some pictures of the holder. I will also mention that I am currently using a couple of non-polarised 30uf motor Capacitors as a temporary replacement for the 2 section electrolytic filter, and that when I put in a 130uf electrolytic photo flash capacitor instead of either of them, the anode voltage fell to about 100v. The schematic calls for 25uf Capacitors, and the one in there when I bought the radio was a 50uf. Could this be contributing to the problem?

[Sideband]

Originally Posted by Montythegreat10

and that when I put in a 130uf electrolytic photo flash capacitor instead of either of them, the anode voltage fell to about 100v.

!! You CAN'T do things like that with a valve rectifier!! There is a maximum reservoir value rating for most rectifiers that varies according to the rectifier. Exceeding the maximum value stated will damage the rectifier. I haven't looked at the maximum allowed for a UY42 but it certainly will be well below 130uF!! It will probably be OK with the 30uF types but it might not have liked the 50uF that were fitted originally.

Anyway hopefully it has survived since you do have HT but when you get the set going it might give low HT.

It's very unlikely that those valveholders will give any trouble but make sure any soldering around the valveholder itself is good and not looking 'dull'.

I'll sort out the valve for you and if you drop me a PM with your name and address I'll get it in the post. I know I haven't got a UY42...they are not common in the UK and had more use on the continent.


Just looked up the data on UY42 and max reservoir capacitance is 50uF so it was probably OK with the 50uF caps fitted.

[ms660]

Post#1 says silicon rectifier fitted.

Lawrence.

[Montythegreat10]

Don't worry! The set is currently running on a solid state rectifier that I built, as the original one was missing when I got it. I'm guessing that overrating the capacitor may cause too much current draw or something, if the voltage was dropping that much. Definitely good info to keep in mind for the future though, thank you! I'm guessing that my non-polarised motor capacitors are fine though, no ESR problems or anything?

[Sideband]

They should be OK as long as they are motor run caps and not motor start caps. If you want to revert back to a valve rectifier, you could use the much more common UY41 which is basically the same valve with a higher voltage rating. OK I missed the solid-state sub so that should be OK.

PM your details and I'll get that UL84 in the post.