Issues with Philips B2F70U.

[Montythegreat10]

I bought this radio a few weeks ago in France at a car boot sale. When I opened it up, the dropping resistor was broken in half, the across the mains capacitor was melted, and the rectifier valve was missing. I replaced all 3, with the rectifier being replaced with a 300 ohm resistor and silicon diode across the plate and cathode, and a 100ohm resistor across the heater. I also replaced the paper capacitor on the plate of the output valve with a value I later realised was 10 times too big! (I replaced it with the correct value later).

When I switched it on on the lamp limiter, it worked perfectly, and it worked fine on mains as well. When I measured the voltage on the plate of the output valve, it seemed a little high, so I started fiddling with the value of the resistor on my rectifier replacement, and this is when the real problems started! I noticed that my resistors were beginning to smoke when I turned on the radio, although the radio was still working. Soon, the plate voltage on the output dropped really low, and the radio stopped working. I reasoned that this issue was the filter capacitor shorting so I clipped in some replacements.

When I did this, the plate voltage on the output shot up to 250v, when it is meant to be 130v, and now I can only hear a slight crackling from the speaker on all wavebands along with the gramophone setting, and there is no buzzing when I touch the centre of the volume control. I would really appreciate some help with this, because I am fast running out of ideas with this!

[ms660]

According to a schematic I'm looking at the rectifier is a UY42, a sub. heater resistance would be 310 Ohms not 100 Ohms.

The voltages shown in the schematic are for 127 VAC mains input, they would be higher for UK mains input.

Lawrence.

[Montythegreat10]

Thanks for replying. Because the radio was made in france, they were in the process of changing to 220V, so in this radio, the dropping resistor is only in use when running on 220V (there is a little tab that you short when running on 127, it is not shorted on mine). I assume that this would keep all voltages the same regardless of your supply voltage. I got the heater resistance off some website, and I am beginning yo think that the heaters were a little bright when running on full voltage, so I will be changing that! It doesn't appear like it would be the cause of the issue though.

I must have got the resistors the wrong way round! I wondered why the heaters were looking pretty bright when running on full voltage, though with the resistors swapped, the plate voltages would be even higher than now. As for the mains input voltage, the radio appears to have a set of pads on the circuit board which need to be jumped together to run it on 127V, and this will effectively bypass the dropping resistor, counteracting the effect of changing the input voltage.

[jhalphen]

Hi to all,
Hi Montythegreat10,

Philips B2F70U photos, schematics, service data here on the docTSF French vintage radio web site.
All downloads free, no inscription required.
Click on each item for full size & download :

https://www.doctsf.com/b2f-70-u/f4606

Hope this helps,

Best Regards
jhalphen
Paris/France

[Montythegreat10]

Thanks for the link, although I have already got schematics for this

[ms660]

Originally Posted by Montythegreat10

the dropping resistor is only in use when running on 220V (there is a little tab that you short when running on 127, it is not shorted on mine). I assume that this would keep all voltages the same regardless of your supply voltage.

The heater dropper resistor should maintain the heaters at the correct voltage for a mains input of 220VAC

The other resistor, the one in series with the anode of the rectifier, is in circuit when 220VAC mains is selected, it's not there to keep the HT the same as it would be if the receiver was set to run on 127VAC mains, it's there to limit the peak surge current to a value that won't destroy the rectifier should a hot switch on event occur.

In the schematic for a near identical circuit and the same valve line up (Philips B2X71U) the mains selector is set to 220VAC with a mains voltage input of 220VAC and the voltage shown on the anode of the audio output valve (UL84) is 173VDC.

Lawrence.

[Montythegreat10]

Thanks for the explanation. I have done a bit of googling, and it seems like there is a massive amount of leeway in the plate voltages of these valves, wow! The voltage I was reading was within tolerances then, although it was still far more than the 173VDC you specified. It is obvious that I made a mistake with the rectifier substitute, but I don't see how this could prevent the set from working, as it was going fine on this setup previously. I will mention that when the set was working, the plate voltage was about 150V with the overrated resistor, which lines up with what you said, meaning that it is still definitely too high for this particular circuit, even if it shouldn't blow the valves up!

[ms660]

Originally Posted by Montythegreat10

Thanks for the explanation. I have done a bit of googling, and it seems like there is a massive amount of leeway in the plate voltages of these valves, wow!

Yes, that's right, the Philips U series valves were designed to be operated in receivers that operated from a wide range of mains voltages and thus HT voltages.

Lawrence.

[Montythegreat10]

I have just checked all of the valves for shorts between grids and cathodes/plates and all seems to be ok on that front. With this the case, I am pretty stuck on why there is no sound, given that there are no longer any old paper or electrolytic caps in circuit.

[Sideband]

Start with basics. No hum when touching centre of volume control. Measure the voltages around the output stage and audio amplifier so anode, (sorry I don't use American terminology), screen grid and cathode voltages of the output valve and the anode voltage of the audio amplifier. That can tell you an awful lot about what is (or isn't) going on.

Is there ANY noise from the speaker at all....hum, crackle, hiss? Lets have those voltages and we can go from there.

[Montythegreat10]

Thanks for saying. The voltage I measured as too high was on the anode of the output valve, but I will measure the others. The speaker occasionally makes crackling noises both on radio and gramophone modes, and these seem to die away in tandem with the heater on the output valve, making it appear that the output is working. The radio/gram switch contacts are easily accessible, so I was also wondering whether it would be a good idea to try connecting a powered speaker up to where the radio comes in on the switch, to see if that side is working (possibly through an isolation capscitor)?

[ms660]

If the the output valve heater isn't working, it's anode voltage will go high.

Lawrence.

[Montythegreat10]

The heater definitely seems to work though, as I can see it glowing. The set ac/dc anyway, so if one heater dies, they will all go off.

[ms660]

Originally Posted by Montythegreat10

The set ac/dc anyway, so if one heater dies, they will all go off.

Not always, it depends on the failure mode.

Lawrence.

[Montythegreat10]

I'm guessing you're talking about heater to cathode shorts. Well in my instance, all of the heaters seem to be functional. Thanks for saying though

[ms660]

Originally Posted by Montythegreat10

I'm guessing you're talking about heater to cathode shorts. Well in my instance, all of the heaters seem to be functional. Thanks for saying though

No, I wasn't thinking heater to cathode short, that aside if the heaters appear to be working ok it would help if you could measure and post the DC voltages on the UL84 anode, g2, g1 and cathode, also the the triode anode, cathode and grid on the UBC81, all measurements WRT chassis.

Just to note that if the heater chain was to go open circuit the HT would increase to the peak value of the mains voltage unless there was a noticeable leakage between HT and ground.

Lawrence.

[Montythegreat10]

I am currently on a bus heading home, and will make sure to make those measurements as soon as I can.

[Katar83]

If you have an ESR meter and a multimeter, isn't this set simple enough to literally recheck every single component in under an hour? If you're proper stuck I'd go from there. It helped me finding other faults in my sets too, which I didn't even realise initially that they were there.
Good luck and post a pic of the set if you can, love the dial on it!

[Montythegreat10]

I have no ESR meter unfortunately, and I don't have access to a valve tester either, making it difficult to diagnose in this way. I may check some of the resistors in the output stage, but I doubt that is the problem. Thanks for the suggestion though

[Sideband]

You should only need a multitester to diagnose most faults. There is nothing special about the circuit....it's all very conventional. So lets get voltages measured first. Of course make sure the heaters are all glowing properly....series chain of course they should be. Voltages as I said before on anode and screen grid of output valve and anode of AF amplifier. That should be enough to tell you what is going on. I don't understand your comment 'The speaker occasionally makes crackling noises both on radio and gramophone modes, and these seem to die away in tandem with the heater on the output valve,....does this mean the output valve heater is going out? So all the other valve heaters are going out?

The circuit couldn't be more helpful....it even gives the expected anode current of each valve so you can always use your meter to check that as well.

One other thing you can check...what is the resistance of the output transformer primary?

Lets have some voltage readings.