Philips B3G75U

[Wage Slave]

Right, and thank you folks. The transformer is definitely an auto transformer as predicted so I will bear that in mind being careful to keep the chassis on neutral.

Capacitors have all been identified and have been ordered. It will take 10 days or so for them to arrive. Once they have I can look to seeing what it does with power applied.

[Wage Slave]

Resistance across the mains with all valves in and power switch on is 623 ohms. HT smoothing capacitor seems OK on both the ESR meter and resistance. One side shoots up to open circuit, the other starts at about .7 MegOhms and then steadily increases to about 1.4 after a minute or so. It was still climbing steadily when the probe slipped off.

[Wage Slave]

Originally Posted by Herald1360

Check whether there is a (much) lower resistance between neutral in / neutral out and line in / line out. For safest operation there should be, other wise the output "neutral" could end up at 100V from ground with the set effectively on the same 100-0-100 as if it were on the aircon supply!

There is only a couple of ohms difference between the two with neutral effectively zero. So, I think it it must be an auto transformer as predicted.

Am I correct in thinking that in that case the key thing is to ensure that active is connected to the circuit and neutral to the chassis? A quick check with a test screwdriver will confirm this before doing anything with the set.

If I use this transformer will the mains RCD protection still work? I got the impression earlier that it would with an autotransformer but not an isolation type. Not that I intend to need it of course.

[Wage Slave]

Just a quick check. THAT capacitor is C57 in the extract from the circuit diagram attached. Or have I misunderstood?

[Alistair D]

Hi

You are correct C57 is THAT cap. All of your assumptions regarding the transformer and RCD are also correct.

One interesting point regarding the RCD. The transformer doubles the voltage and halves the current to keep the power transfer the same. 110volts@30mA = 220volts@15mA so your RCD will trip at half the current due to leakage on the 220 V side. No bad thing.

Al

[Sideband]

Does Japan still use 70 - 90MHz for FM broadcasts? If so, there will be a problem using the Philips (or any other European set) there. Just a thought. Hopefully they have adopted International standards by now.



SB

[Wage Slave]

You are right of course they do still use 70-90 MHz and it is an issue. However, I plan to use this set tuned to a mini car transmitter fed by an ipod or PC output to get Radio 5 which is all anybody (ie me) listens to in this house. I suppose I could also feed the output from a local radio to the car transmitter if there was anything worth listening to.

I could even look at finding a way to feed the audio signal in so I can use it as an amplifier/speaker once I have a working radio.

The MW band will work OK but LW doesn't exist here so that won't be of any use at all. Eventually I may well bring the set back to the UK with me but I believe there is some doubt that there will be FM broadcasts there before too long.

In the meantime I'm just enjoying the challenge of getting it working and restored.

[Sideband]

Originally Posted by Wage Slave

I believe there is some doubt that there will be FM broadcasts there before too long.

We don't want to go there in this thread or it will get closed!! Save to say that this is unlikely to happen in the foreseable future. Nuff said!!


SB

[Wage Slave]

Whoops! Anyway, just waiting on parts now.

May as well think about rigging up my lamp limiter. The set is rated at about 60 watts according to the service sheet and 73 watts on the backplate.

So, I should use a 60watt bulb. Or am I missing something?

The bulb should glow dimly when the set is idling and then brighten as the volume goes up. How dim should it be at idle? Is it like a transistor unit and be very dim or will the heaters etc pull enough power to get a decent glow even idling over?

[Wage Slave]

Just bought the manufacturer's sheet from the link at the top of the page. It has a bit more info on the paper capacitors and I see C61 (in the excerpt in my post above) on the output transformer is rated at 1,300 volts.

Is this really necessary or just what it happened to be? the replacement I ordered is 400V. A few of the others are 630V - would 400V be sufficient for these?

[Sideband]

This is presumably the tone corrector capacitor across the primary of the transformer? It is possible to get high transient voltages so you have the DC component plus whatever AC voltage in the form of audio that's riding on the top. Most of these capacitors have a rating 500v and upwards and it's not unusual to see 1000v or possibly 1500V used. Personally I would say that a modern 630V polyester would probably be the minimum to use and would be quite satisfactory. You are only likely to get excessive voltage here if the speaker is disconnected and the volume is up towards full.



SB

[Wage Slave]

I see. Thanks, I will have to see about getting a 630V one in that case. Annoying as shipping cost is the same for one capacitor as it is for a bunch. Maybe I can find something suitable at my local shop, but I doubt it - there are almost no people restoring valve equipment around here.

Either that or avoid running it at high volume with the speaker disconnected ......

[jjl]

Please try never to run your set with the speaker disconnected. Damage to the output transformer and other components is likely when valve amplifiers are run without a load on the output transformer.

John

[Herald1360]

Lamp limiter- a 100W bulb is fine for typical radios. The function is pretty vague anyway, the real benefit is that a short or low resistance unit under test will have the possible fault current limited to the normal running current of the lamp. The strongly positive tempco of the filament means that most OK sets will run after a fashion since the volt drop across the lamp decreases faster than the current through it for currents less than the nominal rating of the lamp.

Maybe if your set really does take 60W in normal running a parallel pair of 60W bulbs might be better than a single 100W, but it probably won't matter.

[Wage Slave]

Originally Posted by Herald1360

Maybe if your set really does take 60W in normal running a parallel pair of 60W bulbs might be better than a single 100W, but it probably won't matter.

I don't have a 100W bulb so I'll rig up two 60s. Thanks. Should I expect to see them fairly dim if all is well?

[Sideband]

Yes. Dim light is good, bright light is bad (probably). Like most bits of test gear, you need to get the hang of interpreting the results. Remember that when a tungsten bulb is cold, the resistance is low (= maximum voltage to radio), when it's hot, the resistance is high (= minimum voltage to radio). That's why they work so well as limiters as (for instance) a dead short or very low resistance across the mains or HT rail will cause maximum current = bright lamp and minimum voltage to radio but it does mean that you have to use a sensible rating for the lamp depending on what you are connecting to it. Many people have several lamps that can be switched in parallel as required depending on their requirements. Try several good radio's with different lamps to get an idea of what brightness levels to expect.


SB

[Sideband]

Originally Posted by Wage Slave

I see. Thanks, I will have to see about getting a 630V one in that case.

Well needs must and I always think that any hobby should be as cheap as possible.... don't forget that you can connect two higher value capacitors in series to lower the value and increase the voltage. Two 400V capacitors in series will make up 800v....ample for what you want. You can always put some shrink-wrap over the two capacitors to disguise them. A couple of .0022uF 400V in series will give you .0011uF 800V which will do nicely.


SB

[TrevorG3VLF]

I do not like the series capacitor solution to increase voltage capability. If one capacitor has a slightly higher leakage than the other then the voltage will not be equally distributed.

[Herald1360]

Connect a megohm across each to balance it up if you're worried. Any modern dielectric should have a leakage resistance way above 1 megohm. Two megs across the TX primary won't upset anything either.

[Wage Slave]

I don't have 2 .0022s to hand so I think I'll just bite the bullet and order a 1,600V polypropylene. It's only really galling because I could have included it with the earlier order and saved £7.50 in delivery charges. No big deal really but I could easily have avoided it.