Philips B3G75U

[Herald1360]

Before you do that, I have a couple of 2n2 2000V caps you could have. Postage should only be about a couple of quid.

As a bonus they have Philips written on them. Downside is they are bright blue radial package types....

PM if you fancy them.

[Wage Slave]

That would have been great but unfortunately I bit the bullet and ordered about a hour earlier. No biggie but thank you so much anyway.

[Alistair D]

I forgot to mention earlier, always unplug a vale if you are going to solder to its base. The heat can crack the glass.

If the parts have not arrived yet you can always check that the mains switch works ok and check the heater continuity(meter on ohms across the mains input). Doing it that way checks the dropper as well.

Al

[Wage Slave]

Thanks for the steer on removing the valves before soldering. The parts arrived today but I won't have time to work on the set until the weekend.

The mains switch works fine and the meter across the mains input with the power switch on and all valves in gives 623 ohms which sounded OK according to Paul's guide.

I've rigged up a mains lead for the lamps with 2 60W lamps in parallel inserted into the live wire and confirmed that it is indeed live not neutral when plugged into the transformer.

Plan is

Replace the output grid capacitor C57
Replace electro C62 as it measures a bit high for ESR
Connect mains via lamp limiter
Check that chassis is not live
Check for audio stage function if not working via screwdriver to centre wiper
Come on here and beg for more advice and help ;-)

The set is a known non worker so I am not expecting it to work off the bat. I'm kind of thinking that when it doesn't work I should proceed with changing all the paper capacitors, perhaps two at a time checking after each pair to see if anything has altered. Or should I take some valve measurements before doing that?

[Sideband]

If it's full of those large black tar type capacitors which were a favourite of Philips, they go much the same way as Hunts. Likely they will all need replacing. My approach, if I'm just assessing a radio is to cut out the mains filter cap (if fitted) to avoid the contents splattering everywhere and making a mess, replace the audio coupling as a matter of course and also any HT decouplers that are likely to have full (or nearly full) HT across them.

Doing this is likely to cover the capacitors that will do the most damage if they fail (or make the biggest bang)! All I do is identify the above caps and just temporarily solder the replacements in (only need to cut one end of the originals. The mains filter cap can be left disconnected completely during tests. The temporary connections can be tidied later.

Everyone has their own way of doing things and there is no right or wrong way. It's whatever you feel comfortable with.


SB

[Wage Slave]

Many thanks for the continued advice and support.

It is full of those big black caps and I have replacements to hand. The mains filter cap has already had its incident and been cut out. I have an X2 rated replacement to hand as well which will go in last thing.

Audio coupling capacitor? I don't suppose that's C61? Is it in the circuit diagram excerpt in post #24? Same question regarding the HT decouplers - would they be C65, C66, C67, C68 etc?

[Sideband]

The audio coupling ('that capacitor') couples the anode of the UABC80 to the control grid of the UL84 (C57 in post #24). HT decouplers are usually connected directly across the HT rail to chassis or from the screen grids to chassis. C61 is the tone corrector already discussed. C65, 66, 67 and 68 are the heater decouplers...nothing to do with HT! C56 in post #24 is an example.




SB

[Wage Slave]

Right, thanks. Would C59 be another example then?

[Wage Slave]

Good news. It works and sounds pretty good!

Just changed that capacitor and C62 because its ESR was a bit high, hooked up the lamps and powered up. LW sounds as though it is working but there are no stations to pick up. MW picks up 3 or 4 stations which is all there are around here. The volume is not fantastic and there is a bit of distortion and fair bit of whistle and interference but it is quite listenable. There is only a faint hum to be heard on FM but I know there is nothing here to pick up on frequencies I have. I will organise a car transmitter tomorrow to test that. I wasn't expecting to at that stage tonight.

The limiter lamps show no sign of glow at all. Is that a good sign? They are heavily frosted spotlight bulbs because I needed edison screws.

Now I suppose I just go through and change all the black capacitors two by two checking for function each time. How do I discharge the smoothing capacitor before working on the set? Stick an 8 ohm resistor across both sides of it?

[Herald1360]

8 ohms might be a bit brutal! Chances are it will discharge itself as the heaters cool down after the mains is off, but something like 1k to chassis will discharge any remaining energy in a few milliseconds without any serious sparks.

No glow from the series lamps is good. Try removing one of the parallel pair- the other may glow a bit then.

[Alistair D]

Well done. At least you now know where you are starting from.

It is quite possible you wont need to discharge the power supply electrolytics as the heaters continue to glow for a few seconds after switch off. PLEASE measure this on your meter before going in. The best(and safest) way of discharging the caps is to try and pick up a spare test meter lead, cut the plug off then solder the wire to one end of a wire wound resistor. The othe end of the resistor is connected to a croc clip for connection to the chassis. You will have to insulate the resistor and make sure that the resistor leads cant be bent. That could lead to not discharging the cap when you think you have. The resistor can be anything around 1k 5-7watts.

I would have expected some glow from the lamps. Try it with ony 1 lamp.

Al

[Wage Slave]

Right, thanks both. It was only that I just happen to have a wirewound 8 ohm resistor from previous exploits I'll rig up a discharger. Just to clarify - To check the cap is discharged - Meter to Volts DC, negative to chassis, positive to the two positives on the capacitor?

Will try one lamp tomorrow.

[Alistair D]

Hi Herald1360

It took me so long to type my reply we cross posted.

Wage slave

That is the correct to test if the caps are discharged.

Al

[Wage Slave]

With one lamp I get a flash when turning on that quickly dies away to a nice faint glow.

Rigged up a transmitter and can confirm that FM is not working. Nothing more than a faint unchanging hum from the speaker.

MW is working but not nearly as well as it should be. Low volume and poor signal to noise ratio being the problems.

Going to crack on with changing the capacitors now.

[stitch1]

Have you still got the lamp limiter in? You only need this for an initial safty check and it seems OK. I wouldn't expext it to work properly on reduced power. Change 'that cap' and give it full power then you will know for sure what's working and what's not.

[Wage Slave]

Ah, I was wondering about whether the lamp limiter might be keeping it restricted. Anyway, moving on, I have discovered that the components supplier has sent two wrong capacitors after cutting out old ones, so now I am stuck until they send the correct ones. Very frustrating but it will be a couple more weeks until I can test it again. One of the old ones (which I don't have a replacement for) is literally falling apart on the underside.

[Wage Slave]

Anybody know what value this mustard colour capacitor is?

Someone's telling me that it is, or is as good as, a 1.5nF (1500pF) 400V Axial Polyester Capacitor, as ordered. All were ordered from the same range and all the others delivered are like the 100nF bright yellow one. The 100nF one should be 10nF so they've already agreed to replace it. Actually, they have agreed to send a replacement mustard one with better writing on it but swear it is the correct part.

I can't really read writing on the thing but it looks like 83 somethings (nF?) and 160V?

[Wage Slave]

So an update. The replacement caps finally arrived and I fitted them tonight. Seemed fine on the limiter so took it off and gave it full power. Took a while to get into its stride but yes yes yes. It is now picks up all the available MW stations well and sounds good with plenty of volume. There is a bit of hum, well more a kind of buzz really across the whole band. Tuned to a strong station it isn't very noticeable but on a weaker one it is. It seems to suddenly appear at a certain volume control setting but not before. I haven't put the mains cap back in so that may fix it?

FM sounds terrific. I'm listening to the cricket streamed from my ipod touch via a car transmitter and it is flawless.

Just a matter of replacing the mains cap, squirting the volume control (tis a bit crackly), giving the case a bit of a polish, maybe refresh the paint and hopefully it will be re-assemble time.

Any other thoughts on why the hum/buzz on MW is there would be welcome though.

I was thinking of perhaps doing two modifications.

1. Converting it to 100V as suggested earlier by Alistair viz "convert the heater chain to 110V and convert the HT supply to a half wave voltage doubler." How tricky would this be to do?

and

2. Fitting a line level input for the ipod as using the car transmitter seems a bit redundant but on the other hand I like the thought it's still working as a radio. I read something about feeding a signal into the centre wiper of the volume pot. Given the AC/DC design I also think I read that I would also have to put a suitable Y2 rated cap on the earth? The chassis is not live as I am running it now but although the sockets here have slightly different size slots for active and neutral the plugs all defeat that and can be inserted either way round. Perhaps I can find a non standard plug that will only go in the right way round.

[stitch1]

Originally Posted by Wage Slave

Any other thoughts on why the hum/buzz on MW is there would be welcome though.

Good work

If its just on MW it could possibly be interference? I have a bench lamp that does that.

[Wage Slave]

During final tidying up and testing MW all but stopped working. Eventually I found a connection on the switch that had come loose. Once I remade that the volume returned and the buzz disappeared.