An Intimidating Experience

[Nowhere-Man]

Cant see why that is rated at 1000V, unusual for an electrolytic.  Sure it is the reservoir?   If so the 500V ones sold by the BVWS would do you just fine.

Gary

I was indeed thinking of the reservoir cap. I was surprised to see that I think the burnt cap is a 0.1 mfd which I already have. I have a few spare Sprague rated at 0.1 and 300 volts.

One more point is I can now see the reason I was struggling so much with the soldering was my wire was really just a bit too thick. Even a tiny amount of extra thickness can make soldering harder.

Today I had a rummage around and found a whole bunch of perfectly gauged wire I can use. I find if the wire is a bit too thick you tend to get some strands left unsoldered.

[Yorkie]

When you say 'the burnt cap' do bear in mind that if it's the mains input filter, if it's across the mains, it must be a class X  one, or if its from line and/or neutral it must be Class Y. If you don't have the  correct cap, you can remove the filter cap completely - it has no bearing on the performance of the set. The other cap that must be changed is the audio coupling cap to the grid of the output valve. (It would help if you mentioned the model number).

Every success with the restoration!

[Nowhere-Man]

When you say 'the burnt cap' do bear in mind that if it's the mains input filter, if it's across the mains, it must be a class X  one, or if its from line and/or neutral it must be Class Y. If you don't have the  correct cap, you can remove the filter cap completely - it has no bearing on the performance of the set. The other cap that must be changed is the audio coupling cap to the grid of the output valve. (It would help if you mentioned the model number).

Every success with the restoration!

I've just managed to buy some film caps from Maplin. These items can be found at Maplins if you streamline a search so hopefully I now have something that can be used. I think the 0.01 mFD cap is rated at over 1000 volts and it will go exactly between chassis ground and the positive lead of the large reservoir capacitor. The wax one I removed must have been very close to shorting out and is a sticky mess. The fact is almost blew the very day I took the chassis out is perhaps another classic case of sod's law.

I also have a few 0.01 bypass caps and a couple of 0.05's but I tend to replace caps in small stages so I don't get mixed up. When I have fitted these I'll go back and buy some more to be fitted.

It seems lately my soldering has picked up a bit and I've learned not to try and use too thick a wire.

It won't be that long and I'll be able to consider powering up the radio again but only after many careful tests. It seems Maplins don't have a Variac but they did have a Europe/USA converter that was cheap and it would give me 120 volts A.C. My multimeter wouldn't hold a current monitoring process and would probably blow but I know I need an Avometer as well.

I won't be rushing anyway and am very keen not to make a mess of this project. I'll need to very carefully make good connections of the filter cap and I removed the output transformer, ready to solder a filter cap to it as well and I replaced the speaker wires (the speaker has a very small tear, actually).

[Mark Hennessy]

Honestly, you need a lamp limiter, as I mentioned in post #4. Very simple to make, and will save your bacon! Plenty of explanation at the page I linked to.

The thing is, your step-down transformer (probably auto-transformer) will still be able to pass a reasonable current in fault conditions. Whereas the lamp limiter simply does its job. Don't plug the set in without one. Seriously.

[Nowhere-Man]

Honestly, you need a lamp limiter, as I mentioned in post #4. Very simple to make, and will save your bacon! Plenty of explanation at the page I linked to.

The thing is, your step-down transformer (probably auto-transformer) will still be able to pass a reasonable current in fault conditions. Whereas the lamp limiter simply does its job. Don't plug the set in without one. Seriously.

Last night I was looking into this lamp in series precaution. If I'm not mistaken, you should be able to use a basic wall socket. So far as I can fathom, the 100 watt bulb is placed in the hot circuit and then voltage is tapped off the resistance it creates as in a voltage divider. When resistances are in series and resistance varies, so does the voltage drop. So, if current is rushing through the output valve, voltage will drop at the radio end and rise at the bulb end?

I've wired in sockets on my boat and linked two core and earth from socket to socket in a radial circuit. I'm assuming when you want to drop voltage, however, you would need to wire from the drop across the bulb and then to the appliance. I'm not really sure yet as I've not heard of this before but I will be sure to take up your advice on this when I come to power it up. I don't want to see my set go up in smoke for sure.

[Nowhere-Man]

When you say 'the burnt cap' do bear in mind that if it's the mains input filter, if it's across the mains, it must be a class X  one, or if its from line and/or neutral it must be Class Y. If you don't have the  correct cap, you can remove the filter cap completely - it has no bearing on the performance of the set. The other cap that must be changed is the audio coupling cap to the grid of the output valve. (It would help if you mentioned the model number).

Every success with the restoration!

I've had a heck of a day! Ever seen The Incredible Hulk seventies TV series with Bill Bixby? For example, the pilot episode when he's changing his tyre, bangs his head in the middle of a storm and then the clothes start to rip and he turns green?

Basically, the output transformer on the Ultra ACDC set is held into place by screws and nuts so small, it's a nightmare to fit them back. I was at it for well over an hour, clueless as to how to get my hand in the gap so as to somehow hold the microscopic nut one side of the chassis while trying to get the screw to thread into it from the other side. No room for my hands. I think it was the part where the miniscule nut dropped into the speaker diaphragm that nearly drove me up the wall. Horrendous! I also remain clueless as to why the designers used such small bolts when there is ample room to place a proper bolt and create access to it!!

Guess how it resolved? I had to use superglue to glue the miniature nut to the end of a cocktail stick and gently try and hold it steady and ready to take the screw at the other side as it was turned. Meantime the force of the speaker magnet was pulling the screwdriver and clamping it. Yes, I did try to magnetise the screwdriver bolts but that didn't work at all as the bolts were immune. I

The rest of today's work went well. I enclose a pic of the new capacitor that goes across the output transformer quite neatly. They are film caps so what I did was clamp the wires with needle nosed pliers to sink the heat. I replaced both speaker wires and am happy I managed to solder decent joints.

Just one thing has had me worried now. When I tested continuity across the 0.01 mFD cap across the O.T., I was actually getting an Ohms reading. Also, a crackle from the speaker. I figured I must have burned out the capacitor so I shunted a new one across the connection without soldering and tested from the top. It still crackled and showed Ohms but not when disconnected. So, it must be something to do with the windings and magnetism.

[Nowhere-Man]

In the pic you can see where the reservoir filter cap has been snipped out. It actually goes between the the reservoir cap negative ground and a valve pin. I'll fit the new one tomorrow.

[ppppenguin]

@Nowhere-man, I'm not sure I follow your explanation of a lamp limiter. Here's an excellent explantion: http://www.vintage-radio.com/projects/lamp-limiter.html It doesn't have to be that sophisticated. Both switches are completely optional.

When measuring the insulation resistance of a cap you need to make sure that nothing resistive is across it. The primary of your output transformer is simply a coil of wire, it conducts electricity rather well. You can't check a cap when it's across a transformer winding.

[Nowhere-Man]

@Nowhere-man, I'm not sure I follow your explanation of a lamp limiter. Here's an excellent explantion: http://www.vintage-radio.com/projects/lamp-limiter.html It doesn't have to be that sophisticated. Both switches are completely optional.

When measuring the insulation resistance of a cap you need to make sure that nothing resistive is across it. The primary of your output transformer is simply a coil of wire, it conducts electricity rather well. You can't check a cap when it's across a transformer winding.

That would seem to be the case. I was thinking that capacitors block current and only pass voltage. I suppose the probes of my meter tapping across the lugs was just inducting voltage and that was reading on the digital display.

I was pretty quick soldering the film caps so can assume it will do its job perfectly well.

Yes, I am confused about current limiters and bulbs. The only way I understand voltage drop is when you have a battery + wire and, say, one or two resistors in series and back to negative. If the voltage is 12 volts and, say, the resistors are 8 and 4 ohms, the voltage drop across both resistors will vary.

I know valve radio hobbyists have used bulbs as voltage droppers for decades. It's just something I've never actually done to date.

[ppppenguin]

Suggest that you read about and understand Ohm's lawa and Kirchoff's laws.

http://www.bbc.co.uk/schools/gcsebitesiz...rev3.shtml
http://www.electronics-tutorials.ws/dcci...dcp_4.html