My latest addition - Radialva Groom

[Yorkie]

Result:
Dropper Cap - 10.68uF
Surge Limit Resistor - 33 Ohms / 3 Watt

I've found a source of motor run caps and a 10uF one is the largest value that just fits into the large (electrolytic) can.

I'll order up the cap but let me know if I've missed something crucial here.

Cheers, John

Great to see the rapid progress John. If you wished to you could of course add a 0.68uF cap in parallel with the 10uF to get the exact value, but I'd have thought that for all practical purposes, 10uF alone would do fine. After all, the caps aren't close tolerance anyway. As you've concluded, there just isn't enough space for an adequate-sized heat sink for resistive droppers.

Best of luck with the rest of the restoration John.

[Robert Darwent]

Hi John,

First of all, the dial scan;

   

The inside of the dial glass was very dusty and I was unable to remove everything whilst cleaning for fear of also removing the legend. So some of the dust inevitably ended up being prominent on the scan. However, I think I've removed the worst of it digitally.

Second, the point where the aerial coupling capacitor is soldered to;

   

I hope its clear enough in the image (please excuse the dust as I have yet to clean the chassis properly) but the capacitor is soldered to the first tag on the switch wafer below the long mounting rod.

Regarding your capacitive dropper calculation made today, for a 110V supply and a 70V heater at 0.3A with a 10V surge limiter, I get 12.65 uF with 33 ohms, 3.0W for the limiter. You've somehow arrived at 10.68 uF. In any case wouldn't it be more practical for you to design the dropper for a 220V supply (which I believe is the mains in Switzerland) rather than 110V?

For my set, I am intending to use a 240V to 80V capacitive dropper for the heater chain/dial bulbs. With a seperate 240V to 120V diode/resistor dropper to provide the HT, I calculate a 330 ohm resistor at 7.5W will suffice in series with a 1N4004 diode. It's something of a compromise as regards heat, but physically small sized metal clad 330 ohm resistors at 25 or 50W are available relatively cheaply and being vastly over-rated for this application should run quite cool.

Regards

[Geaney]

Thanks Robert, this is extremely helpful. The dial scan will at least let me consider how I might make a replica further down the road. Thanks for the location of the antenna coupling capacitor. It turns out that on my selector switch that lug is 'missing' so I found a blob of solder on the stator piece it was connected to - so I soldered a replacement cap to that - so far so good.

The slight difference in cap values is because I selected an input voltage of 120V versus 110V (apologies for the typo). In any case I've ordered up both 10uF and 12uF motor run caps just in case.

In general I'd say my plan to run the set off 110v is down to lack of experience/confidence with this type of set. As I get more comfortable with these types of design I'm sure I will be more adventurous and jump more than one stage at a time.

I know I would never have made it this far without the help of yourself and the rest of the team - major thanks all round!

Hopefully the bits will arrive on Friday so I will see about getting the dropper done at the weekend (today is the Swiss National Holiday so everything is closed here).

Cheers, John

[Robert Darwent]

Hi John,

The slight difference in cap values is because I selected an input voltage of 120V versus 110V (apologies for the typo). In any case I've ordered up both 10uF and 12uF motor run caps just in case.

OK I see what you intended, in that case you are spot-on with 10.68 uF for the capacitor value.

In general I'd say my plan to run the set off 110v is down to lack of experience/confidence with this type of set. As I get more comfortable with these types of design I'm sure I will be more adventurous and jump more than one stage at a time.

That's fine John, I understand you not wanting to look too far ahead. However something to consider, if you look at the same spreadsheet results for the 'dropper resistor with diode' it gives just 50 ohms at 4.5W to achieve the same as using the 10.68 uF capacitor with 33 ohm surge limiter.

It might be worth experimenting with this as an alternative. It's a far less amount of heat to dissipate than your earlier 2 x 270 ohm in parallel arrangement, would take up less room and so be easier to fit inside the screening can, as well as being considerably cheaper in terms of component cost. A 1N4004/1N4005 type would be suitably rated for the diode.

Regards

[pwdrive]

Just a thought, if using a dropper diode don't forget the crowbar diode as well with a suitable fuse rating. If you already know this please ignore.

Lawrence.

[Geaney]

Just a thought, if using a dropper diode don't forget the crowbar diode as well with a suitable fuse rating.

Lawrence.

Put it this way Lawrence - I know each of the words in your statement but not when put in that sequence

Since I've ordered up the bits for the capacitive dropper I'll fit that first and see how we go. If that all works I think I'll get the rest of the set sorted before looking at more dropper arrangements. There's plenty of mechanical stuff to do on it - dial cord slips, wave change switch linkage needs attention, etc.

Cheers, John

[Geaney]

That's fine John, I understand you not wanting to look too far ahead. However something to consider, if you look at the same spreadsheet results for the 'dropper resistor with diode' it gives just 50 ohms at 4.5W to achieve the same as using the 10.68 uF capacitor with 33 ohm surge limiter.

It might be worth experimenting with this as an alternative. It's a far less amount of heat to dissipate than your earlier 2 x 270 ohm in parallel arrangement, would take up less room and so be easier to fit inside the screening can, as well as being considerably cheaper in terms of component cost. A 1N4004/1N4005 type would be suitably rated for the diode.

Regards

Thanks Robert, the parts for the capacitive dropper are en route - hopefully will be here tomorrow - so I'll crack on with that and if it works I'll see about the remaining aspects of the restore before tackling additional dropper options. Once the mechanical stuff is sorted the dial will be the major challenge at that point. The text being mainly white adds an additional hurdle!

Cheers, John

[pwdrive]

Hello John, the bit about the diode dropper I wrote in haste, I was trying to say that if using a diode dropper and if for some reason it goes short circuit there will be consequences, hence the crowbar diode, for anyone not familiar the crowbar diode takes the full AC negative output from a short circuit diode dropper and blows a suitably rated fuse, the "cathode" of the crowbar diode is connected to the rectified side of the diode dropper, normally the "cathode" The anode of the crowbar diode is connected to chassis/mains return, a suitably rated fuse is connected in line with the "anode" of the dropper diode.
If the diode dropper is conducting on positive peaks as designed then it's business as normal but if the dropper diode goes short circuit then the negative part of the AC waveform will be shorted to chassis via the crowbar diode and thus will blow the protective fuse in the dropper diodes "anode" circuit. From this it can be seen that any additional dropper resistor reqiured is normally connected in circuit after the diode dropper.

Hope this helps.

Lawrence.

[Geaney]

I've done a temporary install of the capacitive dropper and it's looking good. All my voltages seem just a little on the low side; heater volts = 65V and B+ = 90V. Heater chain current is about 270mA. I've set the supply voltage at 120v on the variac. The diagram says B1 is 105V. The set seems to work OK - picking up Hilversum (or at least it's all in Dutch) on shortwave, etc. Since it seems to work I'm not sure how concerned I should be about the volts being a bit low.

One thing I noticed is the surge limit resistor does get quite hot so I will make sure it is fixed to the case of the can to make sure the heat gets dissipated. I used a 5watt resistor (formula indicates 3watt - hopefully 5watt gives sufficient headroom). I may need to drill some discrete holes in the can to allow for air circulation but hopefully I can do that such that it is not too obvious.

Today is the last day of my summer hols so with any luck I'll get the can all finished and installed by the weekend. Back to work on Monday - ho hum!

Cheers, John

[Robert Darwent]

Hi John,

All my voltages seem just a little on the low side; heater volts = 65V and B+ = 90V. Heater chain current is about 270mA. I've set the supply voltage at 120v on the variac.

The voltages are low because the 10 uF capacitor by itself is a little low in value. Even with a +/-10% tolerance, which is typical for motor run types, the maximum value at best could only be 10.1uF and the calculations require 10.68uF.

One thing I noticed is the surge limit resistor does get quite hot so I will make sure it is fixed to the case of the can to make sure the heat gets dissipated. I used a 5watt resistor (formula indicates 3watt - hopefully 5watt gives sufficient headroom).

If you change the surge limiter to 10 ohms instead of the 33 ohms you are using at present you should find the voltage across the heater chain much closer to the 70v you are looking for, doing this also changes the capacitor value needed to 10.03 uF bringing it into tolerance.

Since a 10 ohm surge limiter produces a 3v drop instead of the 10v you originally allowed for, the resistor will only need a 0.9W power rating. So using a 3 or 5W type it should run much cooler and you will probably not need to make ventilation holes in the screening can.

Regards