My latest addition - Radialva Groom

[Robert Darwent]

Hi David, John and Maitiu

Did these sets have painted grooves around the cabinet, or is that my eyes deceiving me?

Yes the paint is in grooves David, the following image shows it a little clearer;

   

Quite a striking looking set. Makes a nice change from the ubiquitous DAC90A!

I must agree, in my opinion they are great looking little sets;

       

As the comparsion image shows, they are perhaps two-thirds the size of a DAC90A;

   

Is that the original back panel on yours Robert?

No it isn't, but its quite a decent attempt by someone. I shall make a more authentic repro in due course for the set;

   

Some of the comments so far have suggested that 100uF is very high for this type of circuit. Are yours definitley 100uF? if yes I'll change mine since I've now got some.

The electrolytics in my set are replacements but are 100 uF, 400 VW so that ties in with the circuit diagram. I've also found a couple of restorations of the set on the internet and both of those use 100 uF capacitors as well. So its probably a safe bet they are the correct value.

Correction re my dial bulbs - they're both marked 6.5v 100mA.

I've checked the two in mine and I've got a 6.0V, 0.1A and a 6.5V, 0.1A fitted! Both replacements of course.

When you get your chassis out of the case I'd be very keen to see a photo of the underneath!

           

Your dial looks in great shape so I may prevail on you to take a photo of it against a black background so I can see about re-creating one for my set.

Please bear with me as my flatbed scanner is not behaving itself at the moment. I'll attach a scan as soon as possible after I've sorted the problem.

Hopefully I can find where it was connected and it's on a selector switch, not directly on a coil!

Again bear with me and I'll have a look at my chassis for you John.

Regards

[Geaney]

Thanks for the great photos Robert. I can see that yours has the metal type 25L6 so it is not fouling anything.

I'm ages away from needing to re-create the dial so please don't worry - the photo you included is already very helpful.

I did a video clip update yesterday so if anyone wants to see how it's been going the link is here:

http://www.youtube.com/watch?v=EQdncc3_ar8

Cheers, John

[Geaney]

Well I got the set more or less done today so I gave it a first taste of power. I used the valves that were in it originally (except the 25L6) and I left the 50uF caps in. Nothing went bang which is always a plus for me! Some serious noise comes from the speaker even with no volume so I'd say more work is needed on the power supply. On a more positive note there does seem to be RF when I turn the volume up. However, the dropper resistors get seriously hot after just a few minutes so my current solution will have to be revised.

I think I'll tack in the 100uF caps and see if the noise diminishes.

With a 110V supply the B+ measures 80V versus the 105V on the drawing. The bias voltage on R12 is about -15v versus the -7.8V on the diagram. The AC volts after the dropper is just under 70V which seems pretty close to what I was shooting for so I suspect the heater chain current is close to OK.

So, plenty to keep me pondering. Getting an effective dropper setup in such a confined space would seem to be the main challenge at this point.

I have to say I've come to like this little set after all the effort gone into it so far.

Cheers, John

[Robert Darwent]

Good progress John!

Regarding the heater/dial bulb chain. I notice you seem to have the original wiring still in place connecting the two bulbs in sereis. I hope you are aware that one of the wires is a piece of resistive cord that makes up the 50 ohm shunt resistance.

If you look at the earlier picture of my chassis again, someone has replaced the resistive cord with normal connecting wire and a 40 ohm resistor (the large black tube at the right hand side). So it is probably a good idea to do similar with yours John, use a 47 or 51 ohm resistor at no less than 5W rating, it will have a bearing on the total voltage taken by the chain.

The resistor in normal usage dissipates just under 2.5W, but if one of the bulbs fails it takes the full 0.3A current hence its power rating needing to be a minimum of 5W, higher to ensure it runs cool.

I should have the dial scan and an image of where that coupling capacitor was attached for you tomorrow.

Regards

[Geaney]

I replaced the 50uF caps with 100uF ones and I replaced the 'substitute' valves with the ones that should be there - and the baby jumped into life! - on shortwave at least for now. There is still noise at no volume but much less than before.

I managed to squeeze the 100uF caps underneath so I can use the large can space on top for a proper heatsink for the dropper resistors. The dropper gets very hot very quickly so I didn't want to leave the set running for more than a few minutes at a time.

Getting there at last!

Cheers, John

[Robert Darwent]

...and the baby jumped into life!

Excellent John! Its just a matter of routing out the 'gremlins' now then.

The dropper gets very hot very quickly so I didn't want to leave the set running for more than a few minutes at a time.

That's was my major worry John, especially with the small size of this set, and the reason I'm going down the capacitive dropper route if at all possible with mine.

I'm intending to fix the 100 uF capacitors under the chassis out of sight, as you have done, and fit the capacitive dropper(s) inside the large can. Although in my case I will have to make something to replace the missing original.

Keep up the good work!

Regards

[Geaney]

Thanks Robert,
I managed to get the 100uF caps fitted underneath the chassis so I have that space on top. As you mentioned about the complications when driving inductive loads from a capacitive dropper I tended towards the resisitive one at least until I could get the set to work.

As of last night it works great on shortwave (Radio China International was coming in loud and clear with no additional antenna) - I haven't checked it out on the other wavebands yet.

I was aware of the resistive wire shunting the dial bulbs and I replaced this with resistors I had on hand (2 x 100 Ohm 10watt in parallel) - this seems to be ok though perhaps a bit of a sledgehammer solution!

I'm very interested in the values you select for the capacitive dropper - for sure I will do the same later on. I had one attempt at calculating them so I guess now that I have a working set I can check key voltages and see if my calculations still apply.

It's great when one of these old sets starts to speak again after so many years being silent!

Cheers, John

[Geaney]

I did some more playing around with the set this afternoon. The 'noise' seems to be the result of bad valve socket connections. After some jiggling of the valves the noise goes away completely. I seem to have reception on all three bands which is nice.

So, there remains the issue of the dropper. I'm doubtful that a resistive solution will ever be good enough (from a heat dissipation perspective in so small a cab) so I'm going to have a go at the capacitive version. I've put the data into the (Paul Stenning) spreadsheet tool and come up with the following:

Input voltage - 110V
Freq - 50Hz
Heater voltage - 70V (actual value measured when set is warm)
Heater current - 0.3A (not measured but I suppose I should!)
Surge Limiter Drop - 10V

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

[tiv48]

my main reason for recomending resistive dropper was the physical size of the cap to enable running on 110 volts
but now you can use the can from the electrolitics there is no problems.
rob t

[Geaney]

Cheers Rob, it was perfectly valid advice. Now that I have the extra space I can try out the capacitive approach and if it doesn't work out at least I have room for a decent heatsink for the resistors.

Cheers, John