Absolute beginner Q's: sensitivity at lower AC input

[pcravero]

Hello.
I recently acquired a "Radiomarelli RD-160 Amico" AM-only in plastic case, built in 1956 or so. Pictures and diagram can be found online, if interested (schematic diagram). Since it is not "domestic", I will keep my question generic.

Note: this is the first all-valve device I own.

I got it as "non working" in a thrift store for 5€. The volume knob slips and the grip is not enough to turn the shaft in the ON position: could that be the only malfunction?

I built a poor man's AC voltage divider (variometer? variac?) and with a 40W light bulb in series the radio slowly came to life. I measured about 150Vac across the RD-160 AC terminals. BTW, the dimmed bulb looks poetically warmer than usual.

I can tune the upper part of the AM band, while it shuts on the lower: maybe the variable capacitor plates short when most of their surfaces overlap. I need to check that. Nevertheless I can hear the Italian RAI broadcast on 999 kHz: it's pretty loud, but nothing else is picked up. Since it was 19h local, well beyond sunset, the AM band was full of signals for sure.

My question: can the receiver sensitivity be affected by the AC voltage input, like loosing "linearity", or this apparent on-off behavior is caused by something else?

I've had it running this way for 15 minutes, probably too little to trigger other stale problems. I am not confident yet to increase the voltage.

Thanks,
Paolo

PS: dilemma ... follow "if it isn't broken, don't fix it" or "it can be improved"?

[Graham G3ZVT]

It all sounds very promising Paolo.

Yes, you should expect lower gain from all the stages at reduced supply voltage.
Run it with the lamp-limiter a while longer hopefully the electrolytics will reform, replace C17 because it could destroy your output tube and transformer if it fails, then I would just connect it normally and see what it's like.

It could be that the variable capacitor shorts when meshed, but maybe the local oscillator stops running because of the low HT and LT, although it's usually the high frequency end of the band that suffers.

[Refugee]

C18 will also need to be replaced. It is the one that burns the output transformer if it fails.

[pcravero]

Thank you both Graham and Refugee. Someone has been into the set before because a chassis screw is certainly missing and a wire to the ferrire antenna has been re-soldered.

Perhaps both capacitors have already been replaced, but I cannot tell until the chassis is out. It will be a chance to learn something new as well as remove the thin layer of dust.

Paolo

[Nuvistor]

The transformer is an auto-transformer, the chassis could have voltage on it, needs care when working on the radio and don’t remove any safety features.

Probably me but is there a wire link missing on that diagram, I am trying to work out how the rectifier has 125vac on its anode?

[Herald1360]

Not just you..... it ain't there!

[Nuvistor]

Thanks for the info Chris, it is a very USA type circuit, it looks as though they have taken a USA chassis and stuck and auto transformer in the set for European 220 volts.

[Herald1360]

Better than a line cord, at any rate!

[pcravero]

Thanks for spotting the error in the diagram! Radiomarelli built many models with the same valve lineup and their diagrams show the correct wiring. Like the one for RD200 or RD229.
I will confirm the actual wiring once I get the chassis out.

Paolo

[pcravero]

If I keep posting we should move the thread into another topic, since this radio is not "Domestic". The picture shows the circuit. All components look original. I am glad it didn't bang! Now I will work out who is who based on the diagram, markings and lots of DVM probing. That's the fun.

Then at least the 2 green and 3 orange cylinders will be replaced.

Paolo

[turretslug]

I think you're on safe ground- "domestic" in the section title meaning in the household or consumer sense, rather than UK made. It's quite interesting to see what other countries got up to with electronics- although the forum's catchment is inevitably largely UK-made stuff, it's not a restriction!

[ms660]

Originally Posted by pcravero

Then at least the 2 green and 3 orange cylinders will be replaced.

So far as I can make out the large green cylinder capacitor is C21 (0.047uF)

The smaller green cylinder capacitor is C18 (0.01uF)

The first orange capacitor on the left hand side is C15 (0.047uf)

The orange capacitor to the right of that (at an angle) is C17 (0.01uF)

The next orange capacitor to the right is C11 (0.047uF)

If that's any help.

Lawrence.

[pcravero]

That's it, Lawrence. While you spotted them with your experience, I reverse engineered the circuit. I took a color picture, did a bit of photo editing (mainly increasing brightness to save on printer ink and to be able to write on it), printed it out and had fun tracing connections. Probably it is a standard procedure for everyone in here. Markings on orange ones have long gone, so I will trust the diagram. On the other hand I found few resistors of different value that made it harder to reverse engineer connections, but I got the most of it now.

I chopped off the two green guys so that I can use their leads to solder replacements. On the DVM they measure a bit high but I haven't bothered measuring the ESR o leakage. Replacements are on their way from UK to Italy.

I wonder if I should replace the two electrollytics or let the original rebuild. I will measure them in any case since they are easily accessible now.

Paolo

[pcravero]

Mission completed!

I have replaced all five capacitors mentioned by Lawrence with "jb" polypropylene axial rated for 630V (bought off ebay from UK seller). I was worried they wouldn't fit, but they are actually much smaller than the originals. Now I see why "axial" is better.

Then I traced the lack of oscillation in the lower part of the MW band. I slid a strip of paper between the blades until I realized that the outermost blade was bent inwards. That was another easy fix.

Hardest part was to fit the circuit back into the case, mostly getting the right spacing without all original screws&C. Some beads from my daughters' toys were put into service as spacers.

I tried powering the RD160 off 220V mains and I confirm it became more sensitive, at least considering the loudness of the background noise (and hum). I am now waiting for sunset to see how it performs with weaker distant signals. It has a small ferrite antenna, so using an external wire is a must.

Left to do: glue the plastic case, restore knobs (they are cracked and slide instead of turning the shafts).

Thanks for the support, in this thread and in all past discussions about restoring!
Paolo