Mark checks over an 80 year old Ducati RR3405 Radiogram. In this episode, the radio chassis gets an overhaul - and a few surprises to come! It appeared to work, but maybe it shouldn't have.
WOW something older than me! I just turned 70 December of 2022 so this Ducati is a decade older than I am! And I'm sure it works better than me too lol
Hello Mark, Those little twisted wires as you surmised are under 10pf and were used extensively in early radio and early TV chassis to peak IF tank circuits. The twisties were called gimmicks. Cheers
Also, I was taught during my apprenticeship in the '60s not to solder the wire straight onto the valve lug, but to loop it around after pushing it through and squeezing it with pliers. This gave it mechanical strength and helped avoid dry solder joints. It also looked way better.
The twisted wires are there to tune or tweak the circuit. You find them in Telequipment scopes amongst others. It's a genuine technique, not a bodge. I'd have replaced the wire going to the top cap, the insulation was damaged, still, we all do things different.
when people say 3w of power output.. People automatically giggle, and think its not much.... I once heard a 5w DIY valve amplifier, and it absolutely rattled the walls... i was like :O So now, I know there is more to audio systems, than just the output wattage :)
proper watts, not these piddly 3w peak music power things 😉 , as a 'rule of thumb' divide pmpo by 4 to get approx rms watts out so, 3w pmpo woild be 0.75w rms 😁
Loved this! I recently re-capped a 1950s Grundig reel to reel using polypropylene caps and it sounded and worked great afterwards. Kinda fun working on old equipment that don’t have PCBs. I also had to map out which caps went where since they were not labelled either.
You would be an excellent teacher Mark. I have no skill in your area but you make it look so easy and furthermore...interesting! You should think about doing online workshops for future electronics repairers. People would sign up in droves I'm sure!
Absolutely agree. Fantastic content, very informative with none of the waffle that some channels have. Brilliant work, I hope we have lots more content to come from Mark, great technician.
brings back memories at my grand dads workshop on my School holidays helping him repair tbs and radios got me to use the mallard valve tester he used to buy components and give me a project out of practical wireless good times
I think the reason old capacitors seem to increase in capacitance is a consequence of the way the measuring instrument works. If it works by measuring the rate the capacitor charges up, and assumes the longer they take to charge the higher the capacitance, then it seems reasonable that if the component is 'leaky', that is, it lets some DC current through (acting like a resistor) then it will take longer to charge, so a higher than specified capacitance indicates a 'leaky' capacitor.
yep, thats pretty much it, leakage will skew the testing unless you use a lcr bridge which has an adjustment for 'loss'/leakage that can cancel it out, these simple testers like in the video cant handle it 😉 but some capacitance drift isnt an issue, or leakage, depends where they are in the circuit , if in a low impedance/low voltage part such as across a cathode bias resistor, no need to replace (except if electrolytic decoupling audio) , similar if ac af negative feedback network from output secondary to bottom end of volume control
should always try to power it up someohow in a controlled manner to make sure no major and possibly unobtainable parts are faulty, before even thinking of recapping, or all that effort and money will be wasted
What a lovely old piece of kit. I remember an old Ferguson Radiogram from the fifties that my parents bought new that had a similar style. But it had FM as well, and played 33 and 45 records. Used to listen to 'Journey into Space' on it. Wonderful! BTW. The reason the old caps you tested were reading 'high', is because they have gone electrically leaky. It effectively puts a resistance in parallel with the capacitor. It means that when the tester charges up the capacitor, it takes longer than it should, so it reads a higher capacitance than the true value. It is just as well you replaced them. If you had left them in, it could have damaged some critical and difficult to replace bits and pieces.
Excellent work, Mark. Don't envy your job on this one. This kind of valve equipment with no PCB is the stuff of nightmares for me. Gives me colds sweats thinking about making the wrong connection and releasing the magic smoke only to find no parts available😲
There will be an old man somewhere shouting "I added them to stop oscillation"(I'm only guessing here) There are some old guy tricks to work around maybe some issues with certain valves back in the day. I'm sure we are now losing some of that knowledge of this old technology. I personally would like to attempt a repair on something like this but I could not guarantee a fix. If this came to me I would pass on it, I wouldn't want to ruin such a beautiful device. Looking forward to the speaker repair.
Yes like the old Big Iron mainframe computers, everything was tweaked in situ based on the actual hardware quirks, similar to overclocking solid state these days
The twisted wires are called gimmick capacitors. In this case they have been connected between the two tuned coils in the IF stages. They will increase the coupling between the two tuned circuits. This will increase the sensitivity a bit and also reduce the selectivity. Connected differently they are also used to provide negative feedback to prevent oscillation. Are they added or were they originally installed and shown on the schematic?
Please note that these old caps will develop significant leakage (DC parallel resistance) over time, which may be the reason why the capacitance reads higher.
You said something wrong in the beginning.Normally top cap of HF tubes like 6A8G is the grid. in this case for HF/Mixer. A critical cap is in the audio section is C2 if it leak bring the output tube 6V6G to go with to high current bring the output transformer to burn. It is not so easy when somebody before you tryed to fix things in this old radios without knowledge. Sorry for my bad english (I am a Swedish ) I like your you tube channel . Keep going with the good work!!
Yeah, the twisted wire cap is called a "gimmick capacitor". The I.F. transformer is a classic "Optimum Coupled, Double Tuned" transformer. It is tuned by means of the two slugs and the fixed caps, so that gimmick cap is entirely redundant. You should have deleted it (although maybe the slug was broken, so the previous Tech couldn't adjust it). However the little "coil of wire" caps were very commonly used as trimmers and are original..
you can still get ducati branded capacitors, our tumble dryer has one for the motor run 😉 had to change it a couple of years ago as it went almost open circuit , common problem with modern over miniaturised caps, too thin metallising film on the plastic dielectric
I worked in Electronics for over 40 years. My early career during the 70s was with Rediffusion Television for 13 years (later taken over by Granada Television). My question is; doing all this 'Transformer Winding' and all sorts :-D how do you "make a living" in our modern world!! Maybe you have a full-time job? :-D
You are quite right. I do have a 9-5 day job (nothing to do with electronics), and I often make more money from the RU-vid video, than the repair itself. I do however, enjoy the repairs, and making the videos!
Marvellous stuff, I was unaware Ducati made electronics, ironic really, when Italian bikes had a reputation for doggy/poor electrics. (the 851 is still the sexiest bike ever made)
2:44 or you can use a DIY kit to build a medium wave Transmitter with a 3.5mm Jack input. There’s a lot on Ali for a few quid It just a bit of solderingwork to do
Are you going to give us a video on reconing a speaker Mark? I hope so as i have up in my loft a 50watt guitar amp from the 60's that needs reconing and maybe i'll give it a try :-)
I think this device is before 1942 (probably 1938) because during the WW2 le production was converted in military radio device and in 1943 the factory of the neiborough of Borgo Panigale was bombed by american B17.
Could you please explain how a twisted wire can achieve capacitance? The other thing you mentioned in waveform-modulator video, was that there were polarised, as against non polarised capacitors and their position in the power circuit governed whether they needed replacing, could you possibly flesh that out.
Hello Mark, nice work as always. I’ve seen a lot of your work and I know that you know what you’re doing, but I can’t for the life of me figure out why you’d even think of keeping those main filters in there. While new 10 uF caps would have been completely fine for the rectifier, they do make and I do have 8.2 uF high voltage electrolytics. The reason those measure over 10 even though rated for 8 is because they’re electrically leaky. I just don’t think it’s worth the risk, especially since it seems like this is a commission for a local business where it will be used. Being as old as they are they could short and then you’ll really have to worry about the rectifier or worse the transformer or the business building where it resides. With less than $5 worth of capacitors it would give peace of mind, not to mention new and perfectly functioning filters.
Yes you are right, old caps that read higher than their rated value on an LCR meter are leaky and should be replaced, it's a false economy to not to. LCR meter does capacitance by seeing how long it takes to charge the cap, and if it's leaky it's going to have more resistance, and will appear to be higher value, it's a shame more people don't realise this.
I've been an electronic hobbyist for years, although I never took it to this guy's level. And, people want me to fix old vacuum tube equipment, on occasion, and I'm like I don't want to get involved in things like this.
What I dont understand is if you have the schematic why not rip it all out and put it back nice and proper with all the caps and wires replaced or updated so it looks nice and proper. Unless it is to preserve the way it was originally wired? I would think all the wires and things need to come out...
3M company engineers invented "modern" vinyl electrical tape just after WWII ended but it wouldn't have looked like that as black became the standard color later (and this radio predates the tape by a few years for sure), and gutta-percha/Chatterton's "tape" would have been around about 100 years earlier but this obviously wasn't that, maybe it had some originally?
That first valve with the wire on top is not anode it is control grid. Pin3 and 6 the anodes pin 3 being the main and pin 6 being the oscillator supply.
Why did a wartime Italian radio use American series valves? Was this before the sanctions were imposed? These tubes all had European octal E (6.3 volt) equivalents.
