Ferranti A1016 bluetooth project

[egerton]

I have obtained a fully working Ferranti A1016 in fantastic condition. The wooden case is immaculate, all glazing intact, all 3 bands work, and the back panel even has the original Ferranti printed ticket Inspected / Tested hand signed and attached with the original old string! The only issues are that there is a small amount of mains borne hum evident on the audio and the magic eye does not close up fully when tuning.... but I can guess the hum is due to smoothing caps. so I can attend to that. Anyway I noticed that is has a Gram input on the back selected from the front switch bank, so it occurred to me this could be a good candidate for bluetooth conversion. So I propose to use a cheap bluetooth module powered by 5V power which I will derive from a cheap 'buck' converter DC-DC module tapping the audio o/p cathode resistor or perhaps the scale display bulb supply? Whatever - using a buck converter gives flexibilty as anything from say 10 - 24V is ok for input giving a regulated 5V o/p. Combining the stereo o/p to mono using resistors and coupling cap and routing out thru one of the heat slots in back panel and attaching a plug so that it can be plugged into Gram i/p and off we go with a fully bluetooth capable unit with a minimum of intrusion to the original set. The set boast 2 speakers and seems to have a great tone on FM band so I think it will work well.
Well I've written rather a lot but I'm rather excited by the idea of this so I just wonder if any of you guys on here would be kind enough to pass comment on my idea for a project?
Many thanks!

[Silicon]

Linear power supplies would be a better choice. 'Switcher' power supplies are often noisy.

[egerton]

Good point. I will test one of these DC-DC modules near the radio to see if interferes. I know switched mode 'choppers' are often very bad for radiating all kinds of harmonics, but have no idea of buck type converters, so I will test. Many thanks.

[Herald1360]

A buck converter is still a "switch mode chopper".

The few mA needed for the module can be supplied by a linear regulator from the OP cathode (provided there's at least 7 volts or so there) or even a crude resistor/zener/capacitor from the HT supply.

[egerton]

Thanks for the heads up. I have just tested out the buck and it does generate a lot of 'white noise' and crud on all bands when placed near the set! So I will try out a simple zener / resistor / cap. as you suggested.

On a separate issue - this set has a mains transformer, the chassis is not 'live' in fact there is an earth chassis connection provided at the rear. I intend to replace the original 2 core mains cable with 3 core and earth the chassis to mains earth. It did appear to improve reception when I connected the chassis to mains earth, and it seems safer.

Many thanks

[Herald1360]

There's a school of thought that says earthing the chassis puts extra stress on an old transformer's insulation, though there's no issue with the added safety. Of course plugging in a direct earth adds the same stress (unless you deliberately make it an RF only earth via a 10n or less capacitor). It's also possible for a mains earth to be just another way in for the noise and RF Crap that abounds these days.

If you can't touch the chassis or any exposed metalwork when the set is as original, the safety issue is out of the picture.

It's your call......

[egerton]

Thanks for the advice. I think I will fit 3 core mains cable and earth the chassis. As there is a chassis earth socket on the back that is exposed I feel it's a good idea.

About the Gram i/p : anyone know what the i/p impedance and sensitivity might be? Would that not be intended for a ceramic pickup? I don't suppose it's a line i/p standard thing. I should get hold of a schematic. Any advice on the best place as I've seen places online that appear to charge quite a lot.

Many thanks

[ms660]

https://www.service-data.com/product...05/2829/m12105

The gram input would be for a medium to high output ceramic/xtal cartridge, a medium output type would suit the volume control adjustment better.

Lawrence.

[egerton]

Thank you. I bought the service sheet and see the gram i/p effectively has a 0.01uF and 820K pot across it, the slider of the pot taps off signal to the audio stage. I want to connect a bluetooth o/p as it's stereo I will combine the L&R channels with resistors 10K should be fine, but wonder if I need match a standard line in signal to this i/p?

[julie_m]

The level of a "standard line input" was chosen to match the output of a crystal or ceramic pickup cartridge (which perform the RIAA equalisation by mechanical means!) anyway, so don't worry too much about it.

[egerton]

I'm back on this and have sourced a bluetooth module that I can easily install inside the set. I'm hoping to power the module from the sets heater supply using full wave bridge, smoothing cap and zener with limiter resistor to get 5V DC. Unfortunately during bench testing of the module I discovered it draws rather more current than expected around 60mA and there is some surge at switch on that may be around 100mA. Anybody know what the transformer heater output rating is typically? I can't find any info . in the trader sheet I have and I don't want to overload it.

[ms660]

A bridge rectifier across the heaters as depicted in the schematic would short out the heater winding if the bridge -ve was connected to chassis, eg bridge -ve connected to Bluetooth output ground which in turn is connected to chassis.

Lawrence.

[egerton]

I will have to check this out. Trader sheet shows one side of the heaters connected to chassis. See crop attached. Transformer connection no.6 'C' goes to heaters and shown with connection to chassis common.

The possibility of taking DC from the cathode resistor of the o/p valve has been mentioned; I think I require too much current to tap the cathode of the EL84.

[ms660]

Yes, if one side of audio out from the Bluetooth is connected to the -ve supply and then connected to chassis there will be in effect a short across heaters via one of the bridge diodes, eg: anode to chassis ("C") and cathode to "A".

Lawrence.

[egerton]

Yes I can see that now. I was trying to avoid using a separate dedicated main transformer but it looks increasing likely I may have too.

[egerton]

Maybe I could use an audio coupling transformer to DC isolate the Bluetooth output from the Gram i/p of the set. Otherwise I'll have to mount a small mains transformer inside and splice into the incoming mains which I hoped to avoid.

[PJL]

Full wave rectification should work. Schottky diodes and a big reservoir capacitor might give you about 7.5-8V and there are some regulators designed for low headroom.

[Ambientnoise]

Do you mean half wave with a single diode and relatively big cap ? That should work though it unbalances the transformer a bit. Worth a try anyway.

Ken

[egerton]

I used components I had to hand - a full wave bridge 4x 1N4001 diodes, 500uF smoothing cap feeding a LM317 regulator. I gave up with the zener as the Bluetooth load fluctuates quite a bit, so a discrete reg. was easier. It works ok and draws typically 50mA from the heater line but switch on surge seems to go as high as double that. No problems encountered but I as my earlier post #11 I don't know the capacity of the sets transformer on the heater winding and I don't like to stress the old set.

Now I just need a audio coupling transformer to isolate the Bluetooth signal ground from the sets signal ground which is linked to the heater ac winding. They seem hard to come by online?

[wd40addict]

They're mainly sold for car audio use. See

https://m.ebay.co.uk/itm/Ground-Loop...0AAOSwLnlWn6kw

For instance. Note I have no personal experience of this unit.