Don't shoot me... but ideas wanted about Bluetooth adaptation of DAC90A

[Nickthedentist]

Hello everyone,

I'm a big fan of Bush sets, and naturally, have a load of DAC90As. A few weeks ago, I purchased and restored an exceptionally nice ivory one. It was filthy and had more faults than these usually do, but it now works really well, except that like all my AM sets at the moment, I'm plagued by interference from SMPSUs (not necessarily in our house). This makes good old R4 LW a chore to listen to, and so this poor set has gone almost unused. I'm not sure a pantry transmitter etc. would help here.

I'm also a big fan of keeping things original, or if not, making sure that any mods are done in such a way that things can be put back to original easily enough.

However... deep breath... I'd like to adapt this one to accept an input from a tablet or phone so that I can listen to programmes of my choice without a constant drone of interference. I envisaged breaking into the audio path before the volume control, leaving the RF/IF bits in place and powered, but unused for now.

Obviously, being an AC/DC set, a Bluetooth interface seems the way to go. Suitable receiver PCBs can be had on eBay etc. for under a fiver, and work well. They're small and light, and could easily be attached somewhere inside the set in a reversible manner.

The one I have to hand at has a couple of series linear voltage regulators onboard, and so claims to require a DC supply of between +7v and +30v. No idea what the current is, but I would guess about 10mA.

Q1. Is there anywhere in a DAC90A from where I could power this module directly? Or would I be better off building a simple PSU with one of those tiny mains transformers , a bridge rectifier and a cap or two, and just taking a 240v feed from after the mains switch? I see that 0.5VA (!) ratings are available for £3 or so. I could also use one of these, but that might be perverse and pointless considering my reasons for doing all this: https://uk.rs-online.com/web/p/embed...-smps/1359215/

Q2. Where could I connect the AF signal from the BT board? Would the top end of the volume control (via a suitable DC blocking cap) work, or does the fact that the control is upstream of the UBC41 make that a non starter?

All help and suggestions gratefully received.

Nick.

[paulsherwin]

Can't you just take power from across the UL41 cathode resistor? I would connect via a 4k7-10k resistor, with a 1000uF smoothing cap at the BT module end. The current requirement is likely to be much less than 10mA but I suggest you just measure it.

You should be able to inject the signal at the top of the volume control, joining the stereo channels by 4k7 resistors.

None of the resistor values are critical of course.

[Nuvistor]

The cathode voltages of the UL41 is only 5 volts, after passing through a filter circuit there is probably not enough voltage for the BT adapter.

Top end of the V/C through resistors should be ok, may bring the grid circuit impedance down a bit but should be enough signal to drive the valves.

In another thread some one suggested using a wall wart to power the BT, but means two plugs, need to make sure the wart is a fully isolated type.

[paulsherwin]

5V should be OK if any onboard regulators are shorted out, though you might want to fit a 5.1V zener for overvoltage protection. A bit of experimentation may be needed.

[high_vacuum_house]

How about going across the last valve in the heater chain before chassis potential. with a 1N4007 diode and suitable resistor/electrolytic capacitor combination with a zener diode to clamp the voltage should its heater go O/C. This should work without upsetting the heater supply. May require a 0.1/0.01 uF capacitor in parallel to decouple any noise.

Christopher Capener

[toshiba tony]

Switch mode PSU's certainly are the worst offenders for MW\LW obliteration. In my house I have been through it and methodically turned them off and still my old radios are unusable. I suppose ham radio users, (those left) must curse the air blue. I have been in the tv trade all my adult life, and OK, they have their advantages. Engineers used to scared of them, some still are. But I would willingly go back the standard method of making voltages. I'm 62 now but can remember when our bands were free from noise on them, now it's a question of hearing the signal over the hash.Good luck with your alternative idea.

['LIVEWIRE?']

Nick, what did you use as a mains connector on the 90A illustrated. I'm currently working on a 90A in which the chassis plug and it's Paxolin panel were all broken, and would like to fit some kind of non-reversible connector, or even hard-wire a mains lead, which would guarantee that the neutral lead went, via the on/off switch, to chassis. It's not my radio, and the owner isn't over-concerned with keeping things 100%original. He simply wants a working, safe radio, which it most certainly will be. I hasten to add that the outward appearance of the 90A will be original, except for the mains connecting method.

[cheerfulcharlie]

Just stick a single mains wall socket in there which has USB sockets.
You can then directly wire the socket to the mains switch and all your modern stuff will connect to the USBs.

https://www.screwfix.com/p/lap-1-gan...er-white/8812f

They are normally rated about 2 amps across the both which should more than enough to drive the Bluetooth

[PJL]

Rectify the voltage across the scale lamps?

[paulsherwin]

You could use any AC source with a capacitive dropper to produce a suitable voltage to feed the module via the regulator after rectification.

[julie_m]

A separate low-voltage supply from a power socket with USB is no good in this case, because it would have to be common with the mains.

[Electronpusher0]

Originally Posted by julie_m

A separate low-voltage supply from a power socket with USB is no good in this case, because it would have to be common with the mains.

Well said, you just beat me to it.
Any separate power supply would have to have sufficient isolation to be able to accept mains voltage on its output.

Peter

[Mr 1936]

I have fitted a couple of inexpensive bluetooth modules into vintage sets for friends.

One non-obvious thing to watch out for is that the negative return for the module's DC supply and the ground of its audio output are commoned up inside the module (at least in the ones I have used). This gives a potential problem with hum loops, and with "digital noise" from the module getting on to the audio output. Neither of these effects is severe, but can be high enough to give annoying background noises especially when the programme material has a quiet passage.

I recommend that you rig up a temporary arrangment to check that all is well before engineering anything more permanent.

With one set, I tried a couple of approaches to "borrowing" DC power but both suffered from the effects above. In the end I gave up and resorted to fitting a miniature mains transformer, rectifier, smoother, regulator etc. to give an independent isolated power source for the module. The mains input was taken downstream of the radio's mains switch so that they powered on and off together. Luckily this was for a 1930's Murphy with lots of spare room to fit the extra components inside.

[terrybull]

Please don't shoot me but.....how about disconnecting the radio's speaker and fitting a BT module and a small ic audio amp. The radio would remain original and intact. Perhaps a couple of led's to illuminate the dial glass. I'm sure a volume control could be accessed on the back.

[Herald1360]

Originally Posted by julie_m

A separate low-voltage supply from a power socket with USB is no good in this case, because it would have to be common with the mains.

Yes, but wasn't the suggestion to put such a socket inside the set?


Its dc output -ve side can happily be connected to HT- to have a common signal ground with the set. No isolation is required at the USB level, that's the job of the Bluetooth (RF) connection.

[paulsherwin]

An external 5V supply was suggested in #8. I agree with Julie that that would be unsafe.

[Herald1360]

Originally Posted by cheerfulcharlie

Just stick a single mains wall socket in there which has USB sockets.
You can then directly wire the socket to the mains switch and all your modern stuff will connect to the USBs.

https://www.screwfix.com/p/lap-1-gan...er-white/8812f

They are normally rated about 2 amps across the both which should more than enough to drive the Bluetooth

Deffo say "in there" and suggests wiring to the mains switch (on the pot....)

Specifies a single socket which would fit inside, too.

[paulsherwin]

OK, my misreading.

[PJL]

Originally Posted by Herald1360

Its dc output -ve side can happily be connected to HT- to have a common signal ground with the set. No isolation is required at the USB level, that's the job of the Bluetooth (RF) connection.

I agree that a little low voltage mains transformer inside the cabinet should be OK even with it's output effectively tied to mains live. However, the capacitor dropper or rectification of the voltage on the lamps seems a better approach given the lack of space.

[boxdoctor]

A fairly simple and cheap approach is to divert some of the heater voltage from the bottom end of the heater chain as has alredy been suggested.
Something along the lines of the attached comes to mind.
I doubt that the UBC41 would miss a few milliamps of heater current,and the parts cost peanuts.
The series resistor (erroneously marked R2) feeding the diode, might need adjustmet depending on what your Bluetooth unit requires in the way of supply current.
Switching the audio may be best done with a s.p.d.t. switch, switching the "hot" end of the volume control between the detector and the B.T. receiver.
If the filament of the UBC41 was to ever go O.C., the zener diode would (sacraficially) protect the B.T. device from full mains voltage being applied to it. Tony.