History.
I've a Pure DRX-701ES DAB tuner that is around 17+ years old and it has become increasing problematic. Failures up to this point have been caps but the latest issue is the small multi winding toroid that has developed some primary winding issue. It blows the mains input fuse and yet the primary itself reads over a 100k ohm. It is a multi winding affair with two 0-12 volt secondaries and a single 8 volt secondary.
I'd been toying with the idea of replacing the tuner as it is DAB only (no DAB+) and this failure persuaded me and a Rotel T14 has taken its place.
It would be a shame to scrap this though as it is in otherwise pristine condition.
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Would it be possible I wondered to use a generic toroid (i.e. cheap) and rearrange the supplies. I didn't want to start hacking the chassis around or making PCB's and so on... so what was possible...
I modelled the original supply in LTspice and felt it was doable. The two rectified supplies shared a common ground point.
The modified version to use a generic toroid would see an addition regulator added to reduce the voltage into the 5 volt regulator which is PCB mounted and not on a heatsink (actually there are two 5v regulators but I only modelled one into the sim. The current draw on the 5 volt line is 400ma when on and around 300ma in standby (which is more than I would have thought actually).
Resistive dropping would work but would be messy and look messy. The PCB board is secured with very tight fitting screws into substantial metal pillars and these looked a possibility to add a 7812 reg. See top right of the image below this one.
The 470uF cap on the positive unregulated supply for the 12 volt regs would be increased to 2200uF and the 2200uF already fitted for the 5 volt reg raw supply would be removed and a 'token' 47uF fitted in its place.
The diode bridge for the -/+12 volt rails comprises a DB102S SMD bridge which seems to be rated at 1 amp with a 50 amp peak surge rating. Not shown in the simulation are two PTH thermistors feeding each side of the bridge. They measure only around 0.15 ohms in circuit.
The final implementation. In practice with the new 30va 12-0-12 volt toroid sees the raw rails come in at +17 volts and -19 volts. Perfect. You can see the additional 12 volt 'pre reg' in the simulation below.
And all seems good so far.
I've attached the simulations as they might be useful for anyone to build on and modify.
I've a Pure DRX-701ES DAB tuner that is around 17+ years old and it has become increasing problematic. Failures up to this point have been caps but the latest issue is the small multi winding toroid that has developed some primary winding issue. It blows the mains input fuse and yet the primary itself reads over a 100k ohm. It is a multi winding affair with two 0-12 volt secondaries and a single 8 volt secondary.
I'd been toying with the idea of replacing the tuner as it is DAB only (no DAB+) and this failure persuaded me and a Rotel T14 has taken its place.
It would be a shame to scrap this though as it is in otherwise pristine condition.
----------------------------------------------------------------------------------------------------------
Would it be possible I wondered to use a generic toroid (i.e. cheap) and rearrange the supplies. I didn't want to start hacking the chassis around or making PCB's and so on... so what was possible...
I modelled the original supply in LTspice and felt it was doable. The two rectified supplies shared a common ground point.
The modified version to use a generic toroid would see an addition regulator added to reduce the voltage into the 5 volt regulator which is PCB mounted and not on a heatsink (actually there are two 5v regulators but I only modelled one into the sim. The current draw on the 5 volt line is 400ma when on and around 300ma in standby (which is more than I would have thought actually).
Resistive dropping would work but would be messy and look messy. The PCB board is secured with very tight fitting screws into substantial metal pillars and these looked a possibility to add a 7812 reg. See top right of the image below this one.
The 470uF cap on the positive unregulated supply for the 12 volt regs would be increased to 2200uF and the 2200uF already fitted for the 5 volt reg raw supply would be removed and a 'token' 47uF fitted in its place.
The diode bridge for the -/+12 volt rails comprises a DB102S SMD bridge which seems to be rated at 1 amp with a 50 amp peak surge rating. Not shown in the simulation are two PTH thermistors feeding each side of the bridge. They measure only around 0.15 ohms in circuit.
The final implementation. In practice with the new 30va 12-0-12 volt toroid sees the raw rails come in at +17 volts and -19 volts. Perfect. You can see the additional 12 volt 'pre reg' in the simulation below.
And all seems good so far.
I've attached the simulations as they might be useful for anyone to build on and modify.