UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

arjoll · 15th Jul 2018, 5:27 am

I've got what seems to be an embarrassingly simple problem, but thought I'd post it here to get some collective wisdom, as what I initially thought was a solution doesn't seem so good after a little thought and research!

In a nutshell - I need to drop from around 30-33 V to about 12 V, at anything from 30 mA to 80 mA.

I have what I think is a late 70s or early 80s Pye "Pro-Lab" receiver that I use in the workshop. Pro-Lab was a brand they used for pre-configured systems; dad had an Akai Pro-Lab made up of Pye-assembled parts in the early 80s, while this one was Pye branded and lower end.

Earlier this year I realised that it was time to replace my Lumia 950 with a new phone (thanks to Microsoft for giving up...), and with some of the options having no headphone jack, I decided to add Bluetooth so I could listen to music out in the workshop. In the end I bought a Nokia 7 Plus which has the jack, but it was always a pain with phone cases and the like plugging things in, so kept going with the Bluetooth idea.

Last week I received the module I'd ordered on eBay - one that was advertised as for car/truck use and supporting 12-24 V. As far as I can tell it has a small regulator on board, and has an unpopulated +5 V header as well as the "V.in" which happily runs on around 8 V or more from my bench supply.

So, today I wired the thing in - it switches audio, so I've just placed it between the output of the phono amp (I don't listen to records in the workshop!) and the input switching, and pinched what appeared to be regulated +15 V from the 4558 opamp in the phono stage.

Everything looked good, until I powered it on - small flash from the on-board LED, but that was it. The rail had collapsed to around 5 V. If I'd bothered to check before that effort, I'd have noticed the resistors, Zener diodes and TO92 transistors feeding the preamp and tone controls.

Damn.

So, rather than trying to reverse engineer the tuner section in hope that it's powered from something around the same voltage but a little beefier, I've decided the best option will be just to provide my own power supply. The rails look to be around +/- 30 V, but unregulated so were heading up to 33 V.

The current drawn by the Bluetooth module seems to range from 30 mA to 70 mA depending on what it's doing, so dropping with a resistor isn't really an option. My other thought was a 7812 - I've got a pile in stock - but 33 V is getting a little close to absolute maximum ratings (35 V for the ST ones I last bought from element14), and there's still the issue of dissipating over a watt.

Next step would be those cheap little DC-DC converters on eBay and Aliexpress, but this is where I'm out of my comfort zone and just not sure how well they perform.

What is the best option here?

15th Jul 2018, 5:27 am	#1
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,457	Dropping voltage I've got what seems to be an embarrassingly simple problem, but thought I'd post it here to get some collective wisdom, as what I initially thought was a solution doesn't seem so good after a little thought and research! In a nutshell - I need to drop from around 30-33 V to about 12 V, at anything from 30 mA to 80 mA. I have what I think is a late 70s or early 80s Pye "Pro-Lab" receiver that I use in the workshop. Pro-Lab was a brand they used for pre-configured systems; dad had an Akai Pro-Lab made up of Pye-assembled parts in the early 80s, while this one was Pye branded and lower end. Earlier this year I realised that it was time to replace my Lumia 950 with a new phone (thanks to Microsoft for giving up...), and with some of the options having no headphone jack, I decided to add Bluetooth so I could listen to music out in the workshop. In the end I bought a Nokia 7 Plus which has the jack, but it was always a pain with phone cases and the like plugging things in, so kept going with the Bluetooth idea. Last week I received the module I'd ordered on eBay - one that was advertised as for car/truck use and supporting 12-24 V. As far as I can tell it has a small regulator on board, and has an unpopulated +5 V header as well as the "V.in" which happily runs on around 8 V or more from my bench supply. So, today I wired the thing in - it switches audio, so I've just placed it between the output of the phono amp (I don't listen to records in the workshop!) and the input switching, and pinched what appeared to be regulated +15 V from the 4558 opamp in the phono stage. Everything looked good, until I powered it on - small flash from the on-board LED, but that was it. The rail had collapsed to around 5 V. If I'd bothered to check before that effort, I'd have noticed the resistors, Zener diodes and TO92 transistors feeding the preamp and tone controls. Damn. So, rather than trying to reverse engineer the tuner section in hope that it's powered from something around the same voltage but a little beefier, I've decided the best option will be just to provide my own power supply. The rails look to be around +/- 30 V, but unregulated so were heading up to 33 V. The current drawn by the Bluetooth module seems to range from 30 mA to 70 mA depending on what it's doing, so dropping with a resistor isn't really an option. My other thought was a 7812 - I've got a pile in stock - but 33 V is getting a little close to absolute maximum ratings (35 V for the ST ones I last bought from element14), and there's still the issue of dissipating over a watt. Next step would be those cheap little DC-DC converters on eBay and Aliexpress, but this is where I'm out of my comfort zone and just not sure how well they perform. What is the best option here?