Guru
Joined: 05/03/2018
Location: NetherlandsPosts: 5700
| Posted: 06:38am 02 Aug 2024 |
Hi Grogster,
Looking at your photo's I notice few things on this power supply.
1/ There is 1 large electrolytic cap, which is after the secondary rectifier. But no inductor of any size, and second capacitor.
2/ There are 3 SO8 (2 nomal, 1 SO8-wide) chips in the secondary side of the power supply.
Since TV amplifiers must amplify AM modulated signals with high dynamic range, the 12V bus must be clean of any ripple and noise. So I expect that there is a 13.6V power supply, followed by a 12V linear regulator (in SO8 ?) or alike.
3/ There are 2 large test pads in the secondary side, potentially for measuring the 12V (or 13.6V), but also 3 smaller test pads exactly where the power supply passes the aluminum wall to thr RF circuit.
That would be a sign of 3 signals going to the analog circuit. One of them is most likely ground, the other 12V. But what is the 3'rd. Is it a "power good" signal, or a +30V, or +5V....?
In case you plan to replace the power supply with your own design. Make sure there is an isolation between input and the 12V output. You do not want the telephone ground cable to be hard connected to the coax cables in the TV system. That would present horor ground loops all over the area. So you need a transformer based dc-dc convertor.
Regards,
Volhout
P.S. Why replace the current PSU ? You work around the telephone line plan. The load is 12V 540mA = 6.5 watt, assume 7 watt. Since you have to work in the capabilities of the telephone line you can make a curve of volts/amps you think the telephone line can handle. There will be a peak (where the line can transfer most power) and that will be close to the maximum voltage you think you can apply to the line. That is not 12V 24V or 36V, but rather 48V or even 90V.
Can the built-in power supply work at that voltage ? When it can't, isn't it simplet to replace the bridge rectifier to a voltage doubler (need an extra E-cap), and drive the power supply with 60Vac or so... from the telephone line...
Just thoughts.
Edited 2024-08-02 16:52 by Volhout