I hope I'm posting this in the right place.
I need some help getting my head around this power supply schematic for a Philips CDM2/10 and boards in my B&O CD3300 cd player. The picture is an extract from the decoder board circuit. The +/-10v feeds go to the servo board. The problem is the supply going to the servo board is actually +12 & -13v. At first, I thought it might be a faulty voltage regulator, but the supply doesn’t use any.
The 25v AC comes in from the connections P41 – 4,5,6 and 7 on the right of the schematic.
My question is, how does it reduce 25v AC down to 10v DC with just rectifiers and capacitors? I assume the capacitors in the circuit are there to smooth out the supply. I think each diode should drop the voltage about 0.7v but can’t see how these give a 15v drop in total.
I’ve tested the input AC and it’s actually 20v, measured across the 2 feeds. Did I measure that correctly?
In case it helps, I’ve changed C2397, which was reading OK, when I re-capped all the board a few weeks ago. I did not change C2396 as I couldn’t find an exact replacement. The original is enclosed in a can with one +ve and three -ve connections. I have a new one with the correct voltage and capacitance with one -ve connection. I assume this will work if I connect this to all the relevant grounds.
I’ve tested each of the diodes, in circuit, and each one gives about uf=704mv, c=93nf and Ir = 7uA. I have some new ones on order, but before I change them I wanted to see if I was wasting my time as the problem is elsewhere.
I need some help getting my head around this power supply schematic for a Philips CDM2/10 and boards in my B&O CD3300 cd player. The picture is an extract from the decoder board circuit. The +/-10v feeds go to the servo board. The problem is the supply going to the servo board is actually +12 & -13v. At first, I thought it might be a faulty voltage regulator, but the supply doesn’t use any.
The 25v AC comes in from the connections P41 – 4,5,6 and 7 on the right of the schematic.
My question is, how does it reduce 25v AC down to 10v DC with just rectifiers and capacitors? I assume the capacitors in the circuit are there to smooth out the supply. I think each diode should drop the voltage about 0.7v but can’t see how these give a 15v drop in total.
I’ve tested the input AC and it’s actually 20v, measured across the 2 feeds. Did I measure that correctly?
In case it helps, I’ve changed C2397, which was reading OK, when I re-capped all the board a few weeks ago. I did not change C2396 as I couldn’t find an exact replacement. The original is enclosed in a can with one +ve and three -ve connections. I have a new one with the correct voltage and capacitance with one -ve connection. I assume this will work if I connect this to all the relevant grounds.
I’ve tested each of the diodes, in circuit, and each one gives about uf=704mv, c=93nf and Ir = 7uA. I have some new ones on order, but before I change them I wanted to see if I was wasting my time as the problem is elsewhere.
The voltage is not reduced to 10V. The bridge rectifier is connected to 20VAC. And the DC is +10 and -10, total 20V. Since you write you measure +12V and -13V this is perfectly normal. The voltage should be (20V - 2 * 0.7 V) * 1.4. Minus some tolerances and current draw this is what you measure.
Philips calls this +/-10V but as it is unregulated it is just a signal name.
I can't stop saying this, but "recapping" just an entire circuit without clue can do more damage than good.
Philips calls this +/-10V but as it is unregulated it is just a signal name.
I can't stop saying this, but "recapping" just an entire circuit without clue can do more damage than good.
You can blindly / have to replace those Philips blue axial caps. They are probably all dead.
Replacement is easily found.
For all early Philips CD players (and AFAIK the B&O CD part has been sourced from Philips) - they need to be resoldered; whatever they did back then, it certainly was not a great solder job. Not uncommon to find a two-digit number of cold solder joints. IMHO it's best to just resolder all joints before even starting to fix a fault.