PSU For The Grundig C100L Cassette Recorder

[camtechman]

Sorry if this is the wrong section, please feel free to move it if it is.

Some of you know that I have several of these machines and so, I have many spares, including the removable battery container (box).

However, I don't have the mains PSU that was an additional item one could buy and, despite searching for one, it still eludes me.

And so, I have the circuit diagram (a basic stabilised PSU) and I'll construct & fit one into one of my spare battery containers.

I shan't need the dual 110/240 AC input, I'll just go for the 240 V AC but, from the circuit (pic #1) what A/C output voltage should the mains transformer be ?

I can see that the indicator bulb, across the tx output is 18V, so will the output voltage need to be 12V or even 15V & what sort of current rating should it be? As you will see from the diagram the DC output rating is given as 9V 500mA DC.

I'd welcome your advice.

Cheers,

Tony

[ricard]

For this type of linear converter the current in is basically the same as the current out, although a bit is lost in the zener diode (ZD10) as well.

Assuming we have a minimum hFE of 50 for the AD152 (which is what I could find in the data sheet), the maximum base current it would need to supply 500mA out would be 10mA.

Let's assume a 12V AC transformer. That would give us 1.4 * 12 = 17V, minus two diode drops in the rectifier bridge, say 15V at the emitter of the transistor. That leaves us with a voltage drop across the 120 ohm resistor of 15 - 10 = 5V, and consequentially the current through it would be 5/120 = 42 mA. This is ample current for the 10 mA required for the transistor, and results in a maximum power of 0.42 W in the zener diode, so a 0.5W or with a bit of margin 1W part would be fine here. The resistor dissipates 42mA * 5V = 0.2W, so the 1W part specified should be fine. In fact, one would suspect that the voltage at the input could be a bit higher given the resistor specification (and also considering that an 18V lamp has been specified), but I can't really see the need.
We also need to consider the voltage drop across the transistor, but under these conditions it's 15-9 = 6V which should be ok, if it goes too low then the current amplification factor drops too much.

Going to a 15V transformer we'd get 19V DC at the transistor, 75mA through the resistor, dissipating 0.675W, with the zener dissipating 0.75W when the output is unloaded.

So, something between 12V and 15V, with a 600mA current rating should be fine.

I haven't taken into consideration the increased output voltage from the transformer when the circuit is not fully loaded, if the voltage rises too much then the resistor and zener will get too hot. At 15V we're getting not too far from the specified 1W. So most likely a 12V transformer should be the best. I would guess that the 500mA rating is at full volume of the player anyway.

[mitajohn]

A similar PSU is the TN12a - Universal. The transformer secondary voltage is 10.8 VAC/at 220VAC/50mA.

[Herald1360]

Unless you really want the vintage circuit, I would think that replacing the regulator bit with a TO220 LM317, two resistors and a capacitor would be simpler and give better dc too. If the TX is 12V a 14V bulb would be ideal or use a LED and resistor on the dc output side. For (15.6 - 9)*0.5 = 3.3W regulator dissipation you'll need about a 10C/W heatsink to run continuous full output, but in practice just a U shaped bit of ally should do.

[camtechman]

I'm not that fussed about keeping it "vintage" & the LM317 alt would be ok but the battery container, although the length is designed to take D (HP2) cells it's narrow (just wide enough for the diameter of a D (HP2) cell.

Therefore, this may restrict the size of the mains transformer that can be used. Although I haven't handled the original Grundig PSU, I can imagine they fit one of those typical double bobbin low profile transformer.

Here's the pics of the actual battery container that gives an idea of the overall size and restriction.

[camtechman]

....and here's photos (from the web) of the actual PSU (unfortunately no internal pics)

[k_yller]

toroidal transformer 9 V will perfectly fit in place + L317 or L200C

[mitajohn]

I found the transformer of the power pack, as in pic.