Vintage Ampro projector problem

[paddy ryan]

Hi, Anybody help me on an amp problem please. I have an old Ampro 16mm sound projector, with its amplifier removed. (Its been converted, and the old amp stripped out). The pencil thin basic PEC that reads the film track is not connected to any circuit, but runs straight through a lead to a 6.5mm jack, to fit into an amplifier.. To get sound an external amplifier has to be used. The exciter lamp on the projector works okay, the PEC obviously works, and is adjusted for best sound response okay. But the problem is, when the jack is connected on a modern amplifier I cannot get any sound at all, or very very low sound.However interestingly I have a CUB 7 Practice guitar amp that does pick up the sound, but obviously a bit distorted,(Guitar Amp!) and not as much volume as I would like. Is the problem the difference in signal from the PEC to a power amplifier or what?. I have a Q240 amplifier with 120 watts output, but can get nothing on it. I have a small, cheap Chinese amp with a microphone output, and have tried that, but still nothing. Advice would be much appreciated.

 

[rjenkinsgb]

The "ideal"circuit it needs depends on the type of photocell it uses.

If it's a light-dependant resistor, it needs a bias voltage via another resistor, then audio taking from the junction of the two via a capacitor; something similar to a two-wire electret mic input circuit.

If it's a photodiode, that may need to be connected in reverse-bias mode in the feedback loop of an amplifier - or with a similar resistor and capacitor arrangement as above, but probably higher resistance and feeding a very high impedance input, such as the guitar amp.

You could try a 10K series feed resistor fed from a 9V battery (+ terminal), and eg. a 1uF coupling capacitor from the junction to the amp input.
Connect photocell connector ground & amp input ground / screen, and also connect the battery negative to those.

With the current limited to less than 1mA by the resistor, you could also try reversing the battery if it does not do anything useful as above.

If you get audio but very low, try eg. a 100K resistor instead & see if that is better or worse.

 

[paddy ryan]

Many thanks for that, it gives me hope! Could I be very cheeky and ask if you could draw me a pictorial schematic on this advice please? I am an amateur on electronics and a picture would be worth a thousand words. It would be very much appreciated, thanks
Patrick

 

[paddy ryan]

Reference your advice on my Ampro problem, could I be very cheeky and ask if you could possibly draw me a pictorial schematic of your advice. I am an amateur at electronics and don't want to make a mistake on this. It would be very much appreciated.