Studer optical sensor

[AudioIngenia]

Here an optical sensor of a reel to reel Studer.
I am not getting the required voltage. The difference open / closed is minimal, perhaps 1 volt.
Through the camera of the mobile phone, I do not see the infrared led lighting, also the resistance associated with this led, is very hot.

 

[Bo Deadly]

Do you have a scope?

If yes, monitor TP2 and use a TV remote or other infrared device to see if you get reaction. That might help narrow things down a little.

 

[AudioIngenia]

Yes. Already did. A little variation, maybe 1 volt. But it requires 12 volts .....
Maybe the infrared led is burned?
There are two LEDs, red and infrared. Why are there two LEDs?

 

[Bo Deadly]

I have no idea. It's obviously a non-trivial circuit.

I can make wild guesses though.

If one of the LEDs is actual visible red as you say, then I would guess that is just a sort of indicator to the operator. A red LED will not (should not) trigger an infrared phototransistor.

The better question is, why are there two phototransistors? I'm inclined to think that they are balancing each other in some way. When they're both "off", clearly the trimmer sets the bias of the op amp to presumably 0V. But when they're on, it would seem they would both cancel each other. However, if one is normally supposed be blocked somehow you might get the correct behavior and it might be that they're two just to compensating for temperature and such. So look closely at where the phototransistors would normally be positioned. Maybe the tape is just supposed to block one phototransistor. That's why there's a line drawn in front of the upper one in the schem. That's the tape.

 

[AudioIngenia]

Description of the service manual:
Function:  a Checks whether or not a tape is threaded and supplies a  message to the TAPE DECK PERIPHERY CONTROLLER 1.820.762  CGRP20/ELM43) by means of the TTL signal TDO-TRSP.  Circuit description:  The sensor consists of a double Light barrier implemented  with two phototransistors in one housing (QP1) and two  LEDs (DL1, infrared / DL2, red). In the absence of tape  the two phototransistors are supplied with Light not only  by the two Light sources but also by ambient Light; no  current flows from the node between the two transistors to  the inverting input of the opamp IC3/pin 2 (manufacturing tolerances are compensated with R28). When tape is present, the upper phototransistor is dark, its impedance
changes to high, and the current equilibrium is upset,
i.e. a differential current flows to the input of the opamp (IC3/pin 2). The output pin 3 of IC3 changes to positive (gain adjustable with R26).
The Schmitt trigger IC2/1 (comparator with open-cotlector
output) buffers the output signal of IC3/1 and pulls it to
TTL level. It is transmitted as the TD-TRSP signal to the
TAPE DECK PERIPHERY CONTROLLER 1.820.762 (GRP2O/ELM43).

 

[AudioIngenia]

I have checked with the dark room. The infrared led is illuminated. But don't get the required voltage difference.
I see strange the resistance in series of the infrared led is very hot.

 

[Bo Deadly]

In the absence of tape  the two phototransistors are supplied with Light not only  by the two Light sources .... When tape is present, the upper phototransistor is dark, its impedance changes to high, and the current equilibrium is upset,

Yup. So just one of the two phototransistors is masked by the tape. That's what cause the output to go positive. It all makes perfect sense now. So it works fine then or are you still having an issue?

 

[AudioIngenia]

It doesn't work properly yet.
The voltage difference is 4 v.
Requires a difference of 12 v.