Unknown transistors in tape deck

[giammyzanna]

Hi, I have an onkyo tape deck (ta-2058 for those interested) where there's this method for calculating the tape left. Basically it reads the speed of both reels with two LEDs and two phototransistors. The output of those photot. goes in two transistors, a double not logic gate (not sure why) and then finally in the control IC. In the attached schematic you can see this circuit on the right.

Unfortunately one of those sensors doesn't work. On the output of TR1 all I get are 5v (the supply) even if the tape is not turning.
On those transistors (sot23 package) I can only read L5 which doesn't tell me anything what to look for a replacement. Of course on the service manual there's no mention of that circuit  Can someone help figuring this out? Thanks!

[T3sl4co1l]

Likely this, https://alltransistors.com/transistor.php?transistor=12164

Any general purpose type will do e.g. MMBT3904.

Tim

[giammyzanna]

Thank you, I'll order some then. I'm not an expert, but the broken part should be that right? Or maybe the phototransistor. Can I troubleshoot which one without unsoldering? I guess just connecting the base of tr1 to gnd should tell me if it's shorted or not right?

[coromonadalix]

Sometimes you can test a transistor in diode mode, but in this circuit  it could interfere wit the pull up resistors r819 r820 ... or maybe not


For smt markings you have some websites who decipher the part numbering(s)  SMD codebook, SMT markings  ...

L5 gives  MMBC1623L5   but give an MMBT3904 as an equivalent.

[giammyzanna]

I haven't considered those resistors at all but thankfully I can disconnect the board (or at least the output of TR1), it has its own connector. Didn't think of that.
I'll look more in how to test a transistor, I'm a bit rusty lol

[Caliaxy]

On the output of TR1 all I get are 5v (the supply) even if the tape is not turning.

So everything else other than the real time counter works fine? I would try to rotate the reels manually (with my fingers) and check with a dmm or (oscilloscope) if I get any pulses on the collectors of TR-1 and TR-2 (pins 3 and 4 of P701). It’s OK to have +5Vdc if the reel is stopped in a position in which the the phototransistor is not illuminated.

[giammyzanna]

On the output of TR1 all I get are 5v (the supply) even if the tape is not turning.

So everything else other than the real time counter works fine? I would try to rotate the reels manually (with my fingers) and check with a dmm or (oscilloscope) if I get any pulses on the collectors of TR-1 and TR-2 (pins 3 and 4 of P701).

Yes everything else works fine. Thankfully it's the other reel that's responsible for the autostop function. I tried with a tape inside and the output is always 5v. if the reel moves or not doesn't make any difference.

[gamalot]

2SC1623-L5

https://www.mouser.com/datasheet/2/258/2SC1623(SOT-23)-276330.pdf

[james_s]

Thank you, I'll order some then. I'm not an expert, but the broken part should be that right? Or maybe the phototransistor. Can I troubleshoot which one without unsoldering? I guess just connecting the base of tr1 to gnd should tell me if it's shorted or not right?

The first thing I'd suspect is actually the IR LED, I have seen them get weak or fail on numerous occasions. It's possible one of those transistors is bad but I think it's unlikely, small signal transistors very rarely fail. Do you see a signal on the phototransistors? This should be very easy to diagnose by monitoring various nodes in that circuit while momentarily blocking the phototransistor, or shining a bright flashlight on it.

[Caliaxy]

You should also suspect a bad contact (pin 3 of P701, unless you measured the TR1 collector voltage directly on the sensor PC board), a dirty (dusty, oxidized) reflective surface on the back of the (supply?) reel or even a bad seated sensor PC board on the back of the transport (which might prevent the LED light from being reflected onto the phototransistor).

[giammyzanna]

Thank you, I'll order some then. I'm not an expert, but the broken part should be that right? Or maybe the phototransistor. Can I troubleshoot which one without unsoldering? I guess just connecting the base of tr1 to gnd should tell me if it's shorted or not right?

The first thing I'd suspect is actually the IR LED, I have seen them get weak or fail on numerous occasions. It's possible one of those transistors is bad but I think it's unlikely, small signal transistors very rarely fail. Do you see a signal on the phototransistors? This should be very easy to diagnose by monitoring various nodes in that circuit while momentarily blocking the phototransistor, or shining a bright flashlight on it.

Thanks for the idea, actually I have no idea as I've only measured the signal on pin3 on the interconnect cable between the boards. I'll try to check if there's an output on the phototransistor, the pcb is in a difficult place behind the mechanism.

[fzabkar]

The output of those photot. goes in two transistors, a double not logic gate (not sure why) ...

Q706 is configured as a Schmitt trigger .. I think.

[giammyzanna]

You should also suspect a bad contact (pin 3 of P701, unless you measured the TR1 collector voltage directly on the sensor PC board), a dirty (dusty, oxidized) reflective surface on the back of the (supply?) reel or even a bad seated sensor PC board on the back of the transport (which might prevent the LED light from being reflected onto the phototransistor).

Yes the first measurement was done on that connector. I'll try to take off the pcb with the leds and phototransistors from the rest of the mechanism.
Thank you

[giammyzanna]

Here I am again! Today I spent like 2hrs taking the thing apart. I suspected the phototransitor to be faulty so I took the whole sensor board out from the mechanism and then I noticed that one of the LED/phototrasnsitor combo was dirty with old belt goop. I have no idea how it got there since the sensor is basically isolated in a 1cm pipe. Far from the belt too. Again, no idea.
Of course cleaning it brought the logic functions back to life.
I also managed to put the mech back together with no screws remaining lol
Thanks everyone from this amazing forum!

[Maurizio1957]

You can check the IR led with the phone camera.
Maurizio

[perieanuo]

just put in some basic npn from your drawer, just gotta make the voltage/current spec which is piece of cake in this case, hfe is not crytical but more is better
or any darlington will tell you if the q is the culprit