Philips N4520 tape counter sensor.

[NorfolkDaveUK]

I have a 4520 and it's been a bit of a thorn in my side for a while.

I ordered a new IC for the tape counter and that came, but now the counter is lit up the darn thing won't transport any more. I've narrowed it down (I think ) to the sensor, because the counter isn't counting, it does what it should in that it lights up with all the dots, does its check thing and then the dots go out all bar the one that stays on, but it doesn't count .

Could someone tell me how to test the sensor please?

I think the photo cell is working because when I shine a good light into it , the ohms increase and once or twice the counter numbers have moved when the light is fired at it . Before I fitted the new IC transport was working OK because the IC had failed in such a way that the machine was just kind of ignoring it I think. Now I've put the new one it it wants to see it moving, and it doesn't.

This is my own machine so would really love to get this going, the sound on this thing is insanely good.

Could someone also point me in the right direction of replacement for it should it need them. I have no idea what they are other than a part number from the Philips manual which has gotten me nowhere.

Sorry if it seems like I've been a bit needy lately ..haha .. I just need to get some of these shelved projects finished ..LOL.

[TonyDuell]

A couple of quick checks :

Firstly, what voltage do you measure across D201? What about D202. That's the voltage between the 2 connections of each IR emitter, not the voltage wrt ground? I'd expect around 2V across each one.

Secondly, what do the output waveforms (as shown on the circuit diagram) look like when you turn the sensor by hand?

[NorfolkDaveUK]

There is 24V across them mate so they are definitely dead I would say. The question is, what can I replace them with I guess? I don't think a standard LED type is going to work because the light comes out of the side of these, and they dont look like IR's to me , I'll take a pic in a bit and show you what I mean.

[NorfolkDaveUK]

It's a in bit of an awkward place to take a pic really, I need to pull it out. I've got a desk full of crap at the min I'll do better this arvo...LOL.

[TonyDuell]

Are you saying you measure 24V across each of the LEDs separately? I can't see how any fault could cause that. I could easily believe 0V across one and 24V across the other if the latter was open-circuit.

You can get the complete assembly of LED + phototransistor as a 'slotted opto switch'. For example see RS components here :

https://uk.rs-online.com/web/c/displ...ical-switches/

You can also get what RS term 'Side Looker' IR LEDs if you want to fit them to the original housing. For example :

https://uk.rs-online.com/web/p/ir-leds/1713858/

[NorfolkDaveUK]

Thats superb. Thanks for that I`ll measure it again in a moment Tony . I`m going on memory and it isnt what it was ..haha . i remember that i measured 24v across at least one of them , the opto switch looks good I`ll do some measuring and see how they look for a fit. Sorry got being slow I`m trying to clear a path to it ...haha

[NorfolkDaveUK]

OK I have 23.59v on one and - 0.753 (negative) on the other .... they are opto switches looking at them Tony. (i did put the green wire back on before testing )

[NorfolkDaveUK]

Measured them up and ordered a couple from RS, fingers crossed that nails it . I know the transistors are good because I changed them out . As you know by no if I pull them to test them , I change them so I know they are ok 548`s came out 550`s went in , the resistors are all ok so has to be those really .

[NorfolkDaveUK]

Can someone please tell me which one of these is the emitter please . One is black one is clear . I think the black one is the emitter but I just want to be sure and not look like a fool putting them in the wrong way ..

Thank you .


Edit , see i was wrong , clear one is the led ....haha ....thanks guys

[NorfolkDaveUK]

OK . They are fitted and the counter now counts HAPPY DAYS...... BUT ,....the transport still will not stay engaged . Anyone have any clues to where I might look please ? If i hold the play button in , it will play happily , soon as I let go , it stops .

[Ted Kendall]

You should be able to consult the data sheet on the RS website - and there should be some sort of mark on the body.

[Ted Kendall]

If memory serves, the pulses from the drum charge up a capacitor which holds the logic in play. I'd start looking there.

[NorfolkDaveUK]

Thanks Ted. Much appreciated . I`ll check that out . The wiring for the drum is horrible . It goes up to the capstan / logic board via a horrible little two core little black wire (solid core) which doesn`t stay in place very well . The drum is working OK because the capstan is spinning at the right speed . So it could be a transistor or cap up on that board (or a bad joint or something ) i completely rewired this thing because the wiring on it was awful (when i bought it someone had completely butchered it ) So if I get it going it will be a great achievement for me , and its staying with me this is my "baby" so to speak .lol .

[NorfolkDaveUK]

Ted , as you are probably well aware by now , following these schematics is not my strong point ..lol

So ....

When I test TC408 on the capstan control board,, in circuit, its short from collector to emitter when I take it out it isn`t so could you tell me if its supposed to be short in circuit or whether this is part of the fault and indeed whether emitter and collector should ever be short in a circuit. Following on from that could you tell me what would be causing that if it is indeed a fault. I really appreciate you spending so much time with me Ted (and Tony and everyone else ) I am learning from you and not just using you as a repair tool I promise you that .

[julie_m]

C415 is wired across the collector and emitter of TS408. It's an electrolytic capacitor, and they can and do fail short-circuit.

[TonyDuell]

If the motor in that diagram is running, then TC408 is not the fault. In any case, from that circuit diagram TC408's emitter and collector are connected to that motor. A motor will have a low resistance (relatively speaking) and may well appear as though TC408 is shorted, when in fact you are seeing the resistance of the motor.

I've had a very quick look at the N4520 service manual. The signal 'Counter Pulses' from the opto-sensor goes to a couple of diode-capacitor charge pump circuits in the mechanism control. That's where I would start looking.

[NorfolkDaveUK]

Thanks so much guys . Just to clarify , we are talking about this Tony ?

David

[Ted Kendall]

That's the bit I meant. Look at the voltage across C310. It should be low in Stop, and increase when the tape is moving. Does the thing stay in FF/REW, or behave in the same way as in Play?

[TonyDuell]

Yes. I think 'Counter Pulses' goes to another point on the same diagram in the service manual too. I've not yet worked out what each does.

[NorfolkDaveUK]

Yeah it does the same thing in FF and REW as it does in play . It developed another fault now though . I put the transistor back in (the correct way round etc) and now on power up both the brakes disengage and the reel motors are spinning full pelt in opposite directions , rew/ff buttons interrupt this but only while I hold the buttons in , play and stop do nothing . Let me see if I can get it back to where it was. If you saw this when I bought it , you would under stand why I`m having all these issues with it , the previous owner tortured it (litterally) but I refuse to give up on it because when it works it sounds incredible. look at the pic to see his idea of a fuse change and you`ll get the idea .....