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Old 12th Feb 2019, 3:54 pm   #1
collisr
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Default Replacement for a 1970s IR LED [ VP

Hi,

I am attempting to fix a 1978 Grundig SVR video recorder and an IR LED that measures the back tension is almost definitely dead. (I have tried pointing my camera phone at it and there is nothing being emitted).

I have the Grundig service manual and the part is listed as a VP190P. This part of the circuit is 15Vdc and it "shines" onto a sensor (a BPW16).

Does anyone by chance have any spec docs that would identify this LED and perhaps recommend a suitable modern replacement?

Rob.
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Old 12th Feb 2019, 4:04 pm   #2
merlinmaxwell
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Default Re: Replacement for a 1970s IR LED [ VP

I would think any IR led of the right size will do, they are only pence (or free from a duff remote control) worth a punt.
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Old 12th Feb 2019, 4:08 pm   #3
jonnybear
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Default Re: Replacement for a 1970s IR LED [ VP

Picture of the device would be of help, perhaps someone has something that would do the job.
John
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Old 12th Feb 2019, 6:39 pm   #4
collisr
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Default Re: Replacement for a 1970s IR LED [ VP

Hi. I attach some pics.

The first shows the extract from the service manual. The second shows the actual device. I have pulled the IR LED from the housing, but you can see the LED (when in the top of the housing) shines through the housing and is detected by the phototransistor at the bottom. A tension bar touches the laced-up tape and moves a plastic bar that fits in the middle gap of the device. I am guessing this regulates the back-tension in the VCR by changing how much of the LED light is received in the phototransistor, which is connected to the take-up reel making it turn faster when it needs more tension.

I see the phototransistor (BPW16) does have some specs on the web. A bit of a novice at this - I know I will need to match the light wavelength between LED and sensor ~950nM . I am also aware I need to insert a dropping resistor in series to ensure I don't blow the LED.

I have some 890nM IR LEDs which have If=100mA, Vr=5v. Assuming the wavelength is good enough, what resistor should I use? NB The circuit seems to show a 680R already in place before the 15v rail.

R.
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Old 12th Feb 2019, 7:01 pm   #5
merlinmaxwell
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Default Re: Replacement for a 1970s IR LED [ VP

Bog standard LED, anything will do.
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Old 12th Feb 2019, 7:18 pm   #6
jonnybear
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Default Re: Replacement for a 1970s IR LED [ VP

Hi Rob
I have a OPE5594 IR 940nm unfortunately it has sort leads, if you want it PM me
John
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Old 13th Feb 2019, 10:11 am   #7
dr_nick
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Default Re: Replacement for a 1970s IR LED [ VP

Rob,

Forgive me; there are far more knowledgeable people on the forum than me.

But if I was looking at this problem I'd be looking at the characteristic of the sensor to match the LED to, not necessarily trying to replace like for like the LED source.

Although the original LED is described as an IR device emitting a specific frequency of light (which will actually be a description of a centre value - there are several possible to choose from - of a roughly Gaussian distribution) - it will in fact cover quite a spectral range. So if you did want to use an IR LED then probably many types would in practice be close enough.

What may be simpler though, is that the datasheet for the BPW16 that I can download shows a wide spectral sensitivity. The coarse definition given is 'suitable for visible or near IR radiation' and the relative spectral sensitivity graph for the BPW16 suggests that an ordinary LED - probably of any colour - would work (as per merlinmaxwell suggestion).

Hope those thoughts help.

Nick.
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Old 13th Feb 2019, 2:00 pm   #8
Vicboduk
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Default Re: Replacement for a 1970s IR LED [ VP

Before condemning the LED try viewing the IR light by another camera.
In my experience recent mobile phones have VERY effective IR filters, particularly Apple, and show no IR light.
I keep an old Nokia phone and a USB camera for this purpose.
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Old 13th Feb 2019, 2:17 pm   #9
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Default Re: Replacement for a 1970s IR LED [ VP

Quote:
Originally Posted by Vicboduk View Post
Before condemning the LED try viewing the IR light by another camera.
In my experience recent mobile phones have VERY effective IR filters, particularly Apple, and show no IR light.
I keep an old Nokia phone and a USB camera for this purpose.
That's right, effective filters started to be installed after the iPhone 3GS. Damn nuisance for those of us trying to test remote control output!
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Old 13th Feb 2019, 3:31 pm   #10
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Default Re: Replacement for a 1970s IR LED [ VP

I will have to try an IR led with a square wave on it and see what happens?
It will most likely tell me that the remote is very naughty or very one-ey
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Old 13th Feb 2019, 4:32 pm   #11
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Default Re: Replacement for a 1970s IR LED [ VP

Thanks again everyone.

I tried my 890nM IR LED and swapped out the 860R (2W) dropping resistor for a 100R one which I believe is correct. I can see from the same camera phone that this now "lights up" very well - at least I think that proves the Motorola camera phone doesn't have any fancy filters. It certainly hasn't fixed the back tension problem on the VCR but it has improved things. I think I need to prove the sensor/LED combo is working by measuring the change in voltage to the take-up/supply reels as I manually manipulate the bar that partially obscures the LED during PLAY. The (german) service manual I have has an alignnment procedure for the back-tension set-up, so will perform these and revert.
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Old 13th Feb 2019, 5:14 pm   #12
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Default Re: Replacement for a 1970s IR LED [ VP

I would try connecting a high impedance input to an amplifier or scope to the photo transistor and see what you get through the speaker or on the screen when you point a remote control at it.
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Old 13th Feb 2019, 6:23 pm   #13
dr_nick
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Default Re: Replacement for a 1970s IR LED [ VP

Rob,

please forgive me for mentioning this, as I'm sure that you've already thought about it; but as the BPW16 is sensitive to the visible light spectrum, as well as IR, is it possible that the unit won't behave properly with the new LED that you've fitted unless all the light screening that would be in place when it is assembled is re-fitted?

If it is misbehaving still after re-assembly or with some kind of temporary light screen then obviously I am talking nonsense and the problem is elsewhere.

Nick.
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Old 13th Feb 2019, 7:28 pm   #14
collisr
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Default Re: Replacement for a 1970s IR LED [ VP

Hi Nick.

All ideas are welcome. I hadn't considered the sensitivity to regular light as a problem. The phototransistor is tucked away inside a housing that should block out most of the "daylight" - and to be sure, I have tested with the VCR covers back on.

The Grundig SVR was a very unique beast and has proved a tricky renovation. Especially as the service manual I have is in German (and I don't speak German!).

I **think** the back tension mechanism is now working. At least sort-of. I need to take some measurements, follow the tracking adjustment (and back-tension adjustment) procedures next.

I have a few of these machines and 3 of them are all showing the same fault (a tape will be stable then go out of tracking for a few seconds). The fact the LEDs appear to all be dead has got to be part of the puzzle but I don't think it's all the problem. I recall these machines were regarded as a nightmare to keep alignment.
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