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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Oct 2015, 11:24 am   #21
Diabolical Artificer
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Default Re: Zener Diode Advice

See what you mean Graham, you mean D2. Could that be a zener as a voltage reference for the speed control? My schematic interpretation skills arn't great as I'm still learning.

I think we can discount D3 and D4 near the motor, can't see them being zeners. Which leaves D1 which looks like a signal diode.

I have a zener tester such as you describe David, very handy. I don't know how you'd test a leaky one though, perhaps with a cap reformer with a neon. Do they leak the same as caps?

Andy.
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Old 21st Oct 2015, 11:43 am   #22
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Default Re: Zener Diode Advice

It strikes me that rather than removing, testing and changing transistors and diodes, it would be much better to use a scope to fault find on this circuit.
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Old 21st Oct 2015, 12:55 pm   #23
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Default Re: Zener Diode Advice

Agreed. Without an oscilloscope, you're as good as searching in a dark room, with a battery-less flashlight, for a black cat that isn't there.

Some of the "old guys" around here love their old analogue 'scopes, but an inexpensive, hand-held, battery-powered digital one probably will do everything you need (and being battery-powered, can be used on live-chassis sets, with care).
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Old 21st Oct 2015, 1:19 pm   #24
Stylo N M
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Default Re: Zener Diode Advice

Hi,

Thank you all for the information and all your input it really is appreciated, it turns out that both diodes are indeed 1N4148 which as Paul Sherwin said are general purpose diodes, the technical data I have which just came with them in a multi bag, refers to them as fast switching diodes. I don't know if that information is technically correct or not.

Graham thank you very much for the links tha'ts great, i thought at first glance that they might have been zeners, beacuse that's what they looked like I didn't know that some ordinary diodes looked exactly the same. I can't post pictures on the forum because i've only got a mobile phone, I have no home computer. However i am very greatfull to everyone on here and the support enables me to get through it.

The new diodes are in they have helped, and I've ajusted the reed switch as I had to replace that some time ago, and in the 1978 versions it was a sealed unit. I cut the side off the unit got the broken glass out, filed a channel and after hours of work got it working again.

And again it is now working fine.

Thank you for all the help.

Paul.
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Old 21st Oct 2015, 2:52 pm   #25
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Default Re: Zener Diode Advice

I reckon adjusting that reed switch would have been the thing that made the greatest difference. From the description in the patent application, the positioning of the reed switch and the permanent magnet (which causes it to respond only to North poles, or only to South poles, on the armature) are critical: if the switch operates at the wrong point in the cycle, the motor will be underpowered (if the electromagnets are not energised for long enough), "jerky and snatchy" (if they are energised for too long), or even run backwards (if they are energised at the wrong times).

A dual trace oscilloscope probably would have shown up straight away what was up with it ..... says she who has only a single-trace one .....

Still, the main thing is, you've got it working now. And direct drive in a kid's toy record player from the 1970s is actually pretty dned impressive .....
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Old 21st Oct 2015, 3:17 pm   #26
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Default Re: Zener Diode Advice

I had one of these as a kid in the 70's and if I remember correctly, the platter will spin quite happily either way round.

This was useful for searching for "satan's voice" on rock music records when played backwards and other such harmless pastimes.
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Old 21st Oct 2015, 4:21 pm   #27
Stylo N M
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Default Re: Zener Diode Advice

Hi Julie,

Yes that reed switch is a nuisance, imagine what it was like cutting a channel for a larger one! I did that about six months ago, didn't expect it to ever work again.
By the way Paul and yourself were spot on BC237 works quite happily and so do a few other BC ranges.

Thank you , Paul.
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Old 21st Oct 2015, 6:17 pm   #28
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Default Re: Zener Diode Advice

Regarding 'scopes, Hameg 'scopes have a handy built-in component tester, which among other things, tests diodes. The traces for correctly functioning diodes are as shown on the three traces below on my Hameg 203-7. The first trace is what to expect from a good Zener - in this case, the 6V2 Zener I tested in my earlier post on my Zener tester. The second trace is of a silicon diode - in this case a 1N4007, and the third trace is a germanium diode (OA47), a similar trace to the silicon diode, but a slower rise time on the trace.

(There are projects on internet to make component testers for use with 'scopes).
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Old 21st Oct 2015, 7:27 pm   #29
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Default Re: Zener Diode Advice

It isn't time on the horizontal axis, it's current. What is shown on the Ge diode is reverse leakage current greater than that for the silicon diode.

Or if the axes are the other way round, it's higher forward slope resistance.
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Old 21st Oct 2015, 8:08 pm   #30
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Default Re: Zener Diode Advice

Thanks for reading the post and for the explanation Chris.
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Old 22nd Oct 2015, 1:24 am   #31
Stylo N M
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Default Re: Zener Diode Advice

Hi David,

All that information you have so kindly posted is staggering. I wish I could do all that, it really is good advice and the things that you have made are great. Well done.

Paul.
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