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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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15th Nov 2015, 11:10 pm
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#1 |
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Hexode
Join Date: Oct 2010
Location: Sandiway, Cheshire, UK.
Posts: 327
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Zener Diode identity
I think I have located the fault on a 'Riccar' sewing machine I am trying to repair. On the motor speed controller PCB I found a Zener diode that is a complete short circuit and the resistor feeding is open circuit.
The diode is marked 20 37R with the 37R text being a much smaller typeface. The Zener and the resistor are across bridge rectified mains. I'm guessing the resistor should be in the region of 220K but does anyone know what the numbers mean on the Zener? Ian I have shown the zener markings at the top of the picture |
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15th Nov 2015, 11:25 pm
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#2 |
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Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Zener Diode identity
Just an inspired guess: a 1N2037 is a 13 v. 1-watt Zener.
Al. |
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15th Nov 2015, 11:31 pm
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#3 |
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Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Zener Diode identity
Are the two grey capacitors in series with the bridge rectifier? If so, the resistor may just be a surge limiter, and the Zener diode a simple shunt stabiliser which passes more or less current so as to maintain a constant voltage across itself. A 13 V, 1 W rating certainly would not be out of place here.
I'm surprised not to see any electrolytics on that board, though ..... I was expecting at least one ..... Unless some of the green caps are Tantalum types?
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
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16th Nov 2015, 1:31 am
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#4 |
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Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,642
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Re: Zener Diode identity
What are the markings on ZD2, they may give a clue to the style of marking?
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16th Nov 2015, 7:49 am
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#5 |
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Octode
Join Date: Mar 2006
Location: Bracknell, Berkshire,UK.
Posts: 1,175
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Re: Zener Diode identity
Note also that there is no smoothing cap and several thyristors. What is the incoming AC voltage, does this come from a transformer or is it raw mains (in which case the zener would be much higher than 13V...). The two grey caps and toroid choke filter will be for EMC filtering. If the zener is purely overload protection the sewing machine would be expected to work once they are removed - if not you have another fault.
Dave |
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16th Nov 2015, 8:46 am
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#6 | |
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Hexode
Join Date: Oct 2010
Location: Sandiway, Cheshire, UK.
Posts: 327
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Re: Zener Diode identity
Quote:
I have just had a look and it is marked in the same style, it shows '12' followed by small '35R'. A quick test with a PSU and limiting resistor shows it is a 12.3 Volts device. It looks likely the faulty one was 20 Volts. I have not traced out the circuit but it seems that the zener regulated part is on the control side of the two thyristors with absolutely minimal smoothing. There are no electrolytics. The two grey caps are 'X' types which have been put in to replace Rifa ones that completely contaminated the board in its enclosure. (all washed off now) The machine belongs to a friend who took it to a sewing machine service centre who after examining it pronounced it unrepairable (as it needed a new PCB and they could not get spares). When I first accessed the board it did not look as if it had been looked at in years but I could not make sense of the random connecting of the coloured wires to the marked pins. I then spotted one end of R1 had been lifted and the cap changes etc. R1 markings were unreadable but I was able to make contact with part of the spiral track and estimate that it must be at least 200K Ian |
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