# Zener diode



## octaneman (May 13, 2009)

I got myself a VCR Model:VR600HF with no schematics.
The power supply on it is screaming like a banshee from hell. Traced the problem to the secondary side of the SMPS transformer to a zener diode connected to a 16v 1000 uf capacitor. Capacitor tested ok. After testing the zener with a DVOM my conclusions are that it's shorted. I understand that the zener can conduct both ways because of it's breakdown voltage. Here's the problem: The SMPS is using a Matsushita transformer which is obsolete (contacted the manufacturer). The zener diode has just the color code on it. I used the method of using the resistance color codes to attempt to identify it's voltage. There are 2 bands. 1st band: black , 2nd band: blue, I'm hypothesizing it to be a 6.0v zener but do not know it's tolerance, and there is no other way I can identify it. Does anyone know a better method of identifying zener color codes because it has to be replaced with little alternatives, the machine is 20+ yrs old.


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## Done_Fishin (Oct 10, 2006)

Hi Theo
Are you really sure that the diode is a zener ? What is its component designation on the board.
Where does the diode fit in the circuit? you say one side is connected to a capacitor, is that the positive or negative side and is it connected via anode or cathode?
where are the other connections going or fed from?

If it is a zener it will be unlikely that it's being fed from the transformer. It will possibly be used to bias a transistor base voltage taking the voltage via a resistor 

What makes you think it's fault? did you measure it in or out of circuit? What readings did you get?

you don't mention whether the diode is smd or conventional although it's possible it could be either considering the age of the machine.

It's rare for a zener to show readings in both forward and reverse directions unless it is a very low value zener ~3 Volts or less

Is this the RCA VR600 VCR? I am wondering whether this might be a model I know of but under a different badge 

to see if a zener is working or not you an connect the anode to ground of a bench power supply 
connect the cathode to a 1Kohm resistor connected to the positive rail of a bench power supply. Assume that the zener will be taking approx 5mA. This means a minimum 5V from the power supply will drop across the resistor as the current goes through the zener. slowly increase the voltage whilst measuring the voltage across the zener. when the voltage stabilises across the zener it should be within +/- 0.3V of the normal working voltage of the zener.


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## octaneman (May 13, 2009)

Yes the vcr is RCA-VR600HF. I am absolutely sure that it is a zener diode, it is the older glass type and not the smd's. The zener anode is connected to pin 9 of the transformer and the cathode is connected to the positive side of the 1000uf capacitor, there is no transistor. I tested the zener both in and out of circuit and this is what I found. With power on I got a 0.6v and figured that the zener was doing it's job. But, off the board the readings were 0.00 in both directions. The off the circuit board readings convinced me that the zener was shorted, and when connected to the circuit it would it get hot causing the 1000uf capacitor to vent out. It didn't make sense. To be conclusive I will have to bench test it on a bread board, but my gut instinct tells me that I will get the same readings as the in circuit test.


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## Done_Fishin (Oct 10, 2006)

It's not logical to have a zener coupled to a transformer in this fashion. The only similar circuit I can remember having anything like this was for the front panel display prior to LCD screens. The ones that had a heater element and required a (I think) negative Voltage around -30Volts wrt ground and lost about 8 Volts over the heater. The heater I believe was operating on AC but with a dc offset. I think they were called Nixie or that may have been the individual old alphanumeric display modules. A lot of American Manufactured diodes, back in the 60's & 70's, were colour coded but I think at that time there may have been no standards. I tried Googling but came up with no links to any source specifying colour codes against values.

I assume when you say that you got 0.00 in both directions you were talking about 0 OHMs or 0 Volts when using the diode function.

I have found a diagram of what i think might be the display power supply and I will edit it in here shortly

http://img684.imageshack.us/img684/4865/displaypsu.jpg

It's too large to show but you'll see the top right hand corner with some editing. Compare this to your old VCR. The markings were made when I repaired the module replacing the small can with a transformer. The Mod came from Hitachi Service Centre Athens, years ago.


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## octaneman (May 13, 2009)

It may not be logical but the designers did it, on pic 1 you can see the zener's position on the board ( forgive the quality I don't have a digital camera so took this with my web cam) the position on the board is D15 and pin 9 of the transformer. It's orientation as I see it is that it's used as clamping the voltage directly from pin 9. On pic 4 the trace goes directly to the positive side of the capacitor, it's location is C12 ( barely visible).

I also googled to get get any info on the color codes and came up empty, so it's only in the traditional method to I.D them come only through schematics.So nothing has changed over the years.

Thanks for the schematic you uploaded and I'll study it ASAP.


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## Done_Fishin (Oct 10, 2006)

Zeners are usually marked as ZD# on circuit boards whilst rectifying diodes are marked D#.

Whilst I don't doubt your findings I so want to make sure that we are not mistaking a normal rectifier with a zener. Like I say it really is unusual to find a zener on a transformer winding, however your analysis and description point to it being a circuit similar to the one that I posted earlier. The circuit feeds the front panel display plus if I remember correctly also plays a role in the biasing circuit for recording .. although it is many years since I needed to investigate and use that diagram. the photo's above do not help me a great deal since I cannot tell what is connected to what and where it all fits into the electronics of the machine.


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## octaneman (May 13, 2009)

Update: 

The problem of 1000uf capacitor to vent out was that once the secondary side of the transformer was under load the capacitor would get hot. I replaced the capacitor for good measure to a higher temperature rating from 85°C to 105°C giving it a higher tolerance. The problem actually got worse with the 105°C because it's ESR changed. After carefully reading through dozens of documents on capacitors I underestimated the capacitors ESR value. Lo and behold that the higher the capacitor's temperature rating was the higher was its ESR ! so, changing to a higher value temp is only good if there is a heat sink near by which in this case none was present. I replaced it with it's original value and problem was solved. 

Thanks again for the diagram it was a huge help.


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## Done_Fishin (Oct 10, 2006)

I'm glad you solved the problem, although strangely enough the lower the ESR the cooler it should have run. Capacitors used to be used as "wattless droppers"

see *this link* and look for "Capacitive Dropper" towards the bottom of the page. 
Copy, using Cntrl C, *Capacitive Dropper*, open the link, Find *Cntrl F* then paste *Cntrl V* to get there quicker.


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## octaneman (May 13, 2009)

Awesome site. 


Thanks.


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