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I am trying to copy a simple PCB I have bought to read an ec sensor.

There is no wiring diagram but the tracks on the PCB are easily visible, so I copied the circuit and upon testing the PCB points, the voltages are too high in places. The PCB consists of some 1k,4.7k,10k,100k resistors, a quad amp, 555 timer, some 103 ceramic capacitors, two 2.2uF caps and two 220uF caps, two 100k trim pots and finally 6 small orange glass diodes which have ST 41 48 written on them.

I know the diagram I created is correct, I literally sat there for 3 days with a multimeter. The resistors and capacitors, trim pots and quad amp are 100% correct, they were labeled nicely. The timer should have been an NE555P but I could only find a NE555N in store, but I was told it should be fine. So it must be these zener diodes or whatever they are, or my wiring.

The diodes I bought were some 3.3V zener diodes. I tested the voltage going in on each diode on the PCB I bought and it's between 150-200mV and 50mV less out and my PCB is getting 2.56V in the places it should be 200mv or less, it seems my voltage stays the same from the two resistors directly above and parallel to the 555 timer, leading me to believe the diodes are doing nothing as there 3.3 and the voltage is 2.56.

So after saying all that what is the value of a orange glass diode marked ST 41 it has a black band on the outside and a small silver ring on the inside? picture of pcb facing upright same orientation as my pcb design pcb detailed

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  • \$\begingroup\$ Please can you try to improve the translation in the paragraph starting "But let's assume it's these diodes [...]". Phrases like "voltage going in", and "resistor networks" are too vague and so we can't use those to help understand the story, and "basic lyrics" doesn't make sense in English, so is probably a mistranslation. It will probably help when you add your reverse-engineered schematic, and can then identify specific circuit nodes on that for the "voltage going in" and specific "resistor networks". Thanks. \$\endgroup\$ – SamGibson Oct 13 '17 at 14:06
  • \$\begingroup\$ I have added the pcb diagram i made, are ST 41 48 standard silicon small-signal diodes the same, my quad amp is 100Mhz so i assume i get the fast switching 100Mhz standard diodes, do i need to get a certain voltage ? \$\endgroup\$ – Edwin Martin Oct 13 '17 at 14:42
  • \$\begingroup\$ Each pair of diodes is in series. This likely means they are functioning to clamp the signal at their common node only a little bit outside of the rails at their unique nodes. \$\endgroup\$ – Chris Stratton Oct 13 '17 at 16:27
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Unfortunately some parts of your description did not translate well, and so cannot be understood to use the "value" in that part of the story.

However, overall I suspect (at least) one mistake is that you bought replacement diodes:

the ones i bought where some 3.3v zeners diodes

However diodes which are labelled "ST 41" (and, perhaps have additional numbers like "48" further round the diode body) are likely to be standard silicon small-signal diodes, not zeners.

If you supply your reverse-engineered schematic, and if it is clear enough, then perhaps readers may be able to confirm/deny whether a zener diode would make sense in any of the locations where you used them.

It is very unlikely that all of the (6?) glass diodes in your photo are zeners, so using zeners for all of those will have been wrong. It is possible (IMHO likely) that none of them should be zeners.

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    \$\begingroup\$ At least 2 of them are in anti-parallel, which corroborates your hypothesis that they are not zeners. \$\endgroup\$ – Wesley Lee Oct 13 '17 at 13:39
  • \$\begingroup\$ Are be home in 10 minutes and are post my pcb etching design, it's double sided but the components are only on one side so it shoUldale be understandable \$\endgroup\$ – Edwin Martin Oct 13 '17 at 13:39
  • \$\begingroup\$ All the orange diodes have ST 41 written on them, and yes two pairs are inverted to each other and joined which you right meand they can be zeners DOH. Before I get home and add the pcb diagram I created dies any one have a link to some ST4148 diode that would be correct \$\endgroup\$ – Edwin Martin Oct 13 '17 at 13:50
  • \$\begingroup\$ @WesleyLee - Thanks, agreed, that's why I thought the two pairs top-left and bottom-right were not zeners, as they both seem to be in anti-parallel. The pair top-centre, I'm slightly less confident about, however, due to the resistor partly hidden by the white wire (in series with a diode?) and where that pair don't seem to be in anti-parallel. I doubt they will be zeners either :-) \$\endgroup\$ – SamGibson Oct 13 '17 at 13:51
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    \$\begingroup\$ @EdwinMartin - Aargh! You've inverted the photo, so earlier comments about "top-left" etc. make no sense now. :-( "they do say ST 41 48" Great, that's what I suspected. I've already told you the generic part number which you can search for. Unfortunately you haven't supplied the schematic that I asked for. It'll take a while before I have time to try to reverse-engineer a schematic from your PCB layout, which is necessary to try to understand it. Didn't you make a schematic yourself? "[If] you see the group of resistors in the top left" No. Let's try using "chat". Please wait. \$\endgroup\$ – SamGibson Oct 13 '17 at 15:11

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