Which 6.8V Zener Diode for VIC-20CR?

[USA_Joe]

It appears that a 6.8V zener diode at CR1 for the cassette/datasette port on one of my VIC-20CR's has failed. Per the service manual for this board, it states "05Z6.8Y" - whatever that means beyond the 6.8V information I found elsewhere.

Is this one a good replacement?

20PCS 1N4736A 1N4736 Silicon Planar Power Zener Diode 1W 6.8V DO-41 (DO-204AL) Axial Diodes 1 Watt 6.8 Volt

https://www.amazon.com/1N4736A-1N4736-S ... 45Q1G?th=1

Thank you!

Joe

[Orangeman96]

Is this one a good replacement?

20PCS 1N4736A 1N4736 Silicon Planar Power Zener Diode 1W 6.8V DO-41 (DO-204AL) Axial Diodes 1 Watt 6.8 Volt

https://www.amazon.com/1N4736A-1N4736-S ... 45Q1G?th=1

I don't think so. (My understanding is this Z-diode is 0.5-watt, not 1.0-watt.) I would check the markings on the diode itself. -OGM

P.S. This illustrates how a Z-diode works, and why it is an important element of a power buss.

[USA_Joe]

I can't see any markings on it while it is soldered onto the board. I'm reluctant to desolder it without having a replacement.

Thank you!

Joe

[Orangeman96]

I can't see any markings on it while it is soldered onto the board. I'm reluctant to desolder it without having a replacement.

Thank you!

Joe

OK. So you aren't sure it's "bad" then? Have you measured the voltage and impedance, as well? -OGM

[USA_Joe]

True, I'm not sure.

I tried, I think, but I'm not sure what I'm seeing or if I'm doing it right with my mulitmeter. I'm still learning this stuff.

What I do know is that by using a logic probe, the zener diode is held LOW the entire time during the diagnostic test. On the working VIC-20CR, it's LOW then flashes HIGH during the test, then back to LOW.

So, it is my guess that it is the zener diode.

Thank you!

Joe

[Orangeman96]

If you measure 92-ohms, it means the Z-diode is 1/2-watt. If you measure 46-ohms, it means the Z-diode is 1-watt. You should be able to trace from Pin C on the Cassette Port to the Z-diode to measure. -OGM

[tlr]

If you measure 92-ohms, it means the Z-diode is 1/2-watt. If you measure 46-ohms, it means the Z-diode is 1-watt. You should be able to trace from Pin C on the Cassette Port to the Z-diode to measure. -OGM

Where are you getting these numbers from, and why would it be important for this particular zener to only withstand half a watt?

[beamrider]

You can't reliably measure the resistance in-circuit. De-solder it and test it with a mult-meter in diode mode. You can always put it back if it's working fine. A 0.5w can be substituted with a 1W without issues.

[Orangeman96]

Where are you getting these numbers from, and why would it be important for this particular zener to only withstand half a watt?

Here's a handy tool that makes it easy to do the conversions taught in basic electronics courses, such that one does not have to do algebra--even "in private." Also, the published technical data identifies this Z-diode as a 1/2-watt versus 1-watt.

(I am operating under the assumption that it would be irresponsible of me to encourage someone to run the risk of damaging--perhaps irreparably--their vintage VIC-20 by doing non-standard/non-spec things to it, especially since I am not in a position to rectify the situation if I was wrong. I most certainly wouldn't be pleased if that happened to me.)

OGM

[Orangeman96]

You can't reliably measure the resistance in-circuit. De-solder it and test it with a mult-meter in diode mode. You can always put it back if it's working fine. A 0.5w can be substituted with a 1W without issues.

Thanks. We already discussed such an approach, and my understanding is he did not want to de-solder it without having a "to-spec" replacement on hand. Also, the published technical data identifies this Z-diode as a 1/2-watt versus 1-watt. -OGM

[beamrider]

If someone thinks de-soldering a diode is risky, perhaps they shouldn't be attempting a repair like this at all.

The 0.5 watts is the maximum rating for normal use. It's not like a fuse, nothing to say it won't handle more power than that.

[tlr]

Where are you getting these numbers from, and why would it be important for this particular zener to only withstand half a watt?

Here's a handy tool that makes it easy to do the conversions taught in basic electronics courses, such that one does not have to do algebra--even "in private." Also, the published technical data identifies this Z-diode as a 1/2-watt versus 1-watt.

Your reasoning is flawed in more than one way:
1: like beamrider says the power rating for a component is normally the maximum power it can take.
2: you are using Ohm's law, but Ohm's law isn't applicable in a useful way for an active (i.e non-linear) component.

(I am operating under the assumption that it would be irresponsible of me to encourage someone to run the risk of damaging--perhaps irreparably--their vintage VIC-20 by doing non-standard/non-spec things to it, especially since I am not in a position to rectify the situation if I was wrong. I most certainly wouldn't be pleased if that happened to me.)

This I agree on. One should know one's limits.

[Orangeman96]

If someone thinks de-soldering a diode is risky, perhaps they shouldn't be attempting a repair like this at all.

The 0.5 watts is the maximum rating for normal use. It's not like a fuse, nothing to say it won't handle more power than that.

Ah, I see: provide a "not-incorrect" answer versus a "fully-correct" answer, while also insulting the soldering skills and stated desire of the one asking the question...

What I do not at all understand is why one would argue installing an "other-than-spec" part when a "to-spec" part is equally available.

OGM

[Orangeman96]

Your reasoning is flawed in more than one way:
1: like beamrider says the power rating for a component is normally the maximum power it can take.

So this could be a suitable replacement as well? Irrespective if it could or could not be, why would one use a "non-spec" 1-watt Z-diode when a "fully-spec" 1/2-watt Z-diode is equally available.

Your reasoning is flawed in more than one way:
2: you are using Ohm's law, but Ohm's law isn't applicable in a useful way for an active (i.e non-linear) component.

I'm "all ears" then; please do explain a "useful" way, noting Joe said they did not yet want to remove the existing Z-diode.

OGM

[beamrider]

Ah, I see: provide a "not-incorrect" answer versus a "fully-correct" answer, while also insulting the soldering skills and stated desire of the one asking the question...
What I do not at all understand is why one would argue installing an "other-than-spec" part when a "to-spec" part is equally available.

It's standard practice to replace diodes with higher rated equivalents if they fit and that's what you happen to have. I was under the impression he already had a 1W, if that's not the case and the correct part is available then by all means order that.