What is this component? GDT/TVS? And why a capacitor instead? - Page 1
Forum section: electronics / beginners

author: SuperSVGA
author_rank: Contributor
timestamp: April 24, 2022, 11:11:31 pm
content: I'm trying to replace a component in a circuit, but I can't quite figure it out:Looking around at images online, it looks like it may be some sort of glass gas discharge tube.My cheap T7 tester identifies it has a zener diode at around 10-11v, regardless of inserted polarity.Now the weird thing is whenever I look up pictures of this board online, that component is missing and it appears an MLCC capacitor is in the unpopulated above.Is it being used for transient voltage suppression? How does the capacitor compare, and how can I pick an appropriate replacement?Here is the relevant section of the schematic, with the component marked as "?":

author: Manul
author_rank: Super Contributor
timestamp: April 24, 2022, 11:40:34 pm
content: Can you provide values of R1, R2 and supply voltage of U4? What kind of battery is 6V battery?

author: SuperSVGA
author_rank: Contributor
timestamp: April 24, 2022, 11:59:58 pm
content: Quote from: Manul on April 24, 2022, 11:40:34 pmI measured R1 around 278kΩ and R2 around 15.35kΩ, though they are printed resistors so I am not sure how accurate they are after 30+ years.Supply voltage of U4 comes from the B Output of U3.V+ on U3 is as much as 7.5V.I don't really understand enough yet to determine what the supply voltage of U4 is, I'm assuming it's around 5-7V.The battery is a 6V SLA battery, likely ranging between 5.6V and 6.5V depending on state of charge.

author: Manul
author_rank: Super Contributor
timestamp: April 25, 2022, 12:30:00 am
content: Well, it is not a gas discharge tube. It is most likely a zener. But in my opinion it is not needed, or I don't understand why it should. Capacitor might provide some low pass filtering for U4 input I guess. Hard to guess the value, there is not enough information of what this circuit does.But if it functions with a zener and no capacitor, then capacitor is also not really needed. But might be preferable. It might be that designer realized this while issuing newer PCB revisions. I would try something small, maybe 10nF there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2022, 03:13:14 am
content: Seems like a glass encapsulated capacitor instead of the chip footprint nearby. How old is this? Glass caps used to be much more common in the 80s-90s I'd say. If new, maybe it is a diode; thermistors are also available in such.Tim

author: SuperSVGA
author_rank: Contributor
timestamp: April 25, 2022, 03:37:29 am
content: Quote from: T3sl4co1l on April 25, 2022, 03:13:14 amIt's likely from 1989.I did try testing it as a capacitor in an LCR meter, but couldn't get a proper reading from it. In auto mode it just said it was a 5kΩ resistor.Quote from: Manul on April 25, 2022, 12:30:00 amI guess all I can really do at the moment is order some PCBs and the components, and see if it works. Assuming I got the rest of the schematic correct...It is odd though, shouldn't it be polarized if it's a zener? I was thinking maybe it was two zeners end to end, but I think the T7 displays "2 diodes" in that case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2022, 08:46:04 am
content: Oh, try heating it up, does resistance change?And I assume diode test is symmetrical (and some fairly small voltage), suggesting resistor over diode?Tim

author: David_AVD
author_rank: Super Contributor
timestamp: April 25, 2022, 09:00:11 am
content: I also think it may be a capacitor with the unpopulated position an alternative footprint.Unless you remove it from the board (might be hard to do without cracking it) you probably won't get a sensible in circuit reading.Why do you think this particular part is faulty? Are you just guessing?

author: SuperSVGA
author_rank: Contributor
timestamp: April 25, 2022, 08:03:09 pm
content: Quote from: T3sl4co1l on April 25, 2022, 08:46:04 amHeating it seems to drop the resistance. I didn't have a hot air tool near so I used a gentle touch of a soldering iron for less than a second, which brought the resistance down to 3kΩ for a few seconds. Other methods of heating it seemed to have comparable results.Quote from: David_AVD on April 25, 2022, 09:00:11 amI don't think it's faulty, though I don't have a way to determine that it isn't yet.I'm trying to recreate the board, so everything has been removed from the board and tested out of circuit.