Zapped Ohmmeter

[NDNDNDND]

I have a Gunson Testune, which I bought as New Old Stock about five years ago. I bought it for the dwell meter function.

Unfortunately it seems to have stopped working. The last time I used it, a couple of years ago, I was using it as a tachometer. This means connecting the red and black power leads to the battery and the blue lead to the coil. I suspect I did what the meter tells you not to do - I turned the dial the wrong way with the battery leads connected, and zapped the meter by putting 12V through the ohmmeter and ammeter functions.

I've opened up the meter and visually inspected everything but I can't see anything visually wrong. The fuse had blown, and has been replaced. The battery has plenty of charge.

When you zap 12V through an unprotected ammeter/ohmmeter, does anyone know which components are most likely to have been damaged?

I'd give up and chuck it away, but try finding an analogue dwell meter these days....

|https://thumbsnap.com/SyqS4dOO[/url][url]

|https://thumbsnap.com/EI8cco53[/url][url]

|https://thumbsnap.com/Qv6IlOvx[/url]

[peterperkins]

Is there continuity through the meter coil?

You might need to remove the coil red or black wire before you test it.

[NDNDNDND]

That's a good suggestion, although if the coil's gone that's going to be heard to replace...

I'll have another tinker with it later!

[NMNeil]

Analog ammeters are all over Ebay. You just need to find the specs on the old movement.
https://www.ebay.com/sch/i.html?_from=R40&_trk...

[NDNDNDND]

Thanks for the suggestion of cannibalising a new ammeter for a movement, although working out the spec might be a minefield in itself - I can't even find a circuit diagram for this thing!

I just tried to check continuity across the coil. Tracing the circuit on my photo, the meter appears to be an open circuit with the dial set to 'off', plus I could see the solder connections on the face of the circuit board. When I tested for continuity using the connections, the needle moves!

The needle moves up to about the 2 volt mark. I'm not sure how healthy the movement is, but as the needle didn't respond at all when using the leads. This suggests to me that the movement is probably fine (ish) and that the problem is elsewhere.

For the benefit of the doubt, I tested the leads and they measured 0.00 resistance.

Does anybody know what the next likely culprit is? I could start desoldering each component and comparing specs one-by-one, but I think I'll just make a mess if i try that...

[peterperkins]

It doesn't exactly ooze quality and look high spec from that pcb..

But if it does the job..

The meter part as mentioned is probably a fairly standard spec movement.

[NDNDNDND]

Well, it did do the job.

I probably won't invest much time or money in it if isn't a reasonably simple soldering fix.

I've put the money into trying to communicate with the ALDL, as I should be able to set the MCS duty cycle through that instead, plus check O2 readings etc.

[crogthomas]

Does any other function of the meter (volts?) still work? That should give you a clue where the problem is.

[NMNeil]

Old magazine on how to build one, so I'm assuming the circuit diagrams will be close. Page 44
https://www.americanradiohistory.com/Archive-Radio...

[NDNDNDND]

crogthomas said:

Does any other function of the meter (volts?) still work? That should give you a clue where the problem is.

Hi, none of the standard multimeter functions (volts, amps, ohms) appear to work. I can't be sure the tach and dwell don't work, as I haven't yet tested it on the car. Are there any simple desk-based ways of testing a dwell meter?

[NDNDNDND]

NMNeil said:

Old magazine on how to build one, so I'm assuming the circuit diagrams will be close. Page 44
https://www.americanradiohistory.com/Archive-Radio...

What a fantastic link! How did you find that?!

I'll have to find the time to read it - although at first glance it doesn't appear to contain a resistance-measuring circuit, which is my primary suspect.

Looks like an interesting article and publication, though!

ETA: Ah, you can search by text, which does bring up an ohmmeter circuit! Still greek to me though, I'll have to do more research...

Thanks very much!

[crogthomas]

If nothing works at all I'd suggest the problem is more fundamental than the dwell circuitry. Either there is a fault between the sockets on the front and the switch sending the signal to the appropriate part of the circuit (unlikely), or, the meter itself is damaged as suggested above.

A circuit for measuring milliamps is so simple there is really nothing that can go wrong, so if that doesn't work it must be the meter itself. I'd suggest making up a simple circuit to test the meter as your first step.

[LordLoveLength]

On your second photo is there a burnt pcb track at the bottom left - next to the black preset?
Difficult to tell if it’s a shadow or burnt.

[NMNeil]

NDNDNDND said:

What a fantastic link! How did you find that?!

I'll have to find the time to read it - although at first glance it doesn't appear to contain a resistance-measuring circuit, which is my primary suspect.

Looks like an interesting article and publication, though!

ETA: Ah, you can search by text, which does bring up an ohmmeter circuit! Still greek to me though, I'll have to do more research...

Thanks very much!

I tinker with electronics as a hobby.
There is a site listing publications that are in the public domain on just about any subject, so no copyright issues.
I just put in electronics as a search.
https://archive.org/search.php?query=electronics&a...
They also have a separate site devoted to manuals, some centuries old.
https://archive.org/details/manuals?&and[]=yea...
https://archive.org/details/manuals

[NDNDNDND]

LordLoveLength said:

On your second photo is there a burnt pcb track at the bottom left - next to the black preset?
Difficult to tell if it’s a shadow or burnt.

Hi, thanks for the suggestion - I've opened it back up to be doubly-sure and yes, that's just a shadow. The track looks fine there.

[NDNDNDND]

NMNeil said:

I tinker with electronics as a hobby.
There is a site listing publications that are in the public domain on just about any subject, so no copyright issues.
I just put in electronics as a search.
https://archive.org/search.php?query=electronics&a...
They also have a separate site devoted to manuals, some centuries old.
https://archive.org/details/manuals?&and[]=yea...
https://archive.org/details/manuals

Wow... what an internet rabbit hole...

https://archive.org/details/gov.ntis.ava13995vnb1

[NMNeil]

NDNDNDND said:

Wow... what an internet rabbit hole...

https://archive.org/details/gov.ntis.ava13995vnb1

Have fun

[66mpg]

I have just spent a while reliving my youth: reading Practical Wireless from 1978. I could almost smell the musty paper!

[NMNeil]

66mpg said:

I have just spent a while reliving my youth: reading Practical Wireless from 1978. I could almost smell the musty paper!

I hope it bought back plenty of good memories

[Robotron-2084]

I cant make out that IC but pretty sure it won’t have survived a reverse voltage up itself.