• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

broken meter

Status
Not open for further replies.

RetiredHAL

New Member
Hi, my favourite meter has developed a fault. it will read DC on the DC AND on the AC scale (correctly). Eg a 9v battery will read 9v DC and 9v AC! Model is Triplett 310c. Does any one have a cct diagram or knows what the problem is? A quick check does not reveal any burnt resistors, splat marks on the slider tracks or other obvious failures.

Moderator if this post should be under datasheets would you please move it?

Cheers RH

Edit; more info, one component worries me. it is a small brown capsule that has V64 written on it (nothing else) it connects to the meter movement.
I'm pretty sure it is a mov (Google says 1 Mohm at .10V and 4ohm at 1V--- schauer V64--- datasheet not available). Using Analogue meter ohms scale it reads 120 ohm. Using Fluke 23 ohms scale it measures 700Kohm. Is that normal? and is it a likely suspect. Also I unsoldered the AC bridge diodes and they all look, read OK, I will replace these in any case with diodes from another meter. With the slider out all other resistors measure roughly what is printed on them.

Cheers Again RH

Edit again, I have found a schematic at the triplett website.
 
Last edited:

mvs sarma

Well-Known Member
There is a set of rectifiers , perhaps selenium type practically appearing like a mechanical assembly. ( few small disks clamped in a bracket and tightened with a screw.and with 3 pins). may be at times two nos of germanium diodes are forming the same. you may replace the with due polarity, by schottkey diodes like 1N5819 or 1N5822.

Once one diode is short, perhaps the meter will read dc in AC range. If the diodes are OK it might read twice its value.

Second possibility, Generally the AC part of the meter will have lower impedance (ohms/volt) compared to DC. perhaps one of the resistors in the series chain has given way and developed high and not full break.

If you have another preferably DMM, you can trace them and locate it.
 

RetiredHAL

New Member
mvs sarma said:
There is a set of rectifiers , perhaps selenium type practically appearing like a mechanical assembly. ( few small disks clamped in a bracket and tightened with a screw.and with 3 pins). may be at times two nos of germanium diodes are forming the same. you may replace the with due polarity, by schottkey diodes like 1N5819 or 1N5822.

Once one diode is short, perhaps the meter will read dc in AC range. If the diodes are OK it might read twice its value.

Second possibility, Generally the AC part of the meter will have lower impedance (ohms/volt) compared to DC. perhaps one of the resistors in the series chain has given way and developed high and not full break.

If you have another preferably DMM, you can trace them and locate it.
Hi, Sarma, No this meter is almost 15 years old and has discrete components.
The schematic I have shows resistor networks and I think will be electrically the same. The outside looks identical to Triplett 310c, but obviously a much earlier version. I will replace the AC Bridge and re-check the resistor chains.

I am still worried about the MOV behaviour. Can anyone tell me if it would behave much differently for a dmm to an Analogue meter on the ohm scale?

Component is out of the meter. DMM (fluke 23 runs a 9V battery) and the analogue runs a 1.5 v for ohms X1,X10,X100 and 15V for X1000 ohms scale.

Cheers RH
 

mvs sarma

Well-Known Member
RetiredHAL said:
Hi, Sarma, No this meter is almost 15 years old and has discrete components.
The schematic I have shows resistor networks and I think will be electrically the same. The outside looks identical to Triplett 310c, but obviously a much earlier version. I will replace the AC Bridge and re-check the resistor chains.

I am still worried about the MOV behaviour. Can anyone tell me if it would behave much differently for a dmm to an Analogue meter on the ohm scale?

Component is out of the meter. DMM (fluke 23 runs a 9V battery) and the analogue runs a 1.5 v for ohms X1,X10,X100 and 15V for X1000 ohms scale.

Cheers RH
i was telling about meters of 25 years old . like Sanwa P3. there may not be a bridge-- but only capacitor blocked full wave or even without capacitor
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It's perfectly normal for a meter to read DC on the AC range, they mostly don't have blocking capacitors, so DC will pass through the bridge just as well as AC.

Try connecting it to an AC source and see if it reads OK at that, in which case it would be fine.
 

mvs sarma

Well-Known Member
Nigel Goodwin said:
It's perfectly normal for a meter to read DC on the AC range, they mostly don't have blocking capacitors, so DC will pass through the bridge just as well as AC.

Try connecting it to an AC source and see if it reads OK at that, in which case it would be fine.
It reads Ac, Nigel, but I remember that it reads DC in AC range, a value near double almost.
 

RetiredHAL

New Member
Sarma, the link is http://jewellinstruments.com/pdffolder/triplettpdfs/84-731.pdf
I cannot upload as it will not scan well enough.

Nigel , looking at the schematic it seems you are right. I am astounded that I have never noticed this before. This would mean that you cannot look at ripple superimposed on DC without an external dc blocking cap if you use an Analogue meter, mind you I, nearly always, would have used a scope to look at ripple and ofcourse I may have used my Fluke DMM which does block the DC component. And this is exactly what brought it to my attention when I was restoring the valve radio. The scope saw almost no ripple and yet I saw 320 v on the AC scale on my Analogue meter. My Fluke confirmed almost no ripple on the AC scale.

I feel quite foolish, just goes to show that you are never too old to fall into a trap.
Cheers
RH
 

Nigel Goodwin

Super Moderator
Most Helpful Member
RetiredHAL said:
I feel quite foolish, just goes to show that you are never too old to fall into a trap.
Really it's the other way round, it's only relatively modern meters which may have a capacitor fitted, old ones were almost exclusively DC coupled on AC ranges.

As also suggested, a normal averaging AC meter won't read anything like the correct voltage if you feed it DC, although a true RMS one (if DC coupled) should do.
 

Hero999

Banned
I've never managed to measure DC on a meter set to AC.

The only thing I would imagine that would cause that is if the DC has a large ripple voltage superimposed on it.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hero999 said:
I've never managed to measure DC on a meter set to AC.

The only thing I would imagine that would cause that is if the DC has a large ripple voltage superimposed on it.
You've obviously only ever used meters with blocking capacitors in them then.
 

Dean Huster

Well-Known Member
I've never known a non-electronic analog meter that would not respond to a DC level on an AC range. The venerable Simpson 260 or Triplett 630 had an extra jack marked "OUTPUT" which did nothing more than insert a 0.1µF 600V cap in series with what would otherwise be the +V/ohm lead to block the DC in plate circuits (for example) so that you could measure only the AC. You had to be careful hooking these up, though, beginning on the highest AC range as the charging of the cap on a 2.5 V range could seriously pop the meter movement over.

The protection around the meter on the Triplett 310C is likely back-to-back diodes, which is why they measure differently on an analog vs. digital meter because of different voltages being applied. Resistance readings on the analog meter will probably vary depending upon the ohms range selected.

Dean
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top