What is this part in VTVM

2/7/2011 5:03:06 PMScott

Potential project-currently evaluating. This is a NRI VTVM / model 11, in pretty good shape. Needs the usual attention. But what is this part in photo with question mark identifier? Looks like wires wrapped in paper.
Got no schematic for this one and can't seem to locate info anywhere so this potential refurb could take some time.

2/7/2011 6:14:58 PMplanigan

Scott, check the schematic for location on the switch that should give you a narrow area to figure out what component is. My guess is that its the low resistor (less than 10 ohms) that consistantly pops up in the resistance circuit. Wire wound and covered with paper? PL

:Potential project-currently evaluating. This is a NRI VTVM / model 11, in pretty good shape. Needs the usual attention. But what is this part in photo with question mark identifier? Looks like wires wrapped in paper.
:Got no schematic for this one and can't seem to locate info anywhere so this potential refurb could take some time.
:
:

2/7/2011 6:28:30 PMLooking for SCH for you.

:
Scott,

I am looking in my files for the schematic for the VTVM. I restored one about six months ago. National Radio Institute. Edd was very helpful with the unit once we obtained the schematic from someone on the Forum. Hopefully the same poster will send you the info if i can't find it, it is out there. It was not a bad unit but not as good as the Eico and RCA VTVM's.

2/7/2011 7:30:28 PMEdd

Great Scott ! ! ! . . . . .

I done don'ts gots that "11" series of NRI units info, but lets 'jes see what we can glean from the "W" series schematic being supplied below.

I can clearly make out that 5.6 meghommeter 1/2 W carbon composition reeee-sister and it being routed down out of sight with one lead, but the more important resistors end, is going to the paper cap and pin 1 of the probable 6X4 AC voltage functions rectifier tube . . . . since this company had a fixation on using up their old Ray-dee-oh / TeeWee tubes supply cache , whereas everyone else would have been using a Sally-5 (6AL5) in their units circuitry designs.

Also I see the P-T formers GREEN wire going to a pin 7 which would be the cathode of a 6X4 and it along with one filament pin of the 6X4 are getting grounded.

Now, your 5.6 meg value unit could be a permissible variant from THIS designs 8.2 meg unit.

Now, when you come back, fill us in on your units actual tube complement.

(12BH7 /or / 12AU7 and a 6X4 and probably a selenium rectifier also.)

ALSO with use of this units schema . . . I think that you should now be able to get some idea of your where your " mystery component " just then might fit into its design scheme.

I can see that the depicted range switch is using that "rotor less" VERY OUTER END wafer, as a means of providing high voltage isolation tie points on the wafer for arranging the series string of voltage divider resistors.

Those series cascaded resistors, then go to the fully functional wafer just below . . . it where you can see the active rotor section associated with that section.Soooooooo . . . . take your " white mystery ? element" and track one lead end to all connections that it makes and then do the same for the other lead and see where they fall into place on the comparative schematic.

To MY eyes, the parts top visible lead seems to go into a small square bakelite "tag terminal" and then goes over to the range switch connection.

The "roll of white insulation" on its periphery, merely seems to be adding outer mass to it to KEEP it dressed AWAY from getting positioned TOO close to any potentially adjunct "arc over" area.

I suspicion that it might be the "equivalent" 8.2 meg value of your sets highest voltage divider resistor that is associated with the highest VOLTAGE range of the switched in 3M--600k--300K--60K--30K--10K, sequentially, lesser voltage ranges as is being given on this units schematic.

Thassit . . . . from all that I was initially given to wuk with . . . . . .tell us 'mo . . . .tell us 'mo . . . .tell us 'mo ! . . . . . . a . . . la . . . West Side Story.

73's de Edd

le SCHEMATIQUE:

::Potential project-currently evaluating. This is a NRI VTVM / model 11, in pretty good shape. Needs the usual attention. But what is this part in photo with question mark identifier? Looks like wires wrapped in paper.
::Got no schematic for this one and can't seem to locate info anywhere so this potential refurb could take some time.
::
::

:Scott,
:
:I am looking in my files for the schematic for the VTVM. I restored one about six months ago. National Radio Institute. Edd was very helpful with the unit once we obtained the schematic from someone on the Forum. Hopefully the same poster will send you the info if i can't find it, it is out there. It was not a bad unit but not as good as the Eico and RCA VTVM's.
:
:

2/7/2011 7:52:32 PMScott

Thanks for the info Planigan/ hope the schematic turns up. Another meter....it is habit forming!
Edd- in researching this prior to posting I found your previous info on the NRI/W model.
This model has a 6x4 and 12AU7(same as 12BH7 in W.)
There is a selinium rectifier. Also the electrolytic is a 10mfd/150v. The paper/wax caps are: 3- .002uf and 1 -.02uf, all 600v. Looks like W is 3-.01uf.
The model 11 has 2/C cells for the ohms part of the meter. the W has 1/1.5 cell.
Haven't really gotten into the circuit tracing yet but will in the near future.

2/8/2011 5:25:29 PMplanigan

Scott/Edd, I find the wiring of the power switch intriguing!

:Thanks for the info Planigan/ hope the schematic turns up. Another meter....it is habit forming!
:Edd- in researching this prior to posting I found your previous info on the NRI/W model.
:This model has a 6x4 and 12AU7(same as 12BH7 in W.)
:There is a selinium rectifier. Also the electrolytic is a 10mfd/150v. The paper/wax caps are: 3- .002uf and 1 -.02uf, all 600v. Looks like W is 3-.01uf.
:The model 11 has 2/C cells for the ohms part of the meter. the W has 1/1.5 cell.
:Haven't really gotten into the circuit tracing yet but will in the near future.
:

2/11/2011 10:01:38 AMScott

Update on the mystery part. Turns out it is a wire wrapped in paper 20 ohm resistor. I wonder why this kind of resistor was used? It is the RX1/3v location.
Also included in the photo is a 1.5meg charred resistor and the reason I removed wafer switch. It was located between 2nd/3rd wafers and rubbing aginst switch center. It looks burnt, there is a chunk out of it, the wafer was black, but the resistor still measured 1.6meg. Unexplainable?

Still searching the schematic for NRI model 11 as resistor values don't mesh up with W model.

2/11/2011 9:02:52 PMplanigan

Scott what are the values of the resistors in the divider circuit? Also what are the tolerances of the resistors. Usually the ohm scale resistors were 5% while those in AC, DC curcuits were 1% and what are the ranges? May be we can make a calculated guess on necessary replacements. {PL

: Update on the mystery part. Turns out it is a wire wrapped in paper 20 ohm resistor. I wonder why this kind of resistor was used? It is the RX1/3v location.
:Also included in the photo is a 1.5meg charred resistor and the reason I removed wafer switch. It was located between 2nd/3rd wafers and rubbing aginst switch center. It looks burnt, there is a chunk out of it, the wafer was black, but the resistor still measured 1.6meg. Unexplainable?
:
:
:Still searching the schematic for NRI model 11 as resistor values don't mesh up with W model.
:
:

2/11/2011 9:10:03 PMplanigan

:Scott what are the values of the resistors in the divider circuit? Also what are the tolerances of the resistors. Usually the ohm scale resistors were 5% while those in AC, DC curcuits were 1% and what are the ranges? May be we can make a calculated guess on necessary replacements.As to the resistor, low ohmage makes it easy to fabricate with resistance wire as there are few turns and not much call for carbon resistors that low ohms. PL
:
:
:: Update on the mystery part. Turns out it is a wire wrapped in paper 20 ohm resistor. I wonder why this kind of resistor was used? It is the RX1/3v location.
::Also included in the photo is a 1.5meg charred resistor and the reason I removed wafer switch. It was located between 2nd/3rd wafers and rubbing aginst switch center. It looks burnt, there is a chunk out of it, the wafer was black, but the resistor still measured 1.6meg. Unexplainable?
::

::
::Still searching the schematic for NRI model 11 as resistor values don't mesh up with W model.
::
::
:
:

2/12/2011 7:17:36 AMScott

The resistor bridge circuit posted below. As far as values most are silver band and some gold.
I have the meter working ok on ohms and DC but AC is not working correctly. The meter wavers and sometimes stablizes.Voltage measures much lower than actual on AC. The measured voltage coming out of selenium rectifier seems low at 85v and filament voltage is low at 3.5 so I have to figure this out.

2/12/2011 4:43:37 PMIgor

First of all, the selenium belongs in the trash can. You can replace it with a Si diode.

Divider resistors for a lot of these old meters were made up of screened combinations, probably using a bridge and precision standards. You can do the same with a modern DVM.

The ohms resistors should add up to the mid scale value for each range. In other words the total resistance in series with the battery should be equal to the mid scale reading for the selected range. The resistor function uses the voltage divider method, so if the unknown resistor is the same as the series resistance, the meter will read half scale, and so forth.

The voltage divider network is setup so that the ratios of resistance correspond to the ratio of each selected range to the maximum range. This is to make the maximum voltage applied to the bridge tube the same for each range. In this case the highest range is 1200 volts and the lowest is 3 volts so the the 1200 volt range requires a division of 400 to 1. Similarly the 300 volt range requires a division of 100 to one, and so forth.

You can easily figure the appropriate divider resistors if you know the overall input resistance of the VTVM. For istance, if the meter has an input resistance of 10 megs, a highest range of 1500 volts and a lowest of 1.5 volts, the divider chain must scale the input down by a factor of 1000. Therefore the lowest resistor in the chain (to lower the input to the tube the most) must be 0.001 of the total or 10k. For the 50 volt range, the input must be scale down by a factor of 0.03, giving a resistance value of 300k for the part of the chain selected for the 50 volt range.

Ten megs was not always the "standard" for VTVMs. Some used 8, some 9, some used other values.

The values you found in your unit seem to be consistent with 9 megs. This gives numbers reasonably close to the marked values of your divider resitors:

22.5K (1200), 67.5K (300), 135K (120), 675K (30), 1.35M (12), 6.75M (3).

Remember that the originals were made up from screened combinations and that you can do the same.

This is one set that gives the proper ratios. There are similar sets for 8 or 10 megs. Happy figuring.

:The resistor bridge circuit posted below. As far as values most are silver band and some gold.
:I have the meter working ok on ohms and DC but AC is not working correctly. The meter wavers and sometimes stablizes.Voltage measures much lower than actual on AC. The measured voltage coming out of selenium rectifier seems low at 85v and filament voltage is low at 3.5 so I have to figure this out.
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