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 Post subject: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 12:40 am 
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Location: Charlotte, NC 28205
In the process of restoring the subject VTVM, I would like to include a [1.5 volt] battery eliminator circuit. Seems simple enough. Using a step-down / isolation transformer, I could come off the line or one of the on-board transformer's secondaries. Comments welcomed. Not interested in restoring to originality.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 2:43 am 
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Location: Seattle WA US
Jack -
I've done that to all my VTVMS. I recommend it. You can pick up a small LM317 regulator kit on ebay with all parts including heat sink and a circuit board for less than it would cost to buy the parts. One change must be made: the ebay kits usually have a bridge rectifier. It must be replaced with a half-wave rectifier (1N4007) for compatibility with the VTVM 6.3v filament circuit which has one side grounded.
-Chuck


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 4:06 am 
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This is what I came up with. Some overkill because I assumed the probes were continuously shorted on the X1 scale to get max load.
You can probably get by with a rectifier off of the existing 6.3V if you are careful not to leave the probes together on the low scale.
If you decide to use the existing transformer, I would disconnect the ground and use a full wave bridge. The tubes shouldn't care about the offset caused by the diodes.
Attachment:
1.5V.jpg
1.5V.jpg [ 47.36 KiB | Viewed 1148 times ]


Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 8:13 am 
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K7MCG wrote:
Jack -
I've done that to all my VTVMS. I recommend it. You can pick up a small LM317 regulator kit on ebay with all parts including heat sink and a circuit board for less than it would cost to buy the parts. One change must be made: the ebay kits usually have a bridge rectifier. It must be replaced with a half-wave rectifier (1N4007) for compatibility with the VTVM 6.3v filament circuit which has one side grounded.
-Chuck

Does the 6.3 volts go to the filaments only & can it float [removed from ground] ?

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 8:56 am 
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Location: Austin, Texas
[quote="Jack Cureton"
Does the 6.3 volts go to the filaments only & can it float [removed from ground] ?

Jack[/quote]

Only to the filaments. Should not be a problem to use a bridge rectifier.
Attachment:
RCA.jpg
RCA.jpg [ 37.56 KiB | Viewed 1133 times ]

Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 2:09 pm 
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Here is the Stock WV-98C power supply schematic, and a proposed OHMS battery eliminator, based on Jay's schematic.

Keep in mind that in the case of the RCA WV-98 meters, the Ohms supply must provide over 150 mA when the probes are shorted to adjust "zero".

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 4:09 pm 
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If not mistaken, the LM317M can handle 400 - 500 mA output. Would a 150-ohm pot work to adjust the output voltage ?

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 4:56 pm 
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Jack Cureton wrote:
If not mistaken, the LM317M can handle 400 - 500 mA output. Would a 150-ohm pot work to adjust the output voltage ?

Jack


Yes, but you will be better off with a fixed resistor adjustment. Mechanical things like pots are some of the most likely parts to cause you trouble.
Build the circuit with fixed resistors and then put a large value resistor across one of the initial parts to tweak the voltage up or down.

Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 5:35 pm 
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Found this on line. It's what the Heathkit Engineers used in there lab V.T.V.M.'s I installed it in my IM-28. Works well and a good junk box should have all the needed parts. Three 1N400X (any diode 50 PIV, 1amp), 100 Uf/25V cap, 27 Ohm / 3 Watt resistor. Built in on a five lug terminal ( Franklin Strip) four isolated and one ground terminals. Easy to install. Note While your inside, make sure al the ground hardware is good and tight.


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VTVM Battery.jpg
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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 6:02 pm 
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Jack Cureton wrote:
If not mistaken, the LM317M can handle 400 - 500 mA output. Would a 150-ohm pot work to adjust the output voltage ?

Jack

Another concern would be, can the transformer supply the additional current?

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 7:19 pm 
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Notimetolooz wrote:
Another concern would be, can the transformer supply the additional current?


I don't see much problem with the full wave bridge and the series regulator. There is only a significant load when in the X1 ohms range and then it is only about 0.15W more transformer power.

On the other hand, I would not recommend this circuit:
Attachment:
VTVM Battery.jpg
VTVM Battery.jpg [ 13.87 KiB | Viewed 1033 times ]

You would have to increase the capacitor to about 5,000uF for it to work in this meter and it would increase the transformer power by about 1.5W continuously. I think it would be a transformer killer.

Forgot to mention that the 27 ohm resistor would also need to be at least 5W. Continuous power would be about 2.5W with the cap increased to 5,000uF.

Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Mon 30, 2020 9:40 pm 
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JnTX wrote:
I don't see much problem with the full wave bridge and the series regulator. There is only a significant load when in the X1 ohms range and then it is only about 0.15W more transformer power.

How do you calculate that to be only 0.15 Watts?

X1 ohms with probes shorted is 150 mA. Using a series regulator, that 150 mA has to come from the 6.3 volt filament supply. So that's at least 945 mW. But this circuit uses a capacitor-input filter, so the RMS current from the transformer secondary is much higher than the DC output current from the regulator. Given a full-wave rectifier, the RMS current is perhaps 2X the DC load current, so that's something on the order of 2 Watts extra load on the transformer.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 12:14 am 
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stevebyan wrote:
JnTX wrote:
I don't see much problem with the full wave bridge and the series regulator. There is only a significant load when in the X1 ohms range and then it is only about 0.15W more transformer power.

How do you calculate that to be only 0.15 Watts?

X1 ohms with probes shorted is 150 mA. Using a series regulator, that 150 mA has to come from the 6.3 volt filament supply. So that's at least 945 mW. But this circuit uses a capacitor-input filter, so the RMS current from the transformer secondary is much higher than the DC output current from the regulator. Given a full-wave rectifier, the RMS current is perhaps 2X the DC load current, so that's something on the order of 2 Watts extra load on the transformer.


The actual power output increase is about 1 watt so I agree with your 945 mW number.
The RMS current increase in the secondary winding is from 0.6A RMS (for the tubes) to about .9A RMS.
I did make an error calculating the additional power dissipation in the transformer due to the higher RMS currents in the primary and secondary windings.
The power dissipation increase in the transformer is about 0.5W, not 0.15W.
The extra load on the transformer is 1W which is 26% increase for the 6.3V output.
The extra power dissipation in the transformer due to the series regulator is 0.5W which is a 115% increase for the 6.3V output.
I don't think the extra 0.5W is going to be a problem even if the probes were continually shorted together in the X1 ohms range.

You are correct in stating that the RMS current is about 2X the DC load current. Notice the increase is .3A RMS which is twice the DC load of .15A.
However, that does not make the load 2 watts. The additional load is still 1 watt.

Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 1:58 am 
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Keep in mind those "HIGH POWER" requirements are only when the leads are shorted together, otherwise the increased power demands are nil. When measuring typical resistances found in tube, or even SS radios for that matter, the X1 scale isn't used all much, and a few seconds measuring low value resistors, probably won't start a fire.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 3:05 am 
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Mikeinkcmo wrote:
Keep in mind those "HIGH POWER" requirements are only when the leads are shorted together, otherwise the increased power demands are nil.


Mike,
Right on. Unless you accidentally leave the probes touching after making an X1 reading, the series regulator circuit is an insignificant increase in the meter power requirements. However, I have to admit that I have let that happen a few times.

The shunt regulator circuit isn't as nice since it makes a significant increase in the meter power and is there 100% of the time that the meter is turned on.

This just gave me a thought about how to do this without any significant load increase on the transformer. We can get 10F, 2.3V, caps for about $2. If one was set up to charge to 1.5V with a 15mA current source, it would work continuously for X10 and higher range measurements. Measurements on the X1 range would have to be kept to few seconds duration but I think I could live with that. Even the simple shunt regulator would probably be OK if it only needed to provide 15mA instead of 150mA.

Edit: I tried some things with super caps and decided that 1F was a better choice to get the ohmmeter working within 30 seconds of AC power on. Getting rid of the cap right after the bridge rectifier greatly reduces the RMS currents. The simple shunt regulator with 150mA capability increases the power dissipation in the power transformer by about 0.7W. I have some 1Farad caps that were used for memory backup on a computer board so I think I will give this circuit a try on my old Lafayette VTVM.
Attachment:
1.5V_1F.jpg
1.5V_1F.jpg [ 36.75 KiB | Viewed 940 times ]


Jay


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 5:29 am 
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If a way could be found to turn on the ohms battery sub circuit
only when measuring resistance it might be good.

The transformer is + 60 years old.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 1:13 pm 
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JnTX wrote:
[The power dissipation increase in the transformer is about 0.5W, not 0.15W.

Sorry, I misinterpreted your response to be the increase in transformer load, rather than the increase in power dissipation in the transformer. Thanks for the detailed explanation.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 1:49 pm 
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Quote:
...If a way could be found to turn on the ohms battery sub circuit only when measuring resistance it might be good...
Care to elaborate?

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 1:56 pm 
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Mikeinkcmo wrote:
Quote:
...If a way could be found to turn on the ohms battery sub circuit only when measuring resistance it might be good...
Care to elaborate?

I think Steve is talking about the shunt-circuit flavor of battery eliminator.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 2:30 pm 
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stevebyan wrote:
Mikeinkcmo wrote:
Quote:
...If a way could be found to turn on the ohms battery sub circuit only when measuring resistance it might be good...
Care to elaborate?

I think Steve is talking about the shunt-circuit flavor of battery eliminator.

I think so too. There probably aren't any extra contacts on the switch but there may be a way to detect the switch position and drive a MOSFET to turn on the shunt regulator. There's always the addition of an extra toggle switch but most of us would go to the series regulator instead of making a new hole in the enclosure.

Jay


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