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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 3:55 pm 
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As you all know, these kits are on our favorite auction site for about $16 postage included...As I don't have any LM317s about (I do have standoffs, small circuit boards, etc), this still seems like a good deal to me (postage for getting some LM317s...)

As I understand these discussions, current draw off of the 6.3 secondary would be negligible unless one leave the leads shorted...correct?

And, BTW...all of us have had the pleasure of corroded battery holders...some catastrophic....wouldn't using harbor freight zinc chloride greatly reduce the risk of leakage?

Steve


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Mar Tue 31, 2020 6:14 pm 
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I have ten of these transformers. I think they would work for building up a separate supply for the Ohms function. If anyone would like one. Send me a PM.


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Wed 01, 2020 8:29 pm 
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Mikeinkcmo wrote:
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".

Image

Image

What would be the value of the two capacitors in this schematic ?

Thanks,

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Wed 01, 2020 9:00 pm 
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Off hand I'd guess pin 3 100/25 and 47/16. Not shown are a couple of .01 ceramics at 2&3 also.

BTW remember NOT to connect the tab to anything without isolating it via a mica or plastic insulator and mount kit.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Wed 01, 2020 10:18 pm 
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Mikeinkcmo wrote:
Off hand I'd guess pin 3 100/25.

More like 470 µF/25 volt at a minimum for pin 3, to ensure the LM317 doesn't drop out of regulation at max load, 10% low line voltage, and worst-case minimum input-output differential. I'd probably up it to 1000 µF.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Wed 01, 2020 10:32 pm 
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Mikeinkcmo wrote:
Off hand I'd guess pin 3 100/25 and 47/16. Not shown are a couple of .01 ceramics at 2&3 also.

BTW remember NOT to connect the tab to anything without isolating it via a mica or plastic insulator and mount kit.

I sent you a reply but don't know where it went. I'll look for it.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Thu 02, 2020 1:26 am 
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Quote:
... I'd probably up it to 1000 µF...
Don't make it too big or you'll have to fold it in half to get it in the box. :wink:

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Thu 02, 2020 8:17 pm 
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Mikeinkcmo wrote:
Off hand I'd guess pin 3 100/25 and 47/16. Not shown are a couple of .01 ceramics at 2&3 also.

BTW remember NOT to connect the tab to anything without isolating it via a mica or plastic insulator and mount kit.

Can I assume that the two ceramics parallel the electrolytics. And can the LM-317 be attached to the frame via a small sheet of plastic sandwiched between industrial grade epoxy ? I have all the components except the 317s, which are on order.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Thu 02, 2020 10:17 pm 
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I'll let Mike clarify that. My only comment is that using plastic as an insulator might be fine, but don't ground the LM317 with the mounting screw. That need to be insulated too.
73,
-marc


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Thu 02, 2020 11:48 pm 
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That's the reason for the epoxy. It might be better if I don't use the plastic sheet. The electrolytic before the regulator will be 1000 uF @ 25 VDC; and after; 100 uF @ 25VDC.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 12:36 am 
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Jack Cureton wrote:
The electrolytic before the regulator will be 1000 uF @ 25 VDC; and after; 100 uF @ 25VDC.

Anything larger than 25 µF on the output of the LM317 will require a 1N4002 protection diode across the regulator. See the LM317 datasheet: https://www.ti.com/lit/ds/symlink/lm317.pdf

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 12:56 am 
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Thanks Steve, I have one of that value also.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 1:27 am 
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It would be possible to jury rig a zener diode out of two 1n4001 diodes in series which would give about 1.4 volts. The only extra parts that would be required would be a resistor in series with the diodes and an electrolytic capacitor in parallel with the diodes.


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 4:41 am 
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Yes the discs parallel the electrolytics, and as Steve mentioned, look at the data sheet, obviously I didn't, but in my defense, I did say it was a guess.

Use the proper mounting hardware and heat sink compound. Mounting kits are cheep on the bay.

Image

------------

There was discussion on page one of the thread regarding diodes, current requirements, resistor values, and transformer limitations.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 5:21 am 
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For what I'm trying to achieve, is transient & ripple rejection a concern ? Remember, I have 1000uF capacitance right at the input of the regulator.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 5:36 am 
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Yes, I have heat sinks & mounting kits. Plus I do not know the thermal conductivity of epoxy; just easy to work with giving nice looking results.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 2:45 pm 
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The LM317's high gain significantly reduces the ripple. Transient should not be a concern in this application; however, the circuitry (capacitor) for this is discussed in the LM317 datasheet.


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Fri 03, 2020 4:23 pm 
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Jack Cureton wrote:
Plus I do not know the thermal conductivity of epoxy; just easy to work with giving nice looking results.

Don't use epoxy, use a thin coat of thermal grease.

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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Sun 05, 2020 7:43 pm 
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Jack Cureton wrote:
Thanks Steve, I have one of that value also.

Jack

I finished the battery eliminator for the WV-98C. Used all junk box parts except for the [ordered] LM-317T. The output voltage I opted for was: 1.6 VDC. I included Texas Instruments' recommendation of a 0.2 ohm res. on the output [which I do not thing was needed]. Pictures included.

Jack


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 Post subject: Re: RESTORATION OF RCA WV-98 VTVM
PostPosted: Apr Mon 06, 2020 4:22 am 
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I was inspired to build a 1.5V regulator for my Lafayette VTVM which is kind of a cheap copy of the RCA. I made a MicroCap simulation of the power supply circuit based on actual component measurements. The LM317 regulator circuit would have raised the power loss in the transformer from 1.5 watts to 2 watts with the probes shorted on the X1 range. However, I noticed that the filament voltage on the tubes was high since the meter is intended to operate on 110VAC to 120VAC and my shop voltage has always been almost exactly 120VAC. The filament voltage is correct at 6.3VAC when the input is about 113VAC. That allowed me to put a 110 ohm, 1W, resistor in series with the AC power and drop the power loss in the transformer by about 0.4W. With the additional resistor, there is very little danger of over heating the power transformer even if I accidentally leave the probes shorted on the X1 range.

This is the circuit I ended up with for the least power loss in the transformer:
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The circuit board is mounted in place of the battery holder.
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I used three 1.3 ohm resistors in series because I didn't have a 3.9 ohm. The 10 ohm resistor was also moved to this board because the circuit trace was damaged on the main board. No heatsink is needed for the LM317. It is good for about 1W without a heat sink and the maximum in this application is less than 0.5W.

The regulator works great with only about 0.002 volts change from 0 to 150mA load and no ripple or stability problems.

Now I have a fully functional VTVM without concerns about a leaking battery.

Jay


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