VTVM BATTERY ELIMINATOR - CAUTION - TRANSFORMER BURNOUT
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- เผยแพร่เมื่อ 16 ก.พ. 2022
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1 - RCA Receiving Tube Manuals -
Get Several to cover different years.
2 - F.M. Simplified - Milton S. Kiver
Third Edition recommended
3 - Radiotron Designer’s Handbook
Published by RCA
4 - Old Time Radios! Restoration and Repair
Joseph J. Carr
Parts Source:
Antique Car Radio Solid State Vibrators
PEKORF.com These work correctly.
Seller Thetuberoom on e-bay
Radio Knobs at reasonable prices as well as
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Leather Handles from:
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I never saw the one with the bridge rectifier. That's scary. I've converted both my Heathkits with the half-wave style circuit and they work fine.
The radio machan your vtm battery eliminatoy is cool
Good tip, thanks Vern.
Keep up the good work
That is a great tip Vern, I may make the nod on my EICO 235
Thank you for the info. I did the mod and just double checked my Eico 232. I did the halfwave version.
Two thumbs way up on this info in case you missed it being covered somewhere else before.
The voice of experience. Now that's good information - there certainly is plenty of bad information out there. Always enjoy your thoughtful observations and tips. Thanks!
Thank you very much for pointing this out. I've added a much simpler (unregulated) battery eliminator to my IM-28, and was considering adding the regulated version, since the Ohms adjustment needs fairly constant tweaking without the regulator in there. I'll watch those filament grounds! (edit: luckily the IM-28 has a neon power indicator, so current draw isn't as much an issue.)
Thanks for the info. I have two Heathkit VTVMs and was going to do this change. Carl
At least the one eBay seller who lists a small PCB battery eliminator for the IM-11 & IMP18 seems to have a critical error in the instructions he provides. This product is just a Chinese made generic LM-317 based regulator with a trim pot to set the output voltage, and he adds an external diode and electrolytic capacitor for the half wave rectifier, I think he charges something just shy of $15 each. Following the provided installation instructions results in the eliminator PCB's 'ground' wire going to the ungrounded side of the filament circuit, and the 'V-in' wire going to the grounded side of the filament circuit. This works to the extent that the eliminator puts out the correct voltage, but the VTVM's Ohms function does not work quite correctly (typically it reads too low). Swapping the eliminator's 'V-in' (Yellow) and 'ground' (Green) wires fixes the problem. I have left that seller know about the issue, but it might not be easy to solve the problem, as it is a generic device that is supposed to be applicable to multiple models of VTVM, and they probably don't all have their filament circuit traces on the PCB routed the same way, and the installation instructions show a photo from the component side of the PCB only. Just throwing this out in case it might be tripping up anyone else.
I also wonder about so many VTVM battery eliminator designs that I have seen online, where they are woefully over-designed and therefore needlessly complicated and expensive to buy or make. If they are LM-317 based designs, they DO require a single diode for the half-wave rectification, and a suitable electrolytic filter capacitor of something on the order of 470uF. And they DO need something like a 1uF electrolytic or tantalum capacitor between the LM-317's output pin and ground. But in this application there is no benefit to ALSO putting a small value capacitor in parallel with the electrolytic. Many of these designs also have a small value capacitor across one of the voltage-setting resistors (it is not needed) and they usually have two diodes, one of which is connected from the output to the input of the LM-317; these would be good practice to include for a general purpose power supply application, where unknown capacitances and/or voltages might be connected to the output of the supply, but on a VTVM battery eliminator application, there is no time when a larger capacitance or external voltage potential will be applied to the LM-317's output, so those two diodes contribute nothing. I also see some of these LM-317 based eliminator designs where the designer did not understand the datasheet and is using voltage-setting resistors with very less-than-optimal values. Here is a tip; use a 220 Ohm 1% 1/4W resistor from LM-317 Output to Adjust, and a 47 Ohm 1% 1/4W resistor from Adjust to Ground, and it will produce a stabile voltage of appropriate level (just slightly higher than 1.5V). No adjustment needed, and the LM-317 will be operating optimally.
Thanks for the heads up. Just pulled out the schematic of my RCA WV-98C Volt Ohmist and see that the filament transformer is not grounded. Have considered adding a battery eliminator but about the only thing I use the VTVM for is the analog meter when doing receiver alignments. I'm lazy enough to use the DMM for resistance measurements.
I make a much simpler half-wave circuit then your; and will be selling it on Ebay in about a month. But kudo's on your design.
Hey Vern I can vouch for the eBay guy's stuff. I have two Eico 249s and have one in each unit. I didn't switch to LEDs but you make a valid point on transformer loading. I'll probably switch them over to. Keep up the good work and absolutely fantastic advice.
Robb
Your proposed modification should work good for your VTVM especially since the negative side of the battery is referenced to ground. To make an eliminator that would work for any make or variety of VTVM (units where the battery might not be directly referenced to ground) I would propose adding a small, separate low voltage transformer so that the eliminator could be completely isolated from ground. Adding a transformer adds cost, but completely eliminates any need to be concerned about the configuration of a particular VTVM. Good video! Thanks!
An alternative to a battery eliminator is a 'battery externalizer' -- meaning move, and attach with double-side tape, the battery carrier to the outside of the meter so that if the cells begin to rot they will not damage the internals of the meter.
I have an RCA Volt Ohmist, I thought maybe installing the battery holder outside the box, and extend the wires might be a solution. Also maybe there's a material to wrap the battery holder in. I prefer batteries over having it plugged in. There are Procell batteries supposed to be better. I haven't tried them yet.
This is good information. I just recently acquired a Heathkit IM-5228 and was thinking about the whole battery eliminator ordeal whether it was worth it or not.
I added a FWB based eliminator to my IM-5228 and simply lifted the filament ground connection. I did consider the additional current draw for the filament transformer. The half-wave rectifier is going to need more current as the input cap is going to draw a pretty big peak surge charging during the half-cycles; could draw maybe 400 mA on peaks; the ohmmeter needs about 150 mA so if you add the 600 mA maybe for the filaments you might be exceeding 1 A which may be pushing the spec on the transformer. Granted, you only need the higher current draw on the lowest current range and only for a short time, but if it were left connected for an extended period it may be problematic. These points were made by the author of the eliminator I put in (PopComm March 2010). I also added two pin jacks to the front panel so I could check the voltage without having to open the case. Thanks for the tips Vern! 73 - Dino KLØS
Not being totally happy with any battery eliminator solution for my VTVM I opted for a lithium AA batery instead, with an adaptor to C size of course. No worries about leakage and expected battery life for at least 20 years...
batteries are much safer, I agree
It is an Eico model 249 please?
Thanks for not calling it a full bridge rectifier.
Something is wrong . I have two thumbs but can only give Vern one thumbs up per video. Doesn't seem right.