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 Post subject: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Fri 15, 2019 10:22 pm 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
Hi,

I recently got a Knight 620 VTVM fully restored from eBay after my old Heathkit IM-5228 got damaged when my brother plugged it into a 220V outlet. It was fully restored by the seller (including the 2 prong AC cable changed to a 3 pin earthed one) and I tested it on on AC & Ohm, both of which works perfectly fine. As for the DC- & DC+ I tested it with a bunch of lead acid batteries I had and there too the readings were fine.

So, now coming to the issue I'm facing, which could be that I'm doing something wrong. Before I that I need to tell you guys the supply source is a Automatic servo voltage stabilizer with 119VAC stable supply(I guess the supply is isolated here). I've an EICO 950B capacitor tester which I was just trying to change the paper capacitor with new film caps. After re-capping I tried to measure the voltage at three points on the EICO as per the schematics with are 525V, -500V & 200V. The EICO has a 2 prong non-earthed plug. Now I tried measuring the voltages with my uni-t multimeter but it can go only unto 500VDC max. So, I plugged my Knight VTVM which is 3 plug to the same source and tried to measure the DC voltage but I get no reading with just touching the DC + probe only to the 500V course on the EICO and also tried connecting the common (-) from the VTVM to the bare body of the EICO and still didn't get anything.

One, thing I need to mention is that as per the original Knight 620 assembly manual it was only a two with AC cord. What is the mistake I'm doing here? Or is it the 3 prong AC on the VTVM that is causing this?


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Fri 15, 2019 10:30 pm 
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Posts: 18297
Location: Albuquerque, NM 87123
1st, you need to get rid of that earth ground connection on the VTVM. It serves no purpose and can be dangerous to earth ground equipment you are testing.

You may have a problem with the 1 meg resistor in the probe, or the probe switch.


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Fri 15, 2019 10:36 pm 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
Johnnysan wrote:
1st, you need to get rid of that earth ground connection on the VTVM. It serves no purpose and can be dangerous to earth ground equipment you are testing.

You may have a problem with the 1 meg resistor in the probe, or the probe switch.


I replaced the 1 Meg resistor in the probe today just to rule that out and the DC readings are perfectly while taking readings from bunch of Lead Acid batteries I’ve which confirms that the DC section is working fine.

As for the power cord I’ll replace the grounded one back with a two wire cord. One question though my old Heathkit 5228 came factory built with a 3 prong. So, it should be okay with this one too right? But you are right if the hot lead gets grounded that’s bad!


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Fri 15, 2019 11:05 pm 
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Location: Littleton, MA
Vintage Collector wrote:
I've an EICO 950B capacitor tester which I was just trying to change the paper capacitor with new film caps. After re-capping I tried to measure the voltage at three points on the EICO as per the schematics with are 525V, -500V & 200V. The EICO has a 2 prong non-earthed plug. Now I tried measuring the voltages with my uni-t multimeter but it can go only unto 500VDC max. So, I plugged my Knight VTVM which is 3 plug to the same source and tried to measure the DC voltage but I get no reading with just touching the DC + probe only to the 500V course on the EICO and also tried connecting the common (-) from the VTVM to the bare body of the EICO and still didn't get anything.

One, thing I need to mention is that as per the original Knight 620 assembly manual it was only a two with AC cord. What is the mistake I'm doing here?

Just using the DC+ probe doesn't make a complete circuit, so the zero volt reading is as expected.

Try connecting the VTVM common to the grounded end of R10, instead of depending on the 950B enclosure to provide a path to circuit common.

Vintage Collector wrote:
Or is it the 3 prong AC on the VTVM that is causing this?

No, the three prong plug is not the problem. And unlike Johnnysan, I prefer to ground the metal case of my VTVM's using a three-conductor AC plug. I'd leave it connected.

If you search around a bit here on ARF, you'll find the reasoning behind the two differing philosophies. You'll just have to decide for yourself.

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 12:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 706
Location: Opelika, AL
the item under test should probably be fed from a separate isolation transformer than whatever you are using to power the VTVM. Is it possible the polarity is reversed on the VTVM, or perhaps there is something wrong with the range switch? I'm assuming the lead acid batteries are getting measured on the 15 or 50V range, and your 500V is getting measured on the 1500V range.

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 12:46 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Assuming the VTVM is working properly, you should have no problem making these measurements. The Eico 950B is kind of an unusual situation. It manages to develop two output voltages by dividing the charge between two filter capacitors. The instrument develops about 200 volts positive DC voltage for the eye tube, and it also develops negative 520 volts DC for the leakage test circuit. The chassis is the common point between the two supplies, so the ground clip of the VTVM goes there. You should be reading the +200 volts at the positive terminal of C-9 and the -520 volts at the negative terminal of C1. The VTVM will of course have to be switched from +DC to -DC to do this.

The grounded cord on the meter is a red herring. It will not make the least bit of difference in this case because the Eico is operated by a power transformer which isolates the chassis and bridge circuitry from the AC line. If you are not getting the expected voltage readings then either the Eico or the VTVM isn't working properly. Do you have another meter, perhaps a DMM or a reasonably sensitive VOM? If so, it should be able to read the voltages in the Eico too.

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 6:16 am 
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Joined: Feb Sun 02, 2014 1:58 pm
Posts: 1083
Location: Tennessee 38058
stevebyan wrote:
Vintage Collector wrote:
I've an EICO 950B capacitor tester which I was just trying to change the paper capacitor with new film caps. After re-capping I tried to measure the voltage at three points on the EICO as per the schematics with are 525V, -500V & 200V. The EICO has a 2 prong non-earthed plug. Now I tried measuring the voltages with my uni-t multimeter but it can go only unto 500VDC max. So, I plugged my Knight VTVM which is 3 plug to the same source and tried to measure the DC voltage but I get no reading with just touching the DC + probe only to the 500V course on the EICO and also tried connecting the common (-) from the VTVM to the bare body of the EICO and still didn't get anything.

One, thing I need to mention is that as per the original Knight 620 assembly manual it was only a two with AC cord. What is the mistake I'm doing here?

Just using the DC+ probe doesn't make a complete circuit, so the zero volt reading is as expected.

Try connecting the VTVM common to the grounded end of R10, instead of depending on the 950B enclosure to provide a path to circuit common.

Vintage Collector wrote:
Or is it the 3 prong AC on the VTVM that is causing this?


No, the three prong plug is not the problem. And unlike Johnnysan, I prefer to ground the metal case of my VTVM's using a three-conductor AC plug. I'd leave it connected.

If you search around a bit here on ARF, you'll find the reasoning behind the two differing philosophies. You'll just have to decide for yourself.

+1

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KN4GGM

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 6:35 pm 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
So, This time I powered the EICO from an old transformer in the garage that I used to convert from 110V to 220V. I guess this is isolated. I've attached a pic of the transformer that I used below. The VTVM range switch was fine because I had connected a bunch of few lead acid batteries to get up to 48V. So, I tested the ranges and it worked fine. I was able to get the DC reading on VTVM this time.

Also an old Heathkit SM-21A that I had purchased 5-6 years ago which had issues with all the ranges except Ohm also worked perfectly fine with the isolation transformer!


Attachments:
rsz_transformer.jpg
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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 8:16 pm 
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Location: Littleton, MA
Vintage Collector wrote:
I guess this is isolated. I've attached a pic of the transformer that I used below.

No, that transformer is not isolated. It's an autotransformer, so it only has one winding with multiple taps. It's going to put out either 230 or 57.5 volts if you put in 115 volts, so I don't see how any equipment you've connected to it could work properly, unless they are set up for 230 volts AC input. Are you located somewhere with 230 volt power distribution?

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 8:54 pm 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
stevebyan wrote:
Vintage Collector wrote:
I guess this is isolated. I've attached a pic of the transformer that I used below.

No, that transformer is not isolated. It's an autotransformer, so it only has one winding with multiple taps. It's going to put out either 230 or 57.5 volts if you put in 115 volts, so I don't see how any equipment you've connected to it could work properly, unless they are set up for 230 volts AC input. Are you located somewhere with 230 volt power distribution?


Sorry, if my explanation sounds confusing. I've 120V 60Hz outlets in my house with split phase power supply system. But I've setup a 220V outlet also in my garage. You know the two 110V lines together US style, that one.


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sat 16, 2019 10:25 pm 
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Location: Littleton, MA
Ah, thanks, that makes sense.

Again, to be clear, you cannot use your transformer as an isolation transformer.

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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sun 17, 2019 12:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18297
Location: Albuquerque, NM 87123
Chris108 wrote:
The grounded cord on the meter is a red herring. It will not make the least bit of difference in this case because the Eico is operated by a power transformer which isolates the chassis and bridge circuitry from the AC line.


The fact that the EICO has a power transformer is immaterial. It is never a good idea to earth ground equipment you are working on. If that is unavoidable, like when using an oscilloscope, then you must use an isolation transformer on the AC line to power the equipment (that is, the device under test).

You should understand that earth grounding most equipment will not help and may hurt you if you come into contact with a hot connection of an AC line. Remember, earth ground is one side of the AC line.


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sun 17, 2019 6:42 am 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
Johnnysan wrote:
The fact that the EICO has a power transformer is immaterial. It is never a good idea to earth ground equipment you are working on. If that is unavoidable, like when using an oscilloscope, then you must use an isolation transformer on the AC line to power the equipment (that is, the device under test).

You should understand that earth grounding most equipment will not help and may hurt you if you come into contact with a hot connection of an AC line. Remember, earth ground is one side of the AC line.


So, do you suggest that I remove the earth ground and go back to the original 2 wire AC cord? Because currently what the previous owner has done is, he has tied the earth to the common ground. Also there is a chance that the live from the outlet could be swapped at certain outlets and the expected neutral could become live and I guess thats bad when grounded!


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Sun 17, 2019 6:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18297
Location: Albuquerque, NM 87123
Yes, remove the ground connection. If you don't want to replace the cord just remove a small section of the green ground wire.


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 Post subject: Re: Problem reading DC voltage on Knight 620 VTVM?
PostPosted: Feb Wed 20, 2019 2:56 pm 
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Joined: Oct Fri 12, 2018 11:07 am
Posts: 85
Johnnysan wrote:
Yes, remove the ground connection. If you don't want to replace the cord just remove a small section of the green ground wire.


All is good now! :) Glad I posted this question, got another Heath also up and running now that was in storage for a long time same issue.


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