Making an RF probe for VTVM - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

Luxman1050 · 9th Feb 2020, 10:43 pm

I think my VTVM is capable of gaining RMS. Looking at the spec sheet. I don't mean it reads that on the scale but can be calculated.
VTVM circuit usually measures either the average value of the input, the positive or negative peak value, or the peak to peak value. The meter scale is then calibrated for the RMS value assuming a sine wave input waveform.

If you want to measure the RMS value of a sine wave and your meter is in calibration, you just read the meter scale. So either your meter is not in calibration or you are not measuring a sine wave.

If you know the wave shape, it would be possible to calculate a correction factor to apply to your meter reading to get the RMS value. But you need to know whether your VTVM has a circuit that reads the average, peak, or peak to peak value of the input which I'm sure mine does.

Luxman1050 · 10th Feb 2020, 12:25 am

Re post 30#
On ohms section you short the test leads and zero the needle then you move ohms to far right onto kind of figure of 8 mark on the scale. Then you can measure resistance. Actually when got it working which seems best when it's reasonably cold the readings are spot on. Having also checked readings on 2 other DVMs I have just to make sure. Problem with this is when it's warmed up then it starts to mess around abit. Anyhow I've replaced all caps for high quality ones just waiting on this electrolytic to be sorted plus waiting on some Metallised polypropylene caps to replace the ceramics bit more stable. I also put two new pots in from donor unit the zero in and ohms adjuster which are nice and tight on turning the originals where like loose swivels. Plus the carbon tracks are newer and less used.
Not sure what you mean clone

?

joebog1 · 10th Feb 2020, 12:34 am

I dont want to get shot!! BUT the power transformer is not up to another 150 mA load.

You will be pushing hard to find a replacement, and rewinding something that small with no doubt thousands of turns is not for the light hearted.

Joe

joebog1 · 10th Feb 2020, 1:46 am

I think ceramic caps will be far more stable than polypropylene!! They will also be less leaky than polypropylene. That kind of figure 8 mark is known as Infinity.
My meter only had the main electro replaced by me, and that's the first time its been worked on since it was made sometime in the early 1960's. I think the "jumping about" syndrome will be caused by dirty pots or switch contacts rather than faulty components.

As I said at the beginning, these meters are real work horses and rarely have problems if correctly operated. I have two meters on my bench as normal. An AVO model 8, and the Kyoritsu VTVM. I have other fancy meters should I need them, but nowadays I mostly do audio and a little bit of restoration on valve radios. My fancy meters rarely get used.

Joe

ms660 · 10th Feb 2020, 10:41 am

Quote:

Originally Posted by cdm1christopher

I think my VTVM is capable of gaining RMS. Looking at the spec sheet. I don't mean it reads that on the scale but can be calculated.
VTVM circuit usually measures either the average value of the input, the positive or negative peak value, or the peak to peak value. The meter scale is then calibrated for the RMS value assuming a sine wave input waveform.

If you want to measure the RMS value of a sine wave and your meter is in calibration, you just read the meter scale. So either your meter is not in calibration or you are not measuring a sine wave.

If you know the wave shape, it would be possible to calculate a correction factor to apply to your meter reading to get the RMS value. But you need to know whether your VTVM has a circuit that reads the average, peak, or peak to peak value of the input which I'm sure mine does.

4th post:

https://antiqueradios.com/forums/vie...p?f=8&t=290783

Lawrence.

Station X · 10th Feb 2020, 11:01 am

I've fallen foul of that when trying to measure the output voltage of a modified sine wave invertor. As my meter isn't a true RMS type the measured voltage was much less than I expected.

ms660 · 10th Feb 2020, 11:19 am

The point I was trying to make to the OP all along was the fact that the OP is proposing to use an external RF probe and the schematic submitted so far shows it to be a half wave peak detector.

So long as he understands that and any implications that he might have to deal with when reading the scales then that's fine, no problem.

Lawrence.

Luxman1050 · 10th Feb 2020, 9:01 pm

Well I'll give up on the probe seems an impossible task!!!!!
Joe the caps had all been replaced funny enough except the electrolytic which was reading over twice it's uf rating. The 3kv cap was substituted for two 1000v at 0.1uf in series so you have your 0.05uf but only 2kv. These where both wrapped in heat shrink tubing and where reading 0.09uf so again nearly double. So I decided to check them and on removal the top cap had overheated and gone brown these are white caps and it had melted slightly plus 1k lower than they should be which is asking for trouble in my book. That's 1000v!!!! There's a 3kv in there for a reason.
The two 0.05uf oil caps where replaced with ITTF caps which I have only seen in crossovers in speakers not in any other circuits valve or solid state that I personally have come across. So they got upgraded to similar spec to the originals. The ceramics I felt might as well be replaced since I'm going the whole hog. I thought ceramics where noisy especially in rf applications? and sensitive to voltage swings whereas I thought Metallised polypropylene capacitors where more stable with voltage swings more self healing and better in rf applications? But no big deal I can always put new ceramics in if the MP,s cause any problems.
Anyhow that's why I've replaced the caps it was not just for the sake of it and yes if someone asks all caps where discharged before any readings where taken.
So what is the mA rating on this transformer as there are no specifications for it on or in the manual. Therefore how do we know whether it can or can't handle an extra 150mA based just on it's size? Thinking out loud.
Yeah I should have copied that link in Lawrence Interesting read though.
The transformer we know is rated at 230v 100v 130v and 6.3v. So how do we work out with those figures it's maximum mA output?

ms660 · 10th Feb 2020, 9:21 pm

What's the voltage and current rating of the pilot bulb?

Lawrence.

joebog1 · 10th Feb 2020, 11:18 pm

The pilot lamp is grain of wheat lamp. Very small light output. Transformer centre leg is 3/8 X 3/8 at best. ( NO I havent measured it ) It is wound for 240 volts primary with a tap for 120 volts. Secondary is 130 volts and 6.3 volts. I have been making transformers for about 50 years and I KNOW that it hasnt got the bottle for another 150 mA.Chris can take the gamble if he wishes.

The .05 uF 3000 volt cap is the AC coupling capacitor. It couples the AC measured voltage into the 6AL5 rectifier. It is rated 3000 volts for safety reasons.

Joe

Luxman1050 · 10th Feb 2020, 11:51 pm

PL on schematic along heater chain thought you would know that?

Luxman1050 · 11th Feb 2020, 4:02 pm

I've decided to take note and will leave tapping into the circuit for an RF probe into the unit. However since I have two of these I have all the components to hand to make an external power supply. How not sure yet but it will get done as I'm determined to get the RF probe made using the EA50.
With the lamp not sure in this circuit if it's for removing the anode dc voltage? Being used as a voltage stabilizer although it's not looking at schematic connected in series with a resistor between unregulated voltage sources. As far as I can tell it's connected to the heaters and pin 9 being heater tap on the 12AU7 valve. Could be wrong but the lamp as Joe said is incredibly small like a minature led. Again just stabbing in the dark.
Yes that's why I put a 3kv in there not 2kv cant get me head round that one what was he/she thinking. I mean when do you put a lower voltage cap in circuit? Beyond me.
Decided to make a slightly decent DC probe pretty happy with result used a sharpie pen.

Luxman1050 · 11th Feb 2020, 7:23 pm

Here ya go new caps installed and two new pots. Not done ceramics yet as replacements not due until 15th.
Managed to dig out a mundorph cap 22uf so installed that for now on removing the sprague. Will fire her up tomorrow and get some volt readings although not anything on the schematic to go by! Typical ha.
Plus pic of the back case which has rusted where the 3k cap and tranny are located I've changed that for the one off the donor unit as it's in mint condition. I wonder why it's rusted there?
Cheers Chris

joebog1 · 11th Feb 2020, 11:35 pm

I would suggest the rust is from a dry cell being left in there at some stage. It's a good idea to periodically check gear containing batteries!! I have left a few things too long with ugly results.

Joe

I should add, very nice little probe you made.

Luxman1050 · 12th Feb 2020, 11:25 am

Thanks Joe I'm quite happy with it.

Luxman1050 · 12th Feb 2020, 1:45 pm

Joe how did you get valve readings on the meter?
Does the scale need to be connected up? If so it would mean dropping out scale from unit. If connected insitu not able to flip board over.
Cheers Chris

Luxman1050 · 12th Feb 2020, 3:58 pm

Sorted that used jumper leads I found.
I did not connect any leads so not sure if that can affect voltage readings.
Heater, pl reading 6.89v
100v tap reading 113v
130v tap reading 156 bit high
obviously 6.3v tap as per heaters 6.89v. Spec sheets can go up to 12.6v.
Awkward as not able to elevate board don't like the valves touching table top as there getting hot.
Anyhow think that's okay.

Luxman1050 · 12th Feb 2020, 4:10 pm

Ignore that 12.6v that's combined ew rushing.

Luxman1050 · 12th Feb 2020, 5:11 pm

sorry guys this thread supposed to be about an rf probe not refurb job.
I'll go to another thread for that.
Cheers Chris

Skywave · 13th Feb 2020, 2:51 pm

When your probe is in use measuring RF, what method to you intend to use for connecting the 0v. / earth from the probe to the earth / 0v. of the EUT?

You've written a great deal about your ideas on this RF probe and I haven't read all of them in detail. So if you have mentioned how you intend to do this, please ignore my above Q.

Al.

9th Feb 2020, 10:43 pm	#81
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM I think my VTVM is capable of gaining RMS. Looking at the spec sheet. I don't mean it reads that on the scale but can be calculated. VTVM circuit usually measures either the average value of the input, the positive or negative peak value, or the peak to peak value. The meter scale is then calibrated for the RMS value assuming a sine wave input waveform. If you want to measure the RMS value of a sine wave and your meter is in calibration, you just read the meter scale. So either your meter is not in calibration or you are not measuring a sine wave. If you know the wave shape, it would be possible to calculate a correction factor to apply to your meter reading to get the RMS value. But you need to know whether your VTVM has a circuit that reads the average, peak, or peak to peak value of the input which I'm sure mine does.

10th Feb 2020, 12:25 am	#82
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Re post 30# On ohms section you short the test leads and zero the needle then you move ohms to far right onto kind of figure of 8 mark on the scale. Then you can measure resistance. Actually when got it working which seems best when it's reasonably cold the readings are spot on. Having also checked readings on 2 other DVMs I have just to make sure. Problem with this is when it's warmed up then it starts to mess around abit. Anyhow I've replaced all caps for high quality ones just waiting on this electrolytic to be sorted plus waiting on some Metallised polypropylene caps to replace the ceramics bit more stable. I also put two new pots in from donor unit the zero in and ohms adjuster which are nice and tight on turning the originals where like loose swivels. Plus the carbon tracks are newer and less used. Not sure what you mean clone?

10th Feb 2020, 12:34 am	#83
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Making an RF probe for VTVM I dont want to get shot!! BUT the power transformer is not up to another 150 mA load. You will be pushing hard to find a replacement, and rewinding something that small with no doubt thousands of turns is not for the light hearted. Joe

10th Feb 2020, 1:46 am	#84
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Making an RF probe for VTVM I think ceramic caps will be far more stable than polypropylene!! They will also be less leaky than polypropylene. That kind of figure 8 mark is known as Infinity. My meter only had the main electro replaced by me, and that's the first time its been worked on since it was made sometime in the early 1960's. I think the "jumping about" syndrome will be caused by dirty pots or switch contacts rather than faulty components. As I said at the beginning, these meters are real work horses and rarely have problems if correctly operated. I have two meters on my bench as normal. An AVO model 8, and the Kyoritsu VTVM. I have other fancy meters should I need them, but nowadays I mostly do audio and a little bit of restoration on valve radios. My fancy meters rarely get used. Joe

10th Feb 2020, 11:01 am	#86
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Making an RF probe for VTVM I've fallen foul of that when trying to measure the output voltage of a modified sine wave invertor. As my meter isn't a true RMS type the measured voltage was much less than I expected. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

10th Feb 2020, 11:19 am	#87
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Making an RF probe for VTVM The point I was trying to make to the OP all along was the fact that the OP is proposing to use an external RF probe and the schematic submitted so far shows it to be a half wave peak detector. So long as he understands that and any implications that he might have to deal with when reading the scales then that's fine, no problem. Lawrence. Last edited by ms660; 10th Feb 2020 at 11:29 am.

10th Feb 2020, 9:01 pm	#88
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Well I'll give up on the probe seems an impossible task!!!!! Joe the caps had all been replaced funny enough except the electrolytic which was reading over twice it's uf rating. The 3kv cap was substituted for two 1000v at 0.1uf in series so you have your 0.05uf but only 2kv. These where both wrapped in heat shrink tubing and where reading 0.09uf so again nearly double. So I decided to check them and on removal the top cap had overheated and gone brown these are white caps and it had melted slightly plus 1k lower than they should be which is asking for trouble in my book. That's 1000v!!!! There's a 3kv in there for a reason. The two 0.05uf oil caps where replaced with ITTF caps which I have only seen in crossovers in speakers not in any other circuits valve or solid state that I personally have come across. So they got upgraded to similar spec to the originals. The ceramics I felt might as well be replaced since I'm going the whole hog. I thought ceramics where noisy especially in rf applications? and sensitive to voltage swings whereas I thought Metallised polypropylene capacitors where more stable with voltage swings more self healing and better in rf applications? But no big deal I can always put new ceramics in if the MP,s cause any problems. Anyhow that's why I've replaced the caps it was not just for the sake of it and yes if someone asks all caps where discharged before any readings where taken. So what is the mA rating on this transformer as there are no specifications for it on or in the manual. Therefore how do we know whether it can or can't handle an extra 150mA based just on it's size? Thinking out loud. Yeah I should have copied that link in Lawrence Interesting read though. The transformer we know is rated at 230v 100v 130v and 6.3v. So how do we work out with those figures it's maximum mA output?

10th Feb 2020, 9:21 pm	#89
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Making an RF probe for VTVM What's the voltage and current rating of the pilot bulb? Lawrence.

10th Feb 2020, 11:18 pm	#90
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Making an RF probe for VTVM The pilot lamp is grain of wheat lamp. Very small light output. Transformer centre leg is 3/8 X 3/8 at best. ( NO I havent measured it ) It is wound for 240 volts primary with a tap for 120 volts. Secondary is 130 volts and 6.3 volts. I have been making transformers for about 50 years and I KNOW that it hasnt got the bottle for another 150 mA.Chris can take the gamble if he wishes. The .05 uF 3000 volt cap is the AC coupling capacitor. It couples the AC measured voltage into the 6AL5 rectifier. It is rated 3000 volts for safety reasons. Joe

10th Feb 2020, 11:51 pm	#91
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM PL on schematic along heater chain thought you would know that?

11th Feb 2020, 4:02 pm	#92
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM I've decided to take note and will leave tapping into the circuit for an RF probe into the unit. However since I have two of these I have all the components to hand to make an external power supply. How not sure yet but it will get done as I'm determined to get the RF probe made using the EA50. With the lamp not sure in this circuit if it's for removing the anode dc voltage? Being used as a voltage stabilizer although it's not looking at schematic connected in series with a resistor between unregulated voltage sources. As far as I can tell it's connected to the heaters and pin 9 being heater tap on the 12AU7 valve. Could be wrong but the lamp as Joe said is incredibly small like a minature led. Again just stabbing in the dark. Yes that's why I put a 3kv in there not 2kv cant get me head round that one what was he/she thinking. I mean when do you put a lower voltage cap in circuit? Beyond me. Decided to make a slightly decent DC probe pretty happy with result used a sharpie pen. Attached Thumbnails

11th Feb 2020, 7:23 pm	#93
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Here ya go new caps installed and two new pots. Not done ceramics yet as replacements not due until 15th. Managed to dig out a mundorph cap 22uf so installed that for now on removing the sprague. Will fire her up tomorrow and get some volt readings although not anything on the schematic to go by! Typical ha. Plus pic of the back case which has rusted where the 3k cap and tranny are located I've changed that for the one off the donor unit as it's in mint condition. I wonder why it's rusted there? Cheers Chris Attached Thumbnails

11th Feb 2020, 11:35 pm	#94
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	I would suggest the rust is from a dry cell being left in there at some stage. It's a good idea to periodically check gear containing batteries!! I have left a few things too long with ugly results. Joe I should add, very nice little probe you made.

12th Feb 2020, 11:25 am	#95
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Thanks Joe I'm quite happy with it.

12th Feb 2020, 1:45 pm	#96
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Joe how did you get valve readings on the meter? Does the scale need to be connected up? If so it would mean dropping out scale from unit. If connected insitu not able to flip board over. Cheers Chris

12th Feb 2020, 3:58 pm	#97
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Sorted that used jumper leads I found. I did not connect any leads so not sure if that can affect voltage readings. Heater, pl reading 6.89v 100v tap reading 113v 130v tap reading 156 bit high obviously 6.3v tap as per heaters 6.89v. Spec sheets can go up to 12.6v. Awkward as not able to elevate board don't like the valves touching table top as there getting hot. Anyhow think that's okay.

12th Feb 2020, 4:10 pm	#98
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM Ignore that 12.6v that's combined ew rushing.

12th Feb 2020, 5:11 pm	#99
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Making an RF probe for VTVM sorry guys this thread supposed to be about an rf probe not refurb job. I'll go to another thread for that. Cheers Chris

13th Feb 2020, 2:51 pm	#100
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Making an RF probe for VTVM When your probe is in use measuring RF, what method to you intend to use for connecting the 0v. / earth from the probe to the earth / 0v. of the EUT? You've written a great deal about your ideas on this RF probe and I haven't read all of them in detail. So if you have mentioned how you intend to do this, please ignore my above Q. Al.