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Hello everyone. I¡¯m picking up a L75 tomorrow. My TR7 is running around 120-140 PEP. If I need to reduce drive power to amplifier is the only way the WB4HFN mod to alc?? When the amplifier spec¡¯s a 50-80 watt drive is that average watts or PEP? Thanks for any answers. Tony N5DIM
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Use amplifier ALC and your mic gain to control the drive. On Sun, 22 Dec 2024 21:53:35 -0800 "Tony N5DIM via groups.io" <tony@...> wrote: Hello everyone. I¡¯m picking up a L75 tomorrow. My TR7 is running around 120-140 PEP. If I need to reduce drive power to amplifier is the only way the WB4HFN mod to alc?? When the amplifier spec¡¯s a 50-80 watt drive is that average watts or PEP? Thanks for any answers. Tony N5DIM -- 73 -Jim NU0C
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Tony,
The manual for the L-75 provides all of the information you should need for connecting and operating the TR-7 with the L-75. Refer to connection diagrams and section on "SSB Tuning and Operation." As Jim pointed out, the L75 has amplifier ALC, which is adjusted by a front panel control labeled "AGC."
Enjoy the new amp.
Jerry, N4JL
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of Jim Shorney via groups.io Sent: Monday, December 23, 2024 2:13 AM To: [email protected]Subject: Re: [DRAKE-RADIO] L75 amplifier /TR-7 Use amplifier ALC and your mic gain to control the drive. On Sun, 22 Dec 2024 21:53:35 -0800 "Tony N5DIM via groups.io" <tony@...> wrote: Hello everyone. I¡¯m picking up a L75 tomorrow. My TR7 is running around 120-140 PEP. If I need to reduce drive power to amplifier is the only way the WB4HFN mod to alc?? When the amplifier spec¡¯s a 50-80 watt drive is that average watts or PEP? Thanks for any answers. Tony N5DIM -- 73 -Jim NU0C
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As a side note, the ALC on the L75 is similar to the L7 and L4B in that it is frequency sensitive. ?Adjusting it for 10-15 meters, for example, will result in overdrive if switching to 80 or 160 without re-adjusting it. ?The circuits in these amps are not identical but the function is the same.
Tony,
The manual for the L-75 provides all of the information you should need for connecting and operating the TR-7 with the L-75.? Refer to connection diagrams and section on "SSB Tuning and Operation."? As Jim pointed out, the L75 has amplifier ALC, which is adjusted by a front panel control labeled "AGC."
Enjoy the new amp.
Jerry, N4JL
-----Original Message----- From: [email protected] < [email protected]> On Behalf Of Jim Shorney via groups.io Sent: Monday, December 23, 2024 2:13 AM To: [email protected]Subject: Re: [DRAKE-RADIO] L75 amplifier /TR-7 Use amplifier ALC and your mic gain to control the drive. On Sun, 22 Dec 2024 21:53:35 -0800 "Tony N5DIM via groups.io" <tony@...> wrote: > Hello everyone. I¡¯m picking up a L75 tomorrow. My TR7 is running around 120-140 PEP. If I need to reduce drive power to amplifier is the only way the WB4HFN mod to alc?? When the amplifier spec¡¯s a 50-80 watt drive is that average watts or PEP? Thanks for any answers. Tony N5DIM -- 73 -Jim NU0C
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You can also ditch the ALC in the amp and use an adjustable negative low voltage source to the radio.
Several simple circuits are out there, some using a 9vdc battery and a potentiometer and others using a Walwart.?
The amps ALC setting not only varies with band selection but also changes when you switch from CW to SSB?
Tom W9TAB?
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On Dec 23, 2024, at 10:16?AM, Evan via groups.io <k9sqg@...> wrote:
?
As a side note, the ALC on the L75 is similar to the L7 and L4B in that it is frequency sensitive. ?Adjusting it for 10-15 meters, for example, will result in overdrive if switching to 80 or 160 without re-adjusting it. ?The
circuits in these amps are not identical but the function is the same.
Tony,
The manual for the L-75 provides all of the information you should need for connecting and operating the TR-7 with the L-75.? Refer to connection diagrams and section on "SSB Tuning and Operation."? As Jim pointed out, the L75 has amplifier ALC, which is adjusted
by a front panel control labeled "AGC."
Enjoy the new amp.
Jerry, N4JL
-----Original Message-----
From:
[email protected] < [email protected]> On Behalf Of Jim Shorney via groups.io
Sent: Monday, December 23, 2024 2:13 AM
To:
[email protected]
Subject: Re: [DRAKE-RADIO] L75 amplifier /TR-7
Use amplifier ALC and your mic gain to control the drive.
On Sun, 22 Dec 2024 21:53:35 -0800
"Tony N5DIM via groups.io" <tony@...> wrote:
> Hello everyone. I¡¯m picking up a L75 tomorrow. My TR7 is running around 120-140 PEP. If I need to reduce drive power to amplifier is the only way the WB4HFN mod to alc?? When the amplifier spec¡¯s a 50-80 watt drive is that average watts or PEP? Thanks for
any answers. Tony N5DIM
--
73
-Jim
NU0C
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I don't see an advantage to that. Adjustable ALC is right there in the amp and you are adjusting it to what is happening at the amp, not to an arbitrary value. On Mon, 23 Dec 2024 16:47:22 +0000 "Thomas W9TAB via groups.io" <electron@...> wrote: You can also ditch the ALC in the amp and use an adjustable negative low voltage source to the radio.
Several simple circuits are out there, some using a 9vdc battery and a potentiometer and others using a Walwart.
The amps ALC setting not only varies with band selection but also changes when you switch from CW to SSB
Tom W9TAB -- 73 -Jim NU0C
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Thank you everybody for the info. I will definitely check and use the information in the manual. Also scored the speech processor today also. Merry Christmas and 73. Tony N5DIM
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On Dec 23, 2024, at 9:34?AM, Jim Shorney via groups.io <jimNU0C@...> wrote:
?
I don't see an advantage to that. Adjustable ALC is right there in the amp and you are adjusting it to what is happening at the amp, not to an arbitrary value.
On Mon, 23 Dec 2024 16:47:22 +0000
"Thomas W9TAB via groups.io" <electron@...> wrote:
You can also ditch the ALC in the amp and use an adjustable negative low voltage source to the radio.
Several simple circuits are out there, some using a 9vdc battery and a potentiometer and others using a Walwart.
The amps ALC setting not only varies with band selection but also changes when you switch from CW to SSB
Tom W9TAB
--
73
-Jim
NU0C
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AGC in the Drake amp is developed from the transmitters ¡°input¡± to the tube(s)
When you key the transmitter it doesn¡¯t see the AGC signal until it goes through the comparison circuit then develops the required negative DC voltage to pull back the RF coming from the transmitter.
So there is a reoccurring front end RF spike that is pulled back by the AGC circuit each time you key the transmitter as the AGC voltage disappears during receive.
AGC control process is ¡°Sample, compare, voltage generation, then transmitter output reduction¡± each time you key the transmitter.
If you set the AGC voltage to cut the transmitters output to 60watts (In the example of my L7) I get 1000watts carrier out.
Looking at the output on a scope the signal is nice and clean with a two tone audio for a test signal and transmitting into a dummy load.
Tom W9TAB
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On Dec 23, 2024, at 2:04?PM, Tony N5DIM <tony@...> wrote:
?Thank you everybody for the info. I will definitely check and use the information in the manual. Also scored the speech processor today also. Merry Christmas and 73. Tony N5DIM
On Dec 23, 2024, at 9:34?AM, Jim Shorney via groups.io <jimNU0C@...> wrote:
?
I don't see an advantage to that. Adjustable ALC is right there in the amp and you are adjusting it to what is happening at the amp, not to an arbitrary value.
On Mon, 23 Dec 2024 16:47:22 +0000
"Thomas W9TAB via groups.io" <electron@...> wrote:
You can also ditch the ALC in the amp and use an adjustable negative low voltage source to the radio.
Several simple circuits are out there, some using a 9vdc battery and a potentiometer and others using a Walwart.
The amps ALC setting not only varies with band selection but also changes when you switch from CW to SSB
Tom W9TAB
--
73
-Jim
NU0C
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ALC overshoot spikes are minimal if ALC is used as intended. ALC is not a speech processing system nor a hard limiter. It is there to catch those occasional higher peaks. Thus the instruction with the TR7 to adjust mic gain so the ALC LED only flashes on occasional peaks. In bench testing a TR7 some years back with a peak reading Wattmeter I had to hit it pretty hard to generate significant overshoot peaks. The conclusion was that if you are seeing significant overshoot then you are overdriving your audio. I have been using amp ALC for years with L4B, L7, NCL-2000, and now an Alpha 76 and it works as desired if used as intended. 73 -Jim NU0C On Tue, 24 Dec 2024 00:31:05 +0000 "Thomas W9TAB via groups.io" <electron@...> wrote: AGC in the Drake amp is developed from the transmitters ¡°input¡± to the tube(s)
When you key the transmitter it doesn¡¯t see the AGC signal until it goes through the comparison circuit then develops the required negative DC voltage to pull back the RF coming from the transmitter.
So there is a reoccurring front end RF spike that is pulled back by the AGC circuit each time you key the transmitter as the AGC voltage disappears during receive.
AGC control process is ¡°Sample, compare, voltage generation, then transmitter output reduction¡± each time you key the transmitter.
If you set the AGC voltage to cut the transmitters output to 60watts (In the example of my L7) I get 1000watts carrier out.
Looking at the output on a scope the signal is nice and clean with a two tone audio for a test signal and transmitting into a dummy load.
Tom W9TAB
On Dec 23, 2024, at 2:04?PM, Tony N5DIM <tony@...> wrote:
?Thank you everybody for the info. I will definitely check and use the information in the manual. Also scored the speech processor today also. Merry Christmas and 73. Tony N5DIM
On Dec 23, 2024, at 9:34?AM, Jim Shorney via groups.io <jimNU0C@...> wrote:
?
I don't see an advantage to that. Adjustable ALC is right there in the amp and you are adjusting it to what is happening at the amp, not to an arbitrary value.
On Mon, 23 Dec 2024 16:47:22 +0000
"Thomas W9TAB via groups.io" <electron@...> wrote:
You can also ditch the ALC in the amp and use an adjustable negative low voltage source to the radio.
Several simple circuits are out there, some using a 9vdc battery and a potentiometer and others using a Walwart.
The amps ALC setting not only varies with band selection but also changes when you switch from CW to SSB
Tom W9TAB -- 73 -Jim NU0C
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On Tue, Dec 24, 2024 at 01:31 AM, Thomas W9TAB wrote:
<AGC in the Drake amp is developed from the transmitters ¡°input¡± to the tube(s)
<When you key the transmitter it doesn¡¯t see the AGC signal until it goes through the comparison circuit then develops the required negative DC voltage to pull back the RF coming from the transmitter.
<So there is a reoccurring front end RF spike that is pulled back by the AGC circuit each time you key the transmitter as the AGC voltage disappears during receive.
<AGC control process is ¡°Sample, compare, voltage generation, then transmitter output reduction¡± each time you key the transmitter.
<If you set the AGC voltage to cut the transmitters output to 60watts (In the example of my L7) I get 1000watts carrier out.
<Looking at the output on a scope the signal is nice and clean with a two tone audio for a test signal and transmitting into a dummy load.
IOW,? you are closing the barndoor......after the horse has bolted, and every time you trip the VOX on SSB.? ? And the ALC time constant in the amplifier has to match the ALC time constant in the xcvr / tx.? Drake had a tech note out about matching the time constants if using a T4XC with a L7....or using a? TR7? with an L4B.? It was? a cap / resistor change.? ?The mic gain in the T4XC is not a PO control....when using SSB.?? The advantage of using the external, adjustable DCV method is..... the ALC voltage has already been developed, and is sitting there all the time, at a constant level.? ?The drawback is, if whispering into the mic, you won't get desired PO out of the T4XC / XCVR. If an external processor is used, or external audio compressor / gating system used, or a boom headset used,? it's a non issue.? Where you would really get into trbl, is when using a TR7 / T4XC on SSB mode, and try and drive an amp that requires very little drive, like most of the SS amps.? You are putting out 130 watts pep initially from the T4XC, until the derived ALC voltage developed in the SS amplifier, is sent back to the TX / XCVR.? On the Merc-3 SS amp, it has a built in event counter, and will know asap, if you ever applied > 60 watts pep into the amp.....warranty denied. If that 5K @ 7 watt resistor below the chassis in the L4PS / L7PS opens up, you will end up with full B+ on the ALC adjustment pot in the L4B / L7.? In my case, the 7K pot, along with the pair of 50K bleeders was tossed, to reduce heat in the L4PS.? Since the yaesu xcvr already has adjustable PO, it's a non issue.? Still, the external DCV can still be used, if the SS xcvr has the dreaded overshoot / initial spike issue...typ of some of the Icom xcvrs, like local buddy's? ICOM? 775DSP, which puts out a 200 watt spike, regardless of? where the PO is set at....which wreaks havoc when driving his Ameritron AL-80B, single 3-500Z amp.? The ICOM 706MK2 series also put out a 140 watt spike, again regardless of where the PO is set.? ?A few other SS xcvr's have a similar issue, to varying degrees.? On a? sorta related sidenote,? I just ordered a? 40 kv? HV probe.? ?It uses a long, large value resistor, in series with a low value resistor.? Cold end of low value resistor is clipped to the B- of the HV supply...via it's 3' test lead + clip.? ? The V drop across the lower valued resistor is fed to my fluke 87 DVM...via a 2 x conductor cable.....with dual plug assy.? ? ?40 kv dc? will be? 40.0 vdc on the? DVM.? (1000:1 ratio).? 2650 vdc? will be 2.65 vdc on the fluke 87.? IF that return test lead ( that goes to the B- of the supply)? ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM.? This freaks me out.?
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It shouldn't unless your Fluke has a metal case. There is no return path with a non-metallic enclosure so both you and the meter should be OK. What could cause damage would be the cold end resistor opening up. I've had one of these probes for years. They used to be standard equipment in TV repair shops. Even given what I wrote above the only thing I touch when doing a measurement is the probe handle and I do not use a mains powered meter. On Tue, 24 Dec 2024 04:52:04 -0800 "Jim VE7RF via groups.io" <jim.thom@...> wrote: IF that return test lead ( that goes to the B- of the supply)? ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM.? This freaks me out. -- 73 -Jim NU0C
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That¡¯s the same combination I use for HV measurements.
Fluke 80K-40 probe with the Fluke 87 DVM
Tom W9TAB
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On Dec 24, 2024, at 7:40?AM, Jim Shorney via groups.io <jimNU0C@...> wrote:
?
It shouldn't unless your Fluke has a metal case. There is no return path with a non-metallic enclosure so both you and the meter should be OK. What could cause damage would be the cold end resistor opening up. I've had one of these probes for years. They used to be standard equipment in TV repair shops. Even given what I wrote above the only thing I touch when doing a measurement is the probe handle and I do not use a mains powered meter.
On Tue, 24 Dec 2024 04:52:04 -0800
"Jim VE7RF via groups.io" <jim.thom@...> wrote:
IF that return test lead ( that goes to the B- of the supply) ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM. This freaks me out.
--
73
-Jim
NU0C
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As a side note with an unclear memory, the ALC circuit in the L4B is different than the one in the L7. ?Failure of that 5K 7 watt resistor (which should be replaced with a 10-20 watt style) will result in a failure of the ALC pot in the L4B; the L7 circuit is different and might not be as sensitive to failure. ?I've replaced L4B pots which are sometimes hard to ?find due to ?the pot having an on/off switch. ?The L7 ?pot is somewhat standard.
Merry Christmas to all...
On Tue, Dec 24, 2024 at 01:31 AM, Thomas W9TAB wrote:
<AGC in the Drake amp is developed from the transmitters ¡°input¡± to the tube(s)
<When you key the transmitter it doesn¡¯t see the AGC signal until it goes through the comparison circuit then develops the required negative DC voltage to pull back the RF coming from the transmitter.
<So there is a reoccurring front end RF spike that is pulled back by the AGC circuit each time you key the transmitter as the AGC voltage disappears during receive.
<AGC control process is ¡°Sample, compare, voltage generation, then transmitter output reduction¡± each time you key the transmitter.
<If you set the AGC voltage to cut the transmitters output to 60watts (In the example of my L7) I get 1000watts carrier out.
<Looking at the output on a scope the signal is nice and clean with a two tone audio for a test signal and transmitting into a dummy load.
IOW,? you are closing the barndoor......after the horse has bolted, and every time you trip the VOX on SSB.? ? And the ALC time constant in the amplifier has to match the ALC time constant in the xcvr / tx.? Drake had a tech note out about matching the time constants if using a T4XC with a L7....or using a? TR7? with an L4B.? It was? a cap / resistor change.? ?The mic gain in the T4XC is not a PO control....when using SSB.?? The advantage of using the external, adjustable DCV method is..... the ALC voltage has already been developed, and is sitting there all the time, at a constant level.? ?The drawback is, if whispering into the mic, you won't get desired PO out of the T4XC / XCVR. If an external processor is used, or external audio compressor / gating system used, or a boom headset used,? it's a non issue.? Where you would really get into trbl, is when using a TR7 / T4XC on SSB mode, and try and drive an amp that requires very little drive, like most of the SS amps.? You are putting out 130 watts pep initially from the T4XC, until the derived ALC voltage developed in the SS amplifier, is sent back to the TX / XCVR.? On the Merc-3 SS amp, it has a built in event counter, and will know asap, if you ever applied > 60 watts pep into the amp.....warranty denied. If that 5K @ 7 watt resistor below the chassis in the L4PS / L7PS opens up, you will end up with full B+ on the ALC adjustment pot in the L4B / L7.? In my case, the 7K pot, along with the pair of 50K bleeders was tossed, to reduce heat in the L4PS.? Since the yaesu xcvr already has adjustable PO, it's a non issue.? Still, the external DCV can still be used, if the SS xcvr has the dreaded overshoot / initial spike issue...typ of some of the Icom xcvrs, like local buddy's? ICOM? 775DSP, which puts out a 200 watt spike, regardless of? where the PO is set at....which wreaks havoc when driving his Ameritron AL-80B, single 3-500Z amp.? The ICOM 706MK2 series also put out a 140 watt spike, again regardless of where the PO is set.? ?A few other SS xcvr's have a similar issue, to varying degrees.? On a? sorta related sidenote,? I just ordered a? 40 kv? HV probe.? ?It uses a long, large value resistor, in series with a low value resistor.? Cold end of low value resistor is clipped to the B- of the HV supply...via it's 3' test lead + clip.? ? The V drop across the lower valued resistor is fed to my fluke 87 DVM...via a 2 x conductor cable.....with dual plug assy.? ? ?40 kv dc? will be? 40.0 vdc on the? DVM.? (1000:1 ratio).? 2650 vdc? will be 2.65 vdc on the fluke 87.? IF that return test lead ( that goes to the B- of the supply)? ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM.? This freaks me out.?
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On Tue, 24 Dec 2024 04:52:04 -0800 "Jim VE7RF via groups.io" <jim.thom@...> wrote: IOW,? you are closing the barndoor......after the horse has bolted, and every time you trip the VOX on SSB. The funny thing is that those of us that have been using Drake ALC with amplifiers for years don't seem to have any problems with it when used properly. I have posted here multiple times that there is not significant overshoot with the TR7 unless you turn the mic gain up to 11 and scream into it on key-down. I assume that would be true of ANY rig. But that's not the way proper ALC is meant to be used. And the ALC time constant in the amplifier has to match the ALC time constant in the xcvr / tx.? Drake had a tech note out about matching the time constants if using a T4XC with a L7....or using a? TR7? with an L4B.? It was? a cap / resistor change.? ?The mic gain in the T4XC is not a PO control....when using SSB. I would like to see a copy of this tech note. I have gathered a lot of Drake information over the years but no one as aver produced this tidbit. That puts it in urban legend status file for me. I've used a L4B with a TR7 and guess what? ALC works and I get good reports. The only things my minty copy of Single Sideband Principles and Circuits has to say about time constants is that they can be "much faster" in the RF domain, and that dual time constant ALC can be used to "take care of gain variations in the r-f exciter over the frequency band". Also that extra filtering is required to filter IM products outside of the passband if you want to try to use ALC as syllabic compression. We know this also from SM5BSZ's research. But Drake's ALC was not intended as a syllabic compressor and I sincerely hope no one is doing this. The advantage of using the external, adjustable DCV method is..... the ALC voltage has already been developed, and is sitting there all the time, at a constant level. Which seems to me to be an invitation to turn the MIC gain up too high. Same with the TR7 SSB power control mod on the HFN web site. If you set it too low you will still be flat topping your amp. Best IMO to set it at the amp. Tune for maximum smoke CW, overcouple the load a tad, set ALC to the peak level you want, and adjust mic gain for proper level. Even a cave man could do it. Where you would really get into trbl, is when using a TR7 / T4XC on SSB mode, and try and drive an amp that requires very little drive, like most of the SS amps. That is off topic for the OP's question. If that 5K @ 7 watt resistor below the chassis in the L4PS / L7PS opens up, you will end up with full B+ on the ALC adjustment pot in the L4B / L7. There are ways to guard against this. K9SQG has a good one. In my case, the 7K pot, along with the pair of 50K bleeders was tossed, to reduce heat in the L4PS.? Since the yaesu xcvr already has adjustable PO, it's a non issue. Again, off topic. What happens with YaeComWood is not germane to the subject of this thread. Drake designed a good system here. Especially with the TR7 where you have a visual indication that ALC is active. When used as intended it does the job. If you want to run without ALC, your amp has the needed headroom not to clip on peaks, and you have the equipment to ensure this, fine. I don't think there is enough margin in the L75 for that to be practical. -- 73 -Jim NU0C
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The L7 uses a 75K pot in series with a 22K resistor. The L4B is 100K with no series resistor. I think your resistor change is still a good idea. Adding a TVS diode with a breakdown rating greater than the maximum voltage expected across the pot might not be a bad idea. Maybe add a picofuse to open the path if the TVS diode shorts. Or maybe I am overthinking this. :) On Tue, 24 Dec 2024 15:16:25 +0000 (UTC) "Evan via groups.io" <k9sqg@...> wrote: As a side note with an unclear memory, the ALC circuit in the L4B is different than the one in the L7. ?Failure of that 5K 7 watt resistor (which should be replaced with a 10-20 watt style) will result in a failure of the ALC pot in the L4B; the L7 circuit is different and might not be as sensitive to failure. ?I've replaced L4B pots which are sometimes hard to ?find due to ?the pot having an on/off switch. ?The L7 ?pot is somewhat standard. -- 73 -Jim NU0C
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On Tue, Dec 24, 2024 at 02:40 PM, Jim Shorney wrote:
<It shouldn't unless your Fluke has a metal case. There is no return path with a non-metallic enclosure so both you and the meter should be OK. What could cause damage would be the cold end resistor opening up. I've <had one of these probes for years. They used to be standard equipment in TV repair shops. Even given what I wrote above the only thing I touch when doing a measurement is the probe handle and I do not use a mains <powered meter.
<On Tue, 24 Dec 2024 04:52:04 -0800
<"Jim VE7RF via groups.io" <jim.thom@...> wrote:
<IF that return test lead ( that goes to the B- of the supply)? ever opened up / fell off....... the full B+ will then appear on the fluke 87 DVM.? This freaks me out.
--
<73
<-Jim
<NU0C
##? The HV probe just arrived yesterday.? ?They stipulate? that if the return lead goes open, yes, you will end up with full B+ on the return test lead.? ?You will also end up with full? B+ on both? the red / black test leads that plug into the fluke 87.? ?And if the cold end, (smaller 1.1 meg resistor) opens up, you will have full B+ on the red lead...that goes to the fluke 87 DVM.? They tell you to connect the grnd return lead 1st, then the dvm red / black leads, then turn on B+ supply, then attach the probe...last.? Drawn out on paper, it's just a 1000 megohm resistor in series with a 1.1 megohm resistor...then the grnd return.? The V drop across the smaller resistor is applied to the DVM.? ? I measured? the resistance of the bigger resistor (Between tip of probe + RED test lead).? ?Then measured the smaller value resistor? ( between the red / black test leads).? ? Then the total resistance ( between probe tip and grnd return).? Point is, you could have any of this stuff go open....... and think there is zero HV...... when actually there is HV present.? ? Mine came with 3 x different types of tips.? Tapered pointed type.? 2nd one is a looped hook.? ?3rd one is a angled flat metal piece, with an arc cut out.... made to engage the threads of machine screws.? ?The 2nd and 3rd types will never slip, or slide off, like the? 1st type.? Using the power on, extra digit option on the fluke 87 DVM,? it will read down to 1 volt increments, like 2650, 2650 vdc etc.? ? Without the power on option, it reads in 10 V increments.? It works good, but I would caution folks using them, to be very careful.?? Jim? ?VE7RF
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On Sat, 28 Dec 2024 08:51:26 -0800 "Jim VE7RF via groups.io" <jim.thom@...> wrote: ##? The HV probe just arrived yesterday.? ?They stipulate? that if the return lead goes open, yes, you will end up with full B+ on the return test lead.? ?You will also end up with full? B+ on both? the red / black test leads that plug into the fluke 87.? ?And if the cold end, (smaller 1.1 meg resistor) opens up, you will have full B+ on the red lead...that goes to the fluke 87 DVM.? They tell you to connect the grnd return lead 1st, then the dvm red / black leads, then turn on B+ supply, then attach the probe...last. Yes, that is all correct. This is old tech that has been around since television sets became consumer commodities and they are robustly built (if you got a good one) for good reason. No one wants to get sued because a TV technician electrocuted himself with a poorly built probe. :) You are only in danger if you are part of the return path. Which you should not be. The meter is only in danger if that low resistor opens up and there is a return path, which could happen if the low resistor opens up while the meter is connected to the DUT. When I first got mine I used it with my Fluke 8050A running on battery power. After I got the 289 I retired the dual power 8050A in favor of one that did not have the battery option so I wouldn't have to deal with NiCAD maintenance. It is still my master bench meter but as often as not I grab the 289 when I am working. Using the power on, extra digit option on the fluke 87 DVM,? it will read down to 1 volt increments, like 2650, 2650 vdc etc.? ? Without the power on option, it reads in 10 V increments. Such precision is really not necessary at those voltage levels. The calibration of the probe may not even support it. Mine does have a calibration pot in the low end. I calibrated it for my Fluke 289 at 120V AC and that is more than good enough. Really it was "close enough" to start with. The input impedance of the meter becomes part of the voltage divider so it is only accurate with meters of similar input impedances. -- 73 -Jim NU0C
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Jim Shorney
Thomas W9TAB