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410C VTVM Test Leads Coax Type Help
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Dear HP Group
I'm starting a new restoration project and am need of any knowledge around the type of coax used on the test leads.? I've seen many videos (Curious Marc's was an excellent one), scoured the repair manual and read many posts here and elsewhere where they've been replaced, but find the type of coax that was used not mentioned.? As typical, mine are in need of replacing.?? Rough measurements show an outer diameter of around .125"/3.1mm.? The closest diameter to this i'm able to find is RG174 at 0.110"/2.79mm which would probably work, but may be quite loose fitting in the strain relief jackets they go through.? I'm also assuming a 50ohm coax would be needed. Any insight would be appreciated. -Adam |
¿ªÔÆÌåÓýCheck out KISS.? Ashley has what you need. 73/jeff/ac0c alpha-charlie-zero-charlie On 1/3/2024 8:59 PM, Adam Attig wrote:
Dear HP Group |
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I see the triax on her page but I don't see the coax.? Maybe she does have it but not on the page? Thanks, Barry - N4BUQ From: "Jeff AC0C" <keepwalking188@...>
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There may be some confusion about various cable requirements. If you're talking about just getting a signal into a VTVM, then almost any wiring should do - it's low frequency, high-Z, and no Z-matching is involved, so you don't care if it's 50, 75, or whatever ohms. It just should be low in leakage, which is almost guaranteed in modern wiring materials.
If you're talking about the RF probe (I forget the model name) that has a little vacuum tube detector diode in the probe head, then you need an extra conductor to supply the heater current, so a three-conductor scheme is needed, whether it's triax, or twisted-pairs, or other - remember the signal to the VTVM is still mostly DC/low frequency past the detector, so almost any three- or more conductor cable type will do. Even CAT-5 would probably work, but it feels better and more RF-like to use a coax form. It's conceivable that various resonances over the cable length could cause errors due to VSWR in the VHF spec'd response range, and the particular CMRR of the structure, so it's probably safest to stick with a coax, so triax or twinax, or shielded twisted-pair (STP) in this case. This also helps to reduce any HF interference that might more easily get into a more open cable structure, but it's probably no big deal - depends on the RF susceptibility of the VTVM's input. You can confirm to your self how fancy things need to be with some simple experiments. Start with two wires or a piece of zip-cord, then work your way up until happy. One thing I don't recall is whether the 410 is a floating, shielded system. If so, then it could be be a three-wire interface, so triax or others are the way to go, where the outer shield is ground (or guard), and the inner lines are the low and high signals that carry the info - then you really would want a true three wire connection, although usually it would end up being hooked up as two where it finally connects to the circuit. The same principles apply as above - study and understand the system, and don't get fancier than necessary. Ed |
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At 500mV and up, the 410C's input impedance is 100M. Barry - N4BUQ From: "Roy Thistle" <roy.thistle@...> On Wed, Jan 3, 2024 at 10:04 PM, Ed Breya wrote: |
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On Wed, Jan 3, 2024 at 10:04 PM, Ed Breya wrote:
If you're talking about the RF probeThe probe that comes with is? 11036A AC Probe (to measure AC from a few 10 of Hz to around 1 GHz?)... but with not very good accuracy at the low and at high end frequencies.... at the 'sweet' spot it matches the input impedance of the 410c. The EA53 UHF diode tube, in the probe body, has one end of the filament connected to the cathode The signal is taken between the plate and the cathode, by two wires inside a shield, and then a separate wire runs from the common point (of the filament, and the cathode) outside the shield and is connected to the shield at the plug.? So there are three wires... two inside the shield, and one outside the shield. I dunno if the part of the cable with the two wires inside the shield is bona fide 'twinax' ... but guessing it has an impedance of 100 ohms (since I remember it wasn't a large diameter cable)... I guess a shield twisted pair (which has about the same impedance) would do? Anyway, if one makes up a cable, then the 'sweet spot' in the band, won't match the 11036A's 'sweet spot'?... if any of the 11036A's were ever manufactured to have that the 'same' anyway. The hardest part, IMO, will be securing the 'cable' to the probe body, and connecting/soldering to the tube socket in there. The EA53 may be unobtainium, if the one at hand is blown. Ordering a 'special run of cable is no fun, and seems expensive for what it is. (when there is plenty of wire in everything everywhere to salvage.)... and probably doesn't match the cable original cable assembly? In any case, I think Ed says it best. |
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Thanks all,? my only interest is in the test leads, not the RF probe.?
I'll just focus on finding a suitably sized and flexible coax to fit, as it does not seem impedance is a concern here.? Was hoping someone has done this already and could provide the RG-XXX number.? Will post my solution, when i find something suitable...for posterity. -Adam |
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On 1/4/24 17:21, Adam Attig wrote:
Thanks all,? my only interest is in the test leads, not the RF probe.Try audio cable. Many musicians have gone to computer based mixers and have plenty to spare. Probably cheap on ebay also. It's super flexible. |
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I'm curious about this coax aspect if it's on the 3 leads - not the RF.? As far as I recall, those 3 are just single wires, no coaxial content.? DCA/DCV/OHMS/COM - all single wire.? Only the AC probe has the multi-wire content and that's the triax thing.
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73/jeff/ac0c alpha-charlie-zero-charlie www.ac0c.com On 1/4/2024 6:44 PM, John Griessen wrote:
On 1/4/24 17:21, Adam Attig wrote:Thanks all,? my only interest is in the test leads, not the RF probe.Try audio cable.? Many musicians have gone to computer based mixers and have plenty to spare.? Probably cheap on ebay also. |
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I thought they weren't coax either but they are. I verified that with my 410Cs. I think they did that to keep down the noise on low value readings.
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Thanks, Barry - N4BUQ I'm curious about this coax aspect if it's on the 3 leads - not the RF. |
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John is right - checkout the Van Damme range, I've used several of their different cables as sensor leads - super flexible, very tough (built to survive roadies slinging things around on tour!) and a range of mixed wire types (coax, cat5, single cores etc) in one overall jacket.
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Offcuts in the 1-5M range are often available on eBay. On 05/01/2024 00:44, John Griessen wrote:
On 1/4/24 17:21, Adam Attig wrote:Thanks all,? my only interest is in the test leads, not the RF probe.Try audio cable.? Many musicians have gone to computer based mixers and have plenty to spare.? Probably cheap on ebay also. |
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Yes - that's the DC VOLTS input impedance spec. Thanks, Barry - N4BUQ From: "Roy Thistle" <roy.thistle@...> On Thu, Jan 4, 2024 at 03:36 PM, n4buq wrote: |
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Good discussion everyone.
Note, from Ashley at KissElectonics "HP used shielded wire in the "C", custom made by HP for these leads. The OHMS shielded cable was colored RED to distinguish it from the DC shielded wire.
We have never been able to source this shielded wire.
? ? ? ? What we DID use was to purchase power supply modules ( they were used for an HP printer) and harvest the shielded output cable. It is flexible, a nice grey color and fits the "C" rubber test lead shrouds." Also, thanks for the tip on Van Damme cables, seem like quality cables - they had one coax close to the size i need, but looks like only sold in EU/UK. Looking into finding a replacment for the strain reliefs which allow for a more standard diameter coax and keep the same look.? Willl keep everyone updated. -Adam |
¿ªÔÆÌåÓýThanks for sharing that Adam.? That's definitely news to me.? GL! 73/jeff/ac0c alpha-charlie-zero-charlie On 1/5/2024 2:34 PM, Adam Attig wrote:
Good discussion everyone. |
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This is from one of the PDF manuals of the 410C I have.? I suppose I should have used "resistance" instead of "impedance".? The AC VOLTMETER section does use "impedance" and rightfully so. Thanks, Barry - N4BUQ From: "Roy Thistle" <roy.thistle@...> On Fri, Jan 5, 2024 at 10:54 AM, n4buq wrote:DC VOLTS input impedance spec.Ok. Yes... we used to call that the "DC input resistance' (and I'm pretty sure that HP did too.)... but, times change. |
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