Using Vintage test gear - Modify or Keep original?

[txNgineer]

I have a lot of test equipment that could be called "vintage" but is still very useable. When it comes to old radios, etc. I always try to keep them as original as possible when repairing/restoring them. But for test gear it is a more difficult choice for me, especially when a simple modern change can enhance the usability. This gear is part of my lab and is used regularly.

I "grew up" using a 6" VTVM for doing alignments and peaking on radios and audio gear and have a very good condition BKPrecision 177 that I want to restore and use. The choice that I need to make is regarding the old 7/8 inch single conductor threaded input connector. Should I just make an adapter to banana jacks or should I modify the front panel slightly to replace the obsolete connector with proper banana jacks? And more broadly what is the general opinion on modernizing versus original for equipment you use?

By carefully researching and selecting proper used equipment I have been able to set up a lab that can operate with reasonably high precision and perform better than anything I could afford new.

I'm probably being too obsessive, but I am curious about other viewpoints.

[tkamiya]

I tend to keep everything as original as possible.  Only exception is I have to repair something and parts are no longer available.  I typically make adapters and use existing holes.  It makes no practical difference and perhaps not even necessary, but that's the way I choose to do it.

[txNgineer]

Thanks, that is my normal approach but sometimes it is not as convenient. The thing that bothered me the most in this case is the meter is specified to 1500 volts and that old phenolic connector seems underqualified for high voltages.

[TimFox]

Can you replace it with a Teflon-insulated "UHF" coaxial connector?

[Tom45]

Some things just have to be done if you plan to use it. For example, replacing ancient power cords with no ground and dodgy insulation.

[CatalinaWOW]

There are several reasons to keep it original.  One is that you may not fully understand the design intent of the original approach and changes may negatively effect some aspect of performance.  Another is to retain collector value.  Another is for personal nostalgic reasons (closely tied to collector value).

The collector market for test equipment is with few exceptions small and in my opinion not worth chasing.  In many cases you may adequately understand the impact of changes so the first reason drops out.  Unless your change requires an exotic and personal interface I see no problem with making modifications in this case.  You can honor originality if you wish by tucking a note on any changes you have made inside the instrument so any future owner will be aware of them, and if necessary keep any unobtanium parts in a small bag attached to the back.  In many cases that will make it relatively easy to restore the instrument to more original state.

That leaves your personal preference as the dominant factor in the decision.  If you use this regularly I would go for making it convenient and useful.

[tkamiya]

Thanks, that is my normal approach but sometimes it is not as convenient. The thing that bothered me the most in this case is the meter is specified to 1500 volts and that old phenolic connector seems underqualified for high voltages.

It's not that unusual.  Chances are, the meter was not intended to be used at 1500 volt.  I'd say half that voltage max.  It was probably a light hearted (meaning not fully thought out) and ill conceived idea to extend the range to 1500V.  I have several "antique" VOM that goes up to 1000 volt.  I would never apply that much potential.  Case in point, none of the components inside were rated for that much, although technically, circuit itself will divide the voltage correctly.

Safety standards and liability situations changed over the decades.  Even after converting the sockets, I wouldn't go that high....

[bdunham7]

I think banana jacks would be a mess and an adapter would be clumsy.

You can replicate the original probe systems, which included a standard probe, an RF detector probe and a HV probe.  I think these are the correct connectors?

https://www.digikey.com/en/products/detail/switchcraft-inc/2501F/1289503

Or, as TimFox suggested  you could use UHF connectors, sold as PL-259, and again just make up a custom lead set that works in place of the originals.  The PL-259 are readily available as connectors or as cables and they have an old-school look and feel that wouldn't seem out of place on your old VTVM.  I think they are only good to 500 volts by modern rating standards, so not much help as to the HV issue.

Personally, I would just keep it original and use the Switchcraft 2501F plugs if those are indeed correct.

[txNgineer]

Yes, that is the correct connector. Thanks for the link! I think I have one in my parts bin, but if not then I will need to buy one.  On the other hand, I agree that the vintage test gear market is small and for a 1960-70 VTVM that is quite plentiful, I don't imagine it would be something I would consider.

The PL269 idea is a possibility, but in further inspection the ground lead is already a banana jack and I suspect that the hole for the old 7/8" connector is probably a round hole about 0.5 in. With an appropriate PLA shoulder washer I could replace it with a banana jack as well and maybe even achieve 3/4" spacing. As mentioned above I would then wrap the old connector and tack it inside the unit when I am doing the proper capacitor-resistor restoration.

The only issue then would be to replicate the switchable 1 meg resistor in the original probe (bypassed for DC and utilized for AC). Ideally there may be a way for the function switch to do that. I will have to investigate and think about it.

I want to give full agreement to the above comments about making power cord changes to modern compliance. I also make sure all line-neutral and line-chassis capacitors are changed to modern X or Y class as appropriate. I have unhappy memories of being "bit"    in my youth by a hot chassis on someone's radio.  Promised myself to never let anything I work on to be able to do that!

[Tony_G]

One is that you may not fully understand the design intent of the original approach and changes may negatively effect some aspect of performance.

As an aside (w.r.t the overall question, I agree with CatalinaWOW's take) a lot of the older HP gear has excellent "Theory of Operation" sections in their manuals. If you're thinking about making changes then I'd recommend taking a read of that first to see what is going on.

Additionally you should look at the options the equipment may have been available with - You might find that the change you want to make is actually an option that they originally thought of so you might even be able to get proper fittings etc.

TonyG

[txNgineer]

Good points. For all my older gear I make sure I have a service manual before or just after I buy it. That is usually easy for the HP and Keithley and Tek stuff. Not as easy for Wavetek, EIP, Solartron etc. but I still have nearly all of them. I also try to find repair videos on YouTube when they exist. That allows me to learn the possible weak points or shortcomings for the instrument.

One common failure that takes a LOT of careful research is fan replacement. Finding a match to CFM is not too difficult, but finding the CFM vs Static Pressure curve (which is arguably more important) is a REAL challenge. I successfully did a replacement/modification for my HP 8660 signal generator and have been very pleased with the result. The only issue was I had to bodge the shroud to handle the new metric diameter Noctua fan. Someday I will get a 3d printer to handle things like that more elegantly.

[vk6zgo]

Thanks, that is my normal approach but sometimes it is not as convenient. The thing that bothered me the most in this case is the meter is specified to 1500 volts and that old phenolic connector seems underqualified for high voltages.

It's not that unusual.  Chances are, the meter was not intended to be used at 1500 volt.  I'd say half that voltage max.  It was probably a light hearted (meaning not fully thought out) and ill conceived idea to extend the range to 1500V.  I have several "antique" VOM that goes up to 1000 volt.  I would never apply that much potential.  Case in point, none of the components inside were rated for that much, although technically, circuit itself will divide the voltage correctly.

Safety standards and liability situations changed over the decades.  Even after converting the sockets, I wouldn't go that high....

AVO Model 8s were originally specified for 2.5kV, but that increased to 3.0 kV.
I have personally used them at 2kV, with appropriate safety measures.

[Tomorokoshi]

I've had to repair most of the equipment I get. Usually it's minor, like capacitors or other components.

Only occasionally have I modified the equipment for a different functionality or user interface.

Here are a couple instances of slightly changing equipment to convert specific connectors used with probe wiring to generic binding posts.

[txNgineer]

Nice work! Looks clean and "on purpose".

[CatalinaWOW]

I agree.  Well done and useful.  I probably would have made different color choices on the banana jacks, but that is strictly a matter of personal preference.

[Tomorokoshi]

I agree.  Well done and useful.  I probably would have made different color choices on the banana jacks, but that is strictly a matter of personal preference.

For the Simpson 303, red and black work well for DC and Ground. Yellow seems to be a somewhat more typical color for AC, so that worked out.

As for the HP 4328A, the colors were supposed to match this:
https://www.datatec.de/pmk-tkl065-messleitung-kelvin-banane

However, the rendering isn't the same as the actual product:
https://www.reichelt.com/de/en/tkl-065-ban-kelvin-measuring-lead-tkl-065-ban-p106224.html?r=1

Which still isn't what I got because the orange heat shrink on black was actually green, which you can just sort of see here:
https://www.eevblog.com/forum/testgear/hp-4328a-milliohmmeter-update-and-repair/

I got in the trap of ordering parts from Digi-Key before the actual test leads arrived.