Floating wire on Racal Dana 9300 Isolation switch

[Radio Surfer]

Hello,

Anyone any idea where the floating detached wire should go? It’s on the centre pin of the Isolation switch on a Racal Dana 9300 True voltmeter. Not a great photo, but hopefully, you can see the floating white wire coming from the centre right pin of the switch in the photo.

Many thanks, Richard

[Station X]

There are circuits for this voltmeter on the radio museum site, but I don't know whether they cover your variant.

https://www.radiomuseum.org/r/racal_...h_version.html

[Radio Surfer]

Thanks Graham, that image is exactly what I was after, and I’ve managed at last to find a circuit diagram for my model that matches your image. I’ve also looked at some on line images. It looks as though someone has “ had a go” at the front bnc. It’s not got the original which I think was insulated from the chassis. The switch next to it seems to allow the bnc to be either grounded or isolated. Having replaced the insulated bnc for a non insulated version, the switch has become redundant, hence the floating wire.

At least that’s what it looks like. I’m trying to find a suitable insulated bnc chassis mount, but struggling for the right size at the moment. The original I assume matched the rear one, which has a square back with 2 screw holes, and you can see the white insulation keeping the metal shield from the chassis.

There should also be some beads on the cable at the back of the bnc, but they are missing too.

Cheers,

[ppalped]

I have a Racal Dana 9300, but it has been a long time since I last looked into it's internals, so I barely remember how it looks. But I'm happy to take some pictures of the internals during the weekend if that helps. Let me know.

Alex

[Radio Surfer]

Hi Alex,

That would be great. The area covering the isolation switch, BNC socket, and bridge rectifier please.

I seem to be missing the 4 ferrite beads on the cable from the BNC, small 1n capacitor on the back of the BNC, and a non insulated BNC.

It still works, but I'd like to reinstate the grounding switch again.

Much appreciated, Richard.

[ppalped]

No problem Richard. I'll take a few pics during the weekend and I'll post them here.

Alex

[ppalped]

Hi Richard,

Here are 5 pictures of the front panel connections for the BNC and isolation switch.

Sorry about the lighting, but given the location of the switch and BNC connector, it is not easy to take a good picture with good lighting - but let me know if you need something in more detail and I'll do my best.

Alex

[Radio Surfer]

Hi Alex,

That's brilliant, thank you. The photos have helped me clarify how the circuit diagram is actually wired, in particular the slight variations of how signal and chassis grounds have actually been done. I often find these differences between the theoretical circuit diagrams and what is actually wired inside the box. I suppose during manufacture signal and chassis ground can get blurred for ease of wiring, or there is actually no detrimental impact on performance. Pity the circuit diagrams aren't always updated.

Fortunately, having spent 2 days searching for a replacement insulated BNC that would fit, I found a used one in good condition this afternoon. Parts seem readily available these days anywhere but the UK. I just need a short length of decent shielded cable now, rather than the bell wire that's been used. I really don't understand why someone has changed the original BNC, bypassed the switch and taken the shielded cable with ferrite cores out. They also seem to have used a blow torch when soldering.

At least, it should now be a simple fix, to get back to original. I'm also replacing R14 for one with narrower tolerance, which seems to be a generally accepted enhancement.

Cheers, Richard

[ppalped]

Another one - probably a lot more useful than the others.

Alex

[ppalped]

Originally Posted by Radio Surfer

Hi Alex,

That's brilliant, thank you. The photos have helped me clarify how the circuit diagram is actually wired, in particular the slight variations of how signal and chassis grounds have actually been done. I often find these differences between the theoretical circuit diagrams and what is actually wired inside the box. I suppose during manufacture signal and chassis ground can get blurred for ease of wiring, or there is actually no detrimental impact on performance. Pity the circuit diagrams aren't always updated.

Fortunately, having spent 2 days searching for a replacement insulated BNC that would fit, I found a used one in good condition this afternoon. Parts seem readily available these days anywhere but the UK. I just need a short length of decent shielded cable now, rather than the bell wire that's been used. I really don't understand why someone has changed the original BNC, bypassed the switch and taken the shielded cable with ferrite cores out. They also seem to have used a blow torch when soldering.

At least, it should now be a simple fix, to get back to original. I'm also replacing R14 for one with narrower tolerance, which seems to be a generally accepted enhancement.

Cheers, Richard

I guess that after severing the original BNC connector, one of the previous owners of your 9300 might just have used what was available in the spares bin - probably the BNC connector used as the replacement already had the cable fitted, and the decision to bypass the original cable was just a matter of lack of patience and/or quickly fix the issue.

Anyway, glad I could help a bit at least.

Alex

[Radio Surfer]

That last photo is a good one,

Many thanks again, tomorrow I'll order the remaining bits I need, and then get this sorted. In the meantime I'll focus on my other topic of finding an attenuator. I know what I'm looking for now, I just need something at a sensible price to come up. I'm also expecting a Sony D3 to arrive in the post any day, which I'm hoping I can combine with another. I definitely need these jobs over the long wet winter. Will it ever stop raining?

Cheers, Richard

[ppalped]

No problem at all - if you ever need any other information or pictures from the 9300 let me know. It's a really useful, and I'd say unparalleled (for the price), piece of gear for precision sub-millivolt true-rms measurements, given that most digital true-rms meters out there won't cut it at these levels.

The downside of the 9300 is that it is a bit of a pain to calibrate if you don't have a good AC and DC reference and precision attenuators, as it requires very precise sub-millivolt levels during the calibration and adjustment procedures. I adjusted mine against a fluke calibrator some time back, but not sure how much it drifted so far. I might check it again one of these days.

Cheers,

Alex

[Radio Surfer]

Thanks Alex,

The replacement insulated BNC arrived over the weekend. I had trouble finding one, but managed to get a decent used one in the end. The floating wire has now been reconnected, and everything is back to standard again. Most importantly, it’s working. I’m not sure yet how accurate it is yet, so I’ll do some testing, and then consider recalibration if needed. I also found a good Marconi attenuator, so I should have everything I need.

This brings to an end this thread, so Moderators, feel free to close.

Thank you,

Cheers, Richard.