Probes needed for a BWD Portascope 321 - Page 1
Forum section: electronics / testgear

author: sausage
author_rank: Regular Contributor
timestamp: April 21, 2019, 11:02:26 am
content: Hello everyone. First post here.In the last few days I have acquired a BWD Portascope 321 made in Australia in the 1960s. It's valve driven. It still turns on and has a trace, but I have no probe.I've been looking at a few options on aliexpress, and read a few posts here that seem to indicate pretty much any probe would be OK on an old oscilloscope provided that the bandwidth is higher (easy) and that the capacitance matches the input (not as easy).But what I am curious about is an Active vs Passive probe, and I think I need to get this bit right.I also have the original manual and schematic in good condition and it states this regarding input options:1. Single ended, Impedance constant at 1M Ohm and 50pf.2. Balanced or differential, impedance constant at 2M Ohm & 25pf.3. Differential rejection ratio greater than 1000-1 at highest sensitivity, approx. 20-1 with attenuator in use, Subject to a maximum input of X200 atten, setting.Then in the list of "Accessories Available" at the back of the manual:1. High Impedance probe - Type P21 - 10M Ohm and 12 pf approx.2. Demodulator probe - Type P10 - 200 Kc's - 100 Mc's 30V p-p3. Screened Test prods - Type P30 - complete with prod & crocodile clips.A few cheap probes I have looked at on aliexpress... they seem in the range but I don't know if they are active or passive and if they would be suitable for this old probe:https://www.aliexpress.com/item/32893269108.htmlhttps://www.aliexpress.com/item/32862760535.htmlhttps://www.aliexpress.com/item/32654976903.htmlWould anyone mind giving me some advice as to what might be suitable?

author: Kean
author_rank: Unknown
timestamp: April 21, 2019, 03:19:29 pm
content: What input connectors does it use? I found a photo but it is hard to tell.Scopes that old and with low bandwidth (this one has BW of 7.5MHz) didn't always use a BNC input like moderns scopes and probes. Some just had binding posts, and I think I've seen some with a UHF connectors.If it has binding posts 0.75 in apart, you could use an adapter from banana plugs to BNC socket - readily available. Same if it uses some other RF connector than a BNC, adapters will probably be available but maybe a bit costly. You can always hack your own adapter together to start.Then just get a cheap probe like those ones you linked (<$10, not the 2nd one), plug the BNC into the adapter, then into the scope, and and use the probe in x1 mode (1M ohm). The probes you linked were 70+pF in x1, and are not usually adjustable in x1 mode. If you can find a probe that meets the 50pF input capacitance in x1 mode that would help if you're trying to accurately measure signals at higher speeds, but I wouldn't stress or spend too much extra money on it.Trying to measure signals in the MHz may be asking a bit much from a scope 50+ years old that wasn't great to begin with, and I don't imagine you're expecting too much from it anyway. It will be a good learning experience as long as you don't try working on the scope internals which might be somewhat unsafe without appropriate experience.

author: sausage
author_rank: Regular Contributor
timestamp: April 22, 2019, 12:14:46 am
content: Hi Ken, thanks for the reply.The connectors are UHF, and I have since ordered a couple of UHF to BNC adapters.I'll look to try and find a probe that has x1 1MOhm. So are you saying get as close to the 50pf as possible, but if I can't get it 100%, close enough will be reasonable?My application will be to measure signals on the chips in old Commodore computers (1mhz cpu). Later I'll be looking to getting a half decent oscilloscope. This is really just an education, and like you said, I don't expect anything too much.After I'm done, this scope and manual should go to a museum. If anyone knows one interested, send them my way.

author: tautech
author_rank: Super Contributor
timestamp: April 22, 2019, 12:26:01 am
content: FYI, one of the best online resources for BWD is Kevin Chant:https://www.kevinchant.com/bwd.htmlHe doesn't have the Portascope 321 manual so maybe if you want to work with him he can add it to his great BWD resource.

author: wilfred
author_rank: Super Contributor
timestamp: April 22, 2019, 05:09:35 am
content: Long shot. You could shoot off an email to Rockby Electronics. They had a stock of surplus BWD probes a while back and still have a few Powerscope probes. Maybe as part of their pile of surplus stuff they have a box with a suitable probe in it.I'm not sure how much was offered to them but It costs nothing to ask. But I don't recall seeing what you want listed for sale.

author: sausage
author_rank: Regular Contributor
timestamp: April 23, 2019, 02:15:43 pm
content: @tautech Will do many thanks. I have been there once before (before I got the manual). I've since scanned each of the pages with the intent to submit it to the radiomuseum site. But I'll let Kevin know too. The final hurdle is scanning the fold out schematic!Thanks @wilfred. I'll get in touch with them and see what they can do, otherwise aliexpress.