Dynamco 71 Series Oscilloscope Help - Page 1
Forum section: electronics / repair

author: Calabrese
author_rank: Contributor
timestamp: October 24, 2017, 05:17:30 pm
content: I picked up a Dynamco oscilloscope a while back for $5. This has the 7110 display, 1Y2 amplifier and a 1X1 timebase. I would like to plug it in and see if it works but a few things concern me. I am an amateur when it comes to electronics. I'm still learning so I don't want to risk any shocks or fires. I know this is a UK scope but this one has a US plug on it. I am in the US. I'm confused as to if it was modified or just sold in the US also. I know at the very least I need to clean out the dust. Any advice on whether I should even try plugging this in or if it might have been modified would be greatly appreciated.

author: Armadillo
author_rank: Super Contributor
timestamp: October 24, 2017, 05:32:04 pm
content: Check that there is continuity of the Earth with the metal enclosure.Measure with a ohm meter that the live and the neutral is not shorted [depending on the watts of the equipment, say above 10 ohms]. Make sure you switch the unit to the On side while measuring the resistance.To be on the sure side, use a incandescent lamp in series to turn the unit on. If there is no short circuit, then you can plug the power direct without the lamp, after that.Also use a isolating transformer if you have one. Not a variac.If you can, open up the unit for a casual visual inspection if anything is missing or obvious hazardous situation that do not justify it to be plug, example missing circuit board.. etc...Hope it provides some insights to you.

author: Calabrese
author_rank: Contributor
timestamp: October 24, 2017, 06:34:16 pm
content: Thank you for your reply. I get continuity between both Earth ports(the amplifier and the timebase), Earth on the plug and the screws on the back of the all three units. The resistance between live and neutral with the display switch turned on but the unit not plugged in is about 25 Mohms. The resistance between both live and neutral and the Earth on the plug start at about 50 Mohms and quickly reach 0.L on my meter. On an inspection inside the unit it seems everything is present. I have a magnifying lamp with an outlet in it. Would that serve the purpose for plugging the oscilloscope into? I do not have an isolating transformer unfortunately. Thanks again.

author: Armadillo
author_rank: Super Contributor
timestamp: October 24, 2017, 06:54:06 pm
content: Something is open circuited with that kind of huge ohms. Please check the fuses and the internal transformer primary ohms. Check wiring to the on/off switch if it is open.Edit: If the fuse has blown, then you need to be extra careful to check for shorts that causes the fuse to blown. If you just turn on with a new fuse then it will more likely exacerbate the problems.

author: Calabrese
author_rank: Contributor
timestamp: October 24, 2017, 08:51:16 pm
content: The resistance does change when I turn on the on switch and varies as I change it to maximum. The only fuse I could locate was at the back of the oscilloscope. It tests fine for continuity. I'm having trouble getting at the transformer windings and I'm a little nervous messing around in there. There are numerous wires that appear to be coming off the transformer. The bars that each are on are numbered. 88, 89, etc. I appreciate all your help.

author: Armadillo
author_rank: Super Contributor
timestamp: October 24, 2017, 08:58:11 pm
content: If there are wires dangling like so, then hands off. It is not worth the danger to you for $5. Likely you bought it for parts, anyway.However, maybe someone here has the experience with this particular oscilloscope and better advise you. So do wait for their replies.For me, I think is too risky for you. Cheers!.

author: Calabrese
author_rank: Contributor
timestamp: October 24, 2017, 09:09:59 pm
content: Update. I plugged it in. I get a ready light but the entire screen is green. I can't seem to get a line or a dot out of this. The focus knob seemed to alter it slightly but nothing like actually focusing it. Any help would be appreciated.

author: Armadillo
author_rank: Super Contributor
timestamp: October 25, 2017, 08:51:43 pm
content: Hi;Bump up for you for the Pros to reply.You will be dealing with 10KV EHT.This circuit is not difficult but I think others are worried for your safety too.https://elektrotanya.com/dynamco_71_series_oscilloscope.zip/download.htmlThe power circuit is on the display section, see page 42, page 44.You will be dealing with very high voltage. If you are not ready, don't do it. Get the pros guidance.

author: Calabrese
author_rank: Contributor
timestamp: October 25, 2017, 10:56:33 pm
content: Thank you so much. I definitely am one who doesn't mess with something unless I am completely comfortable. I am checking everything safe at the moment. If that doesn't solve anything I will recruit professional help. No desire to mess with KVs.