My newest oldest oscilloscope -  GEC  M861B Miniscope - Page 1
Forum section: electronics / testgear

author: dazz1
author_rank: Frequent Contributor
timestamp: March 30, 2024, 07:25:57 am
content: HiMy newest scope is also the oldest thing I own. It is a late 1940's, early 1950's, GEC M861B Miniscope. A marvel of miniaturization at a time when test equipment weighed as much as a sack of coal and used enough power to heat a small orphanage.My research reveals that this model of scope is the direct descendant of a scope (model M861A?) developed to fault-find clandestine radios in WW2 enemy occupied territory. The post war version was sold to traveling TV repair men (probably not many TV repair ladies at that time).It's military origins can be seen in the knobs. This style was used on aircraft to allow crew to turn the knobs while wearing thick gloves.It features an astonishing 70kc/s bandwidth with a 1 1/2 inch CRT and a weight of just 7 1/2 pounds.The plan is to restore it to working condition. I have not attempted to apply power yet because it has probably been in storage for decades. The electrolytic caps won't be caps anymore.If anyone has a spare M862B Double Beam Unit, or a M863B IF Alignment Unit lying around, I'd be a keen buyer.

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2024, 07:37:57 am
content: Oh so you did buy it !Next you'll be joining the TEA thread.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 30, 2024, 08:07:31 am
content: Quote from: tautech on March 30, 2024, 07:37:57 amI couldn't resist.

author: adinsen
author_rank: Regular Contributor
timestamp: March 30, 2024, 08:17:35 am
content: What a beauty, congrats. I love the carrying case

author: dazz1
author_rank: Frequent Contributor
timestamp: March 30, 2024, 11:13:30 pm
content: Quote from: tautech on March 30, 2024, 07:37:57 amMaybe an admin should/could shift this post to the TEA thread?

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2024, 11:58:03 pm
content: Quote from: dazz1 on March 30, 2024, 11:13:30 pmOr you can post there with a link to here.Slightly more complex is Quoting your first post and Copy all content into a new Reply/post in the TEA thread.You could add the images by way of uploading them again or copying their links and pasting them between Image flags like this:Quote this post to examine the syntax used.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 21, 2024, 03:35:59 am
content: I took the Miniscope to the local technical institute where I used a variac to gradually build up the supply voltage to 240VAC to reform the caps, and see if any magic smoke was waiting to escape. I was working in a class with 3rd year apprentices and the instructor asked me to give a mini-presentation on what I was doing. They were all interested. I forgot to take photos.Over a period of about 20 minutes, the voltage was gently eased up and absolutely nothing happened. I had the variac output in series with a 18R 10W resistor and an underated fuse. Only 200mA. At full supply voltage it drew about 130W which is too high.The critical part is the transformer. This is sealed in a metal can, probably with transformer oil for cooling. If this blows, the Miniscope would probably be unfixable.One of the potentiometers is seized solid and two others have high friction. They are 1/4" SS shafts with no flat. I suspect this was done to prevent damage by over turning the knobs. The set screws are BSW 1/8, 40TPI, but one screw was 38TPI and that was enough to strip the thread in the Bakelite knob. I am currently making some replacement set screws (can't buy them here). And I need to repair the striped thread.Now that I know it can take 240VAC, I plan to run it at about 115VAC to go through and check that individual components are doing their job. Before I do that, I need to reverse engineer a circuit diagram. That will take a while because of the way the components are packed in.

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2024, 10:19:58 am
content: Quote from: dazz1 on April 21, 2024, 03:35:59 amPictures of progress or it's all lies.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 15, 2024, 10:50:46 am
content: Quote from: tautech on April 21, 2024, 10:19:58 amI am recording my adventures with the GEC Miniscope on this forum here:https://www.vintage-radio.net/forum/showthread.php?p=1638107#post1638107A specialist forum for antique gear that no one needs any more.I also posted a video on making replacement set screws for the knobs.https://youtu.be/vSnd8RYDzToQuite a few pictures to look at.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 15, 2024, 01:47:50 pm
content: OMG what a finding 1950 ?? damn

author: ebastler
author_rank: Super Contributor
timestamp: May 15, 2024, 06:40:09 pm
content: Beautiful! But where are the teardown photos?

author: dazz1
author_rank: Frequent Contributor
timestamp: June 05, 2024, 05:44:47 am
content: Quote from: ebastler on May 15, 2024, 06:40:09 pmYou can follow along here:https://www.vintage-radio.net/forum/showthread.php?p=1638107#post1638107] https://www.vintage-radio.net/forum/showthread.php?p=1638107#post1638107

author: dazz1
author_rank: Frequent Contributor
timestamp: June 05, 2024, 05:49:56 am
content: Quote from: coromonadalix on May 15, 2024, 01:47:50 pmNormally I look at posts with people showing off a fantastic piece of test equipment they brought for $1 somewhere. I can only turn green with envy.This may be the only time I get to gloat about a rare and interesting instrument that I have and "you" don't.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2024, 08:46:04 am
content: Quote from: dazz1 on June 05, 2024, 05:44:47 amLink incorrectly linked.https://www.vintage-radio.net/forum/showthread.php?t=209669Seems Dazz has made new friends but none of them seem to have the porcelain valve holder I might have.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 05, 2024, 10:04:23 am
content: Just some reference photos.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 05, 2024, 10:06:52 am
content: And some more photos

author: TomKatt
author_rank: Frequent Contributor
timestamp: June 05, 2024, 11:43:16 am
content: That's really coolI've got a late 40's Waterman 'Pocket Scope' - similar design but larger 3" crt. Replaced the dried up 2-prong line cord + a few old paper caps and it still works fine. I use it for audio signal decoration and some ucontroller vector projects.Don't have access to photos of my scope, but here is alink to the same model I have (with scan of operating manual)

author: factory
author_rank: Super Contributor
timestamp: June 05, 2024, 07:12:56 pm
content: Quote from: tautech on June 05, 2024, 08:46:04 amI'm guessing you didn't read post #71 on the radio forum?David

author: dazz1
author_rank: Frequent Contributor
timestamp: June 07, 2024, 11:28:05 am
content: Quote from: factory on June 05, 2024, 07:12:56 pmHe is sending me valve bases and a can all the way from the UK at no charge. Very generous and helpful. No chance of me buying valve bases locally.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 07, 2024, 11:33:56 am
content: HiSome more photos.A selenium diode in pieces. Just disks held together with springs.Some leaking caps.Soaking a seized potentiometer in hot kerosene in an egg cup, in a bowl of boiling water. It worked really well.A close up reference photo before I cut out and replace mica caps, encased in bakelite, covered in wax.

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2024, 12:11:12 pm
content: That's so GEC. Everything about it shouts "thrown together at the last moment", from the mesh covered vent hole, with a name plate blocking half the vent, down.

author: factory
author_rank: Super Contributor
timestamp: June 07, 2024, 12:44:41 pm
content: The piece with the name plate is actually the tilt stand (pictures below stolen from ePay), I'm sure otherminiatureoscilloscopes of the era are just as crammed & chaotic inside.David

author: TomKatt
author_rank: Frequent Contributor
timestamp: June 07, 2024, 12:56:03 pm
content: Most vintage equipment I've owned or played with has that "built in a garage" look... The older you go, the more "rats nest" wiring and assembly you see.It's just the way things were built back then. The very first consumer televisions that test gear like this was used on looked like somebody's home-made ham radio project.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 08, 2024, 09:29:20 am
content: Quote from: coppice on June 07, 2024, 12:11:12 pmMy impression is that quite a bit of design work went into making these things so small. The parts are spread evenly around the enclosure to make best use of the volume. The caps and diodes are tailored into their locations while still leaving volume for cooling. It might look chaotic but there is order in that chaos.

author: Don Hills
author_rank: Regular Contributor
timestamp: June 10, 2024, 03:36:54 am
content: You said over on the UK Vintage Radio forum that you thought your scope was likely the only one in NZ. Well, not only do I have one too, but I'm even in the same city. :-) I acquired it a few years ago at a recycling centre. It's currently waiting for me to retire and have the time to rehabilitate it. I also have a larger and more recent scope in the queue as shown in the other picture.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 17, 2024, 02:11:45 am
content: Quote from: Don Hills on June 10, 2024, 03:36:54 amHiThat is definitely a coincidence. These little scopes would have been expensive to import, including the tax and duty. I suspect many of them would have been thrown out when transistorised scopes became available. The Miniscope was always quite low spec.Does your Miniscope power up? What is the serial number of your Miniscope? Mine is 83246.

author: Don Hills
author_rank: Regular Contributor
timestamp: June 20, 2024, 12:16:35 am
content: It's S/N 83523.I haven't tried to power it up. I suspect it was at least attempted at some stage, because the power cord has been replaced with a 2-conductor molded plug and cable as commonly found on Japanese transistor radios etc. I plan to at least take a look inside and check for replaced components.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 13, 2024, 12:07:39 am
content: Quote from: Don Hills on June 20, 2024, 12:16:35 amI powered mine up with a variac, low current fuse and current limiting resistor. Nothing glowed.Over half the caps in mine showed visible signs of failure. Four had been replaced.About 1/3rd of the carbon resistors were way over spec.Half of the potentiometers were seized solid.I am basically replacing everything inside except the valves and potentiometers.I am not attempting to stuff the old caps with new versions. Too much work for zero return.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 07, 2025, 05:33:03 am
content: HiIt has been a while since I posted an update here for progress on my GEC miniscope so here it is.After replacing every resistor, capacitor and selenium diode, the Miniscope works.The photographic evidence is attached.The sine wave displayed is a bit noisy, probably because 4uF smoothing cap is missing. I forgot to order it.The image on the scope CRT is "cute". Note that the scope is setup so the blanking of the return beam is disabled, as recommended in the user manual.A quick tweak on the knobs indicates they are all functional.I still have a list of things to do on it:I need to purchase/fit the filter cap.The silicon versions of the selenium rectifiers works but I want to refine the design.The CRT is rated for EHT- between -600VDC and -1000VDC. The manual says -600VDC but I have it set to -670VDC with plenty of headroom left. The higher efficiency of the silicon diodes uses less power through the transformer and increases the achievable HT output voltages.So to recap, when I got the scope, it didn't work, all the caps were kaput, it was missing a couple of components, and there were wiring faults. After months of work (off and on) I have functionally restored to operation a 50k c.p.s single channel, AC only scope to sit beside my somewhat more modern oscilloscopes. Probably a few months to go yet before it reaches the completed stage, but getting there.The photos show a close up of the CRT displaying a sine wave.The other photo is power testing with a Variac to check for magic smoke leakage. There was none.The scope is already about 70 years old, so maybe it will still be working in another 70 years. I know I won't be.

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2025, 10:21:41 am
content: Never seen that blue and white DSO thingy when visiting, a new acquisition ?

author: dazz1
author_rank: Frequent Contributor
timestamp: April 07, 2025, 11:12:11 pm
content: Quote from: tautech on April 07, 2025, 10:21:41 amThe DSO on the right is from the local technical institute. That is where I did the testing so I could use their variac.The white box with the LCD digital display is a boxed version of a Aliexpress power meter. I didn't buy it as a item of test equipment, but it is proving to be useful.It is telling me the scope is using less than spec power, so I know the transformer is safe.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2025, 07:54:52 am
content: HiThe GEC Miniscope project is practically complete. Full blog can be seen here:https://www.vintage-radio.net/forum/showthread.php?t=209669It's only taken two years.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 10, 2025, 04:14:31 am
content: HiI have created and posted a YT video on my restored GEC Miniscope.The video includes a demonstration of the working Miniscope.https://youtu.be/LiEK_iy5CyA