Dumont type 245 WWII Oscilloscope - Page 1
Forum section: electronics / testgear

author: graybeard
author_rank: Frequent Contributor
timestamp: November 12, 2021, 09:02:20 pm
content: I was just given aU.S. Navy WWII vintage Allen B. Dumont Laboratories Type 245 Oscilloscope, model OBL-1 Cathode Ray Oscillograph.Click on the image below or link above to see all of the photos.I am interested in it because of it's historical significance as one of the 1stcommercial/military oscilloscopes.As is to be expected in an 80 year old instrument all of the electrolytic capacitors are bad with the cans leaking. All of the non-wire wound resistors measure well out of spec. Two of the original knobs have been replaced. A binding post is missing. The switch contacts all need to be cleaned.However all of the tubes appear to be good. The CRT has no burn marks. It looks like it will make an interesting restoration project.Chris

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2021, 09:59:48 pm
content: Oscilloscopes of this era (excluding "synchroscopes") had asynchronizedtime base, rather than thetriggeredtime base that we all expect now (analog or digital 'scope).The time base is an astable oscillator, whose free-running frequency is determined by the "coarse" and "fine" frequency controls on the front panel. The "sync" signal (through the "sync amplitude" control) restarts the oscillator so that its actual frequency gives you a stable display.Interesting that this unit has only one-range "gain" controls for both axes (no stepped attenuator).Is there an internal connection for the missing binding post for "Z-axis" (brightness modulation)?

author: graybeard
author_rank: Frequent Contributor
timestamp: November 12, 2021, 10:05:25 pm
content: The wire for the Z connection is loose inside.It can be restored.The sweep oscillator is a gas filled thyratron in which the sync signal is connected to the grid. It is essentially a "neon bulb" negative resistance relaxation oscillator with a grid in between the electrodes.Chris

author: bob91343
author_rank: Super Contributor
timestamp: November 12, 2021, 11:01:27 pm
content: I used to own similar units. Historical, maybe, but useful, not.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2021, 11:08:09 pm
content: Heathkit, among others including DuMont, continued production of “synchronized” CROs into the 1960s. I think there were mass quantities of round CRTs available as military surplus.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 13, 2021, 12:50:18 am
content: Quote from: TimFox on November 12, 2021, 11:08:09 pmLittle later than that, I think the last Heathkit synchronized scope was the IO-102 released in 1971, all transistor! The tube in mine is Japanese IIRC.

author: med6753
author_rank: Super Contributor
timestamp: November 13, 2021, 02:27:16 am
content: Quote from: graybeard on November 12, 2021, 10:05:25 pmThe thyratron sweep circuit was very common in that era. Dumont continued to use it into the 1950's. And they laughed when this little company called "Tektronix" had a better idea with triggered sweep. They weren't laughing for long.

author: bob91343
author_rank: Super Contributor
timestamp: November 13, 2021, 04:22:48 am
content: Yes that was Allen B. Dumont himself who laughed. The president of Ford also laughed when asked about the Japanese cars; he said they were 20 years behind and wouldn't ever catch up.

author: graybeard
author_rank: Frequent Contributor
timestamp: November 13, 2021, 05:21:42 am
content: I was looking at the big dog bone resistors and I was wondering about how to read them and what their power rating is. In doing some research I cam across this old illustraion which is quite useful.The three large dog bone resistors inthis photoandthis oneappear to be 27K 2 Watt 10%.Chris

author: med6753
author_rank: Super Contributor
timestamp: November 13, 2021, 08:14:08 am
content: Quote from: BrokenYugo on November 13, 2021, 12:50:18 amThe correct term is "recurrent sweep". And yes, Heathkit did have recurrent sweep scopes until the early 1970's. By the mid-1970's they had been phased out and even the cheapest scope was triggered sweep.The "mass quantities" of surplus CRT's was pretty much used up by the early 1950's. Most new 5 inch CRT's were manufactured by RCA and even Tektronix initially used them but was not happy with their overall quality. That's why Tek started to roll their own CRT's.

author: med6753
author_rank: Super Contributor
timestamp: November 13, 2021, 02:27:23 pm
content: Quote from: graybeard on November 13, 2021, 05:21:42 amThose appear to be the load resistors for the vertical and horizontal output. But I only see 3 and there should be 4. That large black one in the first photo seems to be the 4th one. Might be a more modern replacement where the value is stamped on the body rather than adhering to the older color code.

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2021, 02:59:11 pm
content: I learned the mnemonic of "BED" for "body-end-dot" in that color code.

author: graybeard
author_rank: Frequent Contributor
timestamp: November 14, 2021, 03:59:31 pm
content: I found these to help me read the values on some of the capacitors.This PDF reference is very comprehensive for reading the color codes all almost all old capacitor and resistor types.Chris

author: graybeard
author_rank: Frequent Contributor
timestamp: January 19, 2022, 08:28:38 pm
content: In doing research I was able to track down this military manual "Instruction Book for Cathode-Ray Oscillograph Navy Model OBL-1. Navships 900, 227(A) Restricted. Allen B. Du Mont Laboratories, Inc., Passaic, NJ. Navy Department Bureau of Ships. Contract No. NXsr-33622. Approved 20 September 1945.."I found it inThe Special Collections for Public Servicesat theJoyner Library of East Carolina University.I contacted them and Ralph Scott,Professor, Curator of Rare Books and Maps, Assistant Head of Special Collections for Public Services, had one of his students make an excellent copy for me. They were great to work with!It was sent to me in two parts:DuMont 245 manual part 1 of 2DuMont 245 manual part 2 of 2In my research to find this I also tracked down some otherold DuMont oscilloscope manuals and schematicsyou might be interested in.Chris

author: TimFox
author_rank: Super Contributor
timestamp: January 20, 2022, 02:43:29 pm
content: The sync tube (triode) in the lower left isn't labeled: is it a thyratron (884 or equivalent)? The 884 has that octal base and pinout; the older 885 has a 5-pin base.

author: graybeard
author_rank: Frequent Contributor
timestamp: January 23, 2022, 08:25:39 am
content: Quote from: TimFox on January 20, 2022, 02:43:29 pmOn page 5-11 it says it is an octal 884.