Micronta 22-208A VM - Page 1
Forum section: electronics / beginners

author: SteveMcCroskey
author_rank: Newbie
timestamp: January 12, 2024, 01:28:17 am
content: Hello Gang!I'm new here and am getting into electronic repair as a hobby. I purchased a used/dead Micronta 22-208A voltmeter (late 1970s) for $10 at a garage sale. I took it apart and found a 9.5 ohm resistor that let out the magic smoke. While waiting for the new resistor I'm self-studying the circuit diagram, figuring out what each symbol is, how it works etc. I'm trying to understand as much as possible how the meter works.I'm following advice given in YT videos about how to test individual components, either in or out of the circuit. I identify the component and test it if I can. I'm keeping notes and building my skills. One question I have is the dual FET I.C. chip. I can't identify it. I have attached a picture of it. For grins and giggles does anyone here have any advice on how to identify it? What company is represented by that "B"?The reason why I wish to identify it is to ascertain if there is a modern replacement. I also may pull it and try to test it, following the advice I've seen in YT videos.Any thoughts are appreciated.-Steve

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2024, 02:04:07 am
content: The company is Siliconix. According to the manual, the transistor type is 1S001Z and is some sort of MOSFET or DMOSFET with two transistors on a single substrate, or something like that. You could look in an old Siliconix data sheet book and see if something looks similar. These products are long obsolete and if yours is bad, you'll probably need to be creative. Here's a book from Siliconix 1986.http://bitsavers.informatik.uni-stuttgart.de/components/siliconix/_dataBooks/1986_Siliconix_FET_Databook.pdfEdit: I had looked up a 22-208, I missed the A. The answer below is much better and you'll have a slightly easier time finding a subsitute if needed as well.

author: floobydust
author_rank: Super Contributor
timestamp: January 12, 2024, 02:59:55 am
content: The original did show up in a 1977 Japanese surplus store as "specialty manufacturer" so I think it's Japanese semi?It's a dual N-ch JFET, rare and long obsolete - leave it alone. Pulling out every part to test it is not always a great idea with vintage gear, they get damaged and in-circuit testing with some knowledge of the circuit works as well most of the time. If the multimeter does not work, taking voltage measurements (using another multimeter) of the IC pins etc. and post them here if you need help to troubleshoot, that is also another way to see if there is a problem.Today, it would be a boutique partInterFETIFNU412datasheetbut TO-71 can package or SOIC-8. IDSS0.5mA, VGS(off)-0.5VLinear Systemscould have an offering as well.FET analog VOM's used Japanese semi's:Micronta 22-220 NEC uPA68HMicronta 22-209 NEC uPA63Micronta 22-208A E412S sub Siliconix U412Micronta 22-208 Sharp 1S001ZMicronta 22-206 Sharp 1S001Z2SK150, uPA68 are similar but SIP package.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: January 12, 2024, 02:36:12 pm
content: Good Morning Floobydust, Have you seen this video?:https://www.youtube.com/watch?app=desktop&v=KbyPZuXRN4sIt's a 'Part 2' and I don't see a Part 1 around, but I didn't spend a lot of time looking. Have fun with it and stop back in with your progress.By the way, there's one on E-Bay for $50. See ya

author: floobydust
author_rank: Super Contributor
timestamp: January 13, 2024, 11:08:02 pm
content: Micronta FET multimeters lived from 1975-1982. I could not find the 22-220 or the "A" versions dates. Digital multimeters took over eventually.Micronta 22-208 "28-Range Dual FET VOM" 10Meg/Volt (New for '78) $59.95 or $282 todayMicronta 22-206, "27-Range Dual FET VOM" (New for '75) - $59.95 or $324 todayGreat place to reminisce:Radio Shack Catalogs.com analog VOM's for 1976

author: SteveMcCroskey
author_rank: Newbie
timestamp: January 14, 2024, 05:52:35 am
content: Thanks for all the help!I made some progress. Pictured below are two components I removed from my meter. They were soldered in parallel. The resistor is brown/green/red/silver. From the chart, I assume it is 1,500 ohms with +-10% tolerance. I tested this resistor at 1,640 ohms, pretty close to the upper limit. I've ordered a replacement as I don't trust it. The board is heat-discolored where the resistor was installed. The circuit board and diagram label this resistor as "R16" and should be a90ohm resistor. Can anyone explain this? I may need to order a 90...This resistor was soldered in parallel with what I believe, after research, is a thermal fuse? It is marked "227C" and "95.3". I'm assuming 227C is 227 degrees Celsius upper limit? I'm assuming 95.3 is a resistance value. This device tests at 97 ohms. When these components were soldered in parallel on the board they tested at a combined 537 ohms. The fuse(?) does not appear on the circuit diagram. The resistor I removed is an old carbon type, and the other resistors on the board look more modern, don't they? Do you think the fuse is original, and the diagram wasn't updated?I know automobile circuit diagrams have errors, so I'm assuming this diagram could have errors. Any thoughts from you guys are greatly appreciated.Thanks!

author: floobydust
author_rank: Super Contributor
timestamp: January 14, 2024, 06:42:32 pm
content: The resistor R16 is for the Ohms function Rx10 and is spec'd at 90Ω. What you have is a 95.3Ω resistor in parallel with a 1.5kΩ 10% to trim down and give around 89.6Ω and the "227C" is a manufacturer's model number.If the meter is on Ohms and you accidentally touch a live voltage source, the resistor will burn up.Check the accuracy on Rx10 see if the meter is accurate. You could also use a trimpot in place of the 1.5kΩ resistor.People usually overload the current function so R9, R10, R11 get roasted.Micronta FET-VOM 22-208 Service manual and schematichttps://elektrotanya.com/micronta_radio_schack_22-208_fet_vom_voltohm_mero_sm.pdf/download.html