Tektronix DM254 (Service) Manual  - Page 1
Forum section: electronics / repair

author: Ziltoid
author_rank: Contributor
timestamp: February 21, 2015, 11:20:55 am
content: Hi,i have a broken Tek DM254. I did you found some information about the dmm254 but i dont think they are similar. Maybe someone knows anything about this Multimeter. I will make some pictures from the inside later.The Voltage Ranges are either reading too high or to low and the Ohms only works at the low end.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 21, 2015, 07:47:58 pm
content: Your Tek DM254 is likely some variation of an older Appa Tech 100 series. Seehttps://www.eevblog.com/forum/testgear/iso-tech-(appa)-idm105-dmm/I suggest the following tests.1) If you haven't tried new batteries, put in new batteries and retest.2) On the Appa 105, there are 3 spark gaps (white with pcb designation SG), 2 blue PTCs and 3 power resistors (wrapped in heat shrink and in between the V/ohm jack and the Caddock precision resistor network). These components are used for input protection and could have been subject to high/over voltage?3) You can perform all these measurements in-circuit first.4) The spark gap should measure out of range (0L) in resistance mode.5) The PTCs should measure around 1k ohm (a guess).6) The power resistors should measure around 1k ohm (a guess).7) Report all your measurements for 4, 5 and 6 above.8] Anything obvioulsy out of these ball park ranges is suspect.

author: Ziltoid
author_rank: Contributor
timestamp: February 22, 2015, 05:28:46 pm
content: Thanks for the reply. You are right it looks like the Appa 105. On the PCB is even a marking 105-3B. The Main IC is a NJU9210FC which is a 3 3/4 Digit DMM. But this version is different to firewalker (from the other thread). The 3B has a AD737 True RMS to DC Converter on the PCB.Resistors should be 470R two are blown.One Sparksgap resistance measures 23M which i think is not right.The two PTC are reading around 1.5k.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 22, 2015, 08:39:49 pm
content: Quote from: Ziltoid on February 22, 2015, 05:28:46 pmFor testing purposes, you can replace those 470R resistors with a regular carbon 470 resistors and remove the 23M ohm sparkgap from the pcb. A 23M ohm reading for the sparkgap likely indicates it is bad. The PTC readings sound about right.If the meter works, then you can get the proper replacements.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 22, 2015, 08:41:04 pm
content: BTW, in the spirit of Dave's Don't turn it on, take it apart, I would like to see inside pictures of your meter.

author: Ziltoid
author_rank: Contributor
timestamp: February 22, 2015, 09:22:17 pm
content: I had to improvise with some smd resistors, the sparkgap measurd OL out of circuit so something is in parallel on the board.Seems to be working now, the OL indication is a little bit uncommon. Sorry the lighting isn't the best, i will make the pictures tomorrow.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 23, 2015, 12:17:18 am
content: Without schematics and/or a bill of materials, I don't know what would be the proper 470 ohm resistors.However, since the APPA 105 resembles the Fluke 87 series I and the APPA tech website mentions a fusible resistor, my guess is that the 470 is a fusible resistor. It could be 1W or 2W, but I can't tell from the size in the photos.Possible replacements includehttp://www.digikey.de/product-detail/en/SPP1UL4700JLF/989-1271-1-ND/4215493http://www.digikey.de/product-detail/en/SPP2UL4700JLF/989-1288-1-ND/4215461I attached the Fluke 87 service manual section that talks about input protection as well.

author: Ziltoid
author_rank: Contributor
timestamp: February 23, 2015, 03:42:21 pm
content: I don't know if there are special markings for fusible resistors, and these look like the ones on the picture you provided. I compared the sizes with the data sheet, it seems they are 2W resistors. The SPP2UL4700JLF seem to be right.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 23, 2015, 07:41:31 pm
content: Quote from: Ziltoid on February 22, 2015, 09:22:17 pmI don't have too many resistors at home and when I need one, I go scrounging around dead power supplies looking for the right value. Now that I see your smd bodge, I have another potential method of getting temporary use resistors.BTW, since the fusible resistors are fairly cheap, I would get a few extras in case of another oops moment. Once you get the proper replacements in, check the calibration and see if it is within another meter.PS. Thanks for sharing the pics.

author: retiredcaps
author_rank: Super Contributor
timestamp: March 04, 2015, 07:03:40 am
content: Just saw another rebadged meter that is an Appa 105. This is a Greenlee 93-105RMSA.