Fluke 5440B - Help? - Page 1
Forum section: electronics / repair

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 05, 2016, 02:19:38 pm
content: I'm trying to repair the cold oven on the 5440B's A6 sample string board. The driving transistor for the heater is a NPN MJE3055 (Q2) which itself is driven by a small-signal NPN (Q1). According to the data sheets, the 3055 pinout goes B-C-E from left to right (top view). But when I look at the A6 board, it seems as though the left-most pin is the one that goes to the heater. That would put it on the base lead which makes no sense to me. And the Q1 driver would be hitting the emitter of Q2. In other words, it would all make sense if the 3055 pinout was E-C-B, not B-C-E. Am I missing something? Is the schematic in the manual wrong? Did they change the 3055 pinout 20 years ago?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 05, 2016, 02:56:53 pm
content: The only explanation I can come up with is that Q2 is mounted upside down?

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 05, 2016, 03:09:27 pm
content: Well, it really beats me. I found a picture of someone's board on the web and it is the same as mine. The 3055 is in the upper right corner and you can see the left-most pin going down to the heater (you can see the shadow of the trace which is on the bottom side of the board). The right-most pin goes up to the emitter of the driving transistor. Can't possibly be! I can make it work, of course, by flipping things. But there must be some explanation.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 05, 2016, 03:16:27 pm
content: This is funny, MJE3055 TO220 is BCE, MJE3055 TO127 is ECB

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 05, 2016, 05:13:23 pm
content: To PA0PBZ: Thank you, thank you, thank you! I thought I was going crazy. Turns out that my board has been reworked and the previous "repairman" put the TO220 version in my board.

author: SeanB
author_rank: Super Contributor
timestamp: November 06, 2016, 05:54:11 pm
content: As it is the ISOTOP version you can simply turn it around and it will work, albeit with reduced thermal transfer to the small heatsink, but you could always move the heatsink as well to be on the right side of the device.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 06, 2016, 06:25:31 pm
content: Quote from: SeanB on November 06, 2016, 05:54:11 pmThe board he posted is one found on the web, his board has a TO220 which is not easy to reverse.

author: SeanB
author_rank: Super Contributor
timestamp: November 06, 2016, 07:41:03 pm
content: Still easy to reverse, just use the nut on the tab, and a second one under the board, or a small M4 spacer between the top of tab and the board.

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 06, 2016, 09:08:27 pm
content: It didn't look too easy to flip the TO-220, so I just cut the leads and used two short pieces of wire to cross them over. But I'm glad for the explanation, because the previous owner had also installed a TO-220 MJE2955 incorrectly for another transistor. That one was sticking straight up from the board without a heat sink, so obviously I simply flipped it around.So the oven now works, and the Fluke 5440B seems to be fine. Originally it failed the analog self-test telling me to check the A4 output board current limiting circuitry. The thing was current-limiting into standby, but it was only after I measured things that I found it was limiting at 1.6 times the user-entered amount. Turns out my unit must have had the A4 board replaced with a later output board (after serial number 4-something) where they changed the current limiting resistors from 80 ohms to 50 ohms. So I put it back to 80 ohms and it now passes the self-test.