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| Repair of a Vintage Simpson 461-2 DMM |
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| xrunner:
Yet another project - the repair of this Simpson meter. This unit has red LEDs and is capable of using rechargeable NiCads. The unit came with the original batteries but they are no good. The unit also does not work. When power is applied it draws way too much current (as much as it wants depending on the voltage applied) and the LEDs do not light. Also emitted is the magic smoke - well at least the smell of it, I didn't observe any of the original smoke coming out. Upon inspection I believe the Intersil IC7107 3 1/2 Digit LED Display A/D Converter is bad. I can see a discolored area right in the center of the package and it reeks of the magic smoke smell. I do not observe any other component that looks overheated, and the IC package that supplies/converts additional voltages to the board from the input +5V is working as expected. I went ahead and pulled that guy, and when power is re-applied, the system seems normal and only a few mA is pulled from the supply. The 40 pin IC is now on order, and we'll see what happens next week ... |
| SeanB:
Check the supply rails are correct, I cooked one when the -5V rail did not come up correctly and the chip latched up and cooked. Check that ICL7660 and the tantalum capacitors attached to it that it can deliver -5v and can deliver at least 20mA into a resistive load without dropping below -4V, use a 270R resistor as a test load and check across it for excessive ripple. |
| Smith:
These icl series chip fail quite frequently. Most just show strange values (or strange digits) or show non linear behaviour. Havent seen any draw too much current though. A lot of (older) 3,5 - 4,5 digit lcd voltmeter modules use these chips too. Thankfully you can still get new chips after 15-20 years. |
| xrunner:
--- Quote from: SeanB on February 13, 2015, 06:59:09 pm ---Check the supply rails are correct, I cooked one when the -5V rail did not come up correctly and the chip latched up and cooked. Check that ICL7660 and the tantalum capacitors attached to it that it can deliver -5v and can deliver at least 20mA into a resistive load without dropping below -4V, use a 270R resistor as a test load and check across it for excessive ripple. --- End quote --- Roger - thanks. --- Quote from: Smith on February 13, 2015, 06:59:40 pm ---These icl series chip fail quite frequently. Most just show strange values (or strange digits) or show non linear behaviour. Havent seen any draw too much current though. A lot of (older) 3,5 - 4,5 digit lcd voltmeter modules use these chips too. Thankfully you can still get new chips after 15-20 years. --- End quote --- Yea it's great it's available - getting it from Mouser. I found another problem. Decided to check all the segments of all the display LEDs using a 5V supply and small resistor. Out of all the 7-segment display parts one and only one has a segment that does not light up (the bottom segment). I removed it from the board and it is definitely not working. It would be expected to display a Zero at times so it needs to work. Why just one - who knows, but I can the exact same part off Ebay. Ever watch the Movie "The Money Pit"? :palm: Reference part - |
| xrunner:
Few more pics while I'm waiting for the parts. There is a regulator unit on the board that accepts +5 and supplies +12 and -8V. It's the Astec AA4750. What I think happened to this meter was too much voltage was applied to the battery or charger input. The devices that run off of the +5 may have been damaged (such as the ICL7107 I ordered), but I have yet to find any other defective parts other than one of the LEDs. Other parts of the circuit that use the +12 and -8 were saved because the converter keeps supplying the proper 12v and 8V even if the supply is increased too much. These little wire thingies provide a connection to the LED display when the top board is attached to the bottom. The other connections are made with a proper connector near the back. This seems a little dodgy to me. Here is the LED display board with the defective LED removed. Not ordering it yet until I see if this unit is going to be able to be fixed. |
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