Repair of a Vintage Simpson 461-2 DMM

[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]

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.

Roger - thanks.

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.

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"? 

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.

[xrunner]

Oops - another casualty found. MPS3702 on LED board bad. Subbed with a 2N2907 PNP on hand. Again consistent with a possible high voltage on the power supply input feeding to certain components (including this one), which didn't affect the +12V and -8V supplies.

The pile of dead bugs grows.

[JohnnyBerg]

Is it an idea to replace all of the 7 segment units?
The new ones give a lot more light and as they are multiplexed you end up with one bright 7 segment, and 3 lesser?

[xrunner]

Is it an idea to replace all of the 7 segment units?
The new ones give a lot more light and as they are multiplexed you end up with one bright 7 segment, and 3 lesser?

Yea I understand your concern. I found a supplier on Ebay that has the exact same part - looks identical. Probably from old stock. I plan on buying one next week if the repair looks promising after the main IC is replaced.

[xrunner]

Decided to socket the ADC chip, to enable easier troubleshooting/replacement. Hopefully the chip will show up in today's mail. The postperson is way late - slammed because of the holiday yesterday. 

[xrunner]

Good news - the mail arrived.

I got the chip and inserted into the socket. I then powered on and ...

It works! 

I did find out I'll need to buy two new LEDs, but that's no big deal. It even shows the correct voltage. It should be reading -12.2 V and you can see the LED is missing two segments. Had to bypass the anode voltage on the missing one with a jumper. This is really looking promising now. I think from what I see all I'll have to do is cal it and then do a cosmetic cleanup.

[xrunner]

Waiting on parts again, I thought I'd try to clean up the case. I ran into a problem. It looks like it has specks of paint on it. I've tried all these cleaners -

Alcohol
Simple Green
Paint thinner
Soap and water
Orange degreaser

I can't get rid of the specks.

Any ideas?

[lowimpedance]

Must be some difficult 'Sh!t' to remove if none of what you tried made any impression at all!!.
what about ;
WD40, kerosine, mineral turps. Best to try a small spot first if the case is plastic.
Cant say if you will have any luck with those either, so maybe just live with the specks.
You should be looking at the display not the case anyway  .
If you do get it off post back the magic liquid that did it.

Seems you have a bit of a thing for old multimeters !.
You know the HP3435a had a touch hold probe assembly similar to the Fluke 80T-H which  is used with the 8010, 12 and 50 series.
The 34112a, (the small hole between the V/ohm and COM terminals is for this probe!).
If you want to add to your collection of oldies see if you can get a HP3476a/b, (that's one odd looking beast!).

[robrenz]

Waiting on parts again, I thought I'd try to clean up the case. I ran into a problem. It looks like it has specks of paint on it. I've tried all these cleaners -

Alcohol
Simple Green
Paint thinner
Soap and water
Orange degreaser

I can't get rid of the specks.
Any ideas?

Use a magic sponge with any of the cleaners above.  It may take quite a bit of rubbing and I would be very careful on the printed info as it may take the print off also.  You may also see if the individual specks will pop off with a soft plastic scraper.

It looks a lot like house fly crap which is fairly hard to get off also.

[Excavatoree]

I've had good luck with "goop" brand hand cleaner, but sometimes it works very slowly.  Apply, let set, wait, rub, wait, apply more, wait, rinse off, evaluate, apply more, rub, wait, apply more, and on and on

[cosmicray]

I can't get rid of the specks.

Any ideas?

Can you scrape any of them with a fingernail ? Alternate idea would be an old toothbrush with a bit of toothpaste. Toothpaste, depending on the brand, is a mild abrasive. Might try the baking soda and peroxide kind.

[xrunner]

Must be some difficult 'Sh!t' to remove if none of what you tried made any impression at all!!.
what about ;
WD40, kerosine, mineral turps. Best to try a small spot first if the case is plastic.
Cant say if you will have any luck with those either, so maybe just live with the specks.
You should be looking at the display not the case anyway  .
If you do get it off post back the magic liquid that did it.

It's tough - part of the problem is that the surface is not all smooth. Most of it has a molded-in texture that is really making it hard to get to the "stuff". Most of the scrubbing is hitting the plastic while the stuff remains hiding in the crevices.

Seems you have a bit of a thing for old multimeters !.

Yea it's fun to restore old stuff. And - this stuff is good especially those HP meters. NOT boat anchors.

You know the HP3435a had a touch hold probe assembly similar to the Fluke 80T-H which  is used with the 8010, 12 and 50 series.
The 34112a, (the small hole between the V/ohm and COM terminals is for this probe!).
If you want to add to your collection of oldies see if you can get a HP3476a/b, (that's one odd looking beast!).

Ah those little holes! I was wondering about those. I thought some jerk had drilled a little hole in the front panel, until I saw the other HP meter I got off Ebay had the same little hole. Thanks for solving that mystery! 

Use a magic sponge with any of the cleaners above.  It may take quite a bit of rubbing and I would be very careful on the printed info as it may take the print off also.  You may also see if the individual specks will pop off with a soft plastic scraper. It looks a lot like house fly crap which is fairly hard to get off also.

I've heard of the magic sponge but never got one. Seems now is the time to try it out.

I've had good luck with "goop" brand hand cleaner, but sometimes it works very slowly.  Apply, let set, wait, rub, wait, apply more, wait, rinse off, evaluate, apply more, rub, wait, apply more, and on and on

Yea that's what I meant by orange degreaser - tried it and no effect.

Can you scrape any of them with a fingernail ?

Nope, wish I could. They are embedded in the texturing on the case plastic - it's not smooth. 

Alternate idea would be an old toothbrush with a bit of toothpaste. Toothpaste, depending on the brand, is a mild abrasive. Might try the baking soda and peroxide kind.

OK good idea. Will try that tomorrow.

[lowimpedance]

It looks a lot like house fly crap which is fairly hard to get off also.

 Now that would never have crossed my mind !.
 

[Excavatoree]

I've had good luck with "goop" brand hand cleaner, but sometimes it works very slowly.  Apply, let set, wait, rub, wait, apply more, wait, rinse off, evaluate, apply more, rub, wait, apply more, and on and on

Yea that's what I meant by orange degreaser - tried it and no effect.

Just for the record, and it may help someone else, the old fashioned, plain white stuff works better than the "orange" formula.   You'll probably find something that works better in this case, but the plain white goop works well for sharpie brand marker on Fluke yellow plastic.

[xrunner]

Well ... I managed to get most of it off. And what did I use?

...

Mother's Mag wheel polish. 

That was the only thing that would make a dent in it. I didn't get it all off but that's all I'm willing to do. So, the cabinet is now cleaned up - still waiting for LED parts.

[xrunner]

Still waiting on parts but I ran into another problem traced to dirty switch contacts. If you need to know how to remove the innards from these types of banked switches for cleaning the trick is shown below. 

[Excavatoree]

Thanks.  I've got one of those, and I may need to do that.

By the way, does yours take some strange battery?  I can't figure out what mine is suppose to use.  (So I use alligator clips and a power supply)

[retiredcaps]

If you need to know how to remove the innards from these types of banked switches for cleaning the trick is shown below. 

Good repair and restoration.  Thanks for the tip on these switches.  I will have to remember this thread for the future.

[xrunner]

By the way, does yours take some strange battery?  I can't figure out what mine is suppose to use.  (So I use alligator clips and a power supply)

It came with two NiCad packs, each of which which consisted of two smaller NiCads shrink-wrapped together. Of course they were no good. It will run off of 4 AA batteries but it's not made to hold them as such, unless you shrink-wrapped two together to make on long battery like the original had. I don't plan on ever doing that, just run it off the charger jack, it works just fine that way (assuming it's fixed like mine isn't yet  )

Good repair and restoration.  Thanks for the tip on these switches.  I will have to remember this thread for the future.

Thanks. I'm just going to pull out all the switch insides now, and clean them. Hopefully the LEDs will show up early this coming week.

[xrunner]

And now a plastic piece that holds a brass insert for the rear screw popped off. Glued back on with model cement. 

[xrunner]

The vintage LEDs arrived today. Meter is looking good! I think we're on the downhill side of this project now.