Author Topic: IET resistance capacitance box too good to be true  (Read 2977 times)

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Offline jwm_

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IET resistance capacitance box too good to be true
« on: June 10, 2017, 03:07:08 am »
So, I thought I stumbled on a deal, an IET resistance/capacitance box for $100. normally each of those boxes cost more than that independently. I recieved it, looked fine but wasn't working. I opened it up, and oh my. One of the capacitors exploded with such force it left a melted crater in the back panel and a bunch of resistors and one of the wires are fried. There is no smell, so whatever happened happened a while ago. I just hope the dials are not damaged. The item was sold as 'used' on ebay not 'as is', do you think it is reasonable to ask for a partial refund in this condition?

Album with photos of damage
https://goo.gl/photos/35rsbAaSma3P17LaA

Offline boggis the cat

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Re: IET resistance capacitance box too good to be true
« Reply #1 on: June 10, 2017, 08:55:25 am »
"Used" just means "not new".  If the seller didn't mention that it was damaged, or was selling 'as is' (which typically means "very damaged indeed"), then I think you have a right to complain.
 

Offline SeanB

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Re: IET resistance capacitance box too good to be true
« Reply #2 on: June 10, 2017, 08:59:00 am »
Looks like it was connected across a high voltage supply with that damage, or the mains. Probably you will lose one decade if the switch is damaged, but might be repairable. Definitely not used, but for parts.
 

Offline TimFox

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Re: IET resistance capacitance box too good to be true
« Reply #3 on: June 10, 2017, 03:21:04 pm »
The blown part is a tantalum capacitor.  Maybe it had been connected with reverse DC polarity, where it will conduct and overheat, causing it to burst.  How does the switch look at that location?  The capacitor itself is easily replaced.
 

Offline MadTux

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Re: IET resistance capacitance box too good to be true
« Reply #4 on: June 10, 2017, 07:41:26 pm »
Quite a stupid idea to use tantalum caps in such a place. Don't know whether high capacity foil caps were available back then, but nowadays, I would exchange all these tantalum caps by polyester/MKS foil capacitors. No more problems with polarity/ripple/spikes.
 

Offline rdl

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Re: IET resistance capacitance box too good to be true
« Reply #5 on: June 10, 2017, 07:58:43 pm »
Quite a stupid idea to use tantalum caps in such a place. Don't know whether high capacity foil caps were available back then, but nowadays, I would exchange all these tantalum caps by polyester/MKS foil capacitors. No more problems with polarity/ripple/spikes.

Probably they were used because of size constraints and also cost to a lesser degree. Wima MKS2 10uf is 16mm*11mm*7.5mm
 

Offline HighVoltage

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Re: IET resistance capacitance box too good to be true
« Reply #6 on: June 10, 2017, 11:14:44 pm »
I would first check the switches and make sure they are all ok.
If they are ok, a repair should be possible
If they are not ok, return it to the seller for a full refund.

"used" means on ebay it is in good working condition.
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Offline jwm_

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Re: IET resistance capacitance box too good to be true
« Reply #7 on: June 11, 2017, 12:38:37 am »
Okay, good news, it appears that the switches are doing something. In that they changed the value when moved that seems to be consistent with  shorted resistors (I thought they usually failed open, strange) and missing capacitors.  There are three wonky decades one capacitor one and two resistor ones.

Random observations:
- there are three different failures here, the blown tantalum most likely due to reverse voltage, 3 blown resistors across two decades, and a blown wire heading to the capacitors. Honestly, i'm not sure how that last one is possible, how can you push enough current through the capacitor range to blow a wire like that?

- This thing is built to extremely good tolerances, it practically seemed glued together into a solid unit until I figured out how to jimmy it apart. You can see that where the resistors blew despite being at ground zero, none of the particulates got through to the contacts despite it just being press fit. So, extremely good molds to have things press fit together like that and form a tight seal. The quality of the switches blows those ebay ones out of the water. All contacts are shiny gold, likely ENIG finish?

- The capacitor switches are of the 1 2 4 2 arrangement rather than the 1 2 4 8 you would get if you repurposed BCD switches. so these are custom made for this purpose. Resistors are all 5 resistors per decade.

- They went through the effort to use a different type of plastic for the bendy retainer clips rather than mold them directly into the switches themselves which are of a much harder plastic. It is clearly meant to be able to be servicable. You can assemble and disassemble it many times without breaking the clips due to the two part construction. It would have been so easy to just mold clips into the sides of the switches and make it so you have to pry it out and damage it if you wanted to dissasemble it but they went through effort and expense to make it take apartable.

- the switches are connected to their neighbors by 4 press-fit thick stakes. nice and solid yet wedgeable apart non destructively with a fine screwdriver.

- every time I see that little crater in the back of the case by the piece of tantalum, I can't help but imagine what it would have been like had that been my eye. only the 10uF range is polarized, the rest are film.

- the spec sheet is printed on a mirror sticker for some reason. fancy I guess.

It's pretty clear that when you buy one of these boxes you are paying for the switches. They exude quality.

I decided to go ahead with the repair even though I have not heard anything from the ebay seller since sending them the pictures. For $100 still seems like a deal right? I don't have test equipment fever blinding me do I?

What's my best bet for cleaning. 90% Isopropynol is my goto, anything look like it will be hurt by that in there.
« Last Edit: June 11, 2017, 12:46:12 am by jwm_ »
 

Offline rdl

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Re: IET resistance capacitance box too good to be true
« Reply #8 on: June 11, 2017, 01:30:42 am »
These are nice devices. I have the capacitor only version and I use it almost every time I sit down at the bench to do something. I've been looking for a resistor version. I like how you can read the exact value selected directly off the front panel.

I think if you can get some discount (partial refund) from the seller to cover the repair then you got a good deal. If you bought this through eBay and it was not listed as damaged, then you can request a refund ("not as described"). Since you have already notified the seller of the problem, wait and see what they offer for your troubles before escalating, but don't wait too long.
 

Offline SkyMaster

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Re: IET resistance capacitance box too good to be true
« Reply #9 on: June 11, 2017, 02:28:11 am »
...
 The item was sold as 'used' on ebay not 'as is', do you think it is reasonable to ask for a partial refund in this condition?


http://pages.ebay.com/help/policies/money-back-guarantee.html#description

When an item doesn't match the listing description

If a buyer receives an item that doesn't match the listing description, the buyer needs to request a return from My eBay. The seller should address the buyer's concern and offer a solution, such as accepting a return, offering a replacement or refund.

If the buyer doesn't receive a response or solution, or returns the item but doesn't receive a refund or replacement from the seller, the buyer can ask us to step in and help.

If asked to step in and help, we review the item description, photos of the item, if provided by the buyer or seller, and any other information about the item that the buyer and seller provide. If we can't determine that the item matches the listing description, if the seller has already offered a return, or the seller's stated return window and policy applies, we may ask the buyer to return the item to the seller.



You could asked yourself how much you would have paid for this IET resistance/capacitance box if you had seen the inside before buying it.

 :)
 

Offline charliedelta

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Re: IET resistance capacitance box too good to be true
« Reply #10 on: June 11, 2017, 06:20:11 am »
The usual dishonesty  of test equipment sellers on Ebay.

A good first clue  is when the seller says " I dont know anything about this item, sold as is" Then you look at their other listing and they are selling tons of other expensive test equipment with expert advice on how good it is! They are pretty dumb trying to pull this off claiming not to know anything about test equipment then you find that they are a big time dealer. This is a very common way of ripping off potential buyers.

These days Ebay is one of my least favorite places to buy test equipment and It has become hugely expensive.
« Last Edit: June 11, 2017, 06:22:37 am by charliedelta »
 

Offline jwm_

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Re: IET resistance capacitance box too good to be true
« Reply #11 on: June 11, 2017, 07:06:35 am »
So the seller got back to me. They were really apologetic and said it was their partners and they found it in storage and had no idea it wasn't working. They offered to refund me the whole cost plus shipping or I could ask for a fair partial refund and they would accept my judgement on what that would be.  I asked for $40 and they said cool and are refunding it. I probably could have asked for whatever, they mainly seemed grateful I didn't want to send it back so they would have to relist it.

It is working now, with crappy junk drawer components while actual precision ones are ordered from mouser.

Only sticking point was one of the resistors blew off it's trace. or more correctly, it loosened the trace enough such that i was able to rip it off with the toothbrush i was using to clean it by accident. Luckily only the exposed part was ripped off and not any of the contact part inside the switch. just barely enough was protruding for me to solder a bus wire to it. (That JBC I got on ebay that unexpectedly had a T210 handle came in handy here for the tiny soldering. Two eBay incorrect listings bought on the same day cancelling out!)

Also, oddly enough, isopropynol discolored the black plastic of the switches where it touched them, so if you want to clean yours. keep it in mind.

The resistor arrangement is 1-2-2-2-2 and all decades are identical except the values. A 20 ohm resistor was blown open and a 200 ohm resistor was blown down to 10 ohms. Oddly enough, another 20 ohm resistor completly blew away its casing so it was bare metal foil and the circuit board was charred underneath, yet was still in spec at 20 ohms.

I put a not too accurate electrolytic in because that's all i could find. Are there any good < 5% 10uF film caps that can fit in the cramped space? It would be nice to replace them all and remove the polarity restriction on the 10uF scale. I can fit one big one in to replace that single one, but if i want to replace em all I'm going to need something better. Right now it is 9 tantalums, but i could make do with a 10uF, 2x20uF, and a 40uF if i can get some that fit. Will dig around on digikey more.

    John

Offline HighVoltage

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Re: IET resistance capacitance box too good to be true
« Reply #12 on: June 11, 2017, 08:27:07 am »

I put a not too accurate electrolytic in because that's all i could find. Are there any good < 5% 10uF film caps that can fit in the cramped space? It would be nice to replace them all and remove the polarity restriction on the 10uF scale. I can fit one big one in to replace that single one, but if i want to replace em all I'm going to need something better. Right now it is 9 tantalums, but i could make do with a 10uF, 2x20uF, and a 40uF if i can get some that fit. Will dig around on digikey more.

    John
I have used 5% WIMA capacitors before for a DIY capacitance box and sorted many of them to get a value as close to my target as possible.
If you buy 10 or 20 of each value, you get usually one that is spot on or close to at least.
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