• Help with calibration of a digital lab PSU

    07/27/2017 at 00:23 14 comments

    I just found out that my new shiny cheap lab PSU is 5% to 10% off !

    It has a nice resolution but the readings in both V and A don't match my measurements with an external multimeter...

    The model AX6002P from Axiomet is similar to the better known Velleman or KORAD KA6002P so it might be a known issue....

    Update 20170730:

    My "golden tester"'s battery is a bit low (8.5V) so I changed with a fresh one (curious since I changed it less than a year ago).

    I tested the measurements against other multimeters, checked the power supplies and ... all values are coherent within the respective accuracies.

    Bad battery. Case closed. Sorry for the alarm. But the question is still good and it would be cool to be able to calibrate the device "if something happened" :-)

  • Another eBay horror story

    10/28/2016 at 15:20 0 comments

    I have more than 400 stars and have purchased hundreds and hundreds of items on eBay.

    I have had some occasional troubles and lately, more than before.

    But even though the last story is worth a laugh (at my expense but only some tens of USD and at least I have very nice example of Chinese counterfeit gone wild) today's story is more of the "grieving" kind.

    As some of you know, I've been bitten by the speed demon again. I want to play with discrete ECL gates and I need the fastest scope I can reasonably get (as if I needed an excuse to get the best gear for my lab). No DSO for me, because I don't trust them enough (at my price point at least), and it seems the fastest analog scope can reach 500MHz for some theoretical reasons (damned RC constants !). But even if I can't fully exploit the 500MHz "bandwidth" of the AF240, there can be all sorts of "very fast" circuits to design in the foreseeable future. For example, I have 250MHz SSRAMs and the lousy FPGA I use are rated up to 350MHz.

    Recently I found an amazing deal: a Tektronix 2465 with original probes, anno 1995 is perfect condition, for a very affordable price and the shipping+customs was nice too. Sorry, no name, it was the last one of a batch... Very fast shipping (5 days from order to delivery), well packaged (sheets over sheets of bubble wrap), works great and will do for the coming 20 years (if I can hack the backup battery). Price: 150 USD, shipping: 42.64 USD, customs: 38.49USD. Total : 231.13 USD with expedited "international shipping program". A steal. Tek-love at first sight.

    But 4×300MHz doesn't cut it and I have no idea what signals I'll have to treat in the coming decade or two. I need an analog speed demon. I looked around and the suspect is the Tektronix 11302, a modular "boat anchor" capable of 500MHz with bells, whistles and even a drum. The last of a long lineage of Tektro tradition. I found the service manual, looked at forums to get an idea and browsed eBay.

    I found one in Italy (close to my French home) for about $300 and $60 shipping. Nice but only one 11A32 (2×400MHz) module. That doesn't scratch the surface. Looking at spare modules, I see that the price will climb quicky if I want to populate the other 2 free slots (one slot cover is missing, which is "no-no"-ed in the service manual, but that's a detail).

    Then I found a well-equiped 11302 (not 11302A) with 2×11A32 (same 2×400MHz as above) and one 11A71 (1GHz@50 Ohms, yipeeee) in the USofA (but no probe). Seller writes: "Garanteed" and "use ebay's global shipping program". So I "bid in confidence". And win !

    I'm elated by the 2465 and happy that it has a big brother, gotten for barely more: around 180$ only ! The seller has thousands of stars, the unit works, so what could go wrong ?

    Let's start with the time taken to decide about shipment, which added a bit of suspense.

    Some pictures:

    I find a peace of plastic while I remove the flakes. What can it be ? Oh, a foot...

    The unit still in the box, most of the flakes have been removed: there is only one layer of thin bubble wrap in the frond and in the back. One more layer around.

    Surprise: another foot is missing !

    And the case shows a bump

    The unit sitting on it side. The screen is "falling" inside its frame. Not good!!!

    The broken screen

    The tube is not repairable. Changing it requires a complete recalibration, if I ever find a spare part, which, for price, the would come as another complete unit...

    The scope was obviously improperly packaged.

    The cardboard box sustained most of the damage but was slightly deformed. Sorry I didn't think I should take a picture because I thought it was well protected inside.

    The big mistake was with the filling. The unit should have been copiously wrapped in a lot of thick bubble wrap sheets. The previous scope (2465) came with bubble wrap only. Not only did protected the scope, but also created enough "pressure" inside the box to keep it rigid.

    On the contrary, the flakes behaved like a "fluid" and moved around the...

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  • Oh the difficulty of sourcing P-MOSFET in TO-92

    08/29/2016 at 02:42 4 comments

    If you follow Yet Another (Discrete) Clock, you know this project uses quite a lot of MOSFET in the easy-to-solder TO-92 package. A bit less than 500, including about 300 N-FET (BS170 or 2N7000) and the rest are P-FET (BS250).

    The N-FET are OK: I could source 300×2N7000 for around 10$, 3 cents per piece is a nice deal and a decent price for the project.

    The P-FET are another story :I could find cheap reels in SOT23 but the prices are much higher for the TO-92 package and there seems to be a sort of shortage in the usual cheap markets. Earlier this year, an order never made it to my home (among other postal problems) and a 200pc lot was found to have a reverse pinout (https://hackaday.io/project/9376-yet-another-discrete-clock/log/37927-it-was-too-easy).

    This post is about another escalation in the BS250 horror story. I found a decently priced lot of 100 on eBay (due to Brexit) and this time ordered only one lot (I was not feeling rich, and at 20GBP, or about 24€ for 100, the unit price is almost 10× higher than the N-channel). Since the precedent lot (2×100) was functional (despite the reversed pinout) and was delivered, I bought from the same seller. Delivery was pretty fast (maybe due to summer vacations and less business) so I was ready to leave another good feedback.

    Just to be sure, I tested a few parts from the bag. I don't remember why but you never know, and I wondered what was the pinout this time. Guess what ? I found a MOSET but in N-channel, not P. WTF ? It's marked "BS250" !

    I take another then another and start to find a "mixed bag"

    • Some parts are indeed P-channel but a minority
    • Most are N-channel
    • Some weird arrangements, like dual diodes, or very malformed junctions
    • The surface of several packages look altered, like attacked by acid, and a couple have no marking. Others have a different style of marking...

    After I sent a complaint to the seller, I received the following:

    « Hi,client.So sorry for your trouble.Could you let me know how many pieces are not working as excepted,I will check with our supplier for this issue.Regards, Namedeleted »

    Wait, you send me worse-than-counterfeit parts then you want me to do your job, for which I paid you ?

    Alright, since you wanted it, let's waste 2 hours testing all the parts by hand.

    I could identify:

    • 25×P-channel (more or less BS250, I only tested the simplest parameters)
    • 61×N-channel (some sort of 2N7000 but I didn't care about the pinout)
    • The rest are malformed, non-functional parts.

    Only 1/4 of the order can qualify as conform.

    Ah, China...

    Pictures to come...