Monthly Archives: December 2020

Miscellaneous

Christmas Time: Honigkuchen (honey based cookies)

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It is a long time favorite, and easily stores for some months – honey based cookies. Here is a good recipe.

500 g Honey
125 g Sugar
150 g Water (you may just add some more Honey if you don’t like to add refined sugar, but it makes the dough easier workable)
1 kg wheat flour (can be some coarser type wheat flour)
60-80 g Spice including anise, clove etc., a ready purchased mixture. Adjust quantity to strength of spice and your taste
25 g Ammoniumbicarbonate (“Hirschhornsalz”)
Pottash can also be added, by I didn’t add it, it will make the cookies flat.

First, mix and melt honey, water, sugar, at low heat. Add the other ingredients and work firmly. Let it rest for 2 or 3 days in the fridge (cover to avoid drying out).

Then, prepare cookies, and bake at about 200 degC for 8-10 minutes. These need to be well baked. Underside can be dark brown, but don’t burn them to bitterness.

The Honigkuchen can be stored in a well-closed container for 2 or 3 months no problem.

Various

HP 4140B pA Meter/DC Voltage Source: some incorrect assumptions, but finally, a repair

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Shortly prior to my departure from Japan I started repair of a HP 4140B. A very desirable instrument for semiconductor characterization. The issue remained that output “A” didn’t provide correct voltages, probably due to some issues in the amplifier.
Two month later, I have returned to Germany, and a spare LF256 J-FET opamp had arrived, so I thought it would be a quick fix – but to no avail.

Fitting the LF256 to the board – I usually leave some part of the old wire in the Teflon isolator, because I want to avoid soldering/melting the solder in the Teflon part – it is all difficult to clean up afterwards, so I prefer to solder the new part to some leftover wire, rather than contaminating the isolator.

But- with the new opamp installed, same symptom, no proper output, current limit function of channel A blinking, but the input of the opamp is good. So it must be something else down the chain. Another look at the schematic…

There is an analog switch, followed by a discrete linear amplifier with a dual J-FET input stage.
After some study of the analog switch (cutting a trace and checking it), the switch appears fine. Next in line, the dual J-FET, and in fact, this is dead – found it by measuring the E-B and C-B transition voltages with a diode tester (instrument powdered down and board removed!!), and the transistor around the FET shows largely different values compared to the working B channel. It is 1855-0049 HP part, available in some single piece quantities but expensive!

Looks still shiny and new, but it isn’t working.

Studying some NSN databases, found at least some data of this part, which had been manufactured in equivalent versions by some other manufacturers as well, probably in the 1970s.

It is a rather not so special depletion n-type J-FET. But it is a dual FET part, and while single J-FETs are no problem to get, dual FETs are rare specimens.

Even in their long past days, these didn’t come cheap… maybe something like 40 EUR a piece in today’s money.

So we need to do further study, and there are essentially two kinds of dual FETs – some that have a specially made dual die, with both FETs on one chip and coupled in various ways to keep them from drifting apart with temperature, etc., and the other kind, which is merely just two reasonably matched separate FETs in one case, for convenience more than anything (and for thermal match).

Screening through my inventory I found these 2N5457 FETs which have pretty similar electrical characteristics, in particular, zero-gate-voltage currents.

The parts I have are all quite uniform so there is no need to select a special pair.

With such replacement with similar parts, rather than identical parts, I think it is a good idea to take no risk, so I took the B-channel dual FET and transplanted it to the A channel. And the B channel, which is anyway only a secondary function of the instrument and doesn’t allow the same fast ramps and functions like the A-Channel, it will be definitely good enough to install the two FETs separately (closely together), rather than the original part.

The dual FET of the B channel replaced by two 2N5457.

The B channel dual FET 1855-0049 transplanted to the A channel.

With these repairs, the instrument powdered on just fine, and the output voltages were spot on without any need for alignment. Even the zero bias setting if the LF256, no need to adjust.

Various

Induction range repair – just a couple of IGBTs, and a 20 Amp fuse

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Recently, the induction cook-top of my SMEG range failed, leaving me with potentially expensive repair options quoted at above 1000 EUR, or to do some investigations myself. Surely, the latter option applies in my case. So, after receiving two spare IGBTs by mail, and a high current fuse that matches the “repair option” fuse holder of the cook-top, it took just a bit of soldering to get the thing up and running again.

Mounted the IGBTs, an easy job compared to the tedious mounting of all the coils and cables. It is not quite a service friendly design, and there are many sharp edges that can damage cables and your skin, so better wear gloves and handle everything with care.

After cooking on the range for a while, there is absolutely no problem at all, it’s a 20 dollar fix, if you don’t count your own time – maybe about two or three hours, mostly, to take out the electronics and put them back in.

Miscellaneous

A major move, and a defective CATV splitter

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After returning from Japan to Germany early October, I decided to move to a new place, a larger property with ample workshop space and gardens.

Needless to say, a major effort with tons of heavy test equipment and other stuff piled up…

Apart from the heavy lifting, apart from tap water, electricity and a tight roof, a fast internet connection is the most essential thing. This took quite some more effort than expected. First, the grounding system of the house needed an upgrade (otherwise the internet cable company would refuse to install their amplifiers and cable modem), and, that done, there was not enough signal going in and out the house to allow for the 1 Gb/s connection…. first, they changed all the stuff inside of the house, a full upgrade, but to no avail, finally, they marked a spot on the sidewalk.

Only hours later, strong men showed up, digging up the ground in the search for the internet…

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The connection here is delivered on CATV coaxial line for the last mile, so there is main line about 50 cm underneath the sidewalk, and there are taps/splitters, usually one splitter serving two houses.

From what I can tell, it is a 2x 15 dB tap, a quite common part in the CATV network. It took the crew just about half an hour to install. Great job.

The part, here is a clean shot, it has the connectors already mounted, and in the field, the cables are merely inserted and heatshrink tubing applied.

There are now a few interesting left-behinds, an old CATV house connection box with filters, I will test it out once there is more time.

After the installation job had been done one the road, it took just another two visits of the cable company to get all up and running! At least, they managed to do all this in less than a week.