03 December 2021

Reverse engineering a PCB - visually

In order to increase my "stock" of MG-17G 7-segment gas-filled displays, I bought a clone of the K80W calculator. It looks like many of them were produced and rebranded. The one I got had two (glass) broken tubes and something wrong in the electronics as only the rightmost digit would lit. Last but not least the faux leather case smelled nasty, so this calculator was set for dismantling.

Both PCB sides at once.
 

While I am against tearing apart calculators like these, this one was beyond repair and I already have a K80W needing a couple of new tubes.

As a sign of respect to the designers of this circuit I have decided to reverse-engineer it. In order to limit the handling of the stinky board, I opted for a visual method.

I shot a picture of the front and the back of the board paying extreme care to keep the (smartphone) camera at the same distance. Then using a photo editor program (The GIMP) I flipped the back picture, melted them together making one slightly transparent and the result is shown above. It's like a color X-ray scan.

Now I can work my way through the PCB traces either on the screen or on a printout.

Obviously this method works best with single sided circuits.

 

29 November 2021

Dishwasher problem: solved

The fix to the dishwasher was as easy as buying a replacement cable guide.

I had to restore the broken white wire and add insulation to the other wire with visible copper. The cable guide slides underneath the body: just push it from outside, both sides already wrapped around the cables. Pay attention to position properly the nibble against the body so that the guide does not slide away when moving the lid.

The replacement part costs anywhere between 15 and 25€, depending on your source. I chose a real shop as we have quite a few in town. The human contact was invaluable: "yes, I have it, but I don't know where." According to the shop owner, it doesn't seem to be a common problem on those Whirlpool dishwashers. Or perhaps we have already used it past the design duration time, when a larger fault would force people to buy a new one rather than repair it.

Case closed.


23 November 2021

Dishwasher problem

English version is half-way down.

Domenica pomeriggio la lavastoviglie si è guastata. Si tratta di una Whirlpool WP75/1 da incasso, ma poco importa. Alla chiusura del portellone è scattato il salvavita anziché far partire la pompa di scarico.

In rete c'è il manuale di servizio, che mi ha guidato fino ai morsetti della pompa, che è OK. Lo stesso manuale indica come avviare la procedura di self-test, che ha diagnosticato: "problema allo scarico." Preziosissimo.

Quando ho avuto un momento di calma ho cercato qualche indizio sul più grande forum italiano (plcforum.it) soprattutto per capire come estrarre la scheda elettronica. Però in un post un utente diceva che il continuo movimento del portellone aveva spezzato i cavi di comando tra l'elettronica e gli attuatori.

Vuoi mica che ....

Vista del portellone da sotto.
Vista da sotto del portellone e fondo, dove passano i cavi.

ECCOLO! Devo dire che da qualche mese aprendo tutto il portellone si udiva un "clack!" proveniente dal basso, che non ho mai approfondito. Ora senza il pannello frontale è chiaro come la struttura metallica del portellone:

  1. fa scattare la plastica della (ex) canalina
  2. ha eroso la guaina protettiva dei cavi

 Se tutto va bene me la cavo sostituendo e proteggendo i cavi (sì, ma come?). Il divertimento continua.

 --------------- ENGLISH BELOW ---------------

On Sunday afternoon the dishwasher broke. It is a Whirlpool SP75/1, but that's not very relevant. When I closed the door, the circuit braker cut the electricity to the apartment, while it should have started the drain pump.

I found online the service manual that guided me to check the pump motor (OK) and start the self-test procedure. The result didn't surprise me: "draining failure."

Then I searched the largest Italian forum on these things how to get the electronics board out of its box. One user mentioned that the door movement had deteriorated and broken the control wiring between the board and motors/sensors.

Since the dishwasher is now more accessible from below I shot the picture seen above. GOTCHA!

I need to replace some wiring and protect them from further erosion. Need to find out how to do it. The fun continues.



 

11 November 2021

VHF elliptic filter retune with NanoVNA

Last month I bought a "surprise box" of surplus (radio)electronics. Well, the content was visually known and the box contained at least one wanted item. In between the heavy remaining stuff there were two VHF filters: one centered at 162 MHz and the second at 137 MHz.

At the time being I have no real uses for such a receive filter, but it is a very useful support to experiment with the NanoVNA, and use both of it ports. Educational!

After a little soldering and a bunch of coax adapters the setup looked like this:

Test setup and wiring.

I intended to retune the filter into the 2m HAM band. At the first glance the NanoVNA showed it is an elliptic bandpass filter. With a mix of awe and patience, adjusting the 4 screws following some fuzzy algorithm the bandbass centre moved down to 144 MHz:

Response at the desired frequency.

I am sure there must be a scientific way to tune these filters. I settled for: the two inner capacitors set the frequency, while the outer two match the impedance. With the obvious slight cross-interaction.

I stopped when I reached a 1.2:1 SWR at 144.300 MHz together with the least attenuation. The diagram matches the theory of elliptic filters. I am unsure of the best insertion loss of -4.06 dB (including the cable and coax adapters) as the label said -3 dB at 162 MHz. But I've got another one to retune and I will pay more attention.

By the way. These filters should be symmetrical, so the pattern should look the same if input and output are swapped.

View of 30 MHz span. A pretty elliptic response.

Which use for a receive VHF bandpass filter about 10 MHz wide in 2021? It should help to reduce out-of-band noise when looking for weak signals, but mind the 3-4 dB insertion loss. I will try it in front of an RTL-SDR and a wideband scanner to see/hear if it makes a difference.

 

 

06 November 2021

3R12 flashlight mania?

I have had a chance to visit the Zagreb Hrelic flea market once again in August 2021. The religious holiday, the Summer period and the forecasted high temperatures probably discouraged many sellers, with the result that many empty slots made it easier and faster to walk around.

I bought a working 3R12 metallic flashlight for 1,3€, battery included. It matches the one I bought in a previous visit since it carries the brand "CROATIA" and shares an uncommon colour tone (to be polite...).

Either I search for the wrong keywords, or these items are of no interest to collectors. Nevermind: I might be starting a collection of some sort myself. I do like the incandescent lamp glow and they can be modified to be run on Li-XX cells, let alone the obvious LED conversion.

So far I have two of them. Lacking proper model indications, I will assign my own names with reference to the place+year of purchase and sequence (in case I buy more in a single year).

Note: the white balance of the following pictures is correct, or the closest that shows their real colour in my lab room.

Left ZYWHR19.1. Right ZYWHR21.1. Front sides.


Left ZYWHR19.1. Right ZYWHR21.1. Back sides.

 ZYWHR19.1 couldn't be simpler as there is not even an on/off switch! I think it is meant to stay lit as long as the battery lasts. The Croatia brand logo is stamped on the back cover.

Inside of ZYWHR19.1 flashlight.

In contrast, ZYWHR21.1 has both a permanent on/off slider switch on the side and a N.O. button to emit flashes on top. The Croatia text brand is stamped on the front cover.

Inside of ZYWHR21.1 flashlight, with battery.

Both of them work.



02 November 2021

Surplus GPS "mouse" receiver and AVR input protection

Somewhere in the back of my mental to-do list I have a project with GPS, NMEA and precise clock (as in hh:mm:ss, not in the digital electronics sense). While I could use the ubiquitous Wi-Fi connection to query an NTP server, I prefer a solution that gets the timestamp without needing a user/password. As a bonus, it tells me the position as well.

A local HAM sells surplus GPS-only receivers (no GLONASS, no Galileo). They require 5V (later found that the range is 3.5V to 5.xV) and speak out NMEA at 4800 baud 8N1. The module inside is one of the GlobalSat series, probably BR-355S4. At least that model number provided a spot-on description of the pinout and existing wiring.

I wired it to a serial TTL/USB adapter and the dongle kept disconnecting from the computer. Both dongles I own. Weird. I needed to see what was going on, so I fired up the oscilloscope. Surprise! The NMEA data out of the surplus GPS dongle is RS-232, with -10/+10V! BTW, I couldn't get the data decoder of the Hantek scope to decode anything out of it.

In order to fix the signal levels I assembled a simple BJT adapter, which also inverts mark/space as in RS-232 logic "mark" (or logic "1") is the negative voltage and "space" (logic "0") is positive WRT ground. In TTL serial, "1" is 5V and "0" is 0V. This let me use the dongle and a GPS decoder software on the computer.

The adapter is 10k resistor into the base of any BJT (BC547, 2N2222, whatever). Emitter to ground. 4k7 from +5V to the collector. You can add a diode between ground and the base, with the cathode on the base side. The diode limits the Vbe reverse voltage to 0.7V. TTL output is at the collector.

The seller told me he could read the data with an Arduino just fine. How comes? Well, many Arduinos use an Atmel AVR ATmega328 processor, whose inputs are protected like this:

Atmel AVR input protection
From an Atmel datasheet, it's all inside the chip.

 

Those two diodes clip the -/+10V right into the AVR microprocessor and everything looks fine, but might break in the future.

Even if I will use the GPS mouse with an AVR micro, I will include the BC547 adapter.


31 October 2021

Topward 1302 multimeter fault?

The Topward 1302 multimeter performs as expected.

I left it plugged to AC outlet in OFF position for a few days after the last use. Then I had to move it away from the bench and I got a hot surprise: the transformer was hot enough I could not hold the finger on it.

WHAT? It was plugged but supposedly turned off, the display unlit too. Even if the ON/OFF switch would operate on the low-voltage circuits, why would an unloaded transformer get so hot?

Time to have a look at the actual circuit.

Good news. The PCB reports the product name as "TDM-104", which brings up a PDF manual from the Topward website. The schematic diagram confirms the transformer is always connected. Therefore also the diode bridge and capacitors are active when the AC cord is plugged in.

I probed voltages and they all seem fine. There is no ripple detectable with the multimeter.

Considering this product probably sat in the low-end side of the market, it was cheaper and safer to power off only the logic circuitry rather than the whole AC. Who cared about saving power in the 1980's?