15 January 2018

Time travel to 1956: Radiomarelli RD-160 Amico

I finally did it: I bought a valve AM radio.

Radiomarelli RD-160 "Amico".

It is a Radiomarelli RD-160 "Amico" built on or after 1956. It was sold as non-working for "just" 5 € after nobody bought it for months. No wonder... In the worst case, individual parts are worth more (if bought individually, for other projects, not on the current market).

The first, off-grid, check reveals that the case is clean whereas cracked (it's plastic, can be glued together). The circuit is relatively clean and not rusty, all valves are present. The audio transformer is a bit loose and has one suspicious floating wire. The volume knob turns indefinitely and doesn't click on/off. The tuning knob runs the indicator bar just fine. Looks like a good troubleshooting playground and, at worst, I will retrofit it with some sort of Internet-radio receiver (not my original idea).

13 January 2018

Can't get along with AMS1117 regulator!

There is something wrong between me and AMS1117 linear voltage regulators: I keep frying them! It first happened on two adapters for breadboard, but in that case I suspected I initiated some self-oscillation by using input leads of different length (comments on the same blog post explain other causes, worth reading!).

Then I blew it on a Arduino Nano clone, those that also mount the CH340 USB chip. Maybe I did something wrong, I don't remember. That circuit is running with a 7805 now, and gets pretty warm.

Last event was last night, when testing a PWM voltage booster with the same Arduino Nano clone board ("same batch"). I fed 12V the board and it all became quite warm. Since I wasn't getting the PWM output I resorted to powering it via USB, but this time I used an inline USB tester to see the current consumption: 150 mA. That sounded wrong for a board that was doing nothing. Once I sorted out the firmware to produce PWM, I reconnected everything on 12V and I didn't get any output, nor I could talk to the board over USB. Basically dead. So I phisically removed the AMS1117-5.0 with pliers (I began unsoldering, but it was taking too long). Result? The board still drains 110 mA and it is not recognised on USB. To the recycle bin.


Well, this time I will build my own 5V regulator with the trusty 7805.

16 December 2017

A missing trace in the Schlumberger 1240

Alright. I've gone back and forth the diagram, read the comprehensive manual, looked for possible leaking capacitors both on paper and on the circuit. Nothing.

Then I checked the "modern" 0.1 ohm resistor on the lower side of the PCB fitted in place of a broken trace. One side is grounded. So I followed ground traces to nearby components suspecting a ground loop but this is what I saw:

Almost 5 mm of missing (ground) trace! And it is on the biasing network around the dual-slope integrator. The result was an impedance mismatch causing stray currents and disturbing the integrator.

A piece of resistor leg was promptly used to rebuild the exposed trace and I anxiously powered up the multimeter (still without the AC converter board). Numbers were shown, didn't flicker or runaway but did not make sense. OPS! I forgot to press one of the "mode" buttons. So I got a stable, not null reading.

I shorted V/ohm input and turned the zero calibration pot as described in the manual. I had moved it around when troubleshooting. VoilĂ ! The Weston Schlumberger 1240 multimeter is with me again!

I have no idea how I managed to pull away that piece of track. Perhaps with time it has "glued" to a floating cable coming from the front panel, or .. well, it's fixed now!

02 December 2017

New instrument in the lab: Schlumberger 1240 multimeter

During one of my time travels in the 1970's I brought home a Nixie-based multimeter: Schlumberger 1240. Three and half digits in a compact desktop case. It is the same instrument of Weston 1240, and the Heathkit IM-102 shares a lot with them.

Schlumberger (Weston) 1240 multimeter from circa 1972.
It was given away as working except for the 200mA scale. Not a big deal if you want ot make a clock out of it, no? The first power up confirmed both its working state and my suspicion that the half digit neon bulb was broken. Just feed a variable voltage in the 20 V scale and let it go beyond 9.99 V. Time to open it up, without a manual/diagram/parts list to be found online.

Well, Weston made also model 1242, a 4.5 digits multimeter that is aestetically similar, and the manual is available online (not complete and some pages were poorly scanned). At least it shows how to extract the circuit board.

Top of the board with discrete logic.
Two notches later, I had the two-layer through-hole board on my desk. Meet another 1970's hand-drawn PCB, with charming curvy traces and no ground plane! There are three Burroughs B5855S Nixies.

The "half digit" was a 25 mm tall neon lamp with an illuminated bar of about 15 mm and long leads. Initially I suspected the driver transistor was gone but I begun removing the lamp first: only two solder points to redo in case it works rather than three short leads of the transistor. Well, one lamp leg broke in the process and I couldn't lit it with my high voltage DC source.

Bottom of the board with curvy traces!
Looks like it is not easy to source a neon bulb with this size in 2017, and temporarily a shorter one will do the job.

But something else happened ...

17 November 2017

Linear PSU failing with overvoltage

According to timestamps on components, I have owned this linear power supply for 30 years (rms K135). It powered my first CB station (Midland Alan 48) and who remembers how many other devices. Lately it has been powering a 12V LED strip in the shack-lab.

One of these days I wanted to take a picture of a multimeter (will be feature on the blog, don't worry) and I needed both something to measure and less light in the shack. So I unplugged the LED strip and wired the PSU to the multimeter.

To my great surprise I read 24V. What?! That would have burned the LED strip and everything else I had connected in the meantime. But, with the LED strip powered, it behaved as expected and outputted 12V or so. Of course I cross-checked the reading with several DVMs and they all agreeed.

A bad capacitor? I opened up the PSU and I was greeted by a large amount of dust. And two 78S12 in parallel, as I remembered. Hard to remove the dust, but once components were in the clear, I still had 24V out without a proper load.

Capacitors looked OK. No leaks, no deformation. Unwillingly I reached for the solder side of the PCB and disconnected one 78S12 at a time (just the output pin, lazy me). One regulated at 12.5V, the second at 24V. Well, it didn't regulate at all then!

I temporarily replaced the bad one with a 7812 and the output is still 12.8V.

So, watch out for this simple technology too! Until today I would test the Amps rating of a PSU. Now I will check the open circuit voltage!

07 November 2017

My first HF RTX, in 2017

Got home a bit early tonight from work and didn't want to stare at a screen, so I finally dug out my first HF transceiver for a quick check-up.

It is an Icom IC-728, 100W on HF bands, AM/CW/SSB, triple conversion receiver. I think I bought it in 1993 after the high-school graduation. The price was a bit less than 1'000'000 lire (that's about 820€ in 2017's value).

First I did a visual inspection of the circuits for leaking capacitors: no signs. Then I grabbed the DC power cord from the IC706 and powered it up. It was exactly as I remembered it when it was last used .... 20 years ago or so. Whew!

I reached for the coax that enters my little shack and inserted the hot pole into the antenna socket. At last HF noise filled my room!! Why just the hot end? Because there's no antenna on the other end! It's a coax left over from my balcony experiments 9-10 years ago (seek blog archives if you want). But it does provide few metres of pseudo-antenna. Just don't press the PTT.

I quickly tuned around the bands. I found CW stations on 40m (and I could even make sense out of it!), some AM broadcastings too.

I caught myself few times looking for a "MENU" button, but this radio has no menu: each key has one function (some have two). That's it. All functions are one press away.

Then I wanted to reduce the incoming noise from the open line antenna and ... no DSP, of course! All you get is a Noise Blanker and Pass-Band Tuning: neither of them is effective against modern QRM. Well, I "retuned" my ears and it was fine.

I love the smooth effect of the large tuning knob. I could keep tuning for hours.

I will do my best to put it on the air even if propagation doesn't seem to be helping.

(I didn't take a picture. I will next time I take it out of the box ... hopefully soon!)

29 October 2017

Failing old LCD displays

Following my recent interest in old numeric display technologies I came across old calculators, and started collecting them. In early 1970's there was a lot going on in the research of display technologies, so while Nixies were fading out, VFDs taking their place where there could be a lot of environmental light, LEDs and LCDs were entering the market.

Overheating causes early LED displays to fail faster, especially if run at full brightness.

Sharp EL-5103S
Early LCDs on the other hand lost the "vacuum" inside and became unusable. You can see the failure as a darkened area in the corners/border and there's nothing you can do.

But I wouldn't expect this kind of failure from my Sharp calculator made in late 1980's or the VHF transceiver of early 1990's! Unfortunately replacing an LCD is not easy and you can hardly fit a replacement in the same space. So these devices just turned into (my personal) museum pieces.

The upper area of the display shows signs of air leak.

Too bad for the scientific calculator that served me in high school and at the university. But it looks like a modern replacement costs less than 10€! And I will look for some old battery powered pocket calculator as well, possibly with LED display.