21 February 2018

Old vs modern high voltage capacitors

Side by side comparison of the size of 500+ V capacitors, same values. 1955 vs 2018!

Well, the dielectric is different!

10 February 2018

First MW reception on the RD-160

So, with the lamp limiter just built I could dare to power the Radiomarelli RD-160 medium wave receiver.

I scrounged home for filament light bulbs and I located 40W, 60W and 100W.

I stared with 40W, that lit up and nothing came out of the receiver. Maybe I wasn't patient enough, or its rating is just too low.

One step further to 60W and ... after the first bright flash, the lamp dim dim dim .... until some noise comes out of the receiver! The residual light is very warm and pleasant too! It works!! It receives RAI1 broadcast on 999 kHz and nothing else.

Problems noted:
  1. When the tuning capacitor blades go towards full mesh the local oscillator stops: there's a short.
  2. The volume control crackles
  3. Knobs don't hold tight on the shaft because their plastic is cracked near the screw
Not bad at all! It is electronically fine! Now I will replace capacitors.

06 February 2018

My poor man's AC limiter

Reading around the Net, everyone mentions that powering old valve circuits under restoration should be done gradually by applying a lower voltage and observing what happens. That's until all capacitors are confirmed to be working (or replaced) and transformers are not shorted. This is accomplished with a tapped or variable autotransformer, that I don't have.

One solution would be to wire back-to-back two step-down transformers, like 220-12 and 220-24 that should provide 110V if connected 220-12-24-220. Of course, I don't have such a selection of old-school transformers.

The "poor man's AC voltage transformer" trick comes to the rescue: connect a filament light bulb in series with the D.U.T. and observe what goes on. It will not allow to control the actual voltage, nor to change it on the fly, but it will limit the current through D.U.T. thus reduce the risk of smoke/fire/damage. Lower wattage bulbs provide lower voltage to the D.U.T. and let less current pass through, therefore less damage is done in case of troubles.

As I understand it, the principle is that a fault in the D.U.T causing excess current draw will be tamed by the light bulb which in turn will light up and provide visual feedback of a problem. Begin with lower wattage and replace with higher, step by step, as the home stock of old filament light bulb allows.

In order to build a "safe" transformer device I decided to modify a power strip: it has a power cord, outlets and an embedded switch. The mod is to wire in series two outlets, the first being used to plug the light bulb (installed in an existing lamp, for example), the second for the D.U.T. A third socket, left in parallel with the second, can be used to measure AC voltage across D.U.T.
The light bulb limiter, finished.

Things to do and remember:
  • AC mains is lethal;
  • add visual signs to the power strip that it is NOT a power strip anymore;
  • mark sockets with their intended use;
  • filament light bulbs get hot, so do not hurry to change it for the next!
If all goes well, the first troubleshooting session will not trip my home's AC protection.

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 ...