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lunedì 28 dicembre 2015

Pensiero di fine anno...

Lo affido alle parole del poeta. Con l'augurio che l'anno che sta per arrivare rechi con sé nuova vita e nuove speranze per ciascuno di noi.

E una donna che reggeva un bambino al seno disse: Parlaci dei figli.
Ed egli disse:
I vostri figli non sono i vostri figli.
Sono i figli e le figlie della brama che la Vita ha di sé.
Essi non provengono da voi, ma per tramite vostro,
E benché stiano con voi non vi appartengono.
Potete dar loro il vostro amore ma non i vostri pensieri,
Perché essi hanno i propri pensieri.
Potete alloggiare i loro corpi ma non le loro anime,
Perché le loro anime abitano nella casa del domani, che voi non potete visitare, neppure in sogno.
Potete sforzarvi d'essere simili a loro, ma non cercate di renderli simili a voi.
Perché la vita non procede a ritroso e non perde tempo con il giorno già trascorso.
Voi siete gli archi dai quali i vostri figli sono lanciati come frecce viventi.
L'Arciere vede il bersaglio sul sentiero dell'infinito, e con la Sua forza vi tende affinché le Sue frecce vadano rapide e lontane.
Fatevi tendere con gioia dalla mano dell'Arciere;
Perché se Egli ama la freccia che vola, ama ugualmente l'arco che sta saldo.

[da Kahil Gibran, "Il Profeta", 1923]

domenica 13 settembre 2015

sabato 8 agosto 2015

Moving an old CB walkie-talkie to 29 MHz AM - Part 2

This is the second post of a short series about converting an old CB walkie-talkie for the 10 meter band (around 29 MHz AM). For those interested, previous post can be found at following link:

Moving an old CB walkie-talkie to 29 MHz AM - Part 1

In present post I will try to describe the simple sequence of operations I did to ensure that my old Inno-Hit RT923 handheld transceiver could work on 29 MHz properly.

First I prepared a small testbench with a 12V power supply, an oscilloscope, a frequency counter, a waveform generator. I built a simple 50 ohms dummy load by using two 100 ohms, 5 watts resistors connected in parallel.

By the current indicator of the power supply, I saw the current consumption was about 25 mA in rx mode, with no signal, raising to some 60 mA with a nicely loud signal at the loudspeaker.

The I started to tweak the tx path. First of all I checked the tx frequency by the frequency counter and I saw it was pretty close to the 29.020 MHz which is the nominal frequency of the tx crystal (the crystal in the local oscillator of the rx is 455 kHz lower). So I decided that there was no real need for adjusting the tx frequency.

Then I went to the oscilloscope and I had a look to the carrier waveform. It looked nicely clean and stable.

I applied a 1 kHz signal to the modulator and tried different resistor values in the ALC circuit (namely R29 and R48) to see if it was possible to raise the RF output power a bit. Shorting R29 proven to give a slight improvement, while changing R48 showed no significant effects, thus in the end I decided to leave it unchanged.

Then I moved to the adjustment of L4 ed L6 in the tx output filter (after the tx power stage). I carefully unscrewed the ferrite cores of both coils to get the maximum output level of the carrier on the oscilloscope, while monitoring the spectral contents of the output by a spectrum analyser.
Here below you can see the results of this step, before and after the operation.

As you can see, the output level of the unmodulated carrier raised from less than 14 dBm to more than 18dBm (on the 50 ohms dummy load). The level of harmonics did not get worst than it was initially.
Final check for the tx was to try to measure the actual modulated output power while whistling into the speaker-microphone. Here below you can see a screenshot taken from the oscilloscope during this test:

Apparently, it looks like there are no signs of overmodulation. Based on measured values (as reported at the bottom of picture above), the PEP power should be slightly above 2 watts, which is fully satisfactory to me.
The power consumption (on the power supply current indicator) was up to 230 mA at 12 VDC (about 2.76 watts). Final transistor T8 was normally hot at peak power, not enough to worry about its safety in normal conditions.

Now a few notes about the rx path: I applied a 29.020 MHz signal to the antenna, modulated by a sine wave at 1 kHz, 100% modulation and adjusted the coils and IF transformers along the rx path for the best AF signal level (measured with the oscilloscope, in the AF section of the receiver).
From the antenna towards the AF amplifier, I adjusted L2 (black core, antenna amplifier), L3 (white core, RF mixer) then T1, T2 and T3 (yellow, blue and pink, respectively 1st, 2nd and 3rd IF transformer).
All went pretty OK and now I'm ready to test my renewed talkie on the air. I'll let you know about my results. See you soon.

Moving an old CB walkie-talkie to 29 MHz AM - Part 1

Hello there, after quite a long pause I write again on this blog to describe my recent experience of moving an old CB walkie-talkie to 29 MHz AM in the 10 meter band.
I had found a nice pair of Inno-Hit RT923 talkies for 15 euros, while visiting a junk dealer shop near home, looking for something cheap and unusual for my personal collection of useless things. They looked in a very good condition and I though that probably there were still working too.
In the next few days, I sold the better-looking one on an on-line auction site for 10 euros and I decided to try moving the remaining talkie on 10 meters, 29 MHz AM. Here below a couple of photographs of the "star" of this post.

The Inno-Hit is a 3 channels, 2 watts, crystal-controlled handheld CB transceiver. The receiver is a single-conversion superetherodyne with 455 kHz IF. The tx frequency and the local oscillator frequency in the rx are controlled by two different crystals. The LO crystal is of course 455 kHz below the actual rx frequency.
The same rig was sold with several brands and some differencies (mainly cosmetic); for example Royce 1-402, Brilliant BT-1232, Finetone TNC-203, MacDonald CE-420, Universum BSG-3271 and maybe others. All those rigs came from the mid '70s and had a very robust steel case, with a small opening door on the front to access the battery pack. Some of them had external connectors for an external antenna, earphone, power supply or battery charger. Usually they were sold with only one channel actually equipped with crystals (for CB channel 11).

The astonishing successes of my good friend Franco I5KAP on 10 meters AM, with even very old and simple QRP rigs, inspired me.

A visit to Franco (who live 10 km or so from my QTH) convinced me to go ahead with my plan. Franco was very kind and helped me in deciding which modifications would have made sense to try.
First decision, was to leave the transceiver as much as possible in its original box and appearance. Given the very limited space inside the case, the only sensible modifications to do were to add external connectors for the antenna and power supply (or external battery) and of course to buy a new pair of crystals (for the rx and tx oscillators) for the 29 MHz AM band.
The crystals were ordered to a factory here in the north of Italy (unfortunately, they costed about 25 euros each, including taxes and shipment). Then I bought for 2 euros each some schematic diagrams of the walkie-talkies I mentioned above (in particular, those which had the antenna and power supply connectors) and I proceeded with my work.
I identified the better points in the circuit were to connect the external antenna and power supply, I prepared the small removable plastic panel that was inserted on the right side of the case (like in photographs below) and quickly checked that all was OK, both mechanically and electrically.

Now it was simply a matter of setting a small measurement bench and do some tweaking on the tx path and the rx path to ensure that the talkie could work properly at the new frequency. This will be the subject of next post. Thank you for reading.

sabato 11 aprile 2015

Panasonic RF-2200 test on MW

A video showing a quick scan of medium wave band in the evening with an old Panasonic RF-2200 receiver, before and after my restoration work on it.

Panasonic RF-2200 restoration adventures - Final

This is the last post (here the previous one) of a series about the restoration of my Panasonic RF-2200 vintage receiver. This is about the final cleanup of the radio. In one of posts I spoke about restoring proper behaviour of switches and potentiometers by using the renowned DeoxIT products from CAIG Labs. While these products proven to be very effective in my case, however I suspected I had used a bit too much of them for each device. So before re-mounting the case of the radio, I wanted to try to remove out any excess of DeoxIT from inside switches and potentiometers, by spraying a lot of compressed air into each device. Well, the amount of DeoxIT that came out from potentiometers were really in excess! It spread out over the main circuit board - without causing any damage, luckily - and I had to carefully remove it with compressed air, paper towel and some cotton swabs.
My recommendation is to use only the strictly necessary amount of DeoxIT (or similar product), especially if it is in the liquid form (the spray form usually contains a reduced grade of product, which easily evaporates together with solvents).
I already had cleaned up the front panel, back panel and the knobs in a soapy bath of warm water (see pictures below), using a soft brush for the panels and a toothbrush for the knobs.

Now I had only to re-mount the radio and proceed with renewing the black plastic of the case. I started with trying the Novus 3 Heavy Scratch Remover, but it did not a great job. I have found it too thick and a bit waxy, difficult to remove when it permeates the small wrinkles and scratches on the black case, causing it to look a bit milky and opaque, not that good really.
Then I tried to repair to my error with a normal, typical spray polisher for car dashboards. Boys, it did it! Faster, easier, cheaper and very effective: ideal for my needs. The results are in pictures below. Far from perfect, but good enough in my view.

That's all folks! I hope you have enjoyed your reading (thank you for that!) and may be you have found something useful for your own restoration project. These beatiful old ladies are well worth the effort.

Panasonic RF-2200 restoration adventures - Part 4

So here we go again. Since latest post of this series (you can find it here if you like), I managed to complete my restoration of Panasonic DR-22 (a.k.a. RF-2200BS).
In this post I will try to summarize what happened during the alignment work on the radio and to provide some suggestions that I learnt from this experience. It was quite a time-consuming activity for me, requiring care, patience and attention, because of both my limited experience in this task and the number of adjustment points on the main board of the radio.
Here below you can see the testbench, with the RF-2200, a simple digital oscilloscope and a good RF generator (I borrowed both the scope and the generator from a laboratory at my workplace). For most of the checks, the digital scope was connected to the speaker wires, where I had connected a 8 ohm, 7W resistor just to provide a safe load for the audio amplifier of the RF-2200.

The big coil made of red wire on the gyro antenna of RF-2200 in picture above, was used to transfer the RF signal from the generator to the receiver during MW alignment. I didn't try to open the gyro antenna to expose the internal ferrite rod and get a better match with the RF signal from the generator. I wanted to avoid any risk of damaging the gyro antenna itself, which is a vital part and one of reasons of the very good MW performance of RF-2200. For the same reason, I didn't performed the adjustment of the MW antenna coil, which is one of the steps of the MW RF alignment process.

For the alignment, I relied on the detailed step-by-step procedure described in the RF-2200 service manual (you can easily download a PDF copy of it from the web).
With the exception of a few cases, the procedure is fairly clear and simple to follow, but there are quite many steps to go through. Some of the checks require the front-panel controls (for example, the AFC/bandwidth selector) to be set in a specific way and I found it easy to forget this point, in the middle of a number of adjustment operations. When you see that adjusting a given coil ferrite core or trimmer capacitor does not produce visible changes on the signal level, I suggest to stop for a while and to carefully check that the setup for that specific procedure step is correct. It is happened sometimes to me that one front panel switch was not correctly set. More frequently than desired I also realized I was simply adjusting the wrong coil or capacitor...
Compared to what is suggested by the service manual, often I have had to raise both the signal level and the modulation depth (in particular, during AM tests) from my RF generator, in order to get a good signal reading on the oscilloscope. For higher SW bands (SW3, SW4, SW5, SW6), typically I needed to set at least 75% modulation depth. This could also be caused by the high level of RF noise in the lab (at my workplace) where I was performing my measures.
Sometimes I reduced the level of audio output (by the volume control) to avoid clipping of the signal in the audio amplifier, which made it not possible to look for the point of maximum amplitude during a given adjustment. In similar cases, however, first measure should be to reduce the output level and/or the modulation depth on the RF generator.

If possible, I suggest to use plastic screwdrivers to turn ferrite cores of coils and transformers and for trimmer capacitors. This allows for an easier adjustment, by removing the magnetic/capacitive effects of a metal screwdriver.
In some cases, a plastic screwdriver does not provide the required torque. In such cases, be extremely careful: using a metal screwdriver for adjusting coils and transformers exposes to a non negligible risk of damaging their magnetic cores, which I did actually with L2 and L3 coils of the FM section (luckily not heavily enough to put them out of work).
Adjustment of trimmer capacitors is very touchy. I needed to proceed very slowly and with very small increments of rotation angles in order to be able to identity the position for the maximum signal amplitude.

All in all, I'm pretty satisfied with the results of the alignment. Even if it has been quite a demanding task, I think it was definitely worth to do it. Now my RF-2200 works much, much better than when it came into my hands from eBay. In addition, I learnt a lot and I had a lot of fun while trying to bring it back to its better days.

Tecsun PL-660 vs Panasonic RF-2200

At the end of my Panasonic RF-2200 restoration adventures (more in a dedicated post soon), I wanted to perform a very quick and simple comparative test on medium wave with my brand new Tecsun PL-660. The result is documented in the short video here below:

lunedì 30 marzo 2015

Panasonic RF-2200 restoration adventures - Part 3

This post follows Part 2 on the same subject. As anticipated, I proceeded in the restoration project of my Panasonic DR-22 (also named the RF-2200BS) with trying to clean-up contacts of front-panel switches and potentiometers.
For this purpose I bought some products: a spray "dust remover" (compressed air) and a couple of well-known products from CAIG Laboratories: the DeoxIT D100 for switches and the DeoxIT Fader for potentiometes. Here below a picture of the toolkit, ready for use:

First step was to try to remove as much dust as possible from inside the case, the switches, the potentiometers and the tuning variable capacitor. Using a bottle of spray compressed air was quite appropriate for this operation, as illustrated by pictures below:

Next step was to use DeoxIT D100L cleaner to restore proper behaviour of contacts in front-panel switches. CAIG guidelines recommed to always wipe off excess of their products. However, wiping off excess is sometimes not possible, especially if the cleaner is applied into a semi-sealed part, like in my case. If this happens, CAIG recommend to just apply a small amount of their product into the part to be cleaned. Here is where the "small squeeze tube" type of applicator helps (compared to the more common spray type), as it allows providing small drops to the target device.

Final step was to apply the CAIG Fader F100L to the potentiometers, to restore and lubricate their internal contacts. Here again, the small squeeze tube applicator proven to be perfectly suitable for my needs:

Just after a single application of DeoxIT, I noticed a significant improvement in contact reliability of switches and in noise of potentiometers. Nevertheless, the 125 kHz marker switch will require a second application of DeoxIT, as it still works not perfect. I plan a second round for it just before re-assembling the unit in its case. Stay tuned for more RF-2200 restoration adventures.

domenica 29 marzo 2015

Sulla tragedia del volo Germanwings

Non se ne può più di leggere pagine e pagine, interviste di "esperti" di ogni sorta, sproloqui politici, post nei blog e sui social network e chi più ne ha più ne metta sui particolari - possibilmente torbidi - della vita del co-pilota tedesco dello sfortunato volo Germanwings, precipitato nelle Alpi Francesi lo scorso 24 Marzo. A nemmeno una settimana dalla tragedia, nessuno spende più una parola per le vittime del disastro aereo (in tutto 150); le cui vite spezzate - che continueremo serenamente ad ignorare - sono state destinate da subito a confondersi in un semplice numero, divenuto ormai quasi un punteggio. Così come nessuno sembra provare un minimo di pietà per i familiari di Andreas Lubitz, sospettato di aver causato volontariamente l'incidente, divenuto rapidamente l'oggetto della morbosa attenzione dei media e del loro vasto pubblico, sempre avido di soluzioni semplici e di colpevoli da vivisezionare. Qui in Italia un quotidiano - se così lo si può chiamare - nei giorni scorsi ha avuto la brillante idea di titolare a tutta pagina "Schettinen", accomunando così in un'unica battuta di triste umorismo - uno sfottò che a qualcuno dev'essere parso geniale o almeno spiritoso - due tragedie recenti e le sofferenze che hanno causato in tante famiglie innocenti. Chapeau.

sabato 28 marzo 2015

Surfing shortwaves with a homebrew tube radio receiver

This one will be probably the last in a series of posts around my "next" regenerative receiver (as I called it when the project began). You can find my previous posts on the same subject by looking backwards from here: My "next" regen receiver - Schematic diagram of the RF stages.
Below you will find the link to a YouTube video showing some tests that I did with the radio at night (say around 21:00 CEST), just after having completed the construction. Thank you for reading!

mercoledì 25 marzo 2015

My "next" regen receiver - Schematic diagram of the RF stages

Here below I copy the schematic diagram of the RF stages of my regen receiver in its current configuration. As said in my latest post on the same subject, the design of the regenerative detector was copied from David Newkirk's web page Receivers for Watching 7120 kHz, with only some minor modifications. From very first tests, it seems that the receiver can work acceptably well from day one, apart from normal tweaking and some stability issues (due to non-ideal mechanical construction). I will post soon a short video clip of the radio in operation.
The actual frequency range (as measured by putting the detector in oscillation and looking for the tone with a Tecsun PL-660 portable in SSB mode) is from about 2400 kHz up to about 7960 kHz.
My reverse computation give about 28 pF to 310 pF for the range of the combined bandset and bandspread variable capacitors; and about 14 uH for the inductance of the tuning coil (I expected 8 uH, based on construction data). I will evaluate later on if it could be worth to try reducing the lower limit of the capacitance (by adding a series fixed capacitor to the bandspread varcap or by using only a single section of it) to extend the upper limit of tuning range, including for example the 31m broadcast band.

Quick update: after having posted the schematic diagram above, actually I made some experiments with the tuning range. I ended up with the decision to use only a single section of both the bandset capacitor and the bandspread capacitor. With this arrangement, the receiver can tune from about 3310 kHz to about 9325 kHz, which is fully satisfactory to me.

lunedì 23 marzo 2015

My next regen receiver - Almost completed

This is number four of a series of posts about my new poor-man regen project. Previous posts (ordered from older to newer) were:

My next regen receiver - The power supply
My next regen receiver - The AF amplifier
Quick test of the AF tube amplifier module

Now I have completed the build with the RF stages (an untuned amplifier based on a 6AK5 tube, followed by a regenerative detector based on a 12AU7 tube, wired as a pseudo-tetrode).

The RF part of the design was copied from David Newkirk's Receivers for Watching 7120 kHz, with only minor modifications that I will describe in a schematic diagram as soon as it will be stable enough after some more tests and tweaking.

Overall, the layout and the mechanics look a bit "oddish" (it is a poor-man regen, as I said). For the chassis, I used two pieces of copper clad and some pine wood. For the back panel, I cut a small rectangle of galvanized sheet and for the front panel I used a piece of aluminium sheet which was part of a dismantled surplus equipment.
My very first tests seemed encouraging: with 1-meter long antenna wire I managed to hear quite strong european broadcast signals, in a shortwave band that I couldn't identify (I have still to make a rough calibration of the tuning range/scale).

Some images follow of the receiver as it looks at present. Next post on the same subject will include the schematic diagram and a short video clip showing the radio in operation (hopefully :-)).

The coil is 22 turns of AWG18 enameled copper wire over a 1" 1/4 support.
The Hartley tap wss provisionally set at 7th turn (about 1/3) from the ground side.
The grey (shielded) cable goes to the AF gain pot and the black one is for the AF output jack.

Another view with the main tuning variable capacitor (Hammarlund, 140 pF per section).
Mounted above on the front panel is the band spread capacitor which has an embedded 1:3 reduction drive.

The back panel has inputs for the 6,3 VAC heaters voltage and for the 150 VDC B+ voltage.

The front panel (without knobs). From left to right, top to bottom: AF gain control,
band spread varcap, AF output jack, main tuning varcap, AF output switch (speaker/headphones),
antenna input, RF gain control, regeneration control.
The hidden side, with wiring.

sabato 21 marzo 2015

Panasonic RF-2200 restoration adventures - Part 2

Among first steps of almost any radio restoration projects is to dismantle the unit, both to ease deeper cleaning and to gain access to the circuit for visual inspection and - later on - for realignment and calibration. So I did. Attached below please find a collection of pictures showing the internals of my new old Panasonic DR-22 (refer to Part 1 for some more background on this story).

The solder side of the main board with the front panel controls exposed

The nice old-style mechanics of the tuning scale

The component side of main board with the tuning capacitor well exposed

Luckily, it seems that the unit does not have suffered from any major fault or repair in its past life. Internally it appeared fairly clean and in good order. After having removed the five self-evident screws on the back panel (only one of them is "hidden" inside the battery compartment) and the knobs on the front panel (all but the main tuning knob), both panels can be easily separated from the main circuit board so exposing that nice piece of vintage electronics.
It is very good for us experimenters that the whip antenna, the battery compartment and the loudspeaker can be easily disconnected from the main board without having to use a soldering iron. The radio itself can still stand up without the front and back panels and it can be very easily serviced this way, with a minimum risk of causing mechanical damages. Well done, old Panasonic designers!

The back panel dismounted with connections for antenna and batteries

Unfortunately while separating the front panel from the main board, a piece of plastic detached from around the signal meter (circled in yellow in image below). The other small rectangular piece of black gummy plastic silently dropped from inside the radio while detaching the back panel. I couldn't find out where it came from. I hope to discover its original placement while rebuilding the set.
Now I'm waiting for a stick of the renowned CAIG DeoxIT D100L conctact cleaner and another stick of DeoxIT Fader F5 for potentiometers. After having recovered a good behaviour of switches and pots I will proceed with a realignment. Stay tuned for more RF-2200 restoration adventures!

The rear of the front panel with the loudspeaker

The timestamp above says (I suppose) December 24, 1977

The venerable front panel dismounted and ready for a careful cleaning

Panasonic RF-2200 restoration adventures - Part 1

Some time ago I wanted to buy a vintage portable receiver for trying a bit of MW listening with good performances even without the need of external antennas or the complexities of software defined radio (SDR). I went through the web and read a number of reviews and finally I choose to look for a used Panasonic RF-2200. I liked its look very much and felt it was just the kind of radio I was looking for. I found one on eBay in fairly good conditions and at a reasonable price (compared to how much these sets are usually rated). Also important it was that that National Panasonic DR-22 (also known as RF-2200BS) came from Europe, without the additional costs for shipment and long processing time at customs that affect similar purchases from the U.S.. Being targeted for the european marked, the DR-22 was also equipped with an AC mains selector (220V/50Hz or 110V/60Hz) so it can be used in Italy (where I live) without the need of an additional AC-AC transformer.
Here below a photograph of the set as it was depicted on eBay:

When the radio arrived in my hands, I quickly tested it to find out if it was fully functional, what of course was partially unexpected, given the age of the receiver. While it looked fairly goon on FM (where it was possible to receive many strong signals from local broadcast stations), apparently it was almost deaf on medium wave and shortwave bands. I knew a common problem with these sets is bad contacts in front panel switches and potentiometers due to long inactivity, dirt and oxidation. So I tried to operate them a bit and something started to come out of the speaker. Well, at least it seemed that there were no major faults. The short video below shows these initial tests:

OK, even after having verified that (apparently) the unit didn't suffer from major issues, it was still apparent that MW sensitivity was quite low and the same it was in SW. Probably a realignment was needed to improve the overall performamce. Also, the switches and potentiometers required a deeper and more durable cleanup. The service manual was available in digital format from the web. It was to time to plan a restoration. More on next episodes of my RF-2200 restoration adventures.

sabato 14 febbraio 2015

Quick test of the AF tube amplifier module

Following my recent post on the small AF tube amplifier module that I have built for my next regenerative receiver, please find below a short video showing a quick test of the amplifier using my PC as the audio source.

venerdì 13 febbraio 2015

My next regen receiver - The AF amplifier

After some time from my last post on the subject of my next homebrew regen receiver (that post was about the power supply), I can finally make a little step forward by adding some information about the AF amplifier module.
The design has been entirely copied from the Jon Fleig's Pipsqueak guitar amplifier (here it is the original schematic diagram), with a few little modifications to component values, based on what I had at hand.

Here below I add some pictures of my build of the circuit above. The construction is all but elegant, however results of initial tests seem encouraging.
I expect to get an output power of less that 1 W on a 8 ohms load, more than enough to drive a small loudspeaker for my receiver. I plan to post a video of how it sounds actually quite soon.