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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.