As one of my summer projects, I decided to build a simple MW box loop antenna, just to put into practice the idea of making it using PVC cable ducts like the ones in the image below:
The construction was fairly simple, as documented shortly by the following series of images:
I used some PVC-specific glue to connect the various pieces together. The inductive section is made by 12 turns of AWG 14 electric wire, covering completely the 40 mm internal width of the PVC duct. The size of the square loop frame is 60 x 60 cm.
For the capacitive (tuning) section, I recycled a polyvaricon capacitor and a small fine tuning capacitor that I had salvaged from an old transistor receiver. They proven to perfectly fit the purpose of tuning the whole MW band. It was only necessary to add a small switch to put the two sections of the polyvaricon in parallel when tuning the lower end of the MW band. With the two sections of the capacitor in parallel, the lower limit of the tuning range reaches about 375 kHz.
When it came to perform some tests, I was not expecting any difficulties. I thought to use my XHDATA D-808 portable radio to verify how the MW reception would have improved by inductively coupling the internal ferrite rod of the radio with the box loop antenna.
Well, after several tests with different approaches, still I wasn't able to assess that the external antenna was behaving as expected. For example, I wasn't able to identify the typical, narrow peak in signal intensity that corresponds to the selective nature of this kind of antenna. On the contrary, there was a distinct, unexpected, narrow notch, along the tuning range, where the signal completely disappeared.
After some reasoning, I concluded that probably the XHDATA D-808 was too sensitive on its own to take advantage from coupling with such a box loop antenna. Probably the narrow notch that I was finding during tests was caused by overloading of the input stages of the radio when the antenna was tuned exactly to the received frequency.
So I decided to try with a little, very cheap, poorly sensitive pocket radio and finally things started to behave as expected, as documented in this short video clip:
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.
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.
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:
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
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.
The short video below shows a simple method for accurate tuning of a regenerative receiver, with the help of a small portable radio with digital frequency readout, 1 kHz tuning accuracy (or better) and SSB mode.
In my video, the regen receiver is the one described in some of my recent posts.
The portable radio is the Tecsun PL-660, that I recently bought on the web to use it for my first experiments as a MW DXer.
Please consider that I have made further modifications on my tube regen, always with the kind help of some members of the regenrx list on Yahoo. The up-to-date schematics is here below:
In particular, I have removed completely the wiring that I had added for the band switch, because it had resulted in a negligible improvement as a feature, while it had caused big problems in terms of spurious resonances that led the detector to demodulate strong local FM stations.
Now the tuning range is again the original 1330 kHz to 3735 kHz, but (thanks to a suggetion from Bernd, a member of the regenrx list) I discovered that I can move it down to approx. 730-2035 kHz, without any modification in the circuit or the tuning coil, by simply inserting a small pack of four ferrite rods in the core of the coil former. I suppose that the tuning range could be moved further down in the MW band by adding another ferrite rod in the pack.
That said, please find here below my short video about a simple method for accurate tuning of a homebrewed regenerative receiver. I hope you will find it of some interest for you.
Recently I felt the need for a simpler way to enjoy radio listening, compared to the usual way I do NDB chasing.
I mean, to listen to the NDB band on LF, I usually place my homebrewed "foldable" 3-turns loop in a sweet spot I found in the apartment (yes, I use it indoors); next, I connect the loop terminals to a wideband amplifier, then - by few meters of cable - to the receiver (an AFEDRI SDR-Net) and to the PC running the SDR-Radio software.
Of course both the AFEDRI and the wideband antenna amplifier need a power supply, to be connected and switched on; and finally I'm ready for my listening session, which usually lasts for 3-4 hours in the night, unless propagation conditions suggest to go to bed sooner than planned.
Well, while I'm pretty satisfied of the above-described setup, sometimes I'd like to simply switch on a more "traditional" receiver and have a couple of hours of pleasant, easy, relaxing listening to broadcast transmissions. By the way, this year I also decided to buy my first copy of the WRTH (the famous World Radio Television Handbook), to support my decision to "taste" the broadcast listening.
For the same purpose, a week ago I bought on the Internet one of these small portable radios, the Tecsun PL-660. I had already had the simpler Tecsun PL-600 model and it had worked very well for years. I had tried several modifications on it and it has survived to my soldering iron as well as to repeated disassemblies. It was still fully functional when I decided to sell it (at a fair price, of course). So, when I came to the decision to buy another portable radio, my choice has been for the successor of the PL-600, that is the PL-660.
My new Tecsun PL-660 portable receiver with a copy of WRTH 2015.
Compared to the PL-600, the PL-660 basically adds syncronous AM detection and the AM air band. Ergonomics were excellent on the PL-600 but have been furtherly improved on the PL-660. Overall, the value vs price is very good in my opinion.
I have made some quick tests of course on my new portable receiver as soon I received it. Most important weaknesses I have found so far are: the battery status indicator and the battery charger, which don't behave very well with normal NiMH rechargeable AA batteries (based on the "battery low" indication on the LCD display, it would seem they last much shorter than expected but I suspect the indication be wrong); and the reception in the MW band, which in my opinion is subject to easy overloading.
I have a 100 kW transmitter on 657 kHz (the Coltano station of the italian broadcaster RAI) within 10 km from my home. If I tune the PL-660 on the exact frequency, the reception seems to be not completely good, while it clearly improves if I detune the receiver by a couple of kHz or switch the built-in attenuator on the "LOCAL" position.
Uhm, this is a bit of a problem for my project to spend some time in broadcast listening. I have been a member of the MW Circle for a couple of years now and I was just thinking of medium wave (together with long wave) as the band of choice for my BCL activity (I always thought that shortwaves were too wide, too many kHz to surf, too many different kind of signals, too much stuff for me).
I think that an external tuned loop with good selectivity could help in working around this issue. I have a small square loop, that I had built with a "quick and dirty" approach for the PL-600. It is handy enough to be used as a table-top antenna. It has a 50 cm side and 19 turns of wire, closely wound on a square wooden frame. I also added a single turn to be used as a secondary winding for better impedance matching, in case of direct connection to the antenna socket of the receiver. The tuning range was from about 280 kHz to about 830 kHz, so mainly suitable for exploring the NDB band, but it could probably be moved upwards by removing one or two of the sections of the variable capacitor, that are currently connected in parallel. The Tecsun PL-660 does not enable its antenna input in MW (neither did the PL-600, I had to apply an HW modification to change this behaviour). So I will use the loop by coupling it inductively to the internal ferrite loop of the receiver.
My simple tuned loop antenna when I used it with the Tecsun PL-600
The variable capacitor connected to the primary winding.
The red wires are the terminals of the single-turn secondary winding.
Later on, if the new hobby will reveal to be attractive enough to me, I will think about a better antenna and maybe a better receiver (the system I currently use for listening to the NDB band will be the first candidate). For now, let's keep it easy (and cheap) as it was originally intended.
The video clip below is a quick test that I have performed with the Tecsun PL-660 during a short walk near my home. SSB reception on 20 m USB and 40 m LSB ham bands is shown, as well as in the AM air band and on MW (with the receiver tuned to the strong 657 kHz local station I have mentioned above). In the air band, following stations were recorded: Milano FIC (Milano Information) on 128.925 MHz AM, Roma ACC (Roma Radar) on 124.800 MHz AM, Pisa VOLMET (nominal frequency 128.400 MHz) on 128.390 MHz AM. The IF filter was set to "wide" in all cases.
Dopo mesi e mesi di lavoro, dedicandovi spezzoni di tempo a distanza anche di parecchi giorni uno dall'altro, finalmente ho deciso di considerare completata la costruzione del mio ricevitore a reazione.
Scrivo volutamente che "ho deciso" di dichiarare finita la costruzione, perchè in realtà sarebbe possibile apportarvi ancora diverse modifiche e fare ancora prove, probabilmente una lista molto lunga di possibili sperimentazioni. Ma sono giunto all'esaurimento delle energie e del tempo che ero disposto a dedicare a questa esperienza, per cui eccomi qua con queste note conclusive, a mostrarvi un paio di foto del ricevitore "finito" e un piccolo filmato di una prova appena fatta in tarda serata sulle onde medie.
Particolare interessante: il ricevitore, per come è stato costruito, coprirebbe la gamma di frequenze da circa 1300 kHz a circa 3700 kHz. Per portarlo più in basso, nel pieno delle onde medie, ho sfruttato un suggerimento di Bernd (non conosco il cognome) della lista regenrx di Yahoo: ho inserito un paio di bacchette di ferrite all'interno del supporto della bobina di sintonia e oplà, ottenuto il miracolo senza nemmeno dover accendere il saldatore.
La qualità delle foto non è buona, perchè sono state scattate al chiuso, di sera e senza l'uso del flash.
Lo schema del pannello frontale è quello riportato nel disegno qui sotto.
Nella seconda fotografia si vede anche l'alimentatore costruito per fornire tensione ai filamenti ed alle placche delle valvole del ricevitore.
Nel filmato ho cercato di evidenziare anche il leggero effetto passabasso di un filtro audio L/C passivo, che può essere inserito tramite il piccolo deviatore posto tra la manopola che controlla la reazione e quella che controlla il volume. Il deviatore ha tre posizioni: filtro inserito, filtro escluso e "mute" (che è la posizione centrale).
