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sabato 11 aprile 2015

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.

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