Legendary 1951 Hammarlund SP 600 radio receiver tuning the bands

This SP-600 is the present of a good and generous friend of mine. The poor thing was mute when it arrived, except for a faint hum, 50Hz-like.
First thing was setting it to the right voltage. It had been running for years on the 210VAC tapping of the primary of the main AC transformer, which was certainly taxing on the filaments of the 20 tubes of this venerable receiver, and a number of other components. I set it to the 237 tapping, which agrees much better with the 234VAC of the mains at my home.
Next, I checked the resistance between ground and each of the pins of each of the tubes, and compared it with the chart included in the manual of the receiver. There were a few shorts, and very many values well off tolerance. So I went on to replace each of the paper capacitors. Early SP-600 extensively use a type of paper capacitor nicknamed Black Beauty Of Death, BBOD, very prone to failure. According to experts, all of them must go and be replaced with fresh, modern capacitors. In all, I replaced some 60 of them. I also replaced the electrolytic capacitors, as they looked suspicious to me. They are not as many as the paper caps. Perhaps 10 of them or so.
Unfortunately, some of the paper caps had indeed failed to the point of catching fire. When that happens, usually the resistors and other components around burned caps fail too. It’s a cascaded process of potentially catastrophic consequences, including shorted coils, which is indeed a pain in the neck.
I found quite a few resistors that had been reduced to ash, and many others that were off by more than 10% of their nominal value. I changed those too, some 30 of them.
Then I switched on the receiver, and measured the voltage at each of the pins of each of the tubes, and found them more or less OK as compared to the corresponding chart in the manual. There were a few of them which were quite off though. Then I changed the mica capacitors associated to those points, and the problem was corrected.
At this point the receiver had “resurrected “, so to speak, and I could tune to quite a few stations and pick them up with just a short piece of wire as an antenna, but performance was obviously very low. What I did next was aligning the two IF circuits (455 and 3955 kc), and finally each of the RF circuits of the receiver.
The results are just stunning. I can’t believe this is a 1951 set. It is stable and selective, but it is particularly sensitive, in pair with some modern, professional receivers I have got.
It is not a hundred percent OK yet though. The behavior of the crystal filters is a bit strange in my opinion, but I will get at that at some point later.
I have learned a lot bringing this radio back to life, but the best lesson perhaps is that you can’t be in a hurry, or tired or distracted when working on one of these. The hardest and most delicate part of the process is extracting, checking and reinstalling the RF deck. I promise you it is a daunting thing. I thought I had to do it just once, and on to pleasanter things. Well, I have been there FIVE times… Which proves that men are the only animals that trip twice on the same stone. Or thrice, or in five occasions… Lesson learned, I hope.
This 5 minute video corresponds to the first test with a “real” antenna.