You have to get used to the British where a valve amp would be tube over here. At first I was like "Valve amp?", what are they talking about.
I am under the impression they disliked the S-7200 so much (but liked the similar NAD) is that they don't like the loudness contour (I don't either) and it has it engaged by default. You have to actually look at the front panel icons to figure out you have to push in the loudness button to turn it off. I think they took some time to figure that out, few manufacturers did it that way.
I collected a few pieces based on their reviews. A Yamaha CR-800, have not recapped it should open it up in sound when I do. Recently finished the infamous Sansui 3000A, once overhauled and service mods done so it shouldn't destroy a woofer (based on the speaker A rocker switch and blown fuses, it had likely already murdered one), it is a really good sounding receiver. Like the S-7100 it has interstage transformers which I believe was used in tube amps too. The S-7100A is cap coupled they got rid of the transformers. The 3000A was an Ebay fix and repair missing a knob only had to beat a bottom feeder to get it for only $37. Poor thing bent shafts massive gouge in tuning knob metal barb sticking out could easily draw blood. Looks like it had been tossed out on the curb as garbage. Ended up being in better shape than I thought when I bought it.
Recapping the old ones can really open them up. Electrolytics don't age well, but the solid aluminum and tantalum types don't either. Their ESR seems to go up with age, at least that is what my $20 transistor tester kept telling me. Many tantulums were getting ESR values in the mid teens. Replacing the small value ones with film caps (Kemet R82, Wima MKS2) can enliven them and quiet them down too. I have always found recapping the late 60 and 70s ones it is a substantial before and after difference. My Kenwood KR-7400 did not have noisy transistors just recapping it the background noise is virtually non-existent in the preamp and amp sections. It had a number of tantalums. Fidelity went up too, like it breathed new life into an old tired unit. Looks like the CR-800 has a number of tantalums too.
Noisy transistors can be a problem too. Looking at the schematic there are some potentially problem types in the S-7100A. I have personal experience with the 2SC693F on the tone boards of both the S-7200 (4) and 3000A (2). The 3000A also had its lower gain cousin (looks identical) the 2SC536 as the other pair in the tone section and front end of the amp card. On both receivers replacing those got rid of a lot of background noise. The 2SC693F will hiss in the background much like the noise between fm stations, guessed the similar 536 probably did too. Since they were in the tone section, I found with the S-7200 the hiss from them went up and down in response to the volume control. The 2SC871 design is known for noise issues too. They list two types of transistors in the amp section, the CDC 8002/9002 flying saucers or another pair of transistors. The CDCs were in my S-7200. Sansui used the CDCs too on a few amp models reading forums on those Sansui models they can have issues many like to replace them. For the 2SC693 and 2SC871 the On (formerly Fairchild) KSC1845 works, and for the CDCs the KSA1220 and KSC2690 are recommended replacements. Keep pin arrangements straight so the new ones are put it right. The 2SA666 were in differential pairs at the front of the S-7200 and started making a popping noise on one channel besides causing a high DC offset in that channel. Have found with several units high dc offset (tens of volts) or blown fuses likely a shorted output transistor (may be other components that went with it), smaller more in the range of 200 mV or so the pair at the front of the amp circuit likely culprit.
Basically recap and see how one is from there. Hear audible noise in the background, circuits start making weird noises, sounds increasingly distorted as the volume increases (only sounds right at low volume) there may be bad transistor(s). Have recapped tuner sections, even older discrete ones (no ICs), without noticeable tuner performance degradation. Leave transistors alone in the tuner circuit unless one goes bad and kills tuner performance. Transistor replacement is more likely to require realignment to have the tuner work well again which requires some expensive specialized test equipment and expertise.
The CDCs and tantalums from my S-7200, and the 2SA666 noise.
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