March 21, 2010 at 5:11 pm #1691Ed StoneForum Participant
Here is an interesting item – bought from a local seller on ‘Craigslist’ in the Fall of 2009 by another SPARC museum member. It was donated to myself after I had expressed an interest in building a small collection of tube tuners – particularly those of a UK origin. It all started with my Eddystone S820 and a Leak ‘Toughline’, the latter restored for the museum a couple of years ago. This Chapman tuner is a little later than those ones though, hailing from around 1959. Also, this particular model has four AM bands – standard Broadcast and three shortwave, with variable IF bandwidth – that make it rather interesting.
As is typical of ‘HiFi’ tuners of this era, the Chapman was designed to be fitted into a cabinet that the purchaser would build, into which would also be installed an amplifier and, possibly, a turntable and/or reel-to-reel tape recorder. High-quality FM broadcasts were one of the most popular sources of audio entertainment at that time and the quality often surpassed that from local sources (record and tape), providing a decent VHF aerial was also invested in.
The Chapman Model S5E/FM tuner is a monophonic tube unit that is very similar in design to the Chapman Model CT-100, except it lacks the audio amplifier/cathode follower stage (ECC82/12AX7) that the latter sports. Good job it is similar, as there seems to be a dearth of information on Chapman equipment apart from the CT-100. The S5E/M is a nine tube circuit comprising an EF85 (6BY7) RF stage for FM, followed by an EF80 (6BW7 or 6BX6) converter to the standard 10.7MHz FM IF. The AM RF stage is an EF89 (6DA6) feeding an ECH81 (6AJ8) local oscillator and mixer, part of this tube also acting as the first 10.7MHz IF amplifier stage during FM operation. A second EF89 (6DA6) acts as the second FM IF amplifier/AM IF amplifier, feeding either an EB91 (6AL5), configured as a ratio detector for FM reception, or an EBF80 (6N8) for AM reception, this tube also providing an amplified AGC function. Audio is taken directly from one of these detectors to the output terminals in the S5E/FM, so the audio output level is not great. An EM81 (6DA5) ‘magic eye’ is used as a (bar) tuning indicator. Suitable operating voltages are provided by a standard power supply section, utilizing an EZ80 (6V4) as the full-wave rectifier for the HT.
On removing the baseplate from the tuner, it was nice to see that it was ‘unmolested’, with no replacement components having been fitted in the last 50 years or so – I guessed that it had probably been out of service since the early-1980’s when solid-state tuners were becoming ubiquitous. I cleaned the chassis up – removing the usually grime build-up with rubbing alcohol and lighter fluid where there were greasy/waxy deposits. The end result was a chassis that looked in reasonable shape, albeit not pristine. The tuning capacitor gangs were cleaned using a brush and vacuum cleaner, the congealed grease in the bearings cleaned-out and replaced with molybdenum grease. Cosmetically, I cleaned-up the front panel, washed the glass tuning scale with warm soupy water and cotton wool, and replaced the mongrel knobs with three matching ones that resembled the originals (from a photo on the front of the CT-100 service manual). One of the dial cords was missing and this was re-strung with new cord – always a fiddly operation but at least the method was fairly intuitive in this case.
All the tubes were removed and the power transformer tested for continuity – all seemed ok, so I applied power to it via a Variac and measured the secondary voltages – all seemed ok. The (Hunts) dual can electrolytic smoothing capacitor (2 x 32uF at 350V) looked in good shape (no swelling or cracking of the membrane on the connection end), so I decided to try to re-form it rather than replace. To do this, the rectifier tube was plugged in and the voltage brought up slowly to full voltage over a few hours – all seemed fine. The paper by-pass and coupling capacitors were all Hunts types, some of which had split cases – I decided to replace all of these with new metalized polyester axial capacitors. Other electrolytic capacitors and a few other small ‘dubious-looking’ capacitors were also replaced from stock. I took the opportunity of testing most resistors while I was doing this work and several that tested out of tolerance were also replaced with new metal-film types. I noticed a couple of dry joints and a disconnected wire (on the EBF80 tube base) – these issues were resolved and the range and other switch wafers cleaned using a careful application of De-Oxit with a Q-tip. A line fuse was fitted beneath the chassis and a BNC socket was installed in a pre-existing hole on the rear chassis as an alternative aerial connection (but possibly as an output to an external stereo decoder eventually – see below). Tubes were tested (all ok, but a couple on the weak side) and the a missing tube, an EB91 (6AL5), installed. The tuner was coupled to my small shop amplified speakers (an old pair of powered computer speakers that come in very handy), switched on and stations on the FM band came in fine with three feet of wire connected. The AM bands worked ok also, except the highest frequency shortwave band, where the local oscillator was ‘dying’ around mid-band (above around 12MHz) – this was traced to a weak ECH81 (6AJ8) and was soon cured with a replacement tube. The magic eye also works well and is one of the brightest I have seen.
So, how does the Chapman tuner perform – very impressively! – even without any re-alignment. It is a well-made unit, generally in the same league as the contemporary Leak and Eddystone units. Frequency stability is not as good as the Leak ‘Troughline’, the Chapman having conventional ‘lumped’ tuned circuits and no AFC circuitry. However, after 20 minutes or so stability is perfectly adequate. It is the quality of the sound that is most impressive though – I now have this tuner linked to my QUAD ‘HiFi’ and it sounds very good indeed. I must make a case for it though before it becomes dusty… I also plan to make a stereo decoder and have bought an (obsolete) MC 1310P PLL chip with this in mind – this can be easily added as a reversible modification or as an external unit.
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