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Thrift store tunas! CPU 100 & Crown

gonzothegreat

gonzothegreat

Super Member
The tuner gods were kind to a friend and me. Just as we were about to check out with our prizes for the day (some Allison Sixes for me and a few turntables for him), they wheeled out a huge metal cage stacked to the top with stereo components. I spotted a Hafler 500 amp in the mix so I told him to park our carts and get ready to move fast. :)

Once I got a better look at the goodies, I saw a Sherwood CPU 100 sitting on top of the Hafler and asked to see both. My friend snagged a big mono PA amp (it wasn't anything spectacular like a Crown or Peavey) and a digital Crown tuner (either a FM2 or FM3).

The CPU 100 has some wiring mods strung out the back where the channel spacing switch used to be and it probably has NiCd battery & power supply issues (although I haven't looked inside or powered it up). Cosmetically its in very nice shape and I was surprised it shared its styling with a Sherwood tuner/amp combo I found with a pair of EV Sentry V speakers a few years ago.

The Crown tuner isn't in great shape (missing buttons) but it appears to be something of a mystery according to www.fmtunerinfo.com. This morning I told him to take some pics so the tuner-philes can try & decipher its internal workings. That might not happen soon as he has a busy weekend ahead.
 
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No pictures of the Crown yet but I had a chance to work on the Sherwood. As predicted by others, the power supply has corrosion around the battery that damaged some traces on the underside of board. The foamy corrosion residue was easily removed with a diluted solution of acid toilet bowl cleaner. I haven't done it on the underside of the PCB because it will involve unsoldering at least four connections from the chassis.

It works but it is stuck on even channel spacing. Tracing the wiring back from the switch terminals, the corrosion nuked some of the circuitry for the spacing switch. Hopefully this will be an easy fix.
 

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The Sherwood (built by Draco) was probably very expensive to manufacture. Totally cutting edge 30+ years ago.

Thanks for the pics, specially the innards.
 
Hi,

The Sherwood Micro-CPU was indeed made by Draco Labs and later sold under the Draco Labs banner after a disagreement with Sherwood. In the 1977-1980 timeframe, it was one of the supreme tuners available. Well worth restoring. The Crown FM 1 and FM 2 are very excellent tuners and sound nice and are very resistant to overload and are very selective. One of the stations I engineer for has an FM 2 used as an air monitor less than 50 feet from a tower. Always good sounding and reliable.
 
The wiring mods on the back have me puzzled. The next time I open it up to work on the spacing circuit, I need to trace the ribbon cable to see where they are connected. Since the wires aren't shielded, I don't think its anything RF/IF/audio related. I wonder if it is some sort of computer interface for the tuner (ie serial or parallel). The mod does look clean enough to be factory or a decent tech job. FWIW the leads to the spacing switch are clipped and taped. The switch is nowhere to be found so I can't tell if a closed switch switches it to odd or even spacing. Too bad the schems are essentially unobtanium afaik.
 
the mod might be an external "remote control" but using direct numerical entry rather than presets. My cpu100 battery leakage wasn't nearly as bad. On the switch, if you wire it backwards, have enough wire so you can flip it upside down, then mount it.
 
I had a couple hours to work on a random project and the Sherwood won the toss. Getting to the underside of the power supply pcb was difficult (to be kind). Three ceramic caps and a ground lead had to be unsoldered from the chassis to free the board after I disconnected countless cables. I finally lost my patience and raised the board just high enough to inspect the battery damage and take a few pics. A hit of acid cleaner allowed me to check trace integrity and to my surprise the only trace that was bad was the run from one leg of the switch to R10. I jumpered the trace on the component side and buttoned it back up for a smoke test. Success! Odd spacing is enabled by leaving the switch open (unless my sleep-deprived brain is messing with me).

At a future point I'd like to clean up the marginal joints, replace the spacing switch and a few yellow PSU electrolytics but a really fun ASAP "to-do" popped up this weekend. Three words - plasma monitor upgrade!

While I didn't have a chance to listen to it, the meters gave a hint of impressive performance. I was running the tuner with a cheap dipole that's missing one leg and it was pulling in stations in stereo at 80-90% signal strength that most of my other tuners would struggle to hit 50-60%.

I took numerous pics of the power supply pcb pre and post surgery and they will be posted later in the week.
 
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