#13 Fisher RS-1022 rehabilitation and bling-ification

So i had this old Fisher RS-1022 Studio Standard stereo receiver brought in, with one of the fuses in-line with the speaker outputs blown. Fair enough, that was easy enough to replace, they made it quite easy, with the fuseholders being mounted right on the rear panel. The clamps were nowhere near as elastic (or firm) as they might've once been, but with a pair of needle-nose pliers and a bit of creativity, that was no biggie.

Second issue, five of the six lamps that illuminate the tuning scale and the radio signal level were dead. In four of the dead ones, the fillament was attached only at one end, and in the fifth, it was attached at neither end. Interesting little lamps, 6.3v AC, 250mA, in a similar casing as 6x32mm glass fuses. Good luck finding replacement ones (which may very well just end up dying again anyway); these will get replaced with a few white LEDs recovered from a laptop display's backlight.

Before the LED mod though, i "obviously" decided to power the amp up, to check for DC on the outputs, in case that might've been the cause (or one of the causes) behind the fuse blowing. What greeted me was a whole lot of hissing, buzzing and paper-tearing-sounding noise coming out of the crappy little test-speaker i hooked up, as well as my subwoofer. The strange, or rather, the even more strange part is, the subwoofer was not connected to the amplifier.

The buzzing seemed to come from the transformer. I've seen cases where transformers, connectors and rectifier diodes / bridges had been damaged by electrolytic capacitors that had kicked the bucket. After removing the bottom cover, i had some prime suspects. On what seems to be the "power supply board", as it were, i noticed two ancient vent-less Rubycon caps, sitting in a dried-up puddle of... something. I can't be sure whether that was glue from the factory, or liquid that had leaked out of said capacitors, but at the very least, they inspired no confidence whatsoever. The fact that they seemed to warm up considerably more than i would've expected, for the few seconds i dared keep the power on (due to the above-mentioned omenous noises) sealed their fate, in my book. All this was compounded by the discoloration of the phenolic(?) circuit board they were on.

The two Panasonics on the left edge are the replacements. Unfortunately, that didn't do much, if anything. In addition to those two, i also noticed two Rifa metallized paper capacitors, across the mains power switch. These are notorious for failing with age (quite destructively in some cases), so they had to come out as well. Oddly enough, neither of the cap swaps did anything to do away with the electrical noise. Upon closer auditory inspection, it turned out the transformer noise was merely secondary (no pun intended) - my suspicions turned towards the mains power switch itself, since it was buzzing away as well.

"Funky" design, though, the power switch was actuated by a tang mounted on the shaft of the speaker output selection switch. Either way, after a bit of wrestling around the barely-long-enough wiring (and quite stiff, to boot), i managed to remove the mains switch from said assembly. The sight was... not entirely shocking or unexpected, i should say.

Oh joy - more (damn-near) custom parts, or at the very least, very rare or way obsolete... The markings on the switch show it to be a Noble U12DV, 4A 250v rated. The closest i could find to a drop-in replacement were this and this, respectively, but you guessed it - out of stock / unavailable.

After presenting my solution to the owner, he agreed - whack on a (more conventional) power switch on the back, and that's it. In addition to being simpler than jury-rigging some bodged-together solution to directly and mechanically replace the original switch (ie. less work & headache-inducing fiddling on my part), it also had the added benefit of not needing to mangle the front panel, maintaining the <ahem> vintage (or at least, the original) aesthetics of the unit.

Not having any compact DPST or DPDT mains switches at hand, i went with an SPST one. That meant, one of the mains wires would get connected directly to one of the wires going to the voltage selector, and the other one would go to the switch. It took drilling a few holes, and a bucket or two of elbow-grease (read: manually filing the hole, to fit the body of the switch through it), but it got done.

I know, it looks a bit like an industrial abortion of i-don't-know-what, but i didn't feel quite comfortable leaving the mains connections, pointy as they are, all exposed while being so close to the bottom panel, so i went with a few layers of Kapton tape. Better safe than sorry, right? Mains voltage isn't something you wanna play around with, at least regarding risks and whatnot. But hey, on the outside, it damn near looks appropriate, if i may say so myself...

In other observations, the heart of the unit is an STK-043 hybrid chip - 2x 20W, nothing to write home about. Nearly all the connections are full vintage: wire-wrap, baby! And the power transformer, well... let's just say it's seriously strong with the Force. Quite the master, one might say...

Test-fire results: fortunately, no fire. But also no more noise of any sort! Great, that's the technical / functional side of things dealt with; now, onwards to the aesthetic issues. The original lamps were mounted to this long circuit board; all of them in parallel. In addition to figuring out how to mount the LEDs and associated current-limiting resistors (i'm thinking some slivers of bare circuit-board, soldered to the lamp-holders, i'm also considering employing some sort of light diffusers, to try to even out the light. I think (or at least hope) i can reuse some of the backlight "processing" layers from a cracked laptop LCD.

But before that, since these lamps were supplied with AC, first that'll need to get converted to DC, and smoothed out. Time for a rectifier bridge and a fat capacitor... Now, since these lamps were rated for 6.3v AC, and there were six 250mA ones, that's about 1.5A in total. Unloaded, the secondary dedicated to the lighting shows about 7.2v AC; considering the LEDs will be drawing a small fraction of that, it's more than safe to assume there won't be much of a drop in the voltage. Rectified and smoothed, the resulting voltage works out to 7.2 x 1.414 = 10.18v. A 25v rated cap should suffice no matter what. 2200uF sound alright to you? Thought so...

Since this amplifier is the age it is, i might as well go with a period-accurate rectifier bridge, wouldn't you say? The "7728" datecode on this KBPC104 i found in my parts bin would suggest it's from week 28 of 1977. 3A should be good enough for a few LEDs, i'd reckon.

Speaking of current - the (cold) white LEDs i plan to use have a voltage drop of about 2.6v across them. Thing is, even at 5mA, they're preeeeeeeetty damn bright, so i seriously doubt i should (and/or will) want to drive them any harder than that (even though they can take at least a couple dozen mA). To get 5mA through them, starting from the roughly 10.2v we have, that would mean we're left with 7.6v to drop across the series resistor; dividing that by 5mA gives us 1520 ohms.

Now, since they're more than bright enough even at 5mA, and this is only supposed to provide backlight for the radio scale (as opposed to being a reading light, for instance), i'm going with two 1k resistors in series. Partially because i have a few dozen 0805 SMD ones, and because 3.8mA should still result in more than enough brightness.

On second thought, about the smoothing capacitor, 2200uF's waaaaay too much - the LEDs stay on for well over 5 seconds after you turn the power off. With 100uF, the decay is under 1 second - a nice, smooth but swift "fade out".

A quick test-power-up of the radio signal level meter backlight (boy, was that a mouthful), with the LED installed - just enough light comes through to offer a comforting glow through the blue filter. Absolutely no need to bother with a light diffuser there. Hopefully that'll be the case for the radio scale as well...

Aw yeah... AWWW YEEEEAH!... Ain't she purrrrrrrrdy?..