Thursday, April 26, 2012

Thursday Already??

Wow....time sure flies when you're having fun.
Working on another project for a friend of mine at work that should be interesting.
His younger sister is Autistic, and just loves playing records and singing with them. Her last little portable phonograph, like the type lots of us had a kids, bit the dust last month. I helped him find several suitable replacements on eBay, and he turned them over to me the other day so I could go through them, and make sure they work properly.
These are really simple little devices, with a single tube (25C5 in one, a 25EH5 in the other, both Beam Power Pentode tubes) grid-driven by the output of the cartridge, and feeding a transformer coupled speaker. Both had bad tubes (one very leaky, the other with a burned out filament), and bad filter capacitors. The filters in these are a dual section "cardboard tube" type, of 30uF/50uF @ 150VDC rating. Rather than get *exact* replacement parts (at $30 a pop!), I'm substituting two modern electrolytics of 33 and 50uF@200VDC ratings, for a total cost of about $4. These will work at least as well, and probably last much longer owing to the newer technology employed in their manufacture, and the higher voltage ratings.
I'll also replace both of the coupling capacitors, which are the old style paper caps dipped in wax. These are notoriously UNreliable, so I'll be using modern film types, rated at 630 Volts. As incredible as it may seem to those of us who actually fix things, and understand about the various types of parts used, I've seen these old, crummy paper caps sell on eBay for up to $30 each! Seems the "audiophile" crowd believes they have those magic properties that give you The Tone in your amplifiers.
Hmm....maybe I should list the ones I remove??
One of the players has a Selenium rectifier, and I'll replace that with a modern Silicon rectifier, and add a 33~47 Ohm 5 Watt series resistor to compensate for the lower Forward Voltage drop of the Silicon rectifier.
Selenium rectifiers can have nasty consequences when they fail, besides the stench they release. The Selenium compounds they produce are toxic, and replacing the old rectifier eliminates this hazard.
This should be a fun little project, and since I've never seen a schematic of one anywhere online, I'll draw one up, and post it here, along with a few pictures of the players.

3 comments:

  1. LOL, you're having fun aren't you! :-)

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  2. Wow - I used to mess with those things when I was in school, but didn't know nearly as much about what I was doing. You DO seem to be enjoying it.

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  3. Yeah, I enjoy bringing old stuff back from the grave!
    It's better than sending it to a landfill, and I feel it's for a good cause. My friend's little sister is in a "home", and she doesn't have much to do.
    I'll make sure both of the players are 100% good-to-go.

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Keep it civil, please....