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Topic: Advice on Selling Old Radio

Joel N. Weber II
Expert Film Handler

Posts: 115
From: Somerville, MA, USA
Registered: Dec 2005

posted 08-03-2006 12:23 AM

Modern stereo recievers have an ``RIAA preamp'' on the phono input, which intentionally changes the frequency response to compensate for the characteristics of the phonograph. I don't know when the RIAA preamp first came into existance, but if there is such a thing in your radio, you might look at how to bypass it for the CD player.

Rane's ``why not wye'' tech note is probably a good thing to look at for how to correctly mono the CD player's stereo output. That tech note would seem to say that your CD player won't be happy with the impedance if you just short the two channels together.

Tubes often have different impedance characteristics than transistors (for example, ham radio transmitters based on tubes are a lot more tolerant of varying antenna impedances than transistorized transmitters), and I don't know much about what sort of impedance you'd find on a typical tube audio amplifier. But the CD player is likely expecting a fairly high input impedance on whatever it's feeding; you probably want at least 10k. I think with transistorized circuits, an op amp is typically used to get this input impedance.

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 08-03-2006 12:54 AM

I'm still working on the schematic. It's been a long time since I worked on anything like this. It's taking a while for that part of my brain to start working again because I haven't used it in 15 years.

(Pauses to wait for the joke...)

It looks like th phono taps in right after the detector, into the preamp. It also looks like there is a potentiometer in there, somewhere between the output of the phono cartridge and the input of the preamp. Once I get better information, maybe I can tell if it will just hook right in or, maybe I'll need to adjust the impedance a little.

Right now, I'm concentrating on the caps. There are a few "Micamold" caps in there. People say that they have a habit of popping like popcorn when they get old. Replacing those and the caps in the power supply will be the first order of business.

The second thing I plan to do is patch any wires that have frayed insulation. If I can unsolder one end of a frayed wire and slip a piece of heat shrinkable tubing over it I'll do it that way. Otherwise I'll have to replace the whole wire. Obviously, I want to keep that to a minimum, but if a wire is too frayed, it's gotta' go!

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