Vintage Receiver Issues

[DreadPirate]

I have this Lafayette LR-1500T in not so good shape. Very near the end of its life. It has a single "pop" on each channel right at startup and a loud "pop" at turnoff. I put a .1uF/600V mylar cap across the switched outlet (equivalent to across xformer primary) and interestingly, did not affect the startup pop, it is still there, but now it doesn't pop when turning off. No matter, it has developed another phenomenon in the Tape HD and Phono positions only, an increasingly loud "roar" followed by a "pop" followed by silence, then cycling through the roar,pop,silence over and over. Is this a typical symptom of a bad transistor? Schematic link is attached, but I'm not even sure this old girl is worth the trouble but I figure I might learn something in the process...

http://www.eserviceinfo.com/download..._LR-1500T.html

[unclejed613]

it's real hard to read that schematic. i bogged my machine down and wouldn't even exit the viewer.... i wasn't able to read it, it's a gigantic graphics file and lots of wires faded/missing from the print.

sounds like you have a couple of preamp transistors gone, or maybe part of the power supply that supplies the preamp. the frying noise could be a bad transistor, but bad electrolytics can also cause this.

they were "ok" receivers for their time. not top of the line, but not trash either. i call them "mid-fi". they're probably better "mid-fi" equipment than you see a few years later, because Lafayette was Radio Shack's competitor at the time, so they actually had to sound better and have better specs than Radio Shack's receiver in the same price range. unfortunately, this was also a time when specs were rather loosely defined. max power output was measured in mega-microwatts (or they took the MAX current for the output transistors and multiplied by the total rail voltage, multiplying, you get the Peak Output Power( the amp with the highest POP sells on TOP)). i used to chuckle at adds that stated "200 WATTS!!!!", and then down below "shipping wt. 10lb". 10lb isn't even 50 watts worth of iron,

back to your problem, try injecting some signal at the wiper of the volume control and see if you get any output. it sounds like you may have a working amp section, since it amplifies switch noise coupled through the power supply, but the preamp section isn't providing any audio.

[DreadPirate]

Thanks for the reply. Even "mid-fi" is a bit generous for this one, though these may have been made by Pioneer--it is "Made in Japan." It came attached to a set of Criterion 100As and a decent Onkyo tape deck all for $15 (I paid too much). I gave this receiver to a kid in the neighborhood a month back and it worked fine except for the popping, I think the noise in the pre-amp may have been the result of "creative interconnect experimentation." The aux jack still works without this strange roaring noise, so the amp is still useful but with the popping noise.

I'm afraid it isn't worth spending too much time, will probably identify a few potential problem transistors in the shared phono/tape hd path, freeze them and see if there is any change. Too many other half completed projects laying around...