My first roundie, ctc 16

[ctc17]

Originally Posted by DaveWM

is there a tuning meter on that stereo?

That means no adj for tracking weight on the arm. Mine had a dead cart (good luck finding one btw, hard to comeby and expensive, plus often the NOS ones are DOA).

Nope no tuning eye. The bass is very impressive for 2 6x9s. Why would the cartridge go bad just sitting?

I have 3 of combos now with this exact tuner. One thing i still havent perfected is getting the stereo mpx to work right.

I have a phone that will broadcast mp3 on a radio station. Its designed so you can listen to your music on any fm radio. It sounds very good and the stereo separation is perfect.
I made an mp3 that is music in Left for 30 seconds then music in Right for 30 seconds, back and forth. Then I made another one that was one song in Left and one in Right. If I listen to it in any other stereo the separations is reproduced very well, even my Zenith combos... but these rca tuners.

Sams gives alignment instructions using an over the air signal and I can get the separation close but then the noise floor is so high its like a waterfall when in stereo mode. I have tried both the phone a good radio station with solid separation.

All 3 behave exactly the same so Im either doing something wrong or these things were never designed to work right. I know Zenith invented the Stereo MPX system we use so maybe rca had to deal with patent issues back then.

[DaveWM]

I use a scope to align mine you need to peak the 19khz pilot and the scope it the best way to do it. Also look around for any big .1 or 1uf caps in the path, I have had troubles there. I used a cheap rat shack MPX transmittier, a audio sine wave gen and a two channel o scope to look at L and R. you can feed one channel at a time from the sine wave generator to really see what is going on, its kinda fun after a while. There is a sep pot that balances the amount of L+R that it mixed with the L-R that is pretty critical, best not to mess with it.

One thing to really watch out for is the decoupling caps in the receiver that feeds the B+. I had a real tough dog that turned out to be an open filter (decoupler) can cap. No hum at all, so I just assumed it was ok, well it also decoupled the B+ so any audio signal riding along the plate circuit is bypass to ground. With out it you get a modulated B+ that can wreak havoc on the MPX (think L+R everywhere even in the phase inverter of the MPX).

So if you are having MPX issues and have NOT recapped every larger value cap (typically canned caps for that era), then try jumping a cap in and see if that has any effect. On mine the effect was a stronger R vs L and poor separation, no matter what I did with the MPX, and also a volume control that would not silence the audio from the radio (again the signal not being decoupled, riding on the B+ and going around the preamp tube).


all that being said prob best idea is to not adj anything unless your sure its been messed with, the caps are the most likely suspect.
My RCA's had some odd ball deemphasis on the MPX as well resulting in a more muffled sound, my guess is it was an attempt to quell the MPX "noise" that I guess was a problem, esp on a weaker station. I made a slight mode to the deemphasis caps and got a much crisper sound, if there is a weak FM station just go to mono anyway.

[Kiwick]

Originally Posted by ctc17

Why would the cartridge go bad just sitting?

Because it's based on Rochelle salt crystal piezoelectric elements, which are hygroscopic, and dissolve with dampness just like regular table salt

if the set or a spare cartridge was stored in a less than ideal environment such as an unheated garage or a basement, the Rochelle crystal has likely dissolved.

Some cartridges such as BSRs from the 70s had sealed crystal elements and endure damp storage better, others had poor or no sealing and dissolved even in a moderately damp home environment.

When the crystal has dissolved, the cartridge usually has some green goo around its terminals or traces of corrosion to the metal stylus cantilever.

later ceramic cartridges are totally immune from this problem.

[DaveWM]

Another thing about the cart is it uses a little rubber part that engages the needle (on that one anyway prob an astatic 475) and activates the two elements. on mine the rubber had gotten hard as a rock was bent out of shape. I hope yours is ok, as they are somewhat hard to find and/or expensive (seen them for 30-60$).

The turntable drive uses a singel idler wheel. On one of mine I was able to turn the wheel on a lathe and take off about 1/16 to make it round (had a divot from the motor shaft drive) then I used a wide rubber band like the kind found on broccli (really) that fit around the wheel. Works like a charm. Since the wheel is just an idler, the diameter is not super critical, other than for making sure it falls withing the right size to put pressure on the turntable when engaged.