No selectivity/Sounds like crap

[Jonathan]

I got an RCA K60 with RC-415B chassis. I recapped it and changed the drifted resistors. I rewired the magic antenna the way it's suppose to be wired. On AM I get only one or two stations. I changed the trimmer capacitor that is part of the antenna tuning circuit. Should I try retuning it or do I have weak tubes? SW comes in pretty decently at night though but I feel it's performance could be better.

Also, the speaker. It's an RL-70H6. It sounds terrible. I connected my mp3 player to the TV and Victrola audio inputs of the RC-415B chassis and it sounded quite bad. Very frequency limited. It sounded like I piped the audio through an AM modulator as it sounded very frequency limited like broadcast AM. The cone isn't too brittle, but has come mostly unglued from the perimeter of the speaker frame. Does it need to be reconed or fully glued back on the frame or is that how it's suppose to sound?

Thanks.

Jonathan

[mr_fixer]

Yes the speaker should attached to the outer frame but that should make it buzzy as it vibrates. as far as having a limited frequency range, i've had a few radios that had a .01 mfd cap going from the output plate to ground just for that purpose. i even found one on a e.h. scott 800b. I guess people weren't expected to enjoy sound frequencies above 6-7 khz, the 800b has a 2 way jensen speaker and the tweeter is disconnected except for playing records. Odd since this radio has a fine FM system. Logan

[Jonathan]

Logan,

Hoe did the .01uF cap connected to the output plate to ground improve the sound quality? Schematic for my chassis is here.

Thanks.

Jonathan

[mr_fixer]

It didn't, it was there to roll off the high frequencies. it sounded much better after i removed it. In your schematic it is c29 the .005 mfd cap. from the plate to cathode/ground. also there is a 220 pf cap number c14 that also rolls off the high frequencies in the driver tube. i would try removing it also. I don't know if peoples hearing has changed over the decades. it makes me wonder if you could take a modern stereo back to the 1940's if people would complain about too much treble. The telephone Co. has limited freq response to 4k for over 100 years, i wonder if people were used to that response and radio manufacturers made radio sound curves to match. I dunno. Logan

[richfro]

Be careful disconnecting bypass capacitors. The mfg. put them in there for a reason. Many times it's to prevent the tube from going into self-oscillation or to bypass any RF that could be picked up by the wiring or other components. AM broadcasts don't go beyond around 8-10 KHz in frequency response anyway. The amp was probably designed to make the broadcasts of the day sound good. There was no "hi-fi" back then. As far as the trimmer cap is concerned, if you know which band it's associated with, tune in a station near the high end of the band and adjust it for the loudest signal. Don't adjust anything associated with L1, L2, or L3. They are oscillator tuning coils and caps and determine the frequency range that band receives. As far as the speaker is concerned, you can try glueing the cone back down to the frame. Try to keep it centered as best as possible. If you have (or want to) replace it with something more modern, be aware that the schemetic shows it has a "voice coil" arrangement. They used a coil in the speaker housing as a filter choke in the power supply, which also doubles a magnet for the speaker itself. If you replace the speaker with a standard one, you'll have to replace the filter choke winding with a fixed choke or a resistor, maybe a 470 ohm 5 watt type. Sounds like a fun project.

Rich

[mr_fixer]

After re-reading the above posts i thought about something that may be affecting your am reception. I believe you said that this radio has a loop antenna? might try moving the radio around to see if there are other stations that are nulled out by being on the wrong side of the loop.
As far as those 2 bypass caps , try clipping one side where it would be easy to re-solder them, if it starts to oscillate. Try 1 cap at a time. Start with c29 that is the one i think does the most high freq roll off. It was put there for a reason, it might saved some cost than putting a better transformer or speaker in the audio section. Only your ears can tell.
Logan

[Jonathan]

I used weather stripping adhesive to glue the edge of the cone back on the frame. It works really well. I used the same on the holes in the cone, but there was only one big hole previously glued and 3 small ones. Other than that the cone is in great shape and i see no reason to get it reconed. Now that it's glued down I can see how stiff the cone is, as it doesn't flex that much at all. too much pressure will puncture it. The speaker is electrodynamic so the field coil provides the magnet for the speaker. The field coil has a resistance of 1.06k which I can find a replacement easy. I don't want to use a resistor because it won't filter that well. A Hammond 155J should do the trick; 1.026k ohm ,30mA max, 400VDC max, 15H. I can also use the an audio transformer by Hammond that has multiple output terminals for connecting almost any speaker you want, just use the right terminals to get the proper impedance. But I really don't want to do that because I want to keep everything as original as possible. The .005uF bypass capacitor from the cathode to the plate is required, but possibly the value can be changed to pass everything below 20kHz. It should have a low reactance at audio frequencies. At 20kHz the reactance is 1.5k ohms. Think I can change it without issue?

As for the trimmer capacitor, like you suggested, I'm going to tune to the strongest station and adjust the trimmer for the best signal.

Thanks.

Jonathan

[mr_fixer]

according to my reference guide the output load of a 6f6 should be around 7000 ohms, my online calculator shows 3183 ohms reactance at .005 mfd at 10khz, I've forgotten how to calculate losses db but a load that is half the ohms of the output can't be good. I guess you could pull up a spice simulation program or you could just substitute caps till you found a tone you liked. Logan