Hi all, I don't know if this is the place for this but here goes. Working on a beautiful Magnavox 70s AM/FM 8 track record changer chassis #R321-12CA floor model for a friend. I know it's a dinosaur but it means a lot to him. Anyway, it's a SS unit with lousy AM. I don't know how well these units worked but it should be better than it is. With the volume all the way up, I can barely here 2-3 stations. Did an alignment on it with hardly any improvement. The antenna trimmer has no effect. I have tested all the components in the RF amp section and even replaced the RF amp transistor. Everything leads me back to the antenna circuit which doesn't have much in it. According to the schematic the antenna primary should measure 4 ohms but only measures 2. I know 2 ohms isn't much but in this case it's half of what it should be. The tuning condenser shows no shorts between the plates. If I touch the base of the RF amp transistor it improves greatly. So I guess what I'm asking is anybody have any experience with this unit and how common would it be to have a defective ferrite loopstick antenna? I tried looking on line for a replacement but it is futile. I will be posting in the want adds for this antenna. Thanks!
I would disconnect the loop and clip in one from an old tube set. The resistance is somewhat unimportant it's inductance that matters. As long as you have the proper of inductance L (loop) and capacitance C (tuning cap) to resonate @ the frequency shown of the dial. L/C ratio may be a bit off when using a random loop meaning you may get much better sensitivity @ one end of the band vs the other but still can be a good troubleshooting tool.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Thanks Terry. I was thinking since the resistance measures half of what is posted on the print, the inductance will probably be off so much which explains the poor sensitivity and also why the trimmer has no effect. I like your idea of subbing a loop from a tube set but me thinks they only have two leads, this one has four as it has a primary and secondary.
Since I don't have a schematic it's a little hard to tell but typically the pri wound connect to an external ant and secondary is the tuned circuit. A lot of the tube type loops just have the tuned circuit but also have a small value cap connected to the grid side of the loop for an external ant connection.
Another way of troubleshooting is to use a signal tracer in the rf mode and connect an ant to the input of the receiver. Like a crystal set w/a tracer on the back end of it. Listen @ the ant connection (loop primary) while tuning around for a station then listen @ the secondary of the loop. If the found station signal level is less on the secondary this could be a good indication of a bad loop or the L/C ratio is way off. If the level is about the same or higher use the tracer to listen @ the base of the rf amp. Could be a bad component between the loop and the base of the rf transistor.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 12-23-2018, 01:47 PM by Radioroslyn.)
In this case, the primary is in the tuned circuit. The secondary appears to be a step down, probably to match the lower impedance of the RF amp input. If I touch the primary of the antenna on the tuning cap, it is a little better. If I touch the secondary at the base terminal of the RF amp it is somewhat better. I've used my signal tracer with RF probe before on other front ends with no results. I think I will use my scope and generator and see what I come up with, probably after the holidays. I appreciate all your input, thanks
Thanks Arran, the couple of stations I do receive are on the right place on the dial so I am assuming the oscillator is ok. I have checked all the components in the RF section and all looks good except possibly the loopstick antenna T4 in the above print!
Ok, got back to work on the Maggy and did some testing on the ferrite loopstick antenna. I took capacitance measurements of the tuning gang in parallel with the antenna after disconnecting everything around it. I took measurements at 550kz, 1 MHz, and 1.6 MHz on the dial assuming the dial is accurate. Then using some math, calculated what the inductance the antenna should be to resonate at these frequencies. What I found was the inductance calculated out to be approx. 435 micro-henries. When I measured the antenna, it turned out to be 475 micro-henries, so I am thinking we are in the ball park close enough considering the accuracy of the settings on the dial. This exonerates the primary of the antenna for me. I don't know what the secondary should be but I find it strange that the antenna would be defective being that the windings are only one layer, what could go wrong! So I retested all the components in the RF stage and nothing. I ended up experimenting with changing the bias voltages on the transistor by changing resistor values hoping to get more gain and viola, success! I don't know how good of a performer this unit was in the day but it now plays decently. Now onto the FM where I believe the oscillator is not running!
I thank you so much as it would have been useful. I never should have taken on this project. I was told the woman that owned it told her son the only problem with it was the 8 track player was having crosstalk between tracks. A simple head adjustment right! She had a guy look at it, he took all the guts out of it and had it over 6 months. The son went to get it back from the guy and that's when he asked me to look at it. The guy took most of the components in the radio section off the pc board, then remounted them on the bottom, the foil side. He butchered it. Why he messed with the radio and did that with all the parts baffles me. Plus I got a bag of components with it too. I carefully unsoldered all those components, tested them, and reinstalled them on the top side of the board where they belong. I got a Sams print for it to help me along and identify where things should go. I had trouble with sensitivity of the AM band and tested and retested everything and found nothing. After replacing the RF amp and fiddling with the bias, got it working pretty good. Now onto the FM band where I get one station, the same station all up and down the dial. Looks like the oscillator isn't working so digging into that right now with no luck. The son asked me to look at it because it belongs to his mom who is widowed. It was a wedding present and means a lot to her. I'm a sucker for a good story I guess but ugh!!
