Panasonic
Servicing a 1970s Magnavox 300 am/fm stereo receiver.
Servicing a 1970s Magnavox 300 am/fm stereo receiver. part 2/? preamp channel imbalance
#Servicing #1970s #Magnavox #amfm #stereo #receiver
“Jordan Pier”
Diving into the preamp channel imbalance.
With channel imbalance, anything from a bad switch, to volume control, balance control, input loading, output loading at the preamp, input loading at the power amp, gain stages, and more can be the cause
sheesh!
to top it off, this is not designed…
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Aside from all of your test equipment expertise ..or rather along with it, your trouble-shooting is like a curious kitty, sneakin up on the rodent.
Excellent cost effective repair, nothing wrong with just making it work with something like this.
I have a Hitachi SR-604 receiver I was trying to set the bias on, and it won't come down any lower than 28 mV on the right channel (across a 0.22 ohm emitter resistor) where it should be 8mV +/- 4mV. Since it is only 35 watts per channel and class G it is getting buttoned up and going back to the owner, no charge. Not worth my time, circuit too complicated. Why they would use class G for a little flea powered receiver I have no idea.
Electronics is a hobby for me since I am retired, I don't have to take on what I don't want to. I like that option.😁
Great video Jordan. I was thinking of adding a power on led to my Capacitor Wizard. I can't say how many times I have kicked myself leaving the power on and putting it back in the box!
That job sure would have been easier with a schematic. And I think it would make your troubleshooting technique much easier to understand. I also don't think I agree with your point on a bad capacitor necessarily causing a frequency dependent gain difference. It would depend where the cap is used in the circuit. E.g., if an emitter bypass cap goes open in a collector follower, all you'd see is a reduction in gain across the entire frequency range. Also you were talking about voltage gain while taking waveform O'scope voltage measurements off the emitter of a couple of transistors. But an emitter follower can't have a gain of more than 1, and in practice it's usually no more than .9.
One technique I've used for cap replacement when the bottom of the board isn't accessible, is to simply chop up the cap down near its base, and carefully remove the rest of the body, to gain access to the leads protruding into the body of the cap. There should be enough lead left there to tack solder in a new capacitor.
I don't like working with cramped spaces, it's a pain in the Arse!!. or Ass.
I'm sure the owner will be happy 😀
Happy Thanksgiving! For solder wick I get the cheap Chinese stuff and lace it with liquid flux.
Another great video young man and you did the trouble shooting without a schematic now thats a true tech! Happy Thanksgiving to you and the family buddy!
This is proof that checking all preamp resistors is a good idea
Happy Thanksgiving to you and your family, JP.
That was very interesting to see the work-around on this. It's practical, adequate and cost effective.
I really enjoy when you take us along during trouble shooting. Helps to teach me the logical way to trace problems. AND you did it with out a schematic, pretty cool!
someone[ebay] in McKeesport Pa has solder quite cheap
i replaced a balance control on my 1964 astro sonic and it was quite easy
SO glad you uploaded this
I'm pretty much convinced it's a combination of HFE values in transistors and drifting resistor values. Not much in each stage but all adding up as you go along the stages. So correcting with a padding resistor is quicker and cheaper than replacing a whole bunch of components and stripping down an awkwardly placed pre-amp board, possibly introducing new faults along the way. Great pragmatic, logical trouble shooting. I don't think it gets more real than this.
My 1970s entry level Sansui receiver also lacks any kind power on light. Maybe a good idea to wire the tuning light so that it's always on, since I tend to forget to turn the thing off, thanks for the insight on that.
Has the selector switch been cleaned? That oxide in wafter switches and linear actuated switches hard to cure unless you pull them apart. Takes forever.
I have a Innovative Technologymusic stereo whin i turn it off the Leds stay on but unit is off what could it be i cleaned the switch can u help.
The price of EVERYTHING has skyrocketed. Just got back from Walmart. Prices for basic food and medicine still up 20 to 40%.
I'd just get a trim pot & pad the input to the amp so both channels are equal, box it up & ship it!
It's not worth wasting time on IMO.
I just pulled out my 300 – the dot light for power indication is below the FM stereo light.