Servicing a 1970s Magnavox 300 am/fm stereo receiver. part 2/3 preamp channel imbalance
HTML-код
- Опубликовано: 20 ноя 2023
- 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 for the preamp to be easily serviced.
so can we fix this problem?
Watch me chase this issue around. Boring? Yep. Real troubleshooting is boring. . . until you find the solution to the problem. 
Наука
![Sean Rii, Karyon, Sharzkii - Taungule [My Love] (Official Music Video)](http://i.ytimg.com/vi/eJoMuypbSzQ/mqdefault.jpg)
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.
That was my thoughts too. I left my comment before reading yours, and yes, same as you said. You got to do what makes practical sense, loved his solution.
Well Said!, It's actually mine!, I tried to fix it but after changing Caps and Output Transistors with no luck figured it was time to send it to the Pro!
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!
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 have a strong gut feeling the slight gain mismatch in the pre-amp was a red-herring. It's much more significant thru the power amp stage. I would have checked the negative feedback network for drifting values.
SO glad you uploaded this
This is proof that checking all preamp resistors is a good idea
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.
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!
Happy Thanksgiving! For solder wick I get the cheap Chinese stuff and lace it with liquid flux.
The price of EVERYTHING has skyrocketed. Just got back from Walmart. Prices for basic food and medicine still up 20 to 40%.
The wife and I save a lot by comparison shopping and using a digital flyer ap to find the best deals. An example: Coffee cream 3.99 at the Safeway.......2.75 at a competitor.....then we buy a lot cause we are definitely going to use it by the best before date. Same with other products that have a long shelf life. Only buy protein when its on sale and vac seal it. Having kids to feed you need to do what you can to save. I figure 2 of our monthly bills are cover by this strategy rather than just going to the neighborhood grocer.
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.
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!
Yep. Been there myself many times. Not a bad idea at all
Something abnoxious like bright blue.
i replaced a balance control on my 1964 astro sonic and it was quite easy
I just pulled out my 300 - the dot light for power indication is below the FM stereo light.
Got it. Ill take a closer look. Thanks!
Thanks Alan!, I sent him a Pic from my other one
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.😁
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.
Thats a problem that really needs to be troubleshot in person
@@JordanPier Thank you
i have a nakamichi rx-202 auto reverse deck -that is just starting to jam up a little from old greese turning to glue deck is clean in grate shape -like to send it to you since you are close to me to fix up --i dont know enough to do it and make it worse --like you to do it ok thanks wc
Sure. Goto repairaudio dot com
Click on the ship in link for instructions
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.
maybe it needs its nails did... ? lol HT...
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 don't like working with cramped spaces, it's a pain in the Arse!!. or Ass.
I'm sure the owner will be happy :-D
someone[ebay] in McKeesport Pa has solder quite cheap
I think that's b&d enterprises, whom ive bought the solder wick from before. No stock there though ...
i just noticed they weren't there anymore@@JordanPier
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.