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 Post subject: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 1:45 am 
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I recently came across this Magnavox Concert Grand Console , it has all the bells and whistles , Remote control, auto tune radio, R2R, but channel number 2 is very weak, if you turn the balance over and crank the volume , you can hear it playing. So by process of elimination, I pulled the input off the main amp and connected another sound source and both channels play fine, So I concentrated on the pre amp .. Checking it I found 2 bad or open caps,( the 2 smaller caps on left is what I replaced) I figured that had to be it, but still no sound ... I checked everything else on the board and everything checks out fine as you can see by the picture , there is not much to it, The only thing I did not check is that couplet in the center off the board, it is 11 pins and both channels go though each side of that couplet ... according to the schematic , it is all resistors inside no caps... could this be the culprit ? the only other thing I could think of is the output goes though the remote control box , but have not looked at it yet , just wondering if anyone has come across a bad couplet before?
Thanks


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 2:24 am 
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bad transistors

more precisely, do voltage checks on each of the transistors or use a signal tracer follow the signal.


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 2:37 am 
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Bones007 wrote:
I checked everything else on the board and everything checks out fine ...
I think you might have to go into a little more detail as to how you determined that the transistors are fine. I am inclined to think along the lines of Artcurus. This is not much reason for for a resistor-only couplate to fail unless there has been a short circuit, but it is remotely possible.

Can you post a schematic of just this preamplifier section (preferrably scaled large enough to be easily read).

That couplate looks remarkably symmetrical. I would like to see what the documentation reveals with regard to the pin connections.

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 3:35 am 
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I am no SS man, but I can muster my way through if I have to do so.

I've serviced enough Magnavox units of this era to know that if it has germanium transistors, they would be a good $20 bet that one/some is/are defective.

I had a massive Magnavox that would rattle the walls and blow out the windows.

the story has a sad ending.

viewtopic.php?f=9&t=341478&p=2863741#p2863741

steve

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 3:37 pm 
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Would you be good enough to post a picture of your Concert Grand? I have a tube model. I haven't seen a solid state model. Thanks.

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Wed 21, 2019 11:43 pm 
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Well I am not sure what it is ... maybe a astrosonic 100? ,on another forum (audiokarma) a guy claimed these top o the lines were considered concert grands but never mentions if they are tube or solid state. trying to get info on any particular model is impossible other than vague 40 year old dealer brochures... I pulled each transistor out and checked with a VOM, and even used one of those Chinese testers and they all read fine...checked voltages on both channels while playing and the voltages were just about identical for both channels ... still brings me to the only thing not checked is the couplet. or as I stated this has the remote control feature and both channels come out of the remote box with the relays ... not sure how it controls the volume I think it uses a stepper motor... maybe one side is wonky and not working? I don't have the remote to test it...this unit also has a loudness push button , every time you push it... it gets lower and on the 3rd push it can shut off the power to the whole set???? never seen that before...I will have to figure out a way to show that part of the schematic because all I have is the online version.


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 1:50 am 
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Bones007 wrote:
still brings me to the only thing not checked is the couplet.
Granted that your picture of the couplet isn't exactly ideal (which is why I am interested in a schematic), but it does look remarkably symmetrical with regard to its connections to the rest of the circuitry. In fact it looks like it might be possible to "carefully" remove the couplet, turn it 180 degrees and reinstall it. In so doing all the left channel resistors would then be on the right channel and visa versa (only if it is truly a completely symmetrical device). In so doing you could prove whether or not the couplet has damage unique to one channel (the problem would swap sides).

Before doing this you would have to be sure the couplet is definitely symmetrical and you would have to be able to remove it and reinstall it turned around without damage.

This would require a good desoldering tool and some caution to avoid damage (not breaking circuit traces and not breaking the couplet itself).

Just a thought.

I would be more inclined to to use a circuit tracer (either audio or an oscilloscope or maybe just an AC voltmeter) to trace the audio signals through the preamp first if you have such equipment. You would be just comparing the working channel with the non-working channel and trying to determine where the AC audio signal suddenly degrades.

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 2:01 am 
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It is probably a good bet that yours is an Astro-Sonic 100 if it is TOTL. I know of at least two other solid state Concert Grands, so apparently they do exist. I have never seen one, and would like a picture, if you would care to share. And I thank you.

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 4:20 am 
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Dutch Rabbit wrote:
I am no SS man, but I can muster my way through if I have to do so.

I've serviced enough Magnavox units of this era to know that if it has germanium transistors, they would be a good $20 bet that one/some is/are defective.




+1

Agreed.

To the OP,

SS is trickier, especially early SS. You still very much need a signal tracer or an Oscope to properly troubleshoot this. You can spend hours swapping component sides, risking damage, or follow the signal directly from the source with the Oscope, and find exactly where it's having issues.

Im still leaning toward a bad xsistor further up the chain.

If you can find a schematic, we can go from there.

I will tell you that as others have said, this is a TOTL system, more than likely with a 100 watt amp, and the speakers are probably in foam casings. This thing will shatter glass.

One other thing, we have seen some disc caps fail. so keep that in mind also.


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 6:30 pm 
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Well I was able to figure out how to extract the one page showing the pre amp hope this helps


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 8:01 pm 
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Well figuring I had nothing to lose , I disassembled the remote control box, since both inputs and outputs run though this thing , and sure enough the control wipers were full of crud , after spraying it out with contact cleaner , it now plays strong on both channels … Thank you Guys for trying to help me out with this...
here are a few pics of the remote with the cover off


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 8:51 pm 
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Congratulations. For future reference that couplet is definitely symmetrical.

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 9:11 pm 
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i will just say that if it is NOT a transistor, i will be really surprised.

i've passed off on enough of these magnavox sets from the 60s once i found the trouble.

also, if a NOS germanium or used one can be found, it is very likely that it too could be problematic.

my most often anomaly with the transistors was static, hiss, crackle, pop, frying eggs in sets from this era. it was not the capacitors.

SS is not my thing. i don't like SS and have very little interest in doing anything with it and there is no way i'm going the "convert to silicon" idea.

respect and hats off to those who have the knowledge and the ability.

steve

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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Thu 22, 2019 10:13 pm 
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Yes , I have has another SS Maggie with output transistors that were leaky but once I replaced them it plays fine , I also have tube sets , just sold a little maggie with 6V6 push pull outputs ... I just couldn't pass this one up this being the total package...I have to re-watch Shango066 on YT he seems to be the pro on switching out silicon for germanium's...
Thanks


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 Post subject: Re: Magnavox Concert Grand... Weak Channel
PostPosted: Aug Sat 24, 2019 6:39 pm 
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There are some modern replacements for some germanium transistors.

Once I needed to replace the germanium output transistors in an organ amp because they were quite noisy.

Found a modern NTE replacement which I installed and it has worked just fine.


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