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blast from left channel when powering off


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#1 dan

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Posted 22 April 2019 - 06:37 AM

i've got a situation that others seem to have but i believe the cause maybe something different in my case.

 

 

when i turn off my m70, there is a blast from the left channel. it doesn't matter if it's the speakers or headphones. it also doesn't matter if the volume is all the way down. it even doesn't matter if you put the unit into mono mode. the blast always comes out of the left channel.

 

it's not the power switch because unplugging the unit and not using the power switch to turn it off still presents this issue. i have cleaned the selector switch, record bar, etc. with deoxit and that has fixed other problems but not this. i have also changed the main filter capacitor. the only other thing i was thinking of was my faulty bass fader but then i noticed something.

 

when the unit powers off, the flywheel of the tape deck does a little spin around. if i disconnect the power cables to the cassette deck, the problem disappears!

 

is it normal for the flywheel to do a little spin around at power off?

 

 

any idea what can be happening here?



#2 BoomboxLover48

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Posted 22 April 2019 - 07:39 AM

A small flywheel spin is normal. I've seen that happening on my M70s. 

 

 

I would do a good switch cleaning and that long record bar cleaning with Deoxit.

 

The noise from the speaker could be from bad capacitors.

Our experts here will have better suggestions.



#3 dan

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Posted 23 April 2019 - 06:42 AM

i left the radio running all night, and this morning, the problem disappeared! very strange.  capacitors reforming?



#4 docs

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Posted 23 April 2019 - 11:46 AM

As already stated, all switches and controls contact cleaned? This has to be done as first step. This will then allow for deeper diagnostics if required.

#5 dan

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Posted 28 April 2019 - 01:16 PM

i cleaned it all with deoxit.  switches, faders, select switch, record bar, etc.

 

 

another thing i noticed is when that happens, i'm using the 12vdc to power the unit. if i plug in the 120v ac cable, it doesn't happen. if i then remove the ac cable and run it on 12vdc, it's like doing that fixed the problem for a while. if the unit stays unplugged for a while and i try to run it on 12vdc again, the problem will come back and will be cured if plugging in the ac cable again.

 

i also notice that if i unplug the unit for a day,  if i plug in the ac cable, i hear an equal hum on both channels, even with the volume down all the way and the selector on any setting. usually a few power cycles fixes the hum.

 

i'm wondering if these issues might be related  to the diodes and capacitors on the secondary of the transformer?

 

 

does the switch in the power socket that switches from batteries to ac when the cord is plugged in need to be cleaned with deoxit as well?



#6 Transistorized

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Posted 28 April 2019 - 02:55 PM

There are two leaf switches. One at the AC power plug that bypasses the 12V input (and subsequently the battery compartment since it's downstream) and one in the 12V barrel plug that terminates the battery operation when a barrel plug is inserted.

 

If you insert the power cord into the box without it being plugged into the AC mains and it turns off while supplying the 12V input then it is working correctly although I am sure it's always possible it could have high resistance from corrosion. I have cleaned them before with Deoxit along with the barrel plug with good results. You can also test the 12V leaf switch in the barrel plug by installing batteries and inserting an unplugged 12V power adapter into the 12V port. The box should cut off with batteries installed. This would be an easy test without taking the box apart. 

 

Have you tried the box with batteries? The path the current takes is the same with batteries as with the 12V port. If you find that the box does not do it with batteries then I would suspect that there is something with your wall wart circuitry that the box doesn't like. 

 

And I am curious. If the box works better with the mains and doesn't do this left channel blasting thing, why wouldn't you use the AC input? Just curious :-) 

 

Also the voltage selector on the AC circuit has contacts as well. They can get dirty too.



#7 dan

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Posted 29 April 2019 - 10:48 AM

i sprayed the switches on the barrel jack and the ac cable jack with deoxit and the problems went away for about a day. that left blast came back again just now so i sprayed the ac cable jack and the barrel jack and it went away again.

 

 

 

 

transistorized, see the thread about the voltage on chassis as to why i'd rather power it with dc.

 

 

 

 

update:

 

 

i seem to have triggered the problem again by using the headphone jack.



#8 Transistorized

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Posted 29 April 2019 - 06:53 PM

Gotcha. I use rechargeables in mine and have marked the cable to provide neutral to the top pin :-) I thought you had taken the resistor out of the equation thus eliminating the voltage to chassis.

That's a weird issue to have. Let us know if you figure it out.

#9 Superduper

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Posted 29 April 2019 - 10:04 PM

Well, you've been advised several times to make sure all the controls and switches are cleaned before doing any diagnoses as it basically clears up a lot of problems. Otherwise you are just chasing your tail.  Some oxidation is serious and can't be cleaned satisfactorily to any lasting effect by just spraying.  In those cases, vigorous agitation with the solvent is required and in really bad cases, the control could need disassembly and mechanically cleaned and treated with preservative before reassembly.  The fact that cleaning clears up your condition temporarily but keeps recurring already suggests that oxidation in this unit is severe.



#10 Hajidub

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Posted 30 April 2019 - 06:24 PM

i left the radio running all night, and this morning, the problem disappeared! very strange.  capacitors reforming?

 

I would replace the power board filter caps, they regulate the input voltage so the unit is a constant voltage instead of 120 to zero (as AC normally does). 

 

To verify I would take Transistorizes comment and test this issue with DC/Battery power. 



#11 dan

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Posted 02 May 2019 - 11:39 AM

the blast issue happens even on battery.  i sprayed deoxit in the ac jack and the dc barrel jack and actuated the heck out of the switches in those jacks but this time i didn't connect the ac power cable to mains voltage and the problem didn't go away. 

 

it seems like something happens when 120v goes into the unit and it fixes the issue for about a day.   is the dc back feeding into the transformer and is this blast sound the collapsing magnetic field when the unit is powered off?   i wonder if something is wrong with the diodes and capacitors on the secondary of the transformer?



#12 dan

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Posted 02 May 2019 - 01:50 PM

has nothing to do with the transformer, diodes, capacitors switches on power supply board. removed the power supply board and transformer completely and injected 12v right into the main circuit board and the problem still persists.

 

 

remove all connectors from cassette portion of the unit and the problem goes away. plug them back and the problem returns.  the question is why does running off 120vac and power cycling a few times cure the issue?  when running off ac cord, there's about 15.5v going to the main board. when using the dc adapter or half used batteries, there's about 12v. this is the only difference.



#13 Fatdog

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Posted 02 May 2019 - 03:26 PM

Toss that M70 in the garbage and try another one.  It's not worth all of this effort you are expending.  ;-)



#14 Hajidub

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Posted 05 May 2019 - 04:54 PM

Toss that M70 in the garbage and try another one.  It's not worth all of this effort you are expending.  ;-)

 

Pretty sure there is sarcasm in your post...lol. With any soldering skills, I'd recap the entire unit!



#15 Fatdog

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Posted 05 May 2019 - 06:28 PM

 

Toss that M70 in the garbage and try another one.  It's not worth all of this effort you are expending.  ;-)

 
Pretty sure there is sarcasm in your post...lol. With any soldering skills, I'd recap the entire unit!

 


Unfortunately, no sarcasm. If you read all of Dan's posts, he has the M70 from hell. Everything is wrong with it, and when he attempts to repair one thing, something else is found to be wrong.  That's why I said to toss it.  He would have to replace every IC, capacitor, diode, etc.



#16 Superduper

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Posted 05 May 2019 - 07:24 PM

Or if he’d just clean the switches and connections properly, he just might find much of his problems magically resolve.



#17 Transistorized

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Posted 06 May 2019 - 03:58 PM

He mentions that when using AC the issue is gone. I know that there has been concern over voltage to chassis ground when on AC however, this is proven to be eliminated by removing/taking the resistor out of the equation so, if it were me and I couldn't figure it out, especially since no one has died from plugging them in in 30 years, I would live life on the wild side, plug it into AC and enjoy it  :rock:



#18 Hajidub

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Posted 07 May 2019 - 05:41 PM

 

 

Toss that M70 in the garbage and try another one.  It's not worth all of this effort you are expending.  ;-)

 
Pretty sure there is sarcasm in your post...lol. With any soldering skills, I'd recap the entire unit!

 


Unfortunately, no sarcasm. If you read all of Dan's posts, he has the M70 from hell. Everything is wrong with it, and when he attempts to repair one thing, something else is found to be wrong.  That's why I said to toss it.  He would have to replace every IC, capacitor, diode, etc.

 

 

Minus the IC's, caps and diode's would cost around $14 and a couple day's labor. 



#19 dan

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Posted 08 May 2019 - 02:09 PM

i don't want to jinx myself, but ever since i put the antennas on a few days ago when i got them in the mail,  i haven't been able to recreate the hum on AC or the left channel blast on DC. i know the antennas only receive but i'm guessing that they possibly dissipate some kind of charge buildup in the unit that would otherwise create interference that manifests in different ways whether on AC or DC.  it sounds really far fetched, i know.  i'm gonna keep an eye on it.



#20 Hajidub

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Posted 09 May 2019 - 05:18 PM

i don't want to jinx myself, but ever since i put the antennas on a few days ago when i got them in the mail,  i haven't been able to recreate the hum on AC or the left channel blast on DC. i know the antennas only receive but i'm guessing that they possibly dissipate some kind of charge buildup in the unit that would otherwise create interference that manifests in different ways whether on AC or DC.  it sounds really far fetched, i know.  i'm gonna keep an eye on it.

 

Fingers crossed!!!!  :rock:



#21 dan

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Posted 12 May 2019 - 11:28 PM

never mind. it was just a coincidence. just gonna live with it for now. if i ever find a solution, i will post it for anyone else.



#22 Hajidub

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Posted 13 May 2019 - 05:58 PM

never mind. it was just a coincidence. just gonna live with it for now. if i ever find a solution, i will post it for anyone else.

 

Recap it yourself, or send it off to someone for repair. Simple soldering skills will bring this back to life!



#23 dan

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Posted 14 May 2019 - 12:53 PM

figured it out. the hum on ac and the left channel blast on dc. i noticed tapping on the unit would induce the hum on ac.  took the back off, with a bic pen, i started tapping around everywhere. i noticed tapping on the main PCB around the the power amps would cause the hum. i narrowed it down to the area and then i simply touched the solder points with the pic pen.  pin 18 on the left power amp is the culprit.  causes hum on both channels even with the volume all the way down while on ac and causes the left channel blast while on dc. all you gotta do is touch it with the finest touch with a plastic pen which rules out other solder joints.



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#24 hopey

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Posted 14 May 2019 - 03:01 PM

Good pickup. It must have been due to the lower voltage on DC and voltage drop across the amp pin versus the higher voltage on AC.

How many people replace caps and fix the problem but it was never the caps in the first place.?



#25 Superduper

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Posted 14 May 2019 - 05:41 PM

Are you saying you "fixed" it, or are you saying you think you found the problem because you hear a hum when you touch one of the amp chip leads?



#26 dan

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Posted 14 May 2019 - 06:03 PM

Are you saying you "fixed" it, or are you saying you think you found the problem because you hear a hum when you touch one of the amp chip leads?

 

pretty sure i fixed it. it was a bad solder joint. no more blast or hum. any vibration or flex in the board in that area would start the hum. pressing down on that joint would stop the hum because it would close the gap. putting sideways force on the lead from the chip would open the gap and start the hum. i repeated it over and over.



#27 Fatdog

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Posted 14 May 2019 - 07:08 PM

d4a06e1b2417b9f597babcbbf58f8583--praise

 

:clap: :clap: :bow: :thumbsup: :cool: :hooray: :afro: :afro:



#28 dan

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Posted 15 May 2019 - 11:24 PM

i know. it also cured some minor left channel hum i had on the phono input when i turned up the volume all the way with nothing playing.



#29 docs

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Posted 16 May 2019 - 04:45 AM

Glad you fixed it, thanks for sharing the fix.



#30 ralrein1

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Posted 16 May 2019 - 02:43 PM

Nice job finding the problem. Frequently persistence pays off. Now its time to celebrate, crank it up!!!