Had a disaster withmy GF-777 HELP!

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gld1307

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Jun 16, 2009
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Hi Guys,

I was trying to see what was wrong with one of my lIne-in sockets and had the GF open. One of the output aux-jacks comming from my IPOD came out and touched the back of the large circuit board that resides behind the In\out socket panel. It has knocked out my entire input stage.

Here are the symptoms.

-- No radio, tape or Line-in.
-- VU light comes on but VU meter does not indicate battery power.
-- My volume control is a tiny bit crackly and when I turn it the crackle is still there (so I don't think the amp stage has gone).
-- The tape players function fully but no sound.

I have a feeling that one of the ICs had gone but do you guys have any pointers as to what to check first?

I have dowloaded the service manual (courtesy of Fatdog from another thread) so I'll be studying that. :sad:

thanks.
 

monchito

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May 5, 2009
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:hmmm: i belive there are a few ic in that board as all audio is though those circuits before it gets amplifed, some ic are preamps plus you might nave a few of the regular transistors, and knowing exactly were it touched just to give a clue to were to start hopefully you have some knowleage of electronics ,, superduper can help alot he will post something as soon as he reads this he is good at pinpointing in these areas :yes: :yes: :yes: :yes:
 

gld1307

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Jun 16, 2009
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Thanks Monchito,

I know roughly where it touched and am looking through the circuit diagrams now to identify the area.

Cheers.
 

gld1307

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Jun 16, 2009
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O.K.

I think the RCA jack touched something in and around the area if IC 351 but it could be around an inch to the side and down. But that's the general area.

Thanks
 

monchito

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May 5, 2009
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well it does not take much to short out an ic really even a transistor could take a bit of a zap depending on what voltage is being applied but a ic they tend to short quick..
 

Superduper

Member (SA)
Well, what can I say dude. Boombox shorted out, no sound, what could be wrong? ---- TAKE YOUR PICK. So many places, so many things can go wrong. Additional troubleshooting is in order. Just too vague to be able to offer any kind of guidance. On the other hand, your Batt Meter/Dial lamp symptom is actually helpful. My guess is that the ripple filter, Q110 transistor is probably toast. That's because the dial lamp is taking juice from the collector of that transistor but the Batt meter takes power from the emitter. With it reading nothing, that transistor is likely toast. Is that the only thing wrong? Maybe, maybe not but it's a starting point. Now that you've got the schematic, you get the distinct priviledge of doing the arduous / tedious task of troubleshooting by measuring and comparing voltages with the schematic.

One thing is for certain. On the boombox, there are many areas where the circuit takes 2 parallel paths. Since you are getting no sound from either side, the issue (and there could be more than one) is common to both channels. The power supply is common to both channels but we already know that it's OK because tape plays and dial lamp illuminates. That is your hint.

BTW, powering up a boombox with back open is NEVER a good idea.
 

Alf

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Jul 29, 2009
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I'm gonna throw up! I'm praying you get everything in order!!!!!
 

gld1307

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Jun 16, 2009
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Thanks Superduper,

My first thoughts were that it may be the SNRS circuit (just thoughts) but I'll go through the whole thing. It will take quite some time obviously but I'l let you guys know how I get on.

Cheers

Gary
 

Superduper

Member (SA)
Semiconductors are the components most likely to suffer damage. The SNRS might've been damaged and I'm not looking at the schematic but I'm going to presume that NRS is usually limited to tape circuitry. The tuner and line-in signals shouldn't filter through that area. Right now, my immediate reflex is to check out IC-115. That would be my first place to look and also check out the transistor I mentioned. There may be other items damaged too, especially if the jack bounced around as it touched the circuit board.

Capacitors, resistors, not likely -- failure on those are usually not acute immediate failures. Semiconductors such as IC's, transistors, diodes :yes:
 

gld1307

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Jun 16, 2009
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Hi Guys,

I'm waiting for a transistor (q110) through the post. I tested a number of components including the ICs. Nothing is recieving power because no power is comming through the emitter of the ripple filter transistor. There is power from the collector to base so I thought that it's either the transistor itself or the capacitors that make up the ripple filter. That part of the circuit has to work before I can see what else is wrong, if anything.

I will let you know how I do.

Thanks for your concern.
 

Superduper

Member (SA)
gld1307 said:
Hi Guys,

I'm waiting for a transistor (q110) through the post. I tested a number of components including the ICs. Nothing is recieving power because no power is comming through the emitter of the ripple filter transistor. There is power from the collector to base so I thought that it's either the transistor itself or the capacitors that make up the ripple filter. That part of the circuit has to work before I can see what else is wrong, if anything.
Ahh, it sounds like my initial suspicion might be correct after all.... :-)
 

gld1307

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Jun 16, 2009
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Yes, it very much does! There is a chance that it could be the capacitors around it that are responsbile for turning on the transisitor when current starts to flow. But first things first.

If I'mm sucessful in the changing of this transistor I think I will need to run the boomer towards full loudness for a while to 'flush out' any other circuits that may have been weakened by what had happened.

I will let you know either way.
 

Superduper

Member (SA)
Don't worry -- that transistor is forward biased and turns on by the current it receives through R223. The capacitors have nothing to do with turning on the transistor and unless they are shorting to ground, should not affect the transistor. While capacitors and resistors can and do frequently fail, IF it was working before your "incident" it is not likely at all that they failed in that way. Soooo... 90% sure that changing out that transistor will restore voltage to that rail.
 

gld1307

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Jun 16, 2009
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Hey Super,

I've replaced the transistor and everthing seems to be working! As I said I need to run it relatively loud to see if there are futher weaknesses. There's just one strange thing I need to look into. When running on batteries everything is fine. When running on mains there is interferance in the form of a mid pitch through the speakers. The music drowns it but when there is no music it is noticable. It may be due to some sort of insulation or something but I am as happy as one can be given what happened. I sure I'll find it.


I can't thank you enough. In fact, I may start a new thread in praise of you abilities; rather embarassing for you I know but I'm not sure whether you have much choice. :lol:
 

gld1307

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Jun 16, 2009
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He definately is! And everyone needs to understand the ability of this ability. He's the stuff that great countries are made of. If there more people like him and less Wall Street types we wouldn't have a banking crisis; no gambling just people with real skill meeting real needs.
 
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