gf-777 buzz/interference puzzle

Hey guys, just got my first old boombox two weeks ago and have been loving it. I'm learning it has a puzzling quirk once in awhile. Sometimes when I flip the power switch up the speakers make a pop and there is a buzzing static sound. The first time I noticed it I had taken it with me for the day and due to its awesome size it had to ride on its side in my passenger seat. (seat belt on of course haha) So I flipped it onto its other side and gave it a light tap and it stopped. I figure its old and something is a little loose. Days pass with no symptoms until the other day when I turn it on and the same thing happens, this time it hasn't been moved for days. I tipped it on the tape deck side, it got louder, I tipped it on the tuner side it went away. Both times I gave it a light tap on the side. The puzzle is, if something is loose why would the buzz be triggered if it hasn't moved. Sorry for the long story but I wanted to be as descriptive as possible. Has anyone encountered something like this? I searched the forum and couldn't come up with anything. My thought is I should open it up and take a look for something loose. It would be easiest to have it powered on when manipulating wires to find which one produces the symptoms, but I'm not sure this is possible or safe. Any suggestions guys? Thanks to all in advance. Oh by the way the sound is amplified if its plugged into the wall.

Could be the filter capacitors failing, or it could be that the controls, especially power and function switches or record bar needing good cleaning. The tapping sometimes can temporarily restore a poor connection. When it's buzzing, try flipping the controls or try putting machine into record mode a couple dozen times & see if that helps. If yes, it's a pretty good indicator your rig needs a thorough cleaning. And of course poor connections as you say could be the culprit and wiggling wires & connectors is a good start to locate such issues.

ok I will give that a try thank you.

are the filter capacitors something I can replace?

It could be the black wire, which is screwed on the tape deck...
The earth wire (shielding), or, turn down the record level

and, what Superduper says, clean all switches

jasatc77 said:

are the filter capacitors something I can replace?

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If you have an electronic soldering iron, not an industrial one, and you have experience soldering, you sure can. They are usually the largest capacitors you can find, and for intermittent issues, I have actually found culprits by doing a "tap" test of the caps. Any tapped capacitor that results in any change is highly suspect.

But the tap test, only identifies intermittent failure candidates of a specific nature. Other tests may need to be performed such as resistance, capacitance and esr. You'll need some equipment to perform those tests however.

another thing about the gf 777 is that damn selector switch on the back by the rca jacks acts up and can cause all kinds of issues , cleaning and lubing that switch helps because they dry out and lose contact over time.

Interesting, i tried flipping switches and pressing the record button a few times without a change in the sound. However I didn't try the switch in the back. The sound seems to be independent of the radio or tape playing. It comes on when you turn it on without anything music turned on yet. Hopefully it's a connection and not something about to fail. I will open and clean and keep you guys posted. Thanks again.

Ok guys, the unit is very clean on the inside, all switches and knobs work perfect. Here is the culprit to the off and on buzz static sound. A light tap will make it come or go. Its the silver piece, I don't know what it is Im sorry. It doesn't feel loose and haven't found a broken piece yet. Anyone know what it is?

That looks to be the heatsink for the amplifier modules. Could be the amp modules have poor solder joints, or the internal connections (from leads to internal circuitry of IC) is fracturing. You can try reflowing the solder joints to see if that will help, and if not, you may need to replace the chip IC. It would be the Hitachi HA1392 module. If you change the module, make sure you restore new heatsink compound.

Wow you really know your stuff thanks! I will check the joints. If I should need to replace these modules, where does one find a replacement? At this point touching aluminum which I gather is the heat sink seems to be the trigger. Hopefully I can narrow it down to a weak joint.

That heatsink would be grounded usually and probably via a screw. Losing it's ground could cause it to collect interference to the chip. If the chip is bad, you can find replacements usually on eBay or through the web. It's an obsolete chip and only NOS (new old stock) remains. If you are unable to find genuine Hitachi replacement chips, you can try the generic replacements NTE chips. They make quality products but compatibility is not 100% assured, especially when the chips are operated in bridge mode (the ones in the GF-777 are not). Try here:
http://www.weisd.com/test/GenericParts_WEISD_view.php?editid1=HA1392

IC's, over time and through frequent heating/cooling cycles from powering on/off, the internal connections attached to the external leads fracture. The rate of such failure obviously depends upon how the unit was used. If the poor connection is due to loose heatsink screw or poor solder joints, it can be repaired by retightening the screw and reflowing the solder joints (add a little fresh solder too). If that doesn't fix it, then the internal lead fracturing is highly suspect. Each of those 2 heatsinks control different amp pairs. One pair powers the inside speakers and the other pair powers the outside speakers.

Hey thanks a million Superduper. I really appreciate it and am really hopeful that we can get this gem rocking consistently again. While I have it open I'm going to repair the loose power switch and perhaps replace the cassette belts. I will keep you updated.

Ok, here is the latest update. I checked and cleaned the connections, repaired the loose power switch using the hot nail method I found here on the forum, put her back together and enjoyed a day of static free listening. Unfortunately she is back at it. So I am going to try to resolder the joints on the lower of the two heatsinks. this one powers the sub woofers I figured out. I am however, not going to attempt it until I practice my techinique on a less valuable piece of electronics. I have done a lot of solder work in cars for audio/video installs, but not on anything this intricate. Can anyone recommend a good solder iron for this application? The gun I use in autos seems too big.

My english isnt the best to understand all here , but when you need an GF 777 amp, let me know it.
You dont have to solid something, all connections are easy to plug out...

As super said, it could still be a bad cap.....more likely than the IC....

jasatc77 said:

Ok guys, the unit is very clean on the inside, all switches and knobs work perfect. Here is the culprit to the off and on buzz static sound. A light tap will make it come or go. Its the silver piece, I don't know what it is Im sorry. It doesn't feel loose and haven't found a broken piece yet. Anyone know what it is?

Click to expand...

Cool to see it open inside. Are the outer speakers rated the same as the inner? Thos center subs replacements?