G871DB; F3076 switch issue. Any ideas?

DougBrewster

AK Subscriber
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finally got around to refurbing my G871DB and dis-embowelled it of the full set of (8) lower boards in pretty much one piece. It's had a history of dropped channels that could be cured by a thump on the left when cold and on the right when hot and I'd already decided to disassemble the various control switches. First switch I've opened is the Tape Function selector, 56 pins to de-solder and an hour plus to get it out. An issue was immediately apparent in that one of the contacts is completely worn away as in "disappeared" material.
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I've been mooching around the web trying to find a spare board without luck. Does anyone know where else these Alps switches were used apart from the big G series?
 
I wonder if the G-7700 used the same switch? If so, I may have one.
Perhaps you could check the service manuals and look at the part numbers.

Rob
 
I'll do that Rob. I've realised since posting that I have a spare 9090DB tone board with Alps (defeat) switches of a similar type which, just maybe, have the same contacts.
 
well the Alps switches in my spare 9090DB switches didn't work out nor did the switches in the G-7700 suggested by Rob in post #2. I've bought from epay a pcb from a G401 which looks like it has the same pattern switch so I'm hoping it contains the same contacts. These contacts are very small and exposed at the inboard end to any crap that may fall into them. If it works out I'll post the result; these contacts may prove to be an invaluable resource..
 
The selector switch I have from a G-7700 looks similar. I will open it up and see how it looks. I also have a Marantz board with an Alps switch that looks the same.

Rob
 
Some pictures of the F3079 pcb which switches the various muting/noise cancel functions

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All the contacts in these switches are in good condition apart from being a little dirty. The dis-assembly was necessary because whilst DeOxit can be sprayed in through the hole on the top of the switch, it isn't possible to get the Corrosion X into the contact area because Corrosion X isn't available here in aerosol form so access is necessary to apply it manually. It also provides a good level of confidence in the switches so that when the unit doesn't work when I switch it back on I can pretty much rule switches out...
 
progress so far is about 96 electrolytics replaced, a few zeners and fuse resistors and a couple of smoke damaged components from a previous episode prior to my ownership. Hardest part is building the order for parts from Digikey, very time consuming. The delivery was impressive; from time of ordering to having the parts in my hand was inside 40hrs (from USA to UK) for the two large orders. I managed to get some switches from our friend in Holland (Sansui Parts Shop) out of an AU-X7 that had contacts that while a little different in shape are a perfect fit.


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progress so far is about 96 electrolytics replaced, a few zeners and fuse resistors and a couple of smoke damaged components from a previous episode prior to my ownership. Hardest part is building the order for parts from Digikey, very time consuming. The delivery was impressive; from time of ordering to having the parts in my hand was inside 40hrs (from USA to UK) for the two large orders. I managed to get some switches from our friend in Holland (Sansui Parts Shop) out of an AU-X7 that had contacts that while a little different in shape are a perfect fit.


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That's awesome, the switch thing does not look like fun.....
 
Did you work out the contact issue from post 1? I can still pull apart a coupe switches if you like to see what's in them.

Rob
 
Did you work out the contact issue from post 1? I can still pull apart a coupe switches if you like to see what's in them.

Rob

yes I did (see post #8); they are a perfect fit but wouldn't work in every instance/switch version because of the contact shape but they worked in this one. Thanks for the offer
 
OK so an update's overdue. Had a problem with non-functioning function lamps when first powered up. The input selector switch which had it's contacts replaced with those from a AU-X7 was the first suspect even though I'd spent a lot of time on it. When I opened it up again I noticed what I hadn't seen earlier; the contact slots in the original switch rack have ribs in them which were causing excessive contact pressure/distortion. So rather than waste the new switch racks from Holland (which of course has the correct contact slots) I figured that I'd be able to make a new long rack out of the two of them. The original switch rack is a four gang whilst the AU X7 switches are three gang.Chopped the end off one of them and super glued them together, making sure the distance between slots was accurate and then filled the hollow back of the rack with Araldite to give it mechanical strength. So now I've got a selector switch with all new contacts and 4 spare contacts and there are now 7 spare original contacts, should I ever need them,for the tape function switch.

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And above is the finished board, the switch in question being the lower one.

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And a picture of the tone board nicely cleaned up..


The body count has gone up somewhat to a little over 100 caps, about 40 resistors (including emitter resistors), a dozen or so diodes, zeners and VD1212s. I've also replaced the digital frequency display which was a little pallid and patchy with one I got out of a T80 tuner. Big visual improvement. The channel dropping which is where I started with this project was almost certainly due to one or both of a couple problems found during the exercise; the volume control had a bad patch on the track which has cleaned up and a there was a dry solder joint in the ribbon connector between the power supply boards which was disrupting the 32V rail.

The only anomaly I've seen is that at very high power - say over 100W - with too low a signal input the unit would oscillate. This would be about 150mV rms or less; is this something I should be concerned about? The output transistors have been replaced with MJ21193/94s.

The unit power tested at 225W/230W per channel (one driven) prior to clipping.
 
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