Sansui G7500 HELP.

anderslober

Yikes...not again!
Hello Forum....I bought this unit pretty cheap. Condition is good. When I got it home I DBT'ed the unit......nothing happened. I popped the hood and found the main fuse missing......'FEW', I said to my self...maybe thats the only problem. So, I put in a new 4A fuse and DBT'ed again......bright then dim.....then smoke from R53 on the driver board.......ok, so not an easy fix. I took the bottom plate of and found some horrible tech-work..........broken traces, scraped traces, funny soldering.....etc...........anyway.......I'll try to fix it. (I absolutely hate when someone has been there before me!!!):sigh:

R53 smoked and R57 looks to have been very hot........any ideas?:thmbsp:

Has anybody here on the forum fixed a busted g7500?:smoke:
 
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Well guys/gals........found another problem........took of the heatsink......no bloody TR's!!!:thumbsdn: Checkout the pic!

Would the resistors smoke because of this?

Now I need a fellow with a g7500 to take a picture of his/hers outputs....just to confirm orientation and position. (Sansui manuals are sometimes wrong with regard to this:sigh:)

Thanks.....

This is getting interesting!:thmbsp:
 
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Are the outputs supposed to be like the above post? There are tr's mounted on the INSIDE of the heatsinks.........they are not TO-3 style......rather....mt100 maybe....is that original?
 
What a mess Anders.......:thmbsp:

You are just the man for the Job.....

You might just be best to trace the positions of the transistors right back to the supplies.
NPN Collectors will go to the Positive Supply rail.

PNP Collectors will go to the Negative Supply rail.

The drivers are probably toast as well. These amps can get in a bit of a mess when they fail.
There are Fusible/NIR type resistors on the Output transistors which go hiZ, and the Bias trimmers seem to crap out on these too......

Somewhere for you to start anyway.......

Let us know how you go....
 
Are the outputs supposed to be like the above post? There are tr's mounted on the INSIDE of the heatsinks.........they are not TO-3 style......rather....mt100 maybe....is that original?

Nah, those slots look like they are for TO-3 transistors, perhaps the TO-3 sockets are missing.......
The heatsink should bolt on right over the top, you will need some mica washers as well.....
 
hey Kevin.....how are ya'? No worries......I will try to fix this one, but I am afraid the damage is already so far gone in crappy repairs that it may not be possible to correct.

I found a thread with a g7500 with a VERY similar problem............I have this odd feeling that it is the SAME unit! Which I now have..........

I will give it a go.

The main outputs on this unit are not TO3's...........2sa1106/2sc2581.....they are mt100's or something like that...............I dunno.......anyone with a parts unit???? I will take the driver board!!!
 
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I just de-soldered and tested the 2sa1106 from that channel with the burnt R53,,,,it is toast.
Kevin...do you have any sub recommendations? 2sa1106/2sc2581?
 
Wow. I wonder what the previous person was trying to do with that poor thing.

The first pic is the most shocking. (NO PUN)

If you do trace and through hole pad repairs on that mangled PC board, showing and describbing the process would be truely appreciated.
 
I just de-soldered and tested the 2sa1106 from that channel with the burnt R53,,,,it is toast.
Kevin...do you have any sub recommendations? 2sa1106/2sc2581?

As a matter of fact yes I do.........

They are TO-3P's

NJW3281G (NPN) replaces 2SC2581
NJW1302G (PNP) replaces 2SA1106

You should be able to find those ones easily. I have used these before.....
This series of G is basically a G7700 in a G7000 chassis.....they do fry up a storm when they fail, that much I know.....you will probably have to replace all the transistors back to the differential FET.
Look at ALL the resistors around the OP transistors and drivers, and replace the bias trimpots without hesitation........
 
Hey Kevin.....ok....thanks for the subs. I'll get hold of some. I just tested one of the drivers, 2sb536, ....it was ok! I will test the other driver, 2sd381, too. I may just replace with mj15032/33.

I also took out the burnt resistor at R53....it measured 218 Ohm.....pretty close to its spec of 220 Ohm.... I have put in a new resistor.
 
I'm in the process of doing a g7500 as well. Heat sink on the back looks exactly the same, tr boards look the same too, with the same transistors. Mine is just really dirty with a couple dead resistors. Check the caps on board 2852 c25-26, don't know if it's just mine but it appears they swelled and broke the solder joint, popped right off when i touched them. Also the fr fuses on mine are almost black.
 
I bought a G-7500 off Ebay. Seller said one side did not work. They sure were correct about that. Someone cut almost every wire coming in and going out of the amp left side. It still sits on my shelf. I have bigger fish to fry.
 
i just placed an order with mouser for most of the electrolytics, nir and fr resistors and a couple sets of transistors.
 
I don't know if it will help but I have the user's manual on pdf file if you want it send me a P.M.
 
I have started working on the problem area. I changed the all the tr's and a couple of resistors in the area.

I started measuring with the meter set to ohms...........R58(good channel) measures 152 Ohms.......R57(bad channel) measures 1.48K ohms(new 1.5k resistor as per scat).
What gives? Shouldn't they measure almost the same? can someone confirm the value of the resistor at R57/58? Thanks. Scat could be wrong!??
All the other resistors in the area measure almost the same. All measurements are in circuit.

I also tested the main outputs on the good channel....they were good. I will change to new tr's though.
 
More stuff.......R71/77 on bad channel.......5W/0.33 ohm resistors,....they measured '0' and 117k ohm........both were goners.

Good channel measured close to 0.33 ohm.

Getting there...............slowly!;-)))
 
I have changed the outputs..........all new sa1106/sc2581's(I managed to find some today)......also the new 5w/0.33ohm resistors went in......I did a ton of measurements on both sides and they seem to measure very close now...............I will dbt the sucka' and see if anything blows or smokes.

I tried to fix the crappy soldering/wiring..........it doesn't look to bad now, but it still doesn't look NICE! At least it seems electrically ok....that is the main thing.
 
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