Here we go again... g7500 problems.

anderslober

Yikes...not again!
I brought this banged up unit from the US hoping that I could do another save.

The unit had a blown right channel output. A couple of resistors looked tired. They got changed.

I recapped ONLY the power supply for now. I then dismounted all the main outputs.......proceeded to power up on dbt.......bright then dim......BUT, R52/54 get hot and smoke! These 2 resistors are only connected to Tr16/18....they get hot too.........any ideas?

I tested ALL the tr's on that channel before doing the above. As a main out put did blow.....I changed the 2 main drivers to ksc2073 / ksa940 ......the originals tested ok.
 
Vr4 measured 3.4M ohm!......ouch.......I changed all 4 pots to new 100 ohm' ers. One pot was a 220 ohm...........some fiddler has been here before me .....
 
Got the pots in.......investigated for any funny solder stuff........dbt'ed.....bright...dim and then relay clicked. Thats good I guess.
I measured the bias while on dbt..........left/0.4mv.....right/0.7mv.....both were adjustable.
My voltages are not totally consistent on each channel. The left channel is all original. The right channel has new main outputs and drivers. Could this affect voltages in areas?
 
I dunno'.......power up on normal power? I would just hate it if something fried......this amp takes out a lot of other stuff when it fails........
Whadya' say........full power?
 
I dunno'.......power up on normal power? I would just hate it if something fried......this amp takes out a lot of other stuff when it fails........
Whadya' say........full power?

Go for it. The bias isn't high so I doubt anything will burn up.
 
I have a dead g7500 with a open trans primary. wondering what are you using of o/p devices?
 
I have a dead g7500 with a open trans primary. wondering what are you using of o/p devices?

NJW3281G
NJW1302G

These are the recommended TO3P replacements.

I used Toshiba 2SA3281 and 2SC1302, which are TO247, I had to manufacture my own output section so using TO3P's wasn't entirely necessary...


EDIT just had a look at ONSEMI and ended up here, looks like they have superseded them with something else now ...

These look just fine...

http://www.onsemi.com/PowerSolutions/product.do?id=NJW0302
 
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Anders, take the plunge. The dbt is only a fail safe. I do however make it a habit to have both channels hooked up to my dmm to catch any output runaway if the amp oscillates. If something does let go at least you will have a trail to follow :D
-Lee
 
Anders, take the plunge. The dbt is only a fail safe. I do however make it a habit to have both channels hooked up to my dmm to catch any output runaway if the amp oscillates. If something does let go at least you will have a trail to follow :D
-Lee

^^
This.....:thmbsp:
 
Crud!.......after changing the pots..........I powered up on DBT.....all good......I then decided to take tge plunge...........power ON.....relay clicked.......few seconds...main 4A fuse blew!
The main tr's were HOT........no heat sink installed at the moment.

Man, WHY are these g7500 so friggin' difficult!!!
 
One thing.....before the above no-dbt-power-up......I measured some voltages at the BCE of the main outputs.......the 54v was ok both sides.....but the other voltages were not. The LEFT side was about 1/2 the voltage of the RIght side...........anyway ...no worries...
Back to the initial step! YEY!
 
Crud!.......after changing the pots..........I powered up on DBT.....all good......I then decided to take tge plunge...........power ON.....relay clicked.......few seconds...main 4A fuse blew!
The main tr's were HOT........no heat sink installed at the moment.

Man, WHY are these g7500 so friggin' difficult!!!

Because if there is ANYTHING wrong that might affect setting the bias (read almost everything) you get blown fuses, drivers and outputs. :yes:
 
I dismounted the main outputs.......the right side was blown. Left side outputs tested ok.
I also tested both stv3h's.....both tested good......I powered up on dbt without the main outputs installed....all ok...bright...dim...click.......on we trundle........

The collectors of the NPN's go to '+' voltage.........the collectors of the PNP's go to '-' voltage.......Confirm?
 
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The collectors of the NPN's go to '+' voltage.........the collectors of the PNP's go to '-' voltage.......Confirm?

Correct Anders........

You replaced the trimmers right?

Any slightest intermittent issue with regards to the bias, and it will run away on you so fast you wont know whats hit you....
 
Kevin......yes, I replaced the trimmers. I decided to power up on normal power.....main outputs not in........unit turns on fine....clicks....nothing smoking!.......BUT, it is as if the voltage on the right is 1/2 the voltage on the left in certain areas..........the main voltages are ok on both sides.

I guess I have to track down a shoddy resistor.............
 
I am back working on this unit. Without the main outputs installed, I have good DBT. Relay clicks fine.
On the LEFT side, I have almost exactly 1v more at R59 compared to R60. Also, when I measure the voltages on all three legs of the bias pots, I have exactly 1v more on the LEFT. The voltage on the RIGHT 3 legs is about 0.56v.......LEFT side is 1.56v.....

I am able to adjust the bias voltage....except on the left it wont go below 1v. Any ideas.

I have changed the emitter resistors.....drivers are new.....tr19/20 are new......resistors
R57/58, R67/68, R69/70 have also been changed.
 
Is the voltage at TR15/C the same as TR16/C? And TR17/C = TR18/C? These should all be = B+ or B-.

Also check the "downhill" side of R43,R44,R49 & R50.

The SM schematic of the bias circuitry in my G-6700 does NOT match what is on the F-2980 bd. The SM schematic of the G-7700 does match the bd. Please check your G-7500 schematic and 2980 bd against my revised schematic in post #84 of this thread.
 
Ghazzer....the voltages are equal in those spots.....thats good.
Reg. the scat'......Sansui is a friggin joke! Your new scat has been downloaded!

Anyway....after again measuring a lot of voltages.....I decided to mount the main outputs again.....brand new mje's. With the unit on dbt amd both bias pots at full CC.....I turned the unit ON.....stared at it for some seconds.....the very gently proceeded to adjust te bias pots......slowly turning them more CW......bias was easily adjustable....I set 1mV on both. NOW.........dare I proceed to normal power?
 
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