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Sansui G7500 HELP.

Sansui used plastic push rivets on this unit...........I will have to get some more of those, as I had to de-mount the whole input section to get to the output boards.
 
Well....I tried to dbt the unit...........bright, dim, bright dim.......kinda' oscillating a bit.
I had 2 meters hooked up to the emitter resistors at the same time as the dbt test. The good channel showed around '0'mV...the bad channel showed 350mV! I tried to adjust the vr, but it did nothing! It was constant 350mV. (should be 15mV).......what can this be? Any suggestions are good..................otherwise, I think the unit is ok. Just need to be able to adjust the vr(it is a new Bourns 100 ohm)...............could it be a tr that I have overlooked somewhere? Which tr 'regulates' the vr?

Thanks..........
 
I have looked at the scat......a well hidden tr......is it tr9 that 'controls the vr3?

WOULD A BAD STV-3H GIVE THE ABOVE SCENARIO......(sorry..caps lock..gulp)

Please confirm............thanks.
 
You should be able to check the st3 with a good dmm. For (testing) purposes you also could string three 1n4148's together which should allow the circuit to function although bias setting could be off a bit. As I recall chasing down a fault on that amp driver board I ended up pulling every resistor on the bad channel until I found the culprit which I believe was part of the feedback circuit.
 
ok......feedback circuit......please indicate where and which resistor....thanks. I changed a few tr's........I still have the pulsating dbt.........now the bias also fluctuates.....before it was constant(350mV)............It seems that I am close..........I just cant figure what to change and test next.........
 
R33,37,39,49,51,53 or their complements on the other channel. Often a bad or highs resistor in that loop will mess up the bias trimmers VR03.
 
overunder1.....DONE! I will change them and report back. I changed R53/45, tr7/8/9, and the drivers..............let us hope it is a resistor that is acting up.

I am using a 40W bulb for dbt.............I hope I haven't damaged anything.
 
Last edited:
anderslober said:
overunder1.....DONE! I will change them and report back. I changed R53/45, tr7/8/9, and the drivers..............let us hope it is a resistor that is acting up.

I am using a 40W bulb for dbt.............I hope I haven't damaged anything.
Click to expand...

I think you should use a higher wattage bulb Anders, this could be some of the problem here......
Put a 100watt in and run it up, and report back....

Also you can take the .33Ω emitter resistors out, and use 100Ω in their place, this will protect the outputs while you are diagnosing.....
 
hey Kevin.....ok...the 40W may be to 'small'? I am just worried that I will fry something........this amp is butchered enough already....probably can't take much more.

I try with the 100W bulb.

I will change the resistors............I don't have any 5w/100 ohm on hand....can I use less wattage?
 
Ok...I am on the case......I tested and changed R33/37/39/51/53..........low and behold......R37 should have been 10 ohm....but was 'OL'..........the others were fine(I changed to brand new ones anyway.......................)

Kevin......I don't have any 100 ohm(instead of .33 ohm)...should I still use the 100w bulb for test? OR use the lower wattage 40w?
 
anderslober said:
Ok...I am on the case......I tested and changed R33/37/39/51/53..........low and behold......R37 should have been 10 ohm....but was 'OL'..........the others were fine(I changed to brand new ones anyway.......................)

Kevin......I don't have any 100 ohm(instead of .33 ohm)...should I still use the 100w bulb for test? OR use the lower wattage 40w?
Click to expand...

Have you got a 60watt bulb?

I think you have found the problem anyway, R37 is crucial, and would make the amp oscillate if it was open, Im pretty sure.....

The 100Ω are a good idea in any situation where you want to protect you output transistors when you are chasing faults around the amplifier....
5watt is best to handle the current.....
 
Haaahhh.....you got me again......no 60w bulb.......I'll get one though.

I don't have any 100 ohm/5w's on hand..........so I'll chance it. R37 was definitely bad.

I'll report back after I double-check everything for power-up.

By the way....is there any chance of the outputs blowing at 350mV? This is what came through to them with the 100w bulb. I think they may be ok.

No fuses have blown.
 
anderslober said:
Haaahhh.....you got me again......no 60w bulb.......I'll get one though.

I don't have any 100 ohm/5w's on hand..........so I'll chance it. R37 was definitely bad.

I'll report back after I double-check everything for power-up.

By the way....is there any chance of the outputs blowing at 350mV? This is what came through to them with the 100w bulb. I think they may be ok.

No fuses have blown.
Click to expand...

It will be fine, the 350mV wouldnt have killed the outputs in this situation........
 
Hey Kevin, Overunder1.........SUCCESS! I checked everything a few times......put meters on the emitter resistors..........dbt'ed with a 40w bulb(I was to woos to use 100W)...bright then dim....no pulsating....4-5 sec. later relay clicked on. I anxiously waited a few more seconds....maybe 20-30........no smoke or funny sounds.

Now I'll get my nerves up for a full power test!

I guess this butchered g7500 is ready for a few more rounds! It aint purty on the butchered channel, but it works and is stable.

Once again it goes to show that this forum can fix anything! Thanks for your input and help guys!

I'll post a picture or 2........
 
Kevin......shucks......I gave it a run......hooked up speakers and a cd player......everything worked fine.........I turned up the volume a bit.........POOF! That friggin channel blew again. I checked the outputs .....they were blown....right channel still good. I changed to my last set of outputs......new fuse........tested everything again.......dbt'ed with 40w....back to pulsating dbt! Resistors, drivers all tested good.

I tested the VR's 3/4 in circuit...right channel measured 42 ohm.......left channel measured 245 K ohm............is the pot fried? They are 100 ohm pots.

What could this be????????????? Big caps are toast???

I have not re-capped the unit, as I wanted to fix what was bust first.

I'm 'conundrummed'!...........shucks...I thought we had it.
 
I changed the vr3 to a new Bourns 100 ohms.........now the dbt is back to normal. Bright then dim. 40w bulb.

What is wrong with the amp when it can't take increasing volume? i dunno........
 
The trimmer measured 245kohms??
Take your dmm and a pair of grabber leads and hook up to the vr pins out of circuit. Slowly take readings as you turn it from stop to stop noting minimum/maximum ohms while watching for any sudden drops to 0 ohms or off the scale readings and report back. If a trimmer pot goes bad it will cause destruction downstream. Report back :)
-Lee
 
Can you also take voltage readings while on the dbt for ---
TR08 b,c,e
TR10 b,c,e
Careful here, fully insulated probes, don't want any more magic smoke production.
-Lee
 
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