g-3000 blown channel - Solved

[rickl]

I have a G-3000 with right channel blown. This had blown outputs, missing transistors and blown resistors. All fuseable resistors have either been replaced or measure correctly.

I can't get the bias set. measuring across R70/2 measured 0v.

Left channel is working (bias set, DC offset 5mV, looks good on a scope).
Right channel signal is lower than left channel at 8 ohm load and is distorted (not clipping).



I've replaced these blown or missing transistors:
outputs TR16/18 with mje15032/3
drivers TR12/14 with ksa1013/ksc2383
bias transistor TR10 with ksc945
TR08 with ksc2383
TR06 ksa992
IC02 with a matched pair of ksa992

electrolytic caps replaced

TR14 base -.367 (should be -1.2V), emitter .07v (-0.6V)
TR12 similar
R56/8 junction measures 19v (18.5V)

I also replaced TR19 (voltage pass transistor for R55/56 voltages).

B+ is 32v (should be 28), B- -32 (-28v). This seems close enough and the left channel is working with these voltages.

Some say (kevin?) the outputs will oscillate and so I've ordered KSB1017YTU/KSD1408YTU. they haven't arrived to verify.

I have a scope, DBT, signal generator and a few DVMs.

I can provide voltage measurements and scope pics if it helps.

I'm kind of stuck.

UPDATE:
The amp is working. here are the transistors i used:
outputs TR16/18 with KSB1017YTU/KSD1408YTU
drivers TR12/14 with ksa1013/ksc2383
bias transistor TR10 with ksc1845
TR08 with ksc2383
TR06 ksa992
IC02 with a matched pair of ksa992
electrolytics replaced
replaced fuseable resistors R67/8 100R,R91 22R, R69-72 220R, R73-76 6.8R with MF
TR19 replaced with a to-220 NPN

replaced preamp naughty transistors
TR01/2 KSA992
TR03/4 KSC1845
C01/2 2.2uF Wima

 

[princever]

where are you measuring for the bias? should be across R78/80. But if you were measuring across R70 and 72 you still should be seeing about 1.2V.

Hows the installation of TR10? I seem to recall that the 2SC945 and KSC945 have a different pinout, unless you have the special KSC945C variant.

 

[rickl]

bias measured across R78/80.

I'll verify the KSC945 pin out. There was a 2n5550(?) in there when I got it.

Thanks for your response

 

[rickl]

I'm so happy that Prince posted. The bias transistors (TR09/10) are a center collector (ECB) and I'm using EBC devices.

here is a snip of the board layout



I hope to replace the transistor tomorrow and see if that helps or I have other problems.

Thanks again!

 

[rickl]

got TR10 replaced with a 2n2240 (ECB). I probably could have used a ksc1845. this fixed the bias problem. Thanks again Prince. I need to check voltages but I'm hopeful.

right channel is still low, and no preamp output so i'm going to replace the TR01/2 2sa726 and TR03/4 2sc1313 since they are on the naughty list. this was working so more review is needed.

 

[princever]

Glad that helped. Don't ask me how I know about that pinout but it definitely involved blowing up an unobtanium jfet in an amp I was repairing.

The G-3000 is well worth saving. I had one years ago that I used at the computer desk and that 26W/ch will rock the office when paired with reasonably efficient speakers.

 

[Redkarter]

Not sure if its helpful but I recently repaired a g3000. Heres the parts list I used and its working well now.

Be mindful of the pinout of the a726 and c1313 as the replacements are different.

 

[rickl]

I recently repaired a g3000. Heres the parts list I used

Thanks. I'll take a look at your list.

I'll post my parts list too (once I get it working).