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SA-9500 II stuck in protection

marc brown

marc brown

AK Supporter
I was working on the tone control section of my SA-9500 II and shorted something. Now it won't come out of protection mode.

I've checked the mains voltage and mains voltage is good. Also checked everything in the tone control section. It's outputting clean sine waves on both channels.

I took voltage readings on the transistors hoping that someone could help me diagnose which transistors I've blown. Below are my readings:

Q7: 36V 0mv 1.7mv
Q15: 36V 16mv 1.7mv
Q3: 20V -.63V -12.8mv
Q4: 20V -.63V -12mv
Q6: 0mv 16.2mv .63mv
Q2: 20V -10mv 7mv
Q1: 20V -10mv -7mv
Q5: 18V 20.6V .63V

thanks in advance for the help guys
 
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Well, based on the blowing fuse FU2 and the stuck protection relay, I pulled out Q5 and Q6 to test them.

Q6 is fine, Q5 is blown. Is the next step to replace Q5, put reinstall Q6 and test the amplifier or do I pull more transistors and check them as well?
 
WHAT BOARD NUMBER (zzz-999???) are those transistors on, I am NOT going on a search and destroy mission just to find out where you are in the schematic!!!!

in protect, you must see if the protect is justified.

required readings:
you are delving too deeply into the forest. look at the inputs.
aws-109
pin 40
pin 41
c14 minus to ground

and thus I see the q numbers you measured... still, peering into a diff amp (DC detector) using ground based measurements is an exercise in frustration.

if the voltages are good (40,41 near 0.000v) and c14 is -5v or so, then read the voltages on d21, d22, d23 to see if one is pulling down (it will have 0.6v avross it, the others will not.)

from what I see, I suspect no -5v on c14.
 
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markthefixer said:
WHAT BOARD NUMBER (zzz-999???) are those transistors on, I am NOT going on a search and destroy mission just to find out where you are in the schematic!!!!

in protect, you must see if the protect is justified.

required readings:
you are delving too deeply into the forest. look at the inputs.
aws-109
pin 40
pin 41
c14 minus to ground

and thus I see the q numbers you measured... still, peering into a diff amp (DC detector) using ground based measurements is an exercise in frustration.

if the voltages are good (40,41 near 0.000v) and c14 is -5v or so, then read the voltages on d21, d22, d23 to see if one is pulling down (it will have 0.6v avross it, the others will not.)

from what I see, I suspect no -5v on c14.
Click to expand...

Thanks for your help and I apologize for the multiple posts. I'm taking measurements now and consolidating information.

regards,
David
 
markthefixer said:
required readings:
you are delving too deeply into the forest. look at the inputs.
aws-109
pin 40
pin 41
c14 minus to ground
Click to expand...

Mark,

From AWS-109

Pin 41: -2mv
Pin 40: -8mv
C14: 1.5V

The amp is now coming out of protection but FU1 is still blowing.

Thanks for your help.
 
c14 should be at -5.5 volts, it is the on and off signal for the protection circuit. Usually it comes from the 7.5v ac lamp circuit, but your transformer doesn't have that winding, so they use a voltage divider from a higher ac voltage secondary winding.

check the voltages at the terminals of the 4 big caps, one DMM lead to each cap terminal

check the 8 big diodes (power off, use diode test, lift one end - look for shorts) aws-109 d1 - d4 specifically, but do the other 4 as well.

the pin 40 and pin 41 voltages say the amplifiers are DC balanced, (and happy, yet they could still have problems.)

FU1 is HALF the amp. a powered down amp channel IS at dc balance of zero, but it SURE isn't going to make sound!!
 
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markthefixer said:
check the voltages at the terminals of the 4 big caps, one DMM lead to each cap terminal

check the 8 big diodes (power off, use diode test, lift one end - look for shorts) aws-109 d1 - d4 specifically, but do the other 4 as well.
Click to expand...

Mark,

C6: 50.4V
C7: 50.5V
C8: -12mv
C9: -4mv

all the diodes tested good. Their forward voltage drops are as follows:

D1: 635mv
D2: 628mv
D3: 626mv
D4: 630mv
D5: 628mv
D6: 626mv
D7: 635mv
D8: 618mv

thanks,
David
 
Ok, I think you have figured out that C8 and C9 are not powered due to FU1 being blown.

Check the thermal bias diode's leads, they are soldered into the amplifier driver board, BUT they go over in two white fabric sleeves to the heat sink, where a small component is screwed onto the heat sink. If they break, the outputs will be told to turn on the idle current hard enough to damage themselves and blow fuses. This diode needs to be freed from the heat sink while work is being done, for it's safety. Unscrew the screw holding it just a turn or two.

Tuck the thermal bias diodes someplace safe around or over the printed circuit board, the leads will break off flush with the body of the device if they are flexed much, so unscrew the thermal bias diodes from the heat sink and do the safety tuck. AVOID AT ALL COSTS moving or flexing the last 1/4 inch of lead where it connects to the body of the device!!! grasp the LEADS as a pair at the base of the device with your fingers, NOT THE BODY, to move it, once mechanically free of the mounting screw.

You need to disconnect and remove the output transistors from that channel.

To POSITIVELY identify them, set your DMM to ohms, put one lead of your dmm on C8's POSITIVE terminal, and touch the other lead to either the left or right side's 2sd558a power output transistor's mounting screws on the big heat sinks. One side will show almost no ohms - that's the dead side, dead channel as they are located on opposite sides of the chassis. They are probably what's blowing the fuse.

This is the side. remove the 2sd558a's (both on THAT SIDE of the chassis only) AND also the 2sb618a from that same side.

test the transistors for shorts and gross faults

Insulate the sockets (plastic bags?) and try turning on the unit again and see if the fuse blows now.

When it turns on, if possible without endangering the sockets insulation, check the c8 and c9 voltages.

Then go to the power amp assembly and measure and post all the pin's voltages. IF you can't reach them, measure the voltages at the empty transistor sockets, that will tell me most of what I need to know. Look for burned components on the board.

If the power output transistors are damaged, you have a bit of a fabrication job to do, as the "batwing" configuration isn't made anymore, nor are there reliable supplies for them. Other than the physical shape of the transistors there is nothing exceptional or remarkable about them.

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If you can squeeze a TO-3P device under the 0.960" screw spacing, there are quite a few decent options with good characteristics.

If the space is too tight for a TO-3P device, you might try a TO-220 device, such as:

MJE15033G

MJE15032G

I fit some of these in a dead Kenny 4070 and it's still singing.
 
WH, this was covered a few years ago, and later I will dig it up, but to-220's don't handle the necessary wattage.
 
Thanks for the info. I was thinking that 50 in each spot might eek by, but it appears that there may be some more refitting required to squeeze in the TO-3P or TO-264 packs.

Maybe you were thinking of these TO-3Ps at 130W:

FJA4213OTU

FJA4313OTU

Or these TO-264s at 150W:

FJL4315


FJL4215

Either might just fit between the mounting holes under a pair of pressure clips.
 
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