SX-950 Blew Main Fuse w/o Speakers

[ericson38]

This is the start of a thread on fixing an SX-950 I bought last weekend. It powered up for about 1 minute without speakers, then heard a transformer whine, then all dark. The board that I need to get behind is pictured here, with the four power supply rectifier diodes. Then a zoom out to it and the adjacent board. I was thinking the best way is to also unsnap the adjacent board to the right, and then lay these boards out together, rotating off the large leads that connect to the main caps and xformer. I'm wanting to minimize undoing any wire wraps.

This thing is built like a tank, much heftier than the G5700 I'm also working on. Its 85wpc, versus 75wpc for the G5700. It also has a 4 channel FM detector out port on the back.

 

[Stevesmullen]

This is the start of a thread on fixing an SX-950 I bought last weekend. It powered up for about 1 minute without speakers, then heard a transformer whine, then all dark. The board that I need to get behind is pictured here, with the four power supply rectifier diodes. Then a zoom out to it and the adjacent board. I was thinking the best way is to also unsnap the adjacent board to the right, and then lay these boards out together, rotating off the large leads that connect to the main caps and xformer. I'm wanting to minimize undoing any wire wraps.

This thing is built like a tank, much heftier than the G5700 I'm also working on. Its 85wpc, versus 75wpc for the G5700. It also has a 4 channel FM detector out port on the back.

Hey Charlie.
I was able to do like you said and undo all the wires from the white plastic holders and them prop the board up with a small chunk of wood like off of the end of a 2x4 etc its not a ton of room but it is doable I didnt have to remove any wires.Watch out for the small heatsinks as they are live.
Have you allready diagnosed what is causing the fuse to blow?
Steve

 

[ericson38]

Steve,

Not yet...

...but it seems likely that D5/D6/D7/D8 are shorted (4 wave bridge) or one of the two main caps that filter their output. I haven't lifted a lead of the main caps yet, but I should eliminate these first, as that is a simpler job than lifting leads on the main diodes in the bridge on CB AWR 101 :yes:

The fuse that blows (6a) is inline with the main power switch S1 and the primary windings of the power xformer.

 

[markthefixer]

This is the start of a thread on fixing an SX-950 I bought last weekend. It powered up for about 1 minute without speakers, then heard a transformer whine, then all dark. The board that I need to get behind is pictured here, with the four power supply rectifier diodes. Then a zoom out to it and the adjacent board. I was thinking the best way is to also unsnap the adjacent board to the right, and then lay these boards out together, rotating off the large leads that connect to the main caps and xformer. I'm wanting to minimize undoing any wire wraps.

This thing is built like a tank, much heftier than the G5700 I'm also working on. Its 85wpc, versus 75wpc for the G5700. It also has a 4 channel FM detector out port on the back.

It may not be necessary to lift the power supply board, or even the diode leads, just test the 4 big diodes in the corner. Then the caps to test are the big pair elsewhere on the chassis. Something shorted enough to blow the main fuse will show up easily on the tests. Follow the red wire from pin 18 and the white wire from pin 17 to the caps....

As it is, the power supply board will pivot once the four corners are freed, although a few wiring looms will have to be moved out of their retainers.

The LAST thing you want to do is to unwrap a wire.

 

[ericson38]

This post in error

 

[ericson38]

Mark,

On D5-D8 I get ~0.490 volts forward, open reverse (after a discharge time through the small disc parallel caps).

But, between pins 17 and 18 on the AWR 101 board, which is across the two parallel arranged 22,000 uf filter caps, I have a short.

According to the the SM, the two filter caps are in series measured this way, and I should measure the series resistance of the two bleeder resistors.

 

[markthefixer]

Mark,

On D5-D8 I get ~0.490 volts forward, open reverse (after a discharge time through the small disc parallel caps).

But, between pins 17 and 18 on the AWR 101 board, which is across the two parallel arranged 22,000 uf filter caps, I have a short.

According to the the SM, the two filter caps are in series measured this way, and I should measure the series resistance of the two bleeder resistors.

If a short is showing up across BOTH caps, it is more likely that a pair of output transistors are shorted.

This is where you remove and test the 8 output transistors. Rememeber and WRITE DOWN what came from where and DO NOT power up the unit with the sockets bouncing around. There are some serious voltages and currents available to those sockets.

After we find the shorted output transistors, then we will have to check the amp board for other damage.

 

[ericson38]

Mark,

Right....across the caps separetly they indicate building up a slow charge, so will pull the outputs this weekend.

This is a nice receiver, four gang tuner, heat sinks similar to that on AU717.

Thanks

 

[markthefixer]

Mark,

Right....across the caps separetly they indicate building up a slow charge, so will pull the outputs this weekend.

This is a nice receiver, four gang tuner, heat sinks similar to that on AU717.

Thanks

There are also two "thermal diodes" connected to the heatsinks. They have VERY FRAGILE LEADS, so unscrew the mounting screws, and bending MINMALLY, AWAY FROM ANY OTHER BENDS NEAR THE BODY OF THE PART, tuck them into a safe location.

These leads WANT to break off right at the epoxy, and these are the hardest parts to obtain in the receiver, ranging from merely difficult (stv-3h) to darn near impossible(stv-4h), and guess what, YOURS are the stv-4h's.......

 

[ericson38]

Great tip, its most likely I would have bungled this step.

I really want this thing to work. Its built like the Air Inlet Control Computer I worked on as an airman grunt on the SR-71 in 1977.

My wife, not an emotional person when it comes to audio equipment, took the wooden case this morning to lemon oil it.

 

[markthefixer]

Great tip, its most likely I would have bungled this step.

I really want this thing to work. Its built like the Air Inlet Control Computer I worked on as an airman grunt on the SR-71 in 1977.

My wife, not an emotional person when it comes to audio equipment, took the wooden case this morning to lemon oil it.

:drool: :drool: :ntwrthy:

I took the Moffet field tour may years ago, and saw everything aeronautical I had ever drooled over EXCEPT a blackbird.... U2's yes.. Kelly Johnson outdid himself on the blackbird.


Your avatar ? It looks like I would expect the consoles in the silos to look like, what is it?

 

[ericson38]

Its a shot of one part of launch range controller's console for commercial launch vehicles (ie Delta II, Atlas 2AS, etc), hence able to take a shot.

..on the SR-71, it always came down at ambient temperature, some have told stories about being too hot to touch for awhile. It essentially enjoyed a 15 minute or more subsonic decent from 30K Ft on down, so had ample time to get cool to the touch.

SX-950-

I found that the four left channel transistors are all shorted, right side indicates proper voltage drop with meter in diode check mode. The #s are 2SD427 and 2SB557.

Here are some shots of the power amplifier assembly rotated backwards, after removing 6 hold down screws (3 each side-can be seen in picture).

La Mesa TV Supply in San Diego has them in stock, 4.95 for the NPNs, 8.95 for the PNPs.

 

[EchoWars]

...and they are almost certainly counterfeit Chinese junk. Replace the blown outputs with new On-Semi MJ21193G and MJ21194G bought from Digikey or Mouser.

 

[markthefixer]

Its a shot of one part of launch range controller's console for commercial launch vehicles (ie Delta II, Atlas 2AS, etc), hence able to take a shot.

..on the SR-71, it always came down at ambient temperature, some have told stories about being too hot to touch for awhile. It essentially enjoyed a 15 minute or more subsonic decent from 30K Ft on down, so had ample time to get cool to the touch.

SX-950-

I found that the four left channel transistors are all shorted, right side indicates proper voltage drop with meter in diode check mode. The #s are 2SD427 and 2SB557.

Here are some shots of the power amplifier assembly rotated backwards, after removing 6 hold down screws (3 each side-can be seen in picture).

La Mesa TV Supply in San Diego has them in stock, 4.95 for the NPNs, 8.95 for the PNPs.

Don't use them !!!!!! The chances of counterfeits is wayy too high, even if the shop is a good guy....

use these from mouser.com, they are tried and true, and we can count on mouser and digikey not getting tricked with counterfeits:
863-MJ21193G pnp to-3 250v 16A 250W 4mhz 8-75hfe $3.83
863-MJ21194G npn to-3 250v 16A 250W 4mhz 8-75hfe $3.83
(2sb557........ pnp to-3 120v 8a 80w 7mhz 20-140hfe)
also get:
534-4662 to-3 mica insulators $0.16 each
534-4673 to-220 mica insulator $0.14
532-249 1 oz thermal compound $4.71
I don't think 1 gram is going to be enough: 526-NTE303 1 gram thermal compound $1.77


we have lost count of all the units these have successfully gone into...

get 8, do all the outputs, they are very robust.

BUT, now we have to address WHY they blew.... ALL 4 implies a very serious bias problem.

The simplest way to state this is to just test the transistors on the amplifier board, Q5 - 11 or Q6-12 depending upon the channel, also check for burned up or open resistors around them.

I'm not sure if the drivers (q9-11 or q10-12) are to-66 or to-220 so a picture of the amp board would be welcome.

The circuit (r35, Q5, D5, VR3, Q7, R71 or it's counterpart on the other channel) are CRITICAL in establishing the +1.2v and -1.2v bias voltages, if the circuit is broken, the +/- 1.2v becomes +/- 30 or more, turning on BOTH sides of the output transistors at the same time, making a massive short across the raw dc.

The sx-850 and sx-950 amplifiers are almost identical and use the same parts, so here is a complete semiconductor replacement parts list for a sx-850 I did recently:
http://www.audiokarma.org/forums/showthread.php?p=1875117&highlight=863-MJ21193G#post1875117

edit: notice I said the same thing as EchoWars, but his post came in while I was researching mouser's stock & price and typing in the results...

 

[ericson38]

Mark,

Here's a shot of the driver board, same side as had blown outputs. Also, I removed the four output transistors and now the short across the two filter caps is gone.

Copy on the transistor parts to use. Will do that.

The drivers are T0-220 style.

I'll compare the readings on the good side (right side of driver board, viewing from front) with the bad channel side.

 

[ericson38]

Mark,

Since the short at the power board was eliminated, I powered it up, right side channel only has output transistors. No speaker. Hear it come out of protection. Lights on, selectors change lights as they should. Power it down, hook up portable speaker to right side, speaker A group.

FM channel tuned in, sounds terrific. Power off.

I'm going to start checking the differences between the two driver sides now, as you suggested.

 

[markthefixer]

Mark,

Since the short at the power board was eliminated, I powered it up, right side channel only has output transistors. No speaker. Hear it come out of protection. Lights on, selectors change lights as they should. Power it down, hook up portable speaker to right side, speaker A group.

FM channel tuned in, sounds terrific. Power off.

I'm going to start checking the differences between the two driver sides now, as you suggested.

Since you powered it up already (not quite what I would have recommended, but luckily no smoke) and it came out of protection, read the base voltages for the output transistors, which you can find at the green 4.7 ohm resistors between the big square white 0.5 ohm resistors, or pins 43,45, 48 & 50.

Then clip the voltmeter between the + and - voltage base connections (or 43 black probe, 50 red probe) and try adjusting vr3(or vr4, the vr closer to terminal 50 in the picture) to min and max and read the voltage between the bases at min and max. These results may negate needing to remove and test the driver transistors.


Don't do vr4 adjust on the good side until you understand bias current measurement.

Have you downloaded the service manual from the database?

 

[ericson38]

I have the service manual and did a bias and offset adjustment on the G5700 earlier this year, so am comfortable with that.

I see the circuit on page 60 on the PDF version of the SM. I see how VR3 sets the BE bias voltage. Will do this test tomorrow then and post.

Five measurements, four for Base setpoint to ground, and two (min/max) from base to base (differential) moving VR03 through its range.

Thanks again Mark-

 

[ericson38]

OK-

43 (neg) to 50 (pos) - initial reading is 1.244 V

Full CCW on VR03 - 1.08 V
Full CW " - 1.61 V

Then set Vr03 back to 1.244 V reading

50 (red lead) to 49: 0.61V
48 (red lead) to 47: 0.62 V
45 (red lead) to 44: -0.62V
43 (red lead) to 42: -0.62V

Measuring base leads on the bad (left) channel show floating off ground.

Measuring base leads to ground on right channel indicate -20 mV for the PNPs and +8mV for the NPNs.

On the good (right side) the equivalent 43 to 50 (pin 35 to pin 40) test is 1.21 V.

 

[markthefixer]

OK-

43 (neg) to 50 (pos) - initial reading is 1.244 V

Full CCW on VR03 - 1.08 V
Full CW " - 1.61 V

Then set Vr03 back to 1.244 V reading

50 (red lead) to 49: 0.61V
48 (red lead) to 47: 0.62 V
45 (red lead) to 44: -0.62V
43 (red lead) to 42: -0.62V

Measuring base leads on the bad (left) channel show floating off ground.

Measuring base leads to ground on right channel indicate -20 mV for the PNPs and +8mV for the NPNs.

On the good (right side) the equivalent 43 to 50 (pin 35 to pin 40) test is 1.21 V.

Looks good enough to install the new outputs, plenty of adjustment range and they're symmetrical. The slight difference in pos and neg voltages just reflects the slight mismatch of the driver transistors, it all comes out in the wash when the feedback gets the final output near zero.

When replacing the outputs, turn the bias vr3/4 down to zero before applying power, then bring it up carefully, just in case something wierd is waiting to pounce.

With those on-semi's, and the power level of the amp, your limitations will be the power supply's capabilities and the heatsink's dissapation. It doesn't hurt that they are quite reasonably priced...