SX-1050 Short on Right Channel

[TomHarrison1]

I recently came across an SX-1050 that looks like it's had a hard life. In any event, after a thorough cleaning, dusting and deoxit, I've hooked it up to my DBT and realize I have a short. I unplugged one of the right channel outputs and no more short. Plugged it back in and unplugged the other output on the right channel and again, no more short. Only when both are connected does the short occur.

With Right Outputs unplugged, Left Channel DC Balance Adjustment to 0V and Idle Current Adjustment to 50 mV dial in fine.

With Rights connected, I see -.81mV DC Balance and .44V Idle Current on the Right Channel.

Does this indicate a problem with the Outputs or something else on the Amp board, or elsewhere?

 

[markthefixer]

at 440mA idle current, a bit high. Something else on the amp board. DBT is reacting to the power draw, but it's not a complete short.

This amp doesn't DC balance without the outputs.
Like the SX-1010 (AWH-032), where we developed a resistor method to provide a feedback point to connect up the amp without outputs for troubleshooting.
So leave one pair in for now.

You see, the PNP and NPN drivers are NOT interconnected through each other at each driver stage.They are independent.
First read the voltage across VR3 for that channel - to see if it went open circuit.
THEN try to dial down idle current. Adjust the pot for zero ohms. see if the idle current drops.

if not, then look at q14 - q20 drivers emitter to base voltage on each of them.
If not 0.6v for npn and -0.6v for pnp, that stage is the probable problem.

 

[TomHarrison1]

Hi Mark, so both VR3 and VR4 were both dialed all the way down (counterclockwise) before I even checked things. My problem appears to be on right channel. Adjusting VR4 had no effect on points 15 and 19 when I did try to adjust idle, when both outputs were connected. .44V was all I saw.

I'll check voltage across VR4 and look at Q14-20 tomorrow morning.

While I'm in there I'm going to pull C6 (input cap?) and check it...compare it to left channel C5.

The layout on this amp board is a challenge. I can't get to solder side of board while it's connected to heatsink. And only if I remove the heatsink and outputs can I get at the back side to even attempt soldering, much less taking measurements. What is the best way to measure Q14-20?

 

[markthefixer]

The STV's are loosed, folded somewhere safe,
the amp board board is unmounted from the heat sink,
the heat sink is unmounted,
the heat sink rotated over to clear the board access.
NOTHING is disconnected.
Insulate the board from shorting to anything - a sheet of cardboard?

It won't be the input cap. COUNT on it. Not with high and unresponsive idle current.

 

[zebulon1]

This is slightly unusual.
The problem here is something is not completely toast, yet.
Does the set look like it's been worked on?
Both amp sides share the same looking components?
I ask because someone may of installed the wrong components. Trimmers, transistors ect.
Mark is a wizard but you have to look at see if anything looks strange.
Many times these problems end up as something visual.

 

[TomHarrison1]

Ok, I'll try moving the heatsink out of the way and take those measurements.

As for being worked on, Everything appears to be original, even the outputs. When I removed the heatsink yesterday to dust things really good and inspect things, I did not see any new (or bad) solder joints on the back of the amp board. All components look original. However, I did notice that one wire wrap had been soldered, #19 which is where you put negative lead of volt meter to set Idle Current on Right Channel. That and the 10A fuse was missing are the only signs that someone had been inside.

 

[TomHarrison1]

Here is what I measured after swinging heatsink out of the way and reconnecting outputs.

When I checked voltage across VR4, I see 0V. Same with VR3. But if I connect negative to ground and check voltage on both sides of trimmers, I see around 300mV on left and 400mV on right. Minor differences depending on what side of trimmer I test.

As for the transistors, nothing is in the .6V range for VBEs.

Q12: +400mV
Q14: -190mV
Q16: -26mV
Q18: -1.36V
Q20: +1.31V

Question: with DBT glowing, the receiver is not receiving full 110V from outlet, right? So will my measurements all read lower than normal as a result?

 

[TomHarrison1]

One more new observation. I unplugged the Right Channel Outputs and went back to check DC Balance Adjustment on the Left Channel and it is fluctuating wildly as low as -300mV and as high as +300mV. Does the heatsink need to be grounded while I have it swung up out of the way, laying on a piece of cardboard? Normally, it is attached to the chassis and grounded there.

Update: It only fluctuates wildly on left channel when right outputs are unplugged. When rights are plugged in, I see steady -25mV on left channel DC Adjustment.

(When rights are unplugged, the relays do click)

 

[TomHarrison1]

I pulled a few things this morning to test them and found the following:

1. Q18, 2SB536A tests bad with DVM (Q20, 2SD381A on same heatsink, tests ok).
2. VR4, 470 Ohm tests ok up to about 400 Ohms and then reads OL beyond that.
3. VR2, 2.2K Ohm tests ok up to about 400 Ohms and then reads OL beyond that.
4. All 4 Outputs on Right Channel test bad.
5. All 4 Outputs on Left Channel test ok.

Mark/others can you confirm these equivalents I found searching AK or suggest better ones:

2SB536 PNP equivalent is OnSemi MJE350

Output Equivalents:
2SD287 NPN = OnSemi 2N3055
2SB539 PNP = OnSemi MJ2955

(These 3 are available at mouser.)

 

[tsd71]

2SB536 = MJE15033G or KSA940

2SB539 =MJ21193G

2SD287=MJ21194G

If you are replacing these parts on the amp you might as well change out everything while you are in there.

awh-047 q1 2sa750 0 0 0 0 0 512-KSA992FBU
awh-047 q2 2sa750 0 0 0 0 0 512-KSA992FBU
awh-047 q3 2sa750 0 0 0 0 0 512-KSA992FBU
awh-047 q4 2sa750 0 0 0 0 0 512-KSA992FBU
awh-047 q5 2sa750 0 0 0 0 0 512-KSA992FBU
awh-047 q6 2sa750 0 0 0 0 0 512-KSA992FBU (6)
awh-047 q7 2sc1439 0 0 0 0 0 512-KSC3503dstu
awh-047 q8 2sc1439 0 0 0 0 0 512-KSC3503dstu
awh-047 q9 2sc1439 0 0 0 0 0 512-KSC3503dstu
awh-047 q10 2sc1439 0 0 0 0 0 512-KSC3503dstu (4)
awh-047 q11 2sa858 0 0 0 0 0 512-KSA1381ESTU
awh-047 q12 2sa858 0 0 0 0 0 512-KSA1381ESTU
awh-047 q13 2sa858 0 0 0 0 0 512-KSA1381ESTU
awh-047 q14 2sa858 0 0 0 0 0 512-KSA1381ESTU (4)
awh-047 q15 2sc1439 0 0 0 0 0 512-KSC3503dstu
awh-047 q16 2sc1439 0 0 0 0 0 512-KSC3503dstu (2)
awh-047 q17 2sb536a 0 0 0 0 0 512-KSA940TU
awh-047 q18 2sb536a 0 0 0 0 0 512-KSA940TU (2)
awh-047 q19 2sd381a 0 0 0 0 0 512-KSC2073TU
awh-047 q20 2sd381a 0 0 0 0 0 512-KSC2073TU (2)
awh-047 q21 2sa858 0 0 0 0 0 512-KSA1381ESTU
awh-047 q22 2sa858 0 0 0 0 0 512-KSA1381ESTU (2)
awh-047 q23 2sb539c 0 0 0 0 0 863-MJ21193G
awh-047 q24 2sb539c 0 0 0 0 0 863-MJ21193G
awh-047 q25 2sd287c 0 0 0 0 0 863-MJ21194G
awh-047 q26 2sd287c 0 0 0 0 0 863-MJ21194G
awh-047 q27 2sb539c 0 0 0 0 0 863-MJ21193G
awh-047 q28 2sb539c 0 0 0 0 0 863-MJ21193G
awh-047 q29 2sd287c 0 0 0 0 0 863-MJ21194G
awh-047 q30 2sd287c 0 0 0 0 0 863-MJ21194G

 

[TomHarrison1]

Uh no, I don't think I'll go that far...

 

[rcs16]

Output Equivalents:
2SD287 NPN = OnSemi 2N3055
2SB539 PNP = OnSemi MJ2955

NO, use what was suggested above.

 

[TomHarrison1]

OK, thanks for the confirmation.

So with Q18/Q20 and VR2/VR4 removed, is it safe to power up the receiver? I should have taken main power readings being supplied to the amp board (+60V/-60V) before removing them just to verify I have good supply.

 

[tsd71]

are you using a dim bulb tester?

 

[TomHarrison1]

I am, but it only lights up when the right channel outputs are connected. So I could leave them unplugged. Just didn't know if it was safe to power up with the other components removed.

I can also pull the pre-out jumpers on the back. Doing so even with the right channel outputs connected allows the receiver to power up without a short, relays click, DBT does not stay lit.

 

[petehall347]

I can also pull the pre-out jumpers on the back. Doing so even with the right channel outputs connected allows the receiver to power up without a short, relays click, DBT does not stay lit.

this is where the problem starts ..its looking like the preamp is sending dc to into the main amp or some sort of oscillation .
measure dc and ac voltages at pre outs .

 

[TomHarrison1]

Correction. I still get short on right channel with outputs plugged in and pre-out jumpers removed.

Checked Power In voltages to Amp board and they both settle down to 0 V after a few seconds.

Power supply is feeding -54 and +54 V to Amp board. Similary it takes a few seconds before settling down. I should be seeing +-60V.

 

[TomHarrison1]

One more update. I pulled Q14 and it appears to test good. I've ordered replacements for Right Channel Outputs, Q18, and 4 new trimmers for VR1-4 (for both channels).

 

[BobbyN]

One more update. I pulled Q14 and it appears to test good. I've ordered replacements for Right Channel Outputs, Q18, and 4 new trimmers for VR1-4 (for both channels).

Sometimes transistors can test good on a meter but be noisy and won't function correctly in the circuit. I suggest you replace anyway while you're there. It should only effect your order by a buck or two. If nothing else, order and keep them on hand. It shouldn't add to your shipping cost either.
Bob

 

[TomHarrison1]

Thanks Bob, everyone. I'm trying to identify exact nature of short, not do a full restore, replacement of everything blindly. Once I get it working, I'll pass recommendations on to the owner about those ideas.