SX-1010 Power Amp Board AWH-032 Rebuild Problem

AJD1964

Well-Known Member
This unit was all working 100% until today, during a right channel amp board overhaul. Before this, other than cleaning switches and pots, I overhauled the Power Supply board (054) first. It was working so well after the PS was done, I wanted to get the power amp boards done to prevent the ticking time bombs. I ran this unit for over 2 hours Friday night to test the power supply – all good up until now. Here’s what has been done so far:

Power Supply Assembly (AWR-054-0)
Replaced all electrolytic caps, transistors, and the 3 Zener diodes, D8, D9 & D10.
Main filter caps also replaced; 18000uF 75v. I used the original bleeders.
.. Again – all good after power supply work, and all voltages were in spec at the pins.

Power Amp Assembly (AWH-032-0) replacement parts installed:
C1: .68uF 25v CSSA: 505-MKS20.68/63/5 .68uF 63v
C4: 10uF 16v CEA: 647-UPW1E100MDD 10uF 25v
C8: 330uF 6.3v CEA: 647-UPW1A331MPD1TD 330uF 10v
Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q8: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q9: 2SC1448: 512-KSC2073TU *optional, did it anyway.
Q11: 2SC945: 512-KSC1845FTA *optional, did it anyway.
Q12: 2SA733: 512-KSA992FBU *optional, did it anyway.
Q7: 2SD358; & Q10: 2SA740; Still Original in the Board – Do not have replacements at this time.
D1 – D8; Still Original in Board.
VR1: 2.2K ohm multi offset: 652-3296P-1-202LF 2K ohm 25 turn (At approx. 1K Ohms center when installed).
VR2: 470 ohm single bias: 652-3386H-1-501LF 500 ohm single turn (AT Zero Ohms when installed).

Output Transistors: Still original (in this channel at least); Cleaned out old thermal glue from Heatsink, and cleaned micas; Reinstalled with new white thermal compound.

The symptom:
After re-installing the amp board, and output/heatsinks, I went to power up on the DBT (150W clear bulb) to begin the offset check on VR1. Turned on the switch, the bulb dimmed down to normal, but I noticed it was staying in protection, but bulb still dim. Then the magic dragon puffed – by taking out R17 470 ohm resistor. Obviously, I never got far enough to adjust any offset or idle current with the new pots.

Things I’ve checked:
- Transistor orientation – All of them at least 3 times over (assuming silkscreens are correct on these boards).
- Wire connections at top & bottom of the board – all good, actually had to fix a couple during the job.
- Traces on board – I do not see any lifts, or bad solder joints. A LOT nicer shape than the 054 board!
- I see Q5 & Q6’s emitter resistors tie to my smoked R17.
- I see the STV3’s are straight off the collector of Q5. If I fried them, I have a nice parts car I guess.

Where would you guys begin from here?

 
You did something.
Take a break from it and relax. Enjoy the holiday weekend.
Double check the work.
Missing solder, wrong transistor in the wrong location, wrong position.
The diodes should be checked over. Caps or polarity.
This post sounds terrible but you need to step back and take a rest. You will see things after you come back to it.
Your going to have to check all the resistors and diodes for values and forward voltages.
The 1010 is a chore to work. Taking the heatsinks loose to remove the amp supports. It can be a turd. Triple check after installing the components. The output harness wire wraps can break, check them by tugging and checking for security.
 
Things I’ve checked since 1st post: (it may not seem like much, but this is very slow going on these boards)

1. Pulled the smoked R17 470ohm resistor (Even though it smoked, it still reads 470 on the DVM), and replaced R17 with a new resistor.

2. Pulled Q5, (3503) and did the 6-way check with the DVM in Diode check mode. Q5 good, left it out.

3. Pulled Q6, (3503) and did the 6-way check with the DVM in Diode check mode. Q6 good, left it out.

4. Pulled one leg from R16 47 ohm resistor (emitter resistor from Q5) is good 47.6 ohms, and no sign of heat stress. Replaced the open leg.

5. Pulled one leg from R15 47 ohm resistor (emitter resistor from Q6) is good 46.6 ohms, and no sign of heat stress. Replaced the open leg.

6. The heat sink / outputs are out of the chassis – again.

7. As noted earlier – DBT was still good and dim, but Right amp board wants to fry R17.

8. Left channel board / HS and Outputs are is still all in place, and AFAIK, is still all good.

9. Virtually all of the older 1010 threads seem to have issues caused by improper component placement, sloppy meter lead placement and taking out the outputs. I don’t seem to find any threads with a scenario similar to mine.

Questions:

1. Am I on the right track by beginning to check components around the smoked R17? This board is a challenge to work on, and I do not want to pull too many components unnecessarily.

2. I noticed on the schematic that R17 is tied directly to the -56v supply from 054 PS board. Should I check to see if I still have -56v at pin 3/7? I do not want to assume that I can just power up, even on DBT, and check Power Supply with a “bad” amp board connected. Or just disconnect the power supply from the board either. On the other hand, there are several other resistors connected directly to the -56v pin/s, and have no apparent stress - but I will check more.
 
Pull ALL OF THE TRANSISTOR DATASHEETS for all NEWLY INSTALLED Transistors. Most of them will HAVE DIFFERENT PINOUTS than that of the originals. (except for the TO-220's) The TO-126's are somewhat hard to see which is front or back. Get them under a bright light and a magnifier, then mark the front with a WHITE MARK. BEWARE of the small signal transistors. They will mess you up. LOOK IN THE FINE PRINT for a "C" variant of a transistor, meaning The "C" denotes a CENTER COLLECTOR version of a transistor (ie; ECB or BCE vs. the more common EBC or CBE variants).
 
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I have just finished up my 1010 restoration recently. I replaced all of the transistors on the power amp boards. I used the same replacement transistors that you have listed. Maybe you can use this photo to help ensure that you’ve installed all of the transistor replacements correctly?

index.php
 
Pull ALL OF THE TRANSISTOR DATASHEETS for all NEWLY INSTALLED Transistors. Most of them will HAVE DIFFERENT PINOUTS than that of the originals. (except for the TO-220's) The TO-126's are somewhat hard to see which is front or back. Get them under a bright light and a magnifier, then mark the front with a WHITE MARK. BEWARE of the small signal transistors. They will mess you up. LOOK IN THE FINE PRINT for a "C" variant of a transistor, meaning The "C" denotes a CENTER COLLECTOR version of a transistor (ie; ECB or BCE vs. the more common EBC or CBE variants).
.

Larry,
Yep - got all the datasheet pdf's I will check for that, and triple check orientations again when I put them back in.
 
I have just finished up my 1010 restoration recently. I replaced all of the transistors on the power amp boards. I used the same replacement transistors that you have listed. Maybe you can use this photo to help ensure that you’ve installed all of the transistor replacements correctly?

Hi - I've been watching your threads very closely - I've learned a lot from them, you did a great job! I pretty much cloned your parts list.
Thanks for posting that pic of your amp board. That will serve as a great visual reference, and thanks for the help!
 
Tonight's "Progress":
Pulled all remaining transistors out, I checked them one at a time, hoping I would find a bad or misplaced one - all good on the 6 way tests.
Checked all diodes out of circuit - All Good.

Edit - except for the STV-3's; I'm not sure how to check them yet. I doubt they're bad since nothing else was. I don't want to mess with them if not necessary. Unless you guys think I should.
 
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Follow the leads down to the board. Probe both lead ends on the board and check it as a DIODE! It's something like 3 diodes in series for voltage drop, so probably 1.8 to 2.0v one way and open the other.
 
Follow the leads down to the board. Probe both lead ends on the board and check it as a DIODE! It's something like 3 diodes in series for voltage drop, so probably 1.8 to 2.0v one way and open the other.

The way I read this is with both legs still in circuit board,
I cant get the 1.8-2.0. I only get OL both directions. I even pulled one leg. I tried the STV on the other side of the small Ht. Sink - same thing; OL both directions, with both legs in still board.

Checked the STV-3's on the Left Channel, which I havent done any work on yet - same thing - OL both directions.

I'm wondering if my meter does not have the juice in diode mode to break the STV junctions for voltage drop? I even tried a new meter battery.
I'll have to research on an alternative method of testing these.
 
The way I read this is with both legs still in circuit board,
I cant get the 1.8-2.0. I only get OL both directions. I even pulled one leg. I tried the STV on the other side of the small Ht. Sink - same thing; OL both directions, with both legs in still board.

Checked the STV-3's on the Left Channel, which I havent done any work on yet - same thing - OL both directions.

I'm wondering if my meter does not have the juice in diode mode to break the STV junctions for voltage drop? I even tried a new meter battery.
I'll have to research on an alternative method of testing these.

EXACTLY - not enough voltage to push through the junctions.

You want 7mA current 9v - 3v for diode = 6v 7mA is 857 ohms, 1000 ohms is 6mA so use a 1k resistor and a 9v battery, and measure the voltage across the diode.

Your meter does not push that many mA's through a normal diode test, and the mA's determine the forward voltage across the thermal bias diode.
 
EXACTLY - not enough voltage to push through the junctions.

You want 7mA current 9v - 3v for diode = 6v 7mA is 857 ohms, 1000 ohms is 6mA so use a 1k resistor and a 9v battery, and measure the voltage across the diode.

Your meter does not push that many mA's through a normal diode test, and the mA's determine the forward voltage across the thermal bias diode.

Ok - I got 1.75v across the STV-3H. They're probably OK.
 
Long shot here, but any chance that the outputs got reinstalled into the wrong spots? Not sure if that would cause your issues or not, but it sounds like you have ruled most everything else out already.

Let me know if I can check anything on my 1010, or post a picture that will help you
 
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I think you have a transistor in backwards or like Larry stated, a correct transistor that has different leg configurations.
Did you notice:
When you check for the DC Balance, is it to one side? Is the Balance reading a + or -54 volts at the amp output, pins 19 or 20? This reading, before R17 begins to cook.
 
The other problem is the version of the board and the variations of each one. I've seen it occasionally, but for the life of me I can't remember where right now.
ADJ1964.........Please note the actual part/board number in the box with any/all suffixes please. I'll see if I can locate the 1010 amp board variants and post them up here. Might be a day or 2, and I won't mind someone else looking also.
 
Long shot here, but any chance that the outputs got reinstalled into the wrong spots? Not sure if that would cause your issues or not, but it sounds like you have ruled most everything else out already.

Let me know if I can check anything on my 1010, or post a picture that will help you

Motnick,
Thanks for the help - much appreciated.
Yes - I took a pic of the heatsink so I could keep track of original transistor locations. WOW - 2 NPNs and 2 PNP's together on 1 channel!
I double checked for isolation from the heatsink, just in case - I'm good there.
 
I think you have a transistor in backwards or like Larry stated, a correct transistor that has different leg configurations.
Did you notice:
When you check for the DC Balance, is it to one side? Is the Balance reading a + or -54 volts at the amp output, pins 19 or 20? This reading, before R17 begins to cook.

Yeah - I'ts gotta be something I did. It was running very stable until I took apart this amp board. I even was able to set offset and idle to 25mv, and it was running nice & cool, even with all the old caps & transistors in there.
DC Balance: Haven't been able to get that far yet - since this issue.
 
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The other problem is the version of the board and the variations of each one. I've seen it occasionally, but for the life of me I can't remember where right now.
ADJ1964.........Please note the actual part/board number in the box with any/all suffixes please. I'll see if I can locate the 1010 amp board variants and post them up here. Might be a day or 2, and I won't mind someone else looking also.

The amp boards are AWH032A version for both channels. I've noticed in other threads that there are slight differences.
 
Larry also mentioned the difficulty installing the TO-126 transistor package, These - as in Q6: 2SC1451: 512-KSC3503DSTU are VERY easy to put in backwards.
 
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