SX990 amp on board output caps

[dnpainting]

Thank you Mark for overlooking those proceedures and djmtascam for writing and drawing up the new doc. I will have to open up the 990 I gave to a friend to check and re-adjust.:thmbsp:

 

[djmtascam]

looks good on first pass.

Thanks again Mark!!...those linked Pioneer SM's were pretty vague and a few errors to boot (30mV one place and 30mA in another)..LOL
I did a little chart of each of those and then went thru the schematics of each and found the commonality/method that fit the pinouts and such for the SX990. So after sketching it all out and understanding what was going on it's actually a pretty easy simple procedure to do..but I'm glad I did it as it helped me understand everything better!
I'm not close to power up yet...as it will be a couple days for parts to arrive, so any other errors you can find would be great!

One thing I was curious about is when you initially turn pots VR1/2 (50k) CCW before powering up, that puts the resistance of those at a low value like 1-3 ohms.
But there is no initial setting for the 50ohm pots VR3/4?
What is that doing in the circuit initially?....is that like a safe starting point so they're not set really high?
Is no initial setting of VR3/4 (50ohm) needed?
On my borad both trimpots were set around halfway (25ohms and 14k)

Also is it important to do both channel 0V steps first and then do both channel 20mV steps second in that sequence or could you do one channel at a time?
I sensed that might be an important order?...but maybe not?

Thank you Mark for overlooking those procedures and djmtascam for writing and drawing up the new doc. I will have to open up the 990 I gave to a friend to check and re-adjust.:thmbsp:

I'm curious dnpainting...was there something in my steps that was different..and what do you think is misaligned or incorrect on your unit?
I would make sure my instructions are fully QA'd by Mark!
There may be some embellishments that may be incorrect on my part.
It sounds like the steps are good...and have confidence Mark would have spotted any blatant issues.
Those are pretty straightforward steps and all makes sense to me now.
I've never done this type of adjustment before so that's why writing everything out like I did helps me!
Being in the aerospace industry for many years and having spent excessive amounts of time making sure every T is crossed and every I dotted, my anal retentive attention to detail probably went a little overboard I'm sure! :scratch2:
It was a fun little exercise though while watching football...it can be very therapeutic!...LOL
Enjoy!

 

[dnpainting]

I'm curious dnpainting...was there something in my steps that was different..and what do you think is misaligned or incorrect on your unit?
I would make sure my instructions are fully QA'd by Mark!

I just don't recall using TP1 and TP2. It was a while back and Marc was unavailable at the time. I am sure that everything on the one my friend has is fine but being a newbie at these repairs, I just want to make sure I crossed my T's and dotted my I's is all. Also, I am going to go ahead with changing out the 4 pots in there.

 

[djmtascam]

I just don't recall using TP1 and TP2. It was a while back and Marc was unavailable at the time. I am sure that everything on the one my friend has is fine but being a newbie at these repairs, I just want to make sure I crossed my T's and dotted my I's is all. Also, I am going to go ahead with changing out the 4 pots in there.

Changing the trimpots is probably a great idea. Mine looked pretty tarnished.
I figured cleaning them would have done some good but for the cost of replacements and being sort of critical part, a great thing to update.

One thing that might be interesting would be to open it up and take the measurements "cold" and record what you get. That way you could tell how far off the original adjustment might have been on or off.
Then do the 10-15 minute warm up thing and take another reading.
That way you can test if the thermal conditions and diode are working properly.
And of course if you replace the trimpots you'll be doing bias adjustment again anyway so that will give you a good idea how each of these procedure worked.
I've decided I'm installing those TP1/2 wires instead of a post because it will be way easier to take the reading. It's just too buried in there. It's sort of weird they didn't just have a spot on the front edge of the board for those which would make it a snap to test and measure.
Good luck!

 

[markthefixer]

they brought out test points in later models. they were learning too back then. Tube equipment mindsets adjusting...

NOT bad, just different....

 

[djmtascam]

they brought out test points in later models. they were learning too back then. Tube equipment mindsets adjusting...

NOT bad, just different....

Yep...not a problem it will be all good with the wire.
I just have nothing to do except clean and stare at it right now!...LOL
I will fit the cap on there and that (-) jumper and then maybe see if there's is a good place to add a pin...if the whim strikes!

 

[djmtascam]

Sx-990

First Photo

 

[djmtascam]

OK well here is the completed main amp board.
Ready to be reinstalled in the receiver.
I checked each new part before installing just to make sure they were all good going in...no issues.
I've checked all the existing resistors....no issues
I checked both 50ohm trim pots and preset them to zero (fully CCW)
I checked both 50kohm trim pots and prest them to about 15kohms (centered dial pointing up)
I have all heatsink areas with a nice film of grease and contact.
Checked for shorts to ground on the collectors, emitters and base pins of the output transistors.

Are there any other checks I should do before installing and powering up?

I will certainly power this using my DBTester for first time and if all is well I will plug it in normally and proceed to measure the power supply for 56V.
And then do the bias/current adjustment..
If all that goes well, I will put 2 50W 8ohm dummy loads on the outputs and reinstall the pre-amp jumpers and run some test signal...
Appreciate pointing out anything I missed or overlooked!

 

[markthefixer]

just double checking - mica's between the output transistors and the heat sinks? The sink is (probably) isolated on the board like that so a ground check might fool you.

What I do is to get through the anodizing at two points for a zero ohm reference, then check to each collector etc... winding up with retesting zero ohms while trying throughout NOT to loose contact with the heatsink on my stationary reference point.

looks good

when on dbt set the center point voltage of one collector at 1/2 the voltage of the other collector. Then TRIAL set the idle current to tease out any faults, then back to zero - one channel at a time.
then if successful, go straight to the power line, recheck the center point voltage and do the idle current for real.

 

[djmtascam]

just double checking - mica's between the output transistors and the heat sinks? The sink is (probably) isolated on the board like that so a ground check might fool you.

What I do is to get through the anodizing at two points for a zero ohm reference, then check to each collector etc... winding up with retesting zero ohms while trying throughout NOT to loose contact with the heatsink on my stationary reference point.

looks good

when on dbt set the center point voltage of one collector at 1/2 the voltage of the other collector. Then TRIAL set the idle current to tease out any faults, then back to zero - one channel at a time.
then if successful, go straight to the power line, recheck the center point voltage and do the idle current for real.

Yes new mica washers and grease between the transistors and heatsink.
I will double check the collectors though!

When you say set the center point voltage to half....do you mean adjust VR3/VR4 to 10mV first?
I'm confused by this..

 

[markthefixer]

Yes new mica washers and grease between the transistors and heatsink.
I will double check the collectors though!

When you say set the center point voltage to half....do you mean adjust VR3/VR4 to 10mV first?
I'm confused by this..

center point voltage set first using vr1,vr2

then 10mv vr3, vr4.

 

[djmtascam]

So just to make sure I'm clear...

Before power up....
Still set VR3 & VR4 fully CCW (<1ohm)
And still have VR1 & VR2 set to the center (~15k)
Power up on DBT
Turn up VR3 & VR4 CW until 0V (center voltage?)
Then adjust VR1 and VR2 to 10mV.

Then check for what?....what would be a good or bad result?
Will the 60W change at all or any other indication that I've flushed out any problems?

OK so then if that all is fine, then plug in normally and recheck the 0V and THEN tweak to 20mV.
Then let it burn in for 15-20 min
Then recheck everything for 0V and 20mV

Is that correct?

I would like to add this part with the DBT portion into my little dissertation sheet for the record....LOL
Sorry if this is getting to picky, but I would like to fully comprehend the process and have everything correct.

And...also thanks for this...

 

[markthefixer]

So just to make sure I'm clear...

no, close

Before power up....
Still set VR3 & VR4 fully CCW (<1ohm)
And still have VR1 & VR2 set to the center (~15k)
Power up on DBT
Turn up VR3 & VR4 CW until (read 52v on one output transistor collector, set the OTHER output transistor of the pair's collector to 26 volts) 52 / 2 = 26 volts
Then adjust VR1 and VR2 to 10mV.

Then check for what?....what would be a good or bad result?
the bulb goes bright or the fuse blows while adjusting to 10mV and back down to zero.

Will the 60W change at all or any other indication that I've flushed out any problems? problems = it may get very bright.

OK so then if that all is fine, then plug in normally and recheck the 0V and THEN tweak to 20mV.
Then let it burn in for 15-20 min
Then recheck everything for 0V and 20mV

Is that correct?

I would like to add this part with the DBT portion into my little dissertation sheet for the record....LOL
Sorry if this is getting to picky, but I would like to fully comprehend the process and have everything correct.

And...also thanks for this...

 

[djmtascam]

OK close but I'm still not real clear!...so a couple more questions!
Sorry but maybe it's just the way you explained it....or the more likely explanation I'm dense...:screwy::yes:

Turn up VR3 & VR4 CW until (read 52v on one output transistor collector, set the OTHER output transistor of the pair's collector to 26 volts) 52 / 2 = 26 volts
I think I get what your saying....measure the collector at the load side of the fuse (Q9/Q10) and whatever the value is there take half of that and adjust VR3/4 CW> so that the reading at the collector of Q11/Q12 (junction with R31/32) is half.
Would it be a good idea to use 2 meters for this?

Then adjust VR1 and VR2 to 10mV.
This one I know should be GND and the TP wire
Will I be turning those CW> or <CCW to get to 10mV
Will the preset position of VR1/2 at ~15k still OK for this or too high or too low?

the bulb goes bright or the fuse blows while adjusting to 10mV and back down to zero.
If it does this what is it that might be wrong?...and what is the theory behind this test?
Will this blow any components up?

Will the 60W change at all or any other indication that I've flushed out any problems? problems = it may get very bright.
Should I be using a higher wattage bulb for this test 75W or 100W?

 

[djmtascam]

Rechecked all collectors to ground (using mounting bracket sheet metal screw into heatsink).....no shorts.
Rechecked the entire board from same heatsink screw and got no shorts on any pins or solder pads!
Rechecked all new transistors and caps for correct orientation...all good!
I take it that is a satisfactory result?
I am proceeding to reinstalling the amp board back into the receiver and reconnecting all of the chassis wires and the 2 external output caps!

 

[markthefixer]

Sounds good!

Hope it results in good sounds!!

 

[djmtascam]

Sounds good!

Hope it results in good sounds!!

Thanks Mark....haven't had a chance to get things back together yet or run the tests...holidays are cutting into the project activities.
Hoping I have focus time to be back at it tomorrow evening...I don't want to rush anything at this stage!

Still curious about what the theory is behind the center half voltage part and understand what that might uncover or is doing?

 

[markthefixer]

the positive or negative part of the waveform will be clipped prematurely. You won't get "full swing" (or power) out of the amplifier before it starts to distort.

It (990) is rated at 35w, which means a +/- 17 volt signal sent to the speakers, and you loose a few volts at the transistors from the theoretical maximum of the supply voltage.

 

[djmtascam]

the positive or negative part of the waveform will be clipped prematurely. You won't get "full swing" (or power) out of the amplifier before it starts to distort.

It (990) is rated at 35w, which means a +/- 17 volt signal sent to the speakers, and you loose a few volts at the transistors from the theoretical maximum of the supply voltage.

I think I understand this now...
So that center point rail being adjusted by VR1/2 balances the two halves of the push-pull stage so they're equal in terms of positive and negative peak swing and then when you adjust bias with VR3/4 you're setting the cutoffs points of each half just slightly above and below zero for each waveform to overcome that crossover distortion anomaly.
That way each half of the waveform never really reaches zero before the other half takes over and each half of the voltage peaks are equal...hence no distortion!...or as close as you can that!?!?:yes:
So it's a real balancing act....
Is that a good understanding?

Is that something I could look at that with my scope?
Mines not a super good one 25Mhz dual channel BK Precision...
And I'm guessing I would need to do that AFTER I've done this "cold" setting procedure...(no signal or load) right?
Anyway....thanks Mark for your knowledge and patience with me on this!
I've certainly gleaned quite a bit detail....
Hopefully all will go well with this and there will be more fun after the amp board is up and running!

 

[markthefixer]

Yes, good understanding.

Yes, you can see it on a scope, use load resistors instead of speakers.

no signal while setting center point and idel current, zero volume is fine.