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Starting To Dive Into an SX-1250

I have a question about C11 on the power amp board(s), AWH-048. The original looks to be a polarized Tantalum cap, .22uF/35V and the replacement that I got (off of the BOM) is a Panasonic stacked film poly cap that is .22 and 50 volts. Is it okay to use this? There is no polarity obviously.
Thanx,
BudImageUploadedByTapatalk1352337305.803961.jpgImageUploadedByTapatalk1352337318.181907.jpgImageUploadedByTapatalk1352337329.954805.jpg


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it is a perfect replacement. use the stacked film cap, it'll never have to be replaced.
 
Well, I am banging right along. I have one more board to do and that's it! All switches cleaned and the chassis wiped down. The cabinet is refinished as is the heat sinks. Faceplate cleaned and lens polished. Tomorrow I should be able to hook it up to the DBT and let her rip! I have the bias pots fully CCW and the idle pots at mid-point.
Getting anxious to say the least. All in all I have invested a little less than three-bills in parts (the filter caps were the big hitters!) and the $300 purchase price so for $600.00 I'll have a new SX-1250!
More pics coming. Thanx again to all of the gracious folks who have made this possible.
:)


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:yikes:

I have the bias pots fully CCW and the idle pots at mid-point
Click to expand...

bias and idle are the same thing idle at midpoint could be TOO MUCH idle current....

offset voltage target is 0.000v and can swing + and - so it's pot is set in the center

idle current goes from zero (ideal) or minimum to way past full, so it is started at minimum and that means fully CCW.

offset and idle are both 500 ohm post so that doesn't help

recommended parts are multi turn offset
and
single turn idle

offset is buried near the center of the board
idle is right up at an edge where the transistor cable connectors plug in.
 
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My terminology is skewed. Late at night.
The one-turn pot is zero and the 25-turn is midway.
:(


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69sixpackbee said:
My terminology is skewed. Late at night.
The one-turn pot is zero and the 25-turn is midway.
:(


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Click to expand...

I'm still worried about us not being "on the same page"

so:
recommended parts are multi turn offset
and
single turn idle

offset is buried near the center of the board
idle is right up at an edge where the transistor cable connectors plug in.
Click to expand...



restating it all at once, hopefully unambiguously:



the multi turn 500 ohm trim pot adjusts the amplifier's output voltage, this offset voltage is adjusted to 0.000 volts ,
this multi turn trim pot is buried near the center of the printed circuit board
and this multi turn pot should be at 12 of 25 turns -- centered --
it has THREE separate connections on the foil pattern

and


the single turn 500 ohm trim pot for amplifier internal idle current adjustment
is right up at the edge of the printed circuit board where the transistor cable connectors plug in.
This single turn trim pot should be fully ccw looking at it face on,
giving zero ohms
it has three connections BUT one side and center tap are connected together by the foil pattern,
reading zero ohms from this doubled connection to the remaining connection of the three
 
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No, we are exactly on the same page of the same novel and volume! Guess I should be a bit more precise with my updates. All is EXACTLY as you so thoroughly described, Mark. I wish I had college professors who took the time and explained my subjects half as well as you.
Now, since we have that behind us now, I noticed that not one component was swapped on the AWE-068 tuning board
Is there a reason why that one is not touched?
There are quite a few components worthy of swapping out in there. Is it primarily because of the possible loss of tune if anything is swapped out?

Thanx,
Bud


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exactly. usually ends up taking almost totl performance and turning it into motl or botl until an alignment is done. Mono distortion levels and stereo pilot rejection, separation etc suffer.

others say they don't notice a difference, but my instruments tell a different story.

you need to send in a controlled and precise stereo signal over a RF carrier, then measure the distortion and audio levels while sending in different signals, all the while adjusting them - and that doesn't include the RF adjustments
 
Ready to pull the trigger tomorrow !
Finished putting her all back together after a thorough cleaning. Went together like a dream. I cleaned the RCA shells with a red Cratex cylindrical bob on an arbor attached to my Dremel. I bored into it just a tad smaller than the O.D. Of the shell. A few seconds on each and viola! I use the same method on the header pinsImageUploadedByTapatalk1353215804.114015.jpg
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Here's some nudies:
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I have it ready to fire up on the DBT.
More pics to follow.


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This thing will be welcome atop of the entertainment center next to my Marantz 4400!
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Not so good news. Started this up via the DBT and I have a non-dimming 50W bulb. I started it up with the 5.1K Ohm dummy load into the "power in" jacks with the jumper removed (obviously) I had my DMM attached to pins 9 & ground on the power amp board and had about -5.1VDC on the left amp and I quickly moved to the right amp where I was close to "0" VDC.

Obviously the stabilizer board won't have a good reading running this way so I just shut it off.

Now the fun begins. Not. I triple checked every component I changed out and I am very meticulous in my methodology so now WTF?




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Okay. I pulled all of the connectors off of the boards and started at square one. Put 50W bulb in DBT. All points on power supply good. Straight wall power, still good. Back to DBT. Hooked up stabilizer. Good so far. Straight power. All points are good. Have rock-solid 65VDC. Started to check tuning functions. All seems normal. Now on to the power amps.

....to be continued.


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Okay. RT amp is good. Got a rock-solid "0" VDC at pin 9 & GND and 100mV between 7 & 19. But...... On the LH amp I get the "0" V no problem trimming down VR1 but between pins 7 & 9 I get zilch! Regardless of VR2 movement. Looks like I need to visit this p board now.
At least I'm getting somewhere :)


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I'm kind of stuck. I have been tracing out the power amp board and am trying to see what is responsible for the problem of no 100mV. I have checked all of the diodes and resistors. I pulled them off of the board and checked them out of circuit. The varistors are not open. This leaves the transistors. Which should I be looking at?

Thanx,
Bud


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69sixpackbee said:
I'm kind of stuck. I have been tracing out the power amp board and am trying to see what is responsible for the problem of no 100mV. I have checked all of the diodes and resistors. I pulled them off of the board and checked them out of circuit. The varistors are not open. This leaves the transistors. Which should I be looking at?

Thanx,
Bud


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Click to expand...

these tests:

Bipolar Junction Transistor Testing Basics
and go through to post 11 to get explicit testing and reporting formats.

and do NOT power up that amp module until we talk a LOT more - you really jumped in the deep end with NO discussion AT ALL.
The last I KNEW, you and I were talking about getting the trim pots in correctly. Now this.... (see the smoke curling slowly from under my collar and out my ears?)

you just tap danced through minefield, blindfolded.

The FIRST thing I would have said is to test the outputs C-E first before power.

The second thing is that 50w for a DBT on a 1250 is inadequate - you need 100 to 150 watts - you can run bulbs in parallel to increase the wattage.

If you INSIST on powering things up, do NOT power up that module WITHOUT the damn DBT - NO EFFING WHAT!!!!! UNTIL I SAY SO

sheesh:
I take Sundays off because company's over, I go away and when I come back, one guy's testing a power supply with a current meter to ground, and the other's tickling a sx-1250 dragon's tail....

answer me one thing - was the 1250 amp that doesn't show idle current ever powered up in your presence before?
 
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Okay, okay,..I didn't tickle him. I grabbed him and swung it around!

One of the first things I did was test the OP's when I pulled them from the heat sinks. All eight tested correctly with my good Fluke DMM. I (here we go) "ASSumed" they were good.

I was being conservative with a 50-Watter. I want to start slowly. I found a 150W to be the best as you already eluded to. I was systematically checking this thing. It was ratty when I got it. To answer your question, this unit never functioned prior to my diving into it. When I got to the point of where I am now I stopped and waved my hands in the air. I'm fairly "green" to this, admittedly, but am a conscientious learner. That being said, I am not shy to fixing complex componentry. I am well learned in robotics and PLC implemented system design and integration as well as C+(++) and Fortran not to mention electrical systems in an Industrial environment so I have a good grasp on a lot of things. Sometimes I dive in expecting blazing results but face mediocrity in reality.

So, moving forward, I will carry on with your guidance and hopefully be able to lessen the amount of smoke wafting from your earlobes.
:)
Thanx!
Bud





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read that transistor testing thread. it MAY bear fruit. the rub is that the amplifier strives mightily to do it's job, even when mortally wounded, and does a pretty good job at a static balancing act with some dead transistors (half of the push pull drive circuits).

quick short cut is to read while running the emitter base voltage on each transistor and if it isn't around 0.6 volts (+ or - depending upon pnp or npn) there's part of the problem. but it takes a bit of messing around to be able to GET to the readings.


burned out and dead resistors are the other half, check them ALL with one leg lifted, of course that means the power is off and the board is out.

a warning: without the outputs in circuit, the amp should not be turned on, as there is no active DC feedback path to balance out the amp - it will immediately "rail". with the outputs in circuit you risk blowing the outputs with huge current surges.

best for you right now is to cold troubleshoot with the ohmmeter and diode tests on the transistors. the risks of power on testing and the gyrations necessary to read the transistor voltages as well as the time to DO those gyrations is well beat by the ohmmeter and diode testing of a board that is simply unplugged.
 
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