djmtascam
Active Member
OK...thanks...I'll give it a look see with the scope when I'm done setting bias and idle....see how it looks!
I have some 50W 8ohm dummy loads that I want to put on heatsinks.
Need to find some 1/8" aluminum 12" x 12".
SX990 amp on board output caps
- Thread starter djmtascam
- Start date
OK...thanks...I'll give it a look see with the scope when I'm done setting bias and idle....see how it looks!
I have some 50W 8ohm dummy loads that I want to put on heatsinks.
Need to find some 1/8" aluminum 12" x 12".
find an old cpu heatsink and fan... off a discarded computer.
djmtascam
Active Member
Perfect....I have one of those!
djmtascam
Active Member
So I got everything back in!
I did the DBT and good results....no shorts bulb went dim quickly.
I checked all the power supplies points
B1=64V should be 56V
that seemed higher than I was expecting!
B2=38.7V should be 40V
B3=27.5V should be 25V
B4=20.3V should be 18V
B5=12.5V should be 12V
Check the collectors of the output transistors
Collector of Q10=64V
Collector of Q12=31.4V (close to half)
Collector of Q9=64V
Collector of Q11=31.4V (pretty much exactly the same readings)
I then moved my meter leads to "Out 1" and GND, powered up and read 15V initially on the meter but then it started to drop rapidly and sort of settled to like .200V a little bouncing around down there.
Never seemed to settle solid anywhere.
Same thing on Out 2.
I tried turning the 50K trimpots and it changed a little but very touchy. I could get it to .150V but then it would jump to .300V again really touchy going up or down????
I then connected to the "TP1" and GND powered up and read .1mV solid.
I turned the 50ohm trim pot a little but nothing seemed to change. I was expecting it to go up to 20mV as I turned it??
Same thing happened on TP2.
Maybe I didn't turn it up enough?
So I've shut it off for now and need to know if I'm doing this right or is there something wrong?
I did the DBT and good results....no shorts bulb went dim quickly.
I checked all the power supplies points
B1=64V should be 56V
that seemed higher than I was expecting!
B2=38.7V should be 40V
B3=27.5V should be 25V
B4=20.3V should be 18V
B5=12.5V should be 12V
Check the collectors of the output transistors
Collector of Q10=64V
Collector of Q12=31.4V (close to half)
Collector of Q9=64V
Collector of Q11=31.4V (pretty much exactly the same readings)
I then moved my meter leads to "Out 1" and GND, powered up and read 15V initially on the meter but then it started to drop rapidly and sort of settled to like .200V a little bouncing around down there.
Never seemed to settle solid anywhere.
Same thing on Out 2.
I tried turning the 50K trimpots and it changed a little but very touchy. I could get it to .150V but then it would jump to .300V again really touchy going up or down????
I then connected to the "TP1" and GND powered up and read .1mV solid.
I turned the 50ohm trim pot a little but nothing seemed to change. I was expecting it to go up to 20mV as I turned it??
Same thing happened on TP2.
Maybe I didn't turn it up enough?
So I've shut it off for now and need to know if I'm doing this right or is there something wrong?
everything's fine up to the quote:
The meter has an impedance of ten million ohms, so it will read the slight leakage through the capacitor as a voltage across the 10 million ohms. There is no meaningful measurement here - it really signifies nothing (the capacitor is supposed to BLOCK almost all the dc from getting through, but ten million ohms does not need much leakage current to read a voltage.
) get the vcc/2 point re set correctly
so skip that "step" (it ain't a step) - with that type of amp it is meaningless - now if you read 0.1v across the 8 ohm speaker load, the cap would be leaky enough to replace it.
yes, try further with the 50 ohm pot, there is a "threshold" you need to find and cross. below it there will be no effect, but once you hit it, it could go up fast, so don't crank on it hard, go a few (10 ?) degrees at a time.
The meter has an impedance of ten million ohms, so it will read the slight leakage through the capacitor as a voltage across the 10 million ohms. There is no meaningful measurement here - it really signifies nothing (the capacitor is supposed to BLOCK almost all the dc from getting through, but ten million ohms does not need much leakage current to read a voltage.
) get the vcc/2 point re set correctlyso skip that "step" (it ain't a step) - with that type of amp it is meaningless - now if you read 0.1v across the 8 ohm speaker load, the cap would be leaky enough to replace it.
yes, try further with the 50 ohm pot, there is a "threshold" you need to find and cross. below it there will be no effect, but once you hit it, it could go up fast, so don't crank on it hard, go a few (10 ?) degrees at a time.
Last edited:
djmtascam
Active Member
everything's fine up to the quote:
The meter has an impedance of ten million ohms, so it will read the slight leakage through the capacitor as a voltage across the 10 million ohms. There is no meaningful measurement here - it really signifies nothing (the capacitor is supposed to BLOCK almost all the dc from getting through, but ten million ohms does not need much leakage current to read a voltage.
so skip that "step" (it ain't a step) - with that type of amp it is meaningless - now if you read 0.1v across the 8 ohm speaker load, the cap would be leaky enough to replace it.
yes, try further with the 50 ohm pot, there is a "threshold" you need to find and cross. below it there will be no effect, but once you hit it, it could go up fast, so don't crank on it hard, go a few (10 ?) degrees at a time.
OK...I took the 63VCC and reset the half points to 31.5 (+/- .5V)
Then set the 50ohm to 20.5mV (+/- .5mV) rechecked both sides and it seems to be working fine.
I think part of the problem was I was expecting these readings to be stable and right on, but they fluctuate a bit. But I think it's all good now.
I fed a 1kHz tone into the Aux In and put the dummy loads on.
The waveform on the scope is good...don't know if could actually "see" that crossover distortion but it looked smooth on my scope and both channels were even. I measure about 10.5Vrms on the load resistors just below clip, but only left it there for a short moment and backed it down.
There is a definite faint "whine" when you get it over 7Vrms
All the other controls worked as expected..
I'm guessing everything else ahead of the main amp is good, so I don't know if there are any improvements to be made.
I did notice the Control Amp had some funky looking caps and I have replacements for them so I might go ahead and replace the electrolytic's in that as well...
Any suggestions as to what I could look at next or any "tweaks" or measurements I could take to see how well it's performing?.
A couple of photos of the test setup
Just happen to snap 20mV readings when they were both exactly the same!
The meter in the center had a thermistor attached which I had above the board just to see if the temp went up.
Funky looking Control Amp capacitors...
Thanks Mark for all your help and any more advice you might have would be greatly appreciated.
the 20mv idle current setting eliminated any(most) crossover distortion - IF you want to see it, you have to turn that back down to zero.
re:caps somebody's already been in here... but the newer caps you have are still appropriate
re:caps somebody's already been in here... but the newer caps you have are still appropriate
djmtascam
Active Member
Is it OK for the 20mV to drift up to say 22-24mV?
They seem to stay right around ~21.5mV....
Both sides are very even...and the same with the 32V collector half Vcc..
both right around ~31.6V.
I recapped that control amp board (electrolytic's) as I had all the parts.
I thought those looked quite out of place!..
None of the old ones were bad (open or shorted) but I figured I might as well do it...
yes it is fine.
agree with the cap change. as long as the appropriate caps were used.
agree with the cap change. as long as the appropriate caps were used.
djmtascam
Active Member
Thanks Mark!
I think I'm done!... no blown fuses no smoke no fire...the whole thing went extremely well and finished in time for Xmas gift wrap!
It was a lot of fun for me to delve in deep on this machine
I did change out the "control amp" caps...only 12 and had all the electrolytic's on hand so it was easy and quick! The mylar and ceramic disc caps I didn't have so I just checked them and they were fine.
It didn't change much if anything and none of the old ones were bad, but I figured might as well while I was in there fixing the bad channel.
Recheck the bias and idle current one more time before I closed it up and they were both right at 20.5mV. Checked all the input and speaker outputs.
Been playing it for over an hour now and the tuner (FM Stereo) is very good..
I also ran some CD's into the Aux in and on my speakers sounded really nice, clear.
My personal opinion on the sound quality was a little below my expectations...It didn't blow me away but I think it improved after a little burn in. So I will play it for awhile tonight...
I hope my friends brother will hook this up to some nice vintage speakers to appreciate it's vintage sound.
Thanks for all the help and advice!
