SX-1250 Power Amp VR-2 wont adjust.
- Thread starter Stevesmullen
- Start date
That transistor is marked weird... but it IS at least a to-220. Yes Do Q9 & Q10 to test, at least we can trust the Fairchilds, especially if they have never been installed.
That square, unheatsinked transistor next to it, what is IT marked??? it's NOT original!!! You only did Q4, right?
While Q9 & Q10 are out, test ALL the resistors in that area.
Were / are R24 & R25 different on the bad board?
edit - if we assume it is a competitor to NTE and look up NTE 375, the nte 375 is an npn 200v 2a (10a peak) 25w to-220 transistor which IS in the ballpark..
CHECK ALL the resistors carefully!!
Stevesmullen
Well-Known Member
Yes the different R24 and R25 are on the bad board and the Fairchild Transistors are new.
I will get the board fixed up tonight and test it out tommorrow as long as I can get 1 watt 100 Ohm resistors today.
You said to check all the resistors carefully,I planned on Ohm Checking them with the DMM.
Thanks
I was going to say something about that at posting #33 but that's Mark's job. **OOPS** QUIET CC!!!
ara::bash:
Just ignore me!!
Stevesmullen said:
I was going to say something about that at posting #33 but that's Mark's job. **OOPS** QUIET CC!!!
Just ignore me!!
Are you talking about his replacing R24 & R25 ? Because I would look at R26 & R27 first.
I wasn't going to argue their replacement, because R24 & R25 are not original, BUT the page and a half of mathematical analysis I DIDN'T post because it got too dense convinced me that the difference in the base voltages of Q9 & Q10 to get 0.75v and -0.703v (THAT 23mv offset reflects upon the mismatch of the output transistors) at Q9 & Q10's emitters (ASSUMING R26 & R27 are ok and equal in value) to drive the good output transistors to zero volts, means that Q9 is badly damaged and the collector to emitter leakage along with the feeble attemps of Q9 to operate were enough to get the circuit going, albeit with higher than normal bias voltages resulting in too much idle current.
Q9 is probably not operating, and audio above a certain level will be distorted badly, and below that level the feedback circuit will be working HARD to cancel the distortion generated by the bad Q9.
Yes, I was thinking that until post#39 shows up when Steve posted the transistor number ERS 375 and your statement in post#40 regarding the datasheet. And thinking again at #43 still scratching about #40.:scratch2:
Stevesmullen
Well-Known Member
Hey Guys.
I did not replace R24 and R25 I have only done what we have talked about in the thread which has been Q3 and Q4 so far,that was the persons work before me who obviously couldnt find the problem.
Thanks Again for the on going help.
I did not replace R24 and R25 I have only done what we have talked about in the thread which has been Q3 and Q4 so far,that was the persons work before me who obviously couldnt find the problem.
Thanks Again for the on going help.
Mark and I didn't say you replaced them. We only knew from your statement that they were different and not original.
I have a feeling you are getting very close to the problem, in fact you might be on top of it. zzziiiippppp:zoom:
Yes, I know the resistors are a legacy problem.
As for the R25 question:
Stevesmullen said:
ALL these weird voltages are because the feedback system is skewing things to try to get 0mV at the output (the common point of the emitter resistors).
The skew or offset changes from stage to stage as the individual device characteristics and matches (or lack thereof) between the transistors affect it. That's why at first I was NOT looking for ground based measurements (eliminating some calculations), and then later switched to ground based measurements in the hopes of differentiating between Q9 being bad and Q10 being bad - which assimes the resistors were ok.
Stevesmullen
Well-Known Member
Hey Guys.
I replaced Q9 and Q10 (diode tested good) with the temporary transistors also R24,R25 and R26,27.
While those components were out I went through and tested all the resistors on the board (I only had to pull a couple of legs at this point) they all tested good,my only question is on some of them that were obvious opposing pairs they werent the same exact reading for example.
R42 13k
R43 12.9k
R21 98.5
R22 99.5
R36 5
R37 4.9
I really dont think that it is a problem but I did want to mention it.
(Mark) Q7 and Q8 are the original square transistors.
At this point I think that I (we) are ready to plug the board in OPs and all and see what we get starting with VR2 at the lowest setting.
EDIT.
I got home and hooked up the Power Amp Board with the VR2 set at the lowest setting and turned on the unit with the DMM hooked up to pins #7 and #19 and there was no voltage until I turned up VR2 and low and behold I seem to have control of the idle current,I set it at 35mv so as not to overload out temporary Drivers (Q9,10) and it stayed at 35mv for the 15 to 20 minutes that I let the unit sit there on with no speakers hooked up and Q9,10 heatsink was only luke warm.
So at this point I would think that I would order the proper drivers and put the unit back together.
Thanks Steve
I replaced Q9 and Q10 (diode tested good) with the temporary transistors also R24,R25 and R26,27.
While those components were out I went through and tested all the resistors on the board (I only had to pull a couple of legs at this point) they all tested good,my only question is on some of them that were obvious opposing pairs they werent the same exact reading for example.
R42 13k
R43 12.9k
R21 98.5
R22 99.5
R36 5
R37 4.9
I really dont think that it is a problem but I did want to mention it.
(Mark) Q7 and Q8 are the original square transistors.
At this point I think that I (we) are ready to plug the board in OPs and all and see what we get starting with VR2 at the lowest setting.
EDIT.
I got home and hooked up the Power Amp Board with the VR2 set at the lowest setting and turned on the unit with the DMM hooked up to pins #7 and #19 and there was no voltage until I turned up VR2 and low and behold I seem to have control of the idle current,I set it at 35mv so as not to overload out temporary Drivers (Q9,10) and it stayed at 35mv for the 15 to 20 minutes that I let the unit sit there on with no speakers hooked up and Q9,10 heatsink was only luke warm.
So at this point I would think that I would order the proper drivers and put the unit back together.
Thanks Steve
I wanted to answer last night but I was going to let Mark answer it. Since you have powered them up, I recall your question regarding R25 that you stated that it opens under load because it was having + one end and - other end. It does not mean it's bad nor does it mean it opens under load. If it ohm'd good, then usually it's good. Measurement for R21, 22, 36, 37, 42, and 43 looks good and seems to be within spec. Congratulations,
but you're not out of the woods yet.
I have noticed in post#40 Mark posted 3 different posting and stated that the thread was never completed and the subsitute was never tested. I'd be interested to know if those subsitute were successful.:yes:
but you're not out of the woods yet. I have noticed in post#40 Mark posted 3 different posting and stated that the thread was never completed and the subsitute was never tested. I'd be interested to know if those subsitute were successful.:yes:
Post #40
IN that post, the recommended replacements :
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 863-MJE15032G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
two: 863-MJE15033G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
versus:
(q9, 2sd610) 512-KSC2073TU_Q npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.53
(q10, 2sb630) 512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 863-MJE15032G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
two: 863-MJE15033G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
edit: use the KSA / KSC's not the MJE's - Vbe turnon is a bit lower and results in too much bias at times 4/25/09 mtf
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
two: 512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
Looking at the specs, we MAY just be able to get away with using the Fairchilds PERMANENTLY, other than the 1.5 vs 2 amp current of the 2sb630/2sd610/s. I will calculate....
The 30 mhz on-semi's worry me, in regards to amplifier stability, while the fairchilds are right on at 4 mhz.
Steve:
1. Upon Calculation, Use the fairchilds permanently, the only theoretical limitation could be at the absolute high power end of the usage, an sx-1250 is rated :
160w into 8 ohms which is 35.77v across the 8 ohm speakers, giving 4.47 amps of current
200w into 4 ohms which is 28.28v across the 4 ohm speakers, giving 7.07 amps of current
assuming a worst case gain of ten at 7 amps for the outputs, they will only need less than an amp to make rated power, and that is at 4 ohms, where the danger to the outputs comes in the form of power dissapation increased by sub 4 ohm loads... just the place we would LIKE to limit things a bit.
worrying about 1.5 amps versus 2 amps is not indicated (silly actually).
2. RE: R25, seeing different polarities at the ends of a resistor is NOT an indication of a problem, the zero volt point would be somewhere IN the resistance element, all that matters is that the current is flowing through the resistor. If the current wasn't flowing, the opposite polarity voltages would be a LOT higher. This is a book learning/calculating part, not a winging it part. It was the result of the Q9 / Q10 mismatch/unbalance (dead/damaged/leaky is STILL mismatched)... looking closely, the effect can be found to a far lesser degree all over the amp, at all the pnp/npn push pull stages.... the end result at the output being zero is what matters as long as the internal conditions are not extreme enough to upset OTHER functions (like it did with the idle current).
IN that post, the recommended replacements :
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 863-MJE15032G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
two: 863-MJE15033G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
versus:
(q9, 2sd610) 512-KSC2073TU_Q npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.53
(q10, 2sb630) 512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 863-MJE15032G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
two: 863-MJE15033G to-220 bce 250v 8a 50w 30mhz hfe:70 $0.87
edit: use the KSA / KSC's not the MJE's - Vbe turnon is a bit lower and results in too much bias at times 4/25/09 mtf
drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
two: 512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
Looking at the specs, we MAY just be able to get away with using the Fairchilds PERMANENTLY, other than the 1.5 vs 2 amp current of the 2sb630/2sd610/s. I will calculate....
The 30 mhz on-semi's worry me, in regards to amplifier stability, while the fairchilds are right on at 4 mhz.
Steve:
1. Upon Calculation, Use the fairchilds permanently, the only theoretical limitation could be at the absolute high power end of the usage, an sx-1250 is rated :
160w into 8 ohms which is 35.77v across the 8 ohm speakers, giving 4.47 amps of current
200w into 4 ohms which is 28.28v across the 4 ohm speakers, giving 7.07 amps of current
assuming a worst case gain of ten at 7 amps for the outputs, they will only need less than an amp to make rated power, and that is at 4 ohms, where the danger to the outputs comes in the form of power dissapation increased by sub 4 ohm loads... just the place we would LIKE to limit things a bit.
worrying about 1.5 amps versus 2 amps is not indicated (silly actually).
2. RE: R25, seeing different polarities at the ends of a resistor is NOT an indication of a problem, the zero volt point would be somewhere IN the resistance element, all that matters is that the current is flowing through the resistor. If the current wasn't flowing, the opposite polarity voltages would be a LOT higher. This is a book learning/calculating part, not a winging it part. It was the result of the Q9 / Q10 mismatch/unbalance (dead/damaged/leaky is STILL mismatched)... looking closely, the effect can be found to a far lesser degree all over the amp, at all the pnp/npn push pull stages.... the end result at the output being zero is what matters as long as the internal conditions are not extreme enough to upset OTHER functions (like it did with the idle current).
Last edited:
Stevesmullen
Well-Known Member
Hey Mark.
Thanks for the info and the explanation,Should I go back and check the voltage between the Input and Driver Transistors again or does the fact that the MV setting is working well mean that the circuits are balanced and we are done?
Thanks Again for your help.
Steve
Thanks for the info and the explanation,Should I go back and check the voltage between the Input and Driver Transistors again or does the fact that the MV setting is working well mean that the circuits are balanced and we are done?
Thanks Again for your help.
Steve
Stevesmullen
Well-Known Member
Hey CC.but you're not out of the woods yet.
I have noticed in post#40 Mark posted 3 different posting and stated that the thread was never completed and the subsitute was never tested. I'd be interested to know if those subsitute were successful.:yes:
Are you talking about the tests in post#36,Once we found the problem at Q9 and Q10 I thought that we had moved past any usable info from those tests.
Steve
No. I was talking about the posting below that's listed in post #40.
I was talking about this regarding the other thread. Mark has listed the parts so I was wondering if that subsitute was successful. I though you were going to replace it until now that Mark suggested to stick with your Fairchild transistors you already installed. I'd be interested to know about that as well. How well it'll hold up under extreme volume and how well it sound under low and exteme. I do play my receiver at extreme sometimes but mostly nominal to moderate. All my transistors are original in the SX-1250 amp, I just like to be ready in case it decides to squat.
Stevesmullen
Well-Known Member
(CC)Mark answered that question in post#52 I believe.
(MARK)I wonder if there is a 2 amp version instead of the 1.5 amp version of Q9,10 that I installed.As far as my 1250 if mark is comfortable with the 1.5 amp Fairchilds then so am I.
But if you were placing a order then I would want to know if the 2 amp Transistors were available.
Steve
He did answer it theoretically:
"Upon Calculation, Use the fairchilds permanently, the only theoretical limitation could be at the absolute high power end of the usage"
If I ran across that, I'll let you know. If your current installed transistor hold up well as Mark suggested, then you may not need to worry about replacing them for it may not be necessary.
Stevesmullen
Well-Known Member
Sounds good. I'll be around. Have fun on your vacation. Mine's about to end. :cry:
Not really, that combination of voltage and current isn't reflected in a Fairchild that I could find in a short time. I may poke around in the On-Semi lists a bit, but you have to be careful not to get a transistor optimized for switching, as opposed to general linear performance.
The On-Semi was a bit of overkill, which I would experiment a bit with, if I had it in front of me, where I could check distortion, and response characteristics of the amp in operation, while watching out for instability or outright oscillation, a possibility especially when slower transistors are replaced with faster ones.