Oh Dear...Ohh Boy....SX-1980..ME!!!
- Thread starter omri617
- Start date
If you follow the schematic, (wire labeled as "Y") AWR154-8 (-30V) is loaded only by the meter driver as part of AWM-122. The shematic says it is 3mA.
If you measure the voltage drop across the off board R3(470/3W) AWR154 pins 30 & 32, that way you can determine the current being drawn. Vdrop/470 = ?
If you measure the voltage drop across the off board R3(470/3W) AWR154 pins 30 & 32, that way you can determine the current being drawn. Vdrop/470 = ?
my calculations:
r210 5.6ohms 0.46v 0.082A 82 mA this is both the +80v supply draw and the +34v supply draw
r212 5.6ohms 0.50v 0.089A 89 mA this is both the -80v supply draw and the -34v supply draw
r2 (31-29) (+34) 360 ohms 27.8v 0.077A 77mA
r3 (32-30) (-34) 470 ohms 41v 0.087A 87mA
currents are reasonable except the -34 being drawn down, probably by one or more leaky capacitors.
temperature of big heat sink will be edited in here.
r210 5.6ohms 0.46v 0.082A 82 mA this is both the +80v supply draw and the +34v supply draw
r212 5.6ohms 0.50v 0.089A 89 mA this is both the -80v supply draw and the -34v supply draw
r2 (31-29) (+34) 360 ohms 27.8v 0.077A 77mA
r3 (32-30) (-34) 470 ohms 41v 0.087A 87mA
currents are reasonable except the -34 being drawn down, probably by one or more leaky capacitors.
temperature of big heat sink will be edited in here.
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Such a deceptively simple question, for at least an hour if not several hours of work.
A short circuit would be easier to find.
+80v 30mA
+34v 64mA
-30v 3mA
-34v 64mA
-80v 30mA
Get the 7 meg schematic and use these as a starting point, tracing everywhere the power supplys go, looking at the current arrows drawn into the schematic.
Then total up the various boards, and MEASURE the voltages across all the series resistors in their power lines
(like r41 and r42 on the flat amp awg-058)
(like r41 and r43 on the tone control amp awg-059)
(like r51 and r52 on the equalizer amp awf-030)
calculate the current by measuring the voltage across that resistor, and dividing the voltage by the resistance (0.5v across 220 ohms = 2.27mA)
It may be a cumulative case, where the current draw is "the death of a thousand cuts".
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Earlier you said, pin 11 was
"The -34 being -29.35 = isn't this related to what rick said above the loads not being connected ?"
So that is the source of the issue, so what is the V across D213? I suspect it to be (-29.35 - 1.2 = -30.55V) which actually high since -30V is higher than -34V!!
You can,
1) select/measure a different zener diode
2) Put another lower voltage zener in series to get closer to -35V
What is pin 9 Voltage? the two should be close to each other but of opposite polarity
"The -34 being -29.35 = isn't this related to what rick said above the loads not being connected ?"
So that is the source of the issue, so what is the V across D213? I suspect it to be (-29.35 - 1.2 = -30.55V) which actually high since -30V is higher than -34V!!
You can,
1) select/measure a different zener diode
2) Put another lower voltage zener in series to get closer to -35V
What is pin 9 Voltage? the two should be close to each other but of opposite polarity
Earlier you said, pin 11 was
"The -34 being -29.35 = isn't this related to what rick said above the loads not being connected ?"
So that is the source of the issue, so what is the V across D213? I suspect it to be (-29.35 - 1.2 = -30.55V) which actually high since -30V is higher than -34V!!
You can,
1) select/measure a different zener diode
2) Put another lower voltage zener in series to get closer to -35V
What is pin 9 Voltage? the two should be close to each other but of opposite polarity
Sorry duplicate posts, I was getting server time outs!!
"The -34 being -29.35 = isn't this related to what rick said above the loads not being connected ?"
So that is the source of the issue, so what is the V across D213? I suspect it to be (-29.35 - 1.2 = -30.55V) which actually high since -30V is higher than -34V!!
You can,
1) select/measure a different zener diode
2) Put another lower voltage zener in series to get closer to -35V
What is pin 9 Voltage? the two should be close to each other but of opposite polarity
Sorry duplicate posts, I was getting server time outs!!
gawd I HATE this damn server problem - huge post down the drain... :gigglemad:
check power supply voltyages close to above readings
measure and post new voltage readings for pins 29,31 for r2 (360 ohms, +34v), and for pins 30, 32 for r3 (470 ohms, -34v) voltages for new calculations.
read and post three (e,c,b) voltages each of q215, 217, and q214, q216
also d212, d213
if you want to chase this deeper.
but a KEY part will be to profile all the loads drawing on the -34v supply.
I haven't been able to steal time to do it, to draw out an explicit measurement map.
That's why i wanted to start you on figuring it out, and answer questions as they came up. It's not rocket science.
Rick, the only determining factors between the -80 and -34v is the 80v feedback voltage divider (r220 59k, r222 51k) AND the current drawn through R3 with it's associated voltage drop...
omri - are those original r220 59k 1/4w and r222 51k 1/4w?
check power supply voltyages close to above readings
measure and post new voltage readings for pins 29,31 for r2 (360 ohms, +34v), and for pins 30, 32 for r3 (470 ohms, -34v) voltages for new calculations.
read and post three (e,c,b) voltages each of q215, 217, and q214, q216
also d212, d213
if you want to chase this deeper.
but a KEY part will be to profile all the loads drawing on the -34v supply.
I haven't been able to steal time to do it, to draw out an explicit measurement map.
That's why i wanted to start you on figuring it out, and answer questions as they came up. It's not rocket science.
Rick, the only determining factors between the -80 and -34v is the 80v feedback voltage divider (r220 59k, r222 51k) AND the current drawn through R3 with it's associated voltage drop...
omri - are those original r220 59k 1/4w and r222 51k 1/4w?
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omri617
Super Member
I'd like to post again because Mark asked and also to make it clearer for me :
Pin 18 = -78
Pin 17 = +78
Pin 9 = +34
Pin 11 = -34
Pin 8 = -29.30
Voltage across D213 = 34.7
Voltage across D212 = 35.02
Pins 29 & 31 = -27.94
Pins 32 & 30 = 40.9
q215 -
e = -34.8
c = -38.5
b = - 35.20
q214 -
e = 34.9
c = 50.27
b = 35.44
q216
e = 34.4
c = 50.02
b = 35
q217
e = -34.35
c = -36.4
b = -34.9
Yes , they look original and also their "equals" on the other side R221/R223 are the same type..
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Everything looks fine!
So the current calculated, as drawn from pin 8 is (34-29.30)/1500 = 3mA which is what is written on the schematic. If pin 8 is supposed to be -30V and pin 11 is supposed to be -34V then 4/1500 = 2.6mA, so off by 0.4ma, not an issue to me.
In my mind, good to move on and test further stuff like signal paths.
So the current calculated, as drawn from pin 8 is (34-29.30)/1500 = 3mA which is what is written on the schematic. If pin 8 is supposed to be -30V and pin 11 is supposed to be -34V then 4/1500 = 2.6mA, so off by 0.4ma, not an issue to me.
In my mind, good to move on and test further stuff like signal paths.
Sure, when I get the chassis running on the bench again.
I am STRIVING MIGHTILY to do JUST THAT!!! Again it is way past dawn here and I should have been asleep for hours already.
The board was not made with those thermistors, you may never find additional correct thermistors. So rebuild the board exactly as the schematic shows.
The OTHER board, if you have irretrievably lost the knowledge of which resistor went where, then replace the thermistors with wire jumpers, and copy the values from the board that doesn't have any thermistors...
Yes Omri, you should be able to test out what is working and leave the PA sections till later when you have the PA boards assembled.
So test, inputs into phono, aux to pre-amp outputs, check volume/balance tone etc. Also could check tape monitor paths, then finally check the tuner section. That should keep you busy for a while. Good luck
From what I am to understand, the thermistors are part of the current overload detection ckt and their exact function is to lower the current limit when the amp gets really hot, obv. a after thought, as it was not part of the original design, thus not that important at this point in time. Like I said previously, reference the Spec2 PA as it does the similar thing.
So I agree with Mark's advise to assemble the PA's as documented.
So test, inputs into phono, aux to pre-amp outputs, check volume/balance tone etc. Also could check tape monitor paths, then finally check the tuner section. That should keep you busy for a while. Good luck
From what I am to understand, the thermistors are part of the current overload detection ckt and their exact function is to lower the current limit when the amp gets really hot, obv. a after thought, as it was not part of the original design, thus not that important at this point in time. Like I said previously, reference the Spec2 PA as it does the similar thing.
So I agree with Mark's advise to assemble the PA's as documented.
omri617
Super Member
O.K guys ! Sx-1980 is behaving good as pre-amp.
All inputs were tested and they all work
All I had to do is deoxit some switches and pot's , especially tone and tape sections.
Potentiometers should be screwed and tightened properly to the chassis otherwise they will sound very static and scratchy!
Tomorrow's CHORES :
1. Wiring the outputs .
2. Finishing the amp boards .
I hope everything will go just fine and no surprises so you can get some rest from me guys
LoL
All inputs were tested and they all work
All I had to do is deoxit some switches and pot's , especially tone and tape sections.
Potentiometers should be screwed and tightened properly to the chassis otherwise they will sound very static and scratchy!
Tomorrow's CHORES :
1. Wiring the outputs .
2. Finishing the amp boards .
I hope everything will go just fine and no surprises so you can get some rest from me guys
LoL
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Great news, Omri, you should be very happy.
No rest for the wicked, as they say, so go for it. You are the one doing all the work, I am just an arm chair quarterback
Interesting and I was under the impression that the pot cases are isolated and not required to be grounded. Are yours the old Alps black beauty? I have one, it is a acw-116-0 (100K) the volume. I did not like it, the detents are to far apart, esp on the low end. I understand it tracks better but at a big cost.
No rest for the wicked, as they say, so go for it. You are the one doing all the work, I am just an arm chair quarterback