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Strange muting circuit

The other day I was remembering a Roland synthesizer of mine I repaired a while back, and a strange circuit that I never did figure out the function of. Today on a whim I wired up the circuit in LTSpice, and unfortunately I'm no closer to understanding how it functions. For those who don't want to fuss with the netlist, I have a screen cap of the circuit in question here:

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Two of these circuits are strapped across the main audio outputs of the synthesizer, directly before the master volume control. In the unit I repaired, Q2 on one of the channels had failed shorted, muting one of the outputs. The transistor labels in blue are the types of transistors actually listed in the service manual.

I think it's some kind of muting circuit, that keeps the output off until the rails stabilize and the instrument's microcontroller (an

80C31) finishes its power up sequence, but I can't get it to do anything in LTSpice. I checked and double-checked the polarities of the supplies, diodes, and transistors - they're all hooked up exactly the same as is shown in the service manual.

Version 4 SHEET 1 880 680 WIRE -96 -224 -96 -240 WIRE 0 -224 0 -240 WIRE 320 -224 240 -224 WIRE 448 -224 400 -224 WIRE 0 -128 0 -144 WIRE 240 -112 240 -144 WIRE 448 -80 448 -224 WIRE 384 -32 288 -32 WIRE 80 80 80 48 WIRE 448 80 448 16 WIRE -96 112 -96 48 WIRE 288 112 288 -32 WIRE 80 160 80 144 WIRE 128 160 80 160 WIRE 208 160 192 160 WIRE 80 192 80 160 WIRE 208 208 208 160 WIRE -96 240 -96 192 WIRE 16 240 -96 240 WIRE 448 240 448 208 WIRE 80 320 80 288 WIRE 320 320 80 320 WIRE 448 320 448 304 WIRE 448 320 384 320 WIRE 288 432 288 192 WIRE 288 432 240 432 WIRE 320 432 288 432 WIRE 448 432 448 320 WIRE 448 432 384 432 WIRE 240 528 240 512 WIRE 448 528 448 512 FLAG -96 48 Vcc FLAG 80 48 Vcc FLAG 208 208 0 FLAG 240 528 Vee FLAG 448 528 Vee FLAG 448 208 Vcc FLAG 448 80 0 FLAG -96 -144 0 FLAG 0 -128 0 FLAG -96 -240 Vcc FLAG 0 -240 Vee FLAG 240 -112 0 FLAG 448 -224 AudioOut IOPIN 448 -224 Out SYMBOL res -112 96 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL diode 64 80 R0 WINDOW 3 -101 30 Left 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL pnp 16 288 M180 SYMATTR InstName Q1 SYMATTR Value 2N2907 SYMBOL polcap 128 176 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 -9 28 VBottom 0 SYMATTR InstName C1 SYMATTR Value 4.7µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=34m Rser=4.7 Lser=0 mfg="Nichicon" pn="UPR1J4R7MAH" type="Al electrolytic" SYMBOL diode 384 304 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 320 448 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL res 224 416 R0 SYMATTR InstName R2 SYMATTR Value 4.7k SYMBOL polcap 432 240 R0 WINDOW 3 24 64 Left 0 SYMATTR InstName C2 SYMATTR Value 10µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=16 Irms=35m Rser=2.8 Lser=0 mfg="Nichicon" pn="UPR1C100MAH" type="Al electrolytic" SYMBOL res 432 416 R0 SYMATTR InstName R3 SYMATTR Value 470k SYMBOL npn 384 -80 R0 SYMATTR InstName Q2 SYMATTR Value 2N2222 SYMBOL res 272 96 R0 SYMATTR InstName R4 SYMATTR Value 47k SYMBOL voltage -96 -240 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 44 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMATTR SpiceLine Rser=5 SYMBOL voltage 0 -240 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 44 Left 0 SYMATTR InstName V2 SYMATTR Value -15 SYMATTR SpiceLine Rser=5 SYMBOL res 304 -208 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL voltage 240 -240 R0 WINDOW 3 22 78 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SINE(0 1 1000) TEXT -152 -80 Left 0 !.tran 20 startup TEXT 136 256 Left 0 ;2SA933 TEXT 480 -48 Left 0 ;2SC2878

Reply to
Bitrex
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There have to be errors in the schematic. There is no way for Q1 to be turned on since the base is pulled up as high (or higher) than the emitter. If current did flow in its collector, it would be blocked by D2. Since there is no positive source in the base circuit of Q2, it can't be turned on either (Vbe is always negative). The circuit makes no sense as drawn.

Reply to
krw

rs

turned

f

ere is

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manuals do get things wrong.

NT

Reply to
NT

Sometimes intentionally.

Reply to
krw

I suspect it's to prevent a pop during power supply sequencing. Replace V1 and V2 with Pulse or PWL sources and watch what happens. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Nothing.

Reply to
John S

If you reverse both D2 and D3, it sort-of works. You sure you got the polarities correct?

Reply to
John S

Even then, the only way I see it doing anything is, *perhaps*, on the negative transition of Vcc. C1 holding the emitter up while Vcc decays is the only way to turn on Q1. But even then, the other side of C2 is decaying, as well. IOW, it still doesn't look like it does anything.

Reply to
krw

Actually, you are sort-of correct. Removing D2 has no effect on the operation of the circuit. The "squelching" that I see by reversing D3 is caused by that circuit surrounding D3 and has nothing to do with Q1 and its associated circuitry. I'd say that something is still wrong with the schematic. There are ways to fix it, but that isn't what we are looking for.

Good catch.

Reply to
John S

BTW, I forgot to point out why I said you are sort-of correct. There is some squelching by virtue of C2 yanking its negative end toward Vcc which pulses the base of Q2.

So, the circuit with D3 reversed does *something*, although I doubt I'm on the right track.

Reply to
John S

But (D2 reversed, as assumed) Q1 is supplying current to the negative end of C2, also. It's a mess.

It would be good to know what it's *supposed* to do. ;-)

Reply to
krw

D2 does nothing (in either direction). As Vcc rises, the emitter of Q1 rises slightly slower (due to C1) which keeps Q1 off. Q1 can never turn on.

As I said, Q1 and its surrounding circuit (as shown) does not enter the picture.

It's a mess.

Well, I think Q2 is supposed to turn on to squelch the audio line during the power-on period as Jim Thompson said. However,

it's a mess.

Reply to
John S

Sure, but there is also the turn-off side (I mentioned Vcc decaying). Q1 will turn on because the emitter is being held up by C1, while there is nothing holding the base.

On power on. Q1-Vbe will go forward on power-off.

;-) Who knows, perhaps it's supposed to suppress the pop on power-down? Analyzing a circuit, that doesn't work, without knowing what it's supposed to do, is hard. ;-)

Reply to
krw

Q1 will turn on if Vcc does a quick drop or ripple as it's coming up. C1 Will hold the charge in this state while the base will then be below enough to turn on Q1.

But, I do see a problem with D2 and D3, they look reversed and if switched around, the circuit looks to work perfectly.

If you program the supplies using a signal instead of voltage, you can then simulate rough starting supply which should force the circuit to do its intended job.

Just an observation of mine. Pay no attention to the Maine_niac.

Jamie

Reply to
Jamie

It would have to be a *lot* of ripple, but yes, I've pointed that out (power falling).

That's quite a stretch.

Reply to
krw

There have to be errors in the schematic. There is no way for Q1 to be turned

You can see what's intended. Q2 is meant to be on when either C2 is charging at start or C1 is discharging at shut-down. Overshoot or excess ripple ~ same thing.

There are obvious issues with Q1's ability to do anything while fighting charge on C2. I expect that more than just diode polarity problems in this schematic drawing.

Could be a complete c*ck-up after switching from an original schematic with PNP transistors, based on typical history of this kind of stuff. If it didn't ever work as intended, nothing would have neccessarily been changed, so long as it didn't blow up.

RL

Reply to
legg

On 10/11/2011 3:50 PM, snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

I didn't consider power-off. The only reason for that part of the circuit is, as you say, for power-down.

Yes.

Okay. I changed R4 and changed the wiring (it will be obvious in LTSpice). I also adjusted the sources for what I guessed would be reasonable power-up/-down times.

I tried to minimize changes to the circuit while getting it to work as we suspect it should. It now works a bit for both power up and power down. However, I can no longer find a good reason for R2 and D3.

It's the best I can do without spending more time on it so, I'm done. The LTSpice description follows:

Version 4 SHEET 1 880 680 WIRE -496 -240 -496 -256 WIRE -128 -240 -128 -256 WIRE 320 -224 240 -224 WIRE 448 -224 400 -224 WIRE -128 -144 -128 -160 WIRE 240 -112 240 -144 WIRE 448 -80 448 -224 WIRE 384 -32 288 -32 WIRE 80 48 -96 48 WIRE 80 80 80 48 WIRE 448 80 448 16 WIRE -96 112 -96 48 WIRE 288 112 288 -32 WIRE 80 160 80 144 WIRE 128 160 80 160 WIRE 208 160 192 160 WIRE 80 192 80 160 WIRE 208 208 208 160 WIRE -96 240 -96 192 WIRE 16 240 -96 240 WIRE 448 240 448 208 WIRE 80 320 80 288 WIRE 288 320 288 192 WIRE 288 320 80 320 WIRE 320 320 288 320 WIRE 448 320 448 304 WIRE 448 320 384 320 WIRE 320 432 240 432 WIRE 448 432 448 320 WIRE 448 432 384 432 WIRE 240 528 240 512 WIRE 448 528 448 512 FLAG -96 48 Vcc FLAG 208 208 0 FLAG 240 528 Vee FLAG 448 528 Vee FLAG 448 208 Vcc FLAG 448 80 0 FLAG -496 -160 0 FLAG -128 -144 0 FLAG -496 -256 Vcc FLAG -128 -256 Vee FLAG 240 -112 0 FLAG 448 -224 AudioOut IOPIN 448 -224 Out SYMBOL res -112 96 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL diode 64 80 R0 WINDOW 3 -101 30 Left 2 SYMATTR Value 1N4148 SYMATTR InstName D1 SYMBOL pnp 16 288 M180 SYMATTR InstName Q1 SYMATTR Value 2N2907 SYMBOL polcap 128 176 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 -9 28 VBottom 2 SYMATTR InstName C1 SYMATTR Value 4.7u SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=63 Irms=34m Rser=4.7 Lser=0 mfg="Nichicon" SYMBOL diode 384 304 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 320 448 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL res 224 416 R0 SYMATTR InstName R2 SYMATTR Value 4.7k SYMBOL polcap 432 240 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 10u SYMATTR InstName C2 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=16 Irms=35m Rser=2.8 Lser=0 mfg="Nichicon" SYMBOL res 432 416 R0 SYMATTR InstName R3 SYMATTR Value 470k SYMBOL npn 384 -80 R0 SYMATTR InstName Q2 SYMATTR Value 2N2222 SYMBOL res 304 208 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 4.7k SYMBOL voltage -496 -256 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 44 Left 2 SYMATTR SpiceLine Rser=5 SYMATTR InstName V1 SYMATTR Value PULSE(0 15 0 30m .1 1) SYMBOL voltage -128 -256 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 44 Left 2 SYMATTR SpiceLine Rser=5 SYMATTR InstName V2 SYMATTR Value PULSE(0 -15 0 30m .1 1) SYMBOL res 304 -208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL voltage 240 -240 R0 WINDOW 3 22 78 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value SINE(0 .7 1000) SYMATTR InstName V3 TEXT -152 -80 Left 2 !.tran 2 startup TEXT 136 256 Left 2 ;2SA933 TEXT 480 -48 Left 2 ;2SC2878

Reply to
John S

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