Author Topic: Question about the function of a capacitor in a circuit  (Read 5363 times)

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Offline fedaalis84Topic starter

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Question about the function of a capacitor in a circuit
« on: March 01, 2019, 11:02:55 pm »
I'm a newb here & I know some basic things about capacitors. But wondering specifically what this guy here is doing in this part of the circuit?

This is a schematic for the main board on the Sony STR-DH130.



Pretty sure this part of the circuit is part of the Protect system, to detect a voltage overload from the output transistors on the right audio channel.

Long story short, those two output transistors on the right channel shorted out (bringing the emitter resistor and an adjacent 100ohm resistor down with it). Anyway, when desoldering the emitter resistor, my dumb self also desoldered C761, a teeny... tiny SMD 220PF capacitor.

I also jacked up the trace from that capacitor to Q761 (the current detector).

After desoldering the output transistors, and even my SMD capacitor/trace mishap, the system will still start - and was not going into protect mode.

Fast forward to today, I installed a new emitter resistor, and also placed a jumper between R761 and Q761 EDIT: From the bottom of R761 to the base of Q761 (eventually once my SMD 220PF capacitors come in the mail, I'll get one of those bad boys in, too.)...

But afterwards (and checking my work with the multi-meter for any kind of weird solder bridges anywhere, everything checked out OK)

But now, the system will go into protect again after powering up.

So I'm wondering, is it because I still need to get this capacitor back in?

Thanks for the help!
« Last Edit: March 01, 2019, 11:34:22 pm by fedaalis84 »
 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #1 on: March 02, 2019, 01:49:06 am »
It's unlikely that the missing capacitor is the cause. To be certain, solder in something else, no matter if it's larger than 220pF.
Is there DC on either channel's output?
What is the voltage on the collectors of Q762 and Q712?. Normally they should be almost the same as the negative rail.
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #2 on: March 02, 2019, 02:26:56 am »
It's unlikely that the missing capacitor is the cause. To be certain, solder in something else, no matter if it's larger than 220pF.
Is there DC on either channel's output?
What is the voltage on the collectors of Q762 and Q712?. Normally they should be almost the same as the negative rail.

Thanks xavier60,

I'll try adding some random cap.

There should be no DC on the right channel's output, I have the output transistors out of circuit at the moment. I'll check the collectors on Q762 and Q712, but the left channel was not having issues before - so I think the protect has to be from my tinkering today.

Before, when it was going into protect when I had the right channel's transistors in, it would hit protect much faster. Now it takes some time, and almost seems like it won't. Then bam, Protect.

I might try desoldering the jumper if a random cap doesn't resolve the issue.

Thanks for the input!
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #3 on: March 02, 2019, 05:04:48 am »
Update:

Tried an unknown value SMD ceramic cap. Still goes into protect.

Tried unsoldering the jumper, still goes into protect.

BUT something that I hadn't considered:

I replaced the bad emitter resistor (which was previously behaving like an open circuit). So that could be causing protect to surface via some means that I have not identified.

Didn't have time tonight, but next I will try removing that & see what happens.

Couldn't be something else in the protect circuit that, because I have the right channel transistors out, is now being wonky because there's a connection via the emitter resistor that wasn't there previously?
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #4 on: March 02, 2019, 03:49:46 pm »
I don't have a DC power supply, so I can really only measure voltages on top of the board, because I have to use the units power supply.

Anyway, on J469, which connects to the emitters of Q762, and Q712 (with a 68k resistor between either emitter and the junction)  I'm reading -53V DC, and drops down to almost nothing once it goes into protect.

Because the right side is open (transistors are out), this voltage must be coming from the left channel, right?

 :o

Stumped at why it's going into protect now, but didn't before I messed up C761 and the trace to the base of Q762.. if the excess voltage is coming from the left channel, which seemed fine before.
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #5 on: March 02, 2019, 04:27:10 pm »
I am reading 60 some-odd volts after the bridge rectifier. It takes in about 45 volt AC. Now, that should be normal. But I'm pretty sure the filter capacitors should smooth it down to 42 some odd volts like the main rails.. right? But I am still getting +-62 some odd volts DC after the filter caps.

Does that point at those being bad?

Am I drawing completely incorrect conclusions here due to inexperience?

I do see -62 and 62 volts on pins 15 and 14 of IC700 which according to the schematic should be reading something like -43.6 and 43.6 respectively..
 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #6 on: March 02, 2019, 04:29:34 pm »
I don't have a DC power supply, so I can really only measure voltages on top of the board, because I have to use the units power supply.

Anyway, on J469, which connects to the emitters of Q762, and Q712 (with a 68k resistor between either emitter and the junction)  I'm reading -53V DC, and drops down to almost nothing once it goes into protect.

Because the right side is open (transistors are out), this voltage must be coming from the left channel, right?

 :o

Stumped at why it's going into protect now, but didn't before I messed up C761 and the trace to the base of Q762.. if the excess voltage is coming from the left channel, which seemed fine before.
That is a bit difficult to follow. If  the voltage on the collectors of Q762 and Q712 are the same at about -53V, it means that the overload detect circuitry isn't causing the shutdown. Voltage readings are valid only while the power relay is on.
An overload protect detection will cause a voltage across R326, about 0.6V.
A DC protect detection will cause a voltage across R323, about 0.6V.
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Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #7 on: March 02, 2019, 04:34:27 pm »
Some amplifiers have a 4Ω/8Ω switch on the back that changes the main rail voltages.
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Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #8 on: March 02, 2019, 04:40:56 pm »
Rectified and filtered 45 VAC will be close to 62VDC
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #9 on: March 02, 2019, 04:44:21 pm »
Rectified and filtered 45 VAC will be close to 62VDC

Thanks, OK - so that's not abnormal?

I'll look for a switch. I suppose it could be set to something odd that would cause the main rail voltages to be about ~20V higher than they should be?

I think I'm in a bit over my head knowledge wise. Will have to keep learning.
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #10 on: March 02, 2019, 05:56:12 pm »
So I think I'm going to wait until I get the right-channel transistors in before worrying too much.

The leads on the emitter resistor on the left channel read at about -20V, whereas on the right they read -4V (because the transistors are out, and not pumping any voltage).

The busted emitter resistor from before was reading a few mega ohms between the left and center pins. Now it's a functional resistor there. So those -4 volts are getting over to Q762. Could be what's leading to the protect, and explain why a random cap, and desoldering the jumper did nothing. But that protect did not start until I put in the new emitter resistor...
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #11 on: March 03, 2019, 02:40:06 pm »
Definitely the protector circuit causing the shutdown. JW463 runs to the protect pin on IC702 and should be reading 3.3V, but instead is giving -150mV. Slowly rises back to 3.3V once in protect due to a big cap in the area
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #12 on: March 03, 2019, 03:13:05 pm »
An overload protect detection will cause a voltage across R326, about 0.6V.
A DC protect detection will cause a voltage across R323, about 0.6V.

So, xavier60, if you're talking about only a 0.6V drop across R323/R326, and I'm looking at -150mV on JW463/Pin 87 on IC702 (Protect pin), should I be suspecting a short in one of Q323/Q324/Q325?
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #13 on: March 03, 2019, 04:00:09 pm »
So I'm still seeing something strange with the main power rails.

On R545, the resistor for the positive power rail, I'm reading about -20VDC. On R546, the negative power rail, I'm reading -62VDC.

R545 should be giving me positive 62VDC, correct?

Also, something weird... The positive leg of the bridge rectifier diode reads a steady 62VDC even after the AC is turned off by the relay in protect mode. It doesn't drop, it just sticks at 62VDC... Weird, no?
 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #14 on: March 03, 2019, 05:05:42 pm »
So I'm still seeing something strange with the main power rails.

On R545, the resistor for the positive power rail, I'm reading about -20VDC. On R546, the negative power rail, I'm reading -62VDC.

R545 should be giving me positive 62VDC, correct?

Also, something weird... The positive leg of the bridge rectifier diode reads a steady 62VDC even after the AC is turned off by the relay in protect mode. It doesn't drop, it just sticks at 62VDC... Weird, no?
R545 must be open. The positive rail capacitor is staying charged because no current is being drawn from the rail. R545 and R546 function as fuses.
Don't bridge out R545. replace it with the same value or something close and hope that there is no permanent overload present.
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #15 on: March 03, 2019, 05:10:32 pm »
So I'm still seeing something strange with the main power rails.

On R545, the resistor for the positive power rail, I'm reading about -20VDC. On R546, the negative power rail, I'm reading -62VDC.

R545 should be giving me positive 62VDC, correct?

Also, something weird... The positive leg of the bridge rectifier diode reads a steady 62VDC even after the AC is turned off by the relay in protect mode. It doesn't drop, it just sticks at 62VDC... Weird, no?
R545 must be open. The positive rail capacitor is staying charged because no current is being drawn from the rail. R545 and R546 function as fuses.
Don't bridge out R545. replace it with the same value or something close and hope that there is no permanent overload present.

Thanks!!

R545 is reading at 0.00 ohms when I check resistance, is tha4 because it's still in circuit?

Also, R546 is registering at 0.7 ohms, when its rated at 0.47, that seems pretty far out of spec?

I'll try maybe replacing them both.

 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #16 on: March 03, 2019, 05:18:33 pm »
The charged up rail capacitor is likely to be causing a false reading, It will need to be discharged before you start handling the board to avoid further damage. Don't directly short out large capacitors. use a low value power resistor or anything with suitable resistance.
« Last Edit: March 03, 2019, 08:02:06 pm by xavier60 »
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #17 on: March 03, 2019, 11:35:27 pm »
The charged up rail capacitor is likely to be causing a false reading, It will need to be discharged before you start handling the board to avoid further damage. Don't directly short out large capacitors. use a low value power resistor or anything with suitable resistance.

Definitely the case. Reading high voltage on the capacitor side of the resistor & very little on the other.

Replacements on their way.

I think it might have been my fault. When I was measuring voltages on IC700 I think I may have briefly shorted pins 14 & 15 together...  :-[  probably why R545 is toast.

So I don't think that's what's causing the protect.

 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #18 on: March 04, 2019, 09:22:38 pm »
Found the source of the protect.

Whenever I have the system set to output *just* to speakers A, or just speakers B, it will short out & go into protect - and I get DC on both output channels. If the system is set to output to both speakers A and B, it will not go into protect - output on left & right look good with +- ~50ish mV; and the relays also are closing the circuit successfully to the speaker outputs. Also does not go into protect when not outputting to any speakers.

 :-DD

Something any electronics repair guy/gal worth his or her salt would've caught a LOT earlier... But - hey, this is my FIRST ever attempt at electronics repair . So if I can get this old hunk of junk up and running, I'm happy - and it was a success.  :-+

Got some new relays on order. New 0.47 ohm resistors for between the filter caps and IC700... And still waiting on heat paste for the right-channel transistors, and a 100 ohm resistor to replace one that went bad on the right channel, and she should be up and running...

Wish me luck!
« Last Edit: March 04, 2019, 09:28:01 pm by fedaalis84 »
 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #19 on: March 04, 2019, 09:44:27 pm »
I'm not sure about all of that. With one of the 0.47Ω open, that is a reason to have a DC protect detection.
Some thought needs to be given to the possible reason for the output transistors failing. Like being overloaded by driving too low impedance or shorted speaker lead.
The circuitry(2 transistors and resistors) that sits between IC700 and the output transistors  has the function of controlling the idle bias current through the output transistors. This circuitry needs to be carefully checked on both channels for cracked joints and damaged transistors.
A problem with this circuitry can destroy the output stage again.
« Last Edit: March 04, 2019, 09:57:26 pm by xavier60 »
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #20 on: March 04, 2019, 09:57:53 pm »
Good call. I'll give that circuitry a check before firing her up again once R545 & R546 are back.

I don't think it's the biasing transistors/resistors, because -VOUT2, +VOUT2, -VOUT1, and +VOUT1 on IC700 would read some ridiculous voltages when the relays were in a "shorted" state (i.e. just A, or just B). But I'm getting good voltage at the base (and emitter) on the left channel, and all of the VOUTs look good when the relays were in A&B.

I'm fairly certain the original cause of the output transistors shorting out was a speaker wire short. When I re-did the speaker wires on the right speaker, they were very frayed and little strings of wire could have easily been touching.

But I'm still weirded out by how the relays can be causing a short when just in A, or just in B though?? I popped them both out, and already ordered new ones, so I gutted one of them, and from the way the relay is setup, I'm not getting how it could cause a short...

Learning as I go here.
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #21 on: March 06, 2019, 03:16:26 pm »
I'm not sure about all of that. With one of the 0.47Ω open, that is a reason to have a DC protect detection.
Some thought needs to be given to the possible reason for the output transistors failing. Like being overloaded by driving too low impedance or shorted speaker lead.
The circuitry(2 transistors and resistors) that sits between IC700 and the output transistors  has the function of controlling the idle bias current through the output transistors. This circuitry needs to be carefully checked on both channels for cracked joints and damaged transistors.
A problem with this circuitry can destroy the output stage again.

I checked out the joints on Q701, Q702, Q751 and Q752 - and that looks OK.

Q752 and Q702 both check out as they should for NPN transistors.

Q701 and Q751 are giving an interesting reading. I get a diode in either direction between the base and emitter (on both 701 & 751), but only getting a diode between base and collector when the positive lead is on the base, and the negative lead is on the collector.

Data sheet for Q701/751:
https://toshiba.semicon-storage.com/info/docget.jsp?did=19351&prodName=2SA1162

They're both *identical* on each side, so I'm going to go ahead and wager that everything is OK - also, I'm getting a resistance between base and emitter on 701 & 751 that is identical to R708/R758 respectively.

So I think that all checks out good there

Edit - Lol, this would be why:

« Last Edit: March 06, 2019, 03:21:57 pm by fedaalis84 »
 

Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #22 on: March 06, 2019, 11:05:07 pm »
Well it wasn't the relays - turns out, when the system is in just speaker set A, or just speaker set B, it operates on the upper tap (~45VAC which is then rectified to ->>> ~60VDC). When it's in speaker sets A&B, or no speaker output, it operates on the lower tap (~33VAC which is then rectified to ->>> ~43.9VDC).

When either (or both) of R545, or R546 are burned out, it causes a wacky voltage output from the VOUTs on IC700, which - when on the lower tap, does not cause enough voltage issues for the protect to trigger, but when on the upper tap - it does pass enough voltage through the protect circuitry to trigger a protect.

Whew.

OK.

I went against your advice and jumpered R545 and R546 just for the heck of it... And I don't get protect regardless of speaker output chosen (WHEW).

HOWEVER; I get good voltage on the VOUTs on the left channel on IC700 - but I'm getting the positive & negative rail on the VOUTs on the right channel. I'm going to GUESS that this is because I don't have transistors in circuit on the right channel (nor do I have R759 in) and I'm getting the positive & negative rail because the biasing transistors on the right channel are not functioning due to an open circuit???

Any thoughts on that one?

I don't want to toast a set of transistors to test (I already toasted an "extra" set today playing around with various things).

Anything I could do to perform a test WITHOUT transistors in circuit on the right channel????

All of the other circuitry there looks good.

THANKS!
 

Offline xavier60

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Re: Question about the function of a capacitor in a circuit
« Reply #23 on: March 06, 2019, 11:57:40 pm »
Well it wasn't the relays - turns out, when the system is in just speaker set A, or just speaker set B, it operates on the upper tap (~45VAC which is then rectified to ->>> ~60VDC). When it's in speaker sets A&B, or no speaker output, it operates on the lower tap (~33VAC which is then rectified to ->>> ~43.9VDC).

When either (or both) of R545, or R546 are burned out, it causes a wacky voltage output from the VOUTs on IC700, which - when on the lower tap, does not cause enough voltage issues for the protect to trigger, but when on the upper tap - it does pass enough voltage through the protect circuitry to trigger a protect.

Whew.

OK.

I went against your advice and jumpered R545 and R546 just for the heck of it... And I don't get protect regardless of speaker output chosen (WHEW).

HOWEVER; I get good voltage on the VOUTs on the left channel on IC700 - but I'm getting the positive & negative rail on the VOUTs on the right channel. I'm going to GUESS that this is because I don't have transistors in circuit on the right channel (nor do I have R759 in) and I'm getting the positive & negative rail because the biasing transistors on the right channel are not functioning due to an open circuit???

Any thoughts on that one?

I don't want to toast a set of transistors to test (I already toasted an "extra" set today playing around with various things).

Anything I could do to perform a test WITHOUT transistors in circuit on the right channel????

All of the other circuitry there looks good.

THANKS!
That makes sense with the speaker A/B switching changing the rail voltages. It automatically functions as the 4Ω/8Ω switch I mentioned earlier.
The VOUT pins are actually the Collectors of the NPN and PNP  transistors which are the output  stage of a rail to rail transcondctance amplifier that can be seen in most discrete audio power amplifiers designs.
They source/sink a few mA and will pull to full +/- rail voltages if left open circuited.
The function of the idle current bias circuitry is to clamp the difference between the VOUTs to a precise temperature compensated voltage that slightly turns on the output transistors, idle current.
There must be a problem in this area.
If both VOUTs happened to be both at about the same voltage either full + rail or Full - rail, this would be normal because of the missing output components causing the feedback loop to be open.
Think of it as a very large op-amp.
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Offline fedaalis84Topic starter

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Re: Question about the function of a capacitor in a circuit
« Reply #24 on: March 07, 2019, 03:04:14 am »
Well it wasn't the relays - turns out, when the system is in just speaker set A, or just speaker set B, it operates on the upper tap (~45VAC which is then rectified to ->>> ~60VDC). When it's in speaker sets A&B, or no speaker output, it operates on the lower tap (~33VAC which is then rectified to ->>> ~43.9VDC).

When either (or both) of R545, or R546 are burned out, it causes a wacky voltage output from the VOUTs on IC700, which - when on the lower tap, does not cause enough voltage issues for the protect to trigger, but when on the upper tap - it does pass enough voltage through the protect circuitry to trigger a protect.

Whew.

OK.

I went against your advice and jumpered R545 and R546 just for the heck of it... And I don't get protect regardless of speaker output chosen (WHEW).

HOWEVER; I get good voltage on the VOUTs on the left channel on IC700 - but I'm getting the positive & negative rail on the VOUTs on the right channel. I'm going to GUESS that this is because I don't have transistors in circuit on the right channel (nor do I have R759 in) and I'm getting the positive & negative rail because the biasing transistors on the right channel are not functioning due to an open circuit???

Any thoughts on that one?

I don't want to toast a set of transistors to test (I already toasted an "extra" set today playing around with various things).

Anything I could do to perform a test WITHOUT transistors in circuit on the right channel????

All of the other circuitry there looks good.

THANKS!
That makes sense with the speaker A/B switching changing the rail voltages. It automatically functions as the 4Ω/8Ω switch I mentioned earlier.
The VOUT pins are actually the Collectors of the NPN and PNP  transistors which are the output  stage of a rail to rail transcondctance amplifier that can be seen in most discrete audio power amplifiers designs.
They source/sink a few mA and will pull to full +/- rail voltages if left open circuited.
The function of the idle current bias circuitry is to clamp the difference between the VOUTs to a precise temperature compensated voltage that slightly turns on the output transistors, idle current.
There must be a problem in this area.
If both VOUTs happened to be both at about the same voltage either full + rail or Full - rail, this would be normal because of the missing output components causing the feedback loop to be open.
Think of it as a very large op-amp.

Thanks xavier60!!

Yes, the VOUT- and VOUT+ pins on the right channel are equal to the - rail and + rail respectively.

Just to be sure, before toasting another set of transistors, I'll try pulling the left channel transistors out of circuit and take a look at the VOUT pins on that side. And if we're looking at normal voltage, I'll probably grab a new IC700. And if i see the positive and negative rails, I'll try tossing in all of the transistors.

But starting to look like:

A speaker wire short toasted R545, which in turn wonked up the biasing on the right channel, frying the transistors there.

When I took out the right channel transistors, it didn't go into protect because it was set to output to speaker sets A and B, operating on the lower tap without enough power to trigger a protect on the left channel - looking as if it was simply the transistors that caused the problem...




All of that doesn't explain why the right channel transistors fried out, but the left have successfully been protected by protect mode even throughout all my dinking around with it. :wtf:
 


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