Fault Tracing a Carver M-500t

[bob p]

I've got another dead Carver M-500t on my hands, and this one is a little harder than the last one. Its been "serviced" by someone before, and God only knows if they repaired it or not. I suspect there's a strong possibility that its been half-repaired, so that some of the telltale evidence of what's gone wrong is now missing. I think that the amp is now FUBAR'd, which has made diagnosing the problem rather difficult. Please read on if you think you can help.

First the easy part:

This amp seems to have a normal PSU. It powers on and idles without popping fuses. All of the voltage rails have appropriate voltages on them. I've used the PSU trimpot to calibrate the PSU output voltage to 74 VDC on the amp board's test point (R179), as listed on the amp's calibration sheet. I've also biased both channels to idle at 6 mA across each channel's TPs at the OT ballast resistors. When the amp powers on, I get the relay clicking on after about 3 seconds as you'd expect. But here's a funny observation: the relay isolates the PT from the speaker outputs when the amp is off (Z measures in megohms), but instead of having the Z between the OT and the speaker posts drop to zero, it drops to about 500 Kohm on each channel when the relay is activated. OK, let's put the relay on our list of things to be worried about.

Examination of the heatsinks shows that all of the OTs and drivers appear to be un-shorted. That is to say, they all pass my diode chirp tests. Its hard to tell if the OTs have been changed or not. The solder joints may seem a little shiny and new, but they work looks old enough to have been an old repair if the OTs were ever replaced. All of the OTs are OEM Toshibas, and they all have matching lot numbers. If they were replaced, they were all replaced. So far so good.

Examination of the amp board shows some obvious problems. There are three blatantly obvious areas on the board where the board has been discolored from green to brown due to excessive heat. There are two more subtle areas of minor discoloration which are harder to appreciate. None of the traces are actually burned -- only the green color of the board has been turned to dark brown/black. Here's how the discolored areas look:

First the 3 blatantly obvious areas:

1. Scorched earth around R182 and R183. These are 6k8, 2W metal oxide power resistors that attach to Pins 4 and 8 of the TL072 input OpAmp, IC101. Pin 4 is the Vcc- pin, and Pin 8 is the Vcc+ pin. It looks like R182 and R183 had LOTS of current passing through them, as the board is blackened at both ends where the leads mount to the board, though the solder joints remain intact. R182 is mounted close to C103 and scorched its outside. C103 connects to one of the opamp inputs. R183 is mounted close to R179 and looks like its charred the spaghetti insulation off of one of the R179 leads. In this section of the board, I'm assuming that R182, R183, C103 and R179 are all bad from thermal damage. the TL072 is also bad, as are other parts yet to be determined.

2. Scorched earth around R212 and Q207. R212 is a 270R 1/2W resistor between ground and Q207E. Its burned so badly that its body is fractured. There is a small scorched area around the solder connection between R212 and Q207E that's about the size of a dime. No other parts appear to be physically damaged, though the following parts are physically close enough to be on the suspect list: R211, R218, D201, D202, D203. Tracing the circuit path, it looks like the fault current probably came toward Q207 and went through R212 to ground. Where did it come from? Maybe D203 and the relay? Things are starting to add up.

3. Scorched earth around IC601 and R601, R602. IC601 is the IC that monitors the amp's outputs to control the power meters. There is a huge darkened area around IC601 and the two 2W metal oxide power resistors: R601 1k8 2W and R602 3k8 2W. The scorched area on the board extends from IC601 to R601/R602, to R612/R613, and to the two meter trimpots. ITs a big scorched area, maybe 2-inches in diameter.

There are also two areas near the inputs that are subtly discolored, though not so seriously scorched: Q105/Q107 on one side, Q106/Q108 on the other.

Now that I've got the physical observations out of the way, I'll put the preliminary test results in the next post.

Edit: Almost forgot: Hacked up ballast resistors: The OT ballast resistors have additional resistors soldered atop their terminals, as if they had all faulted open and someone was trying to jump replacement resistors atop their carcasses. The amp has a total of eight 0.47R, 2W resistors mounted atop the bodies of the ballasts.

more to follow...

[bob p]

Some measurements:

Trans    E       C       B
Q101     1.7    72.4   2.2
Q102     1.7    72.6   2.2
Q103    -1.7   -72.7  -2.2
Q104    -1.7   -73    -2.2
Q105     73.0   1.7   72.5
Q106     72.7   1.7   73.2
Q107    -73.4  -1.7  -72.9
Q108    -73.6  -1.7  -73.1

Trans    E       C       B
Q201    43.1   -0.59     43.1
Q202    .007    12.1     .014
Q203    12     .007       6.6
Q204    .014    6.5      .007
Q205    11.5    43.2     12.1
Q206    0       12.1     -0.584
Q207    10.2    18.9     10.8

Q301   .0005   +74.1   .0007
Q302   .0007   +74.1   .0008
Q303   .0007   -78.7   .0005
Q304   .0007   -78.4   0

[bob p]

D201   .597 / .495
D202   .597 / .480
D203   .184 / .184 *
R218  10K  (way off)

[bob p]

Oh yes, the relay housing seems a bit yellower than it should be.

I can see how a fault current that might have originated in the power meter circuit might have passed backwards through the relay's snubber diode to the protection circuit Q207 to ground. That is to say, at least it looks like there's a path through there somewhere. But Q207 doesn't test with an obvious E-C short.

I'm still at a loss to explain how these two areas are related to the problem with the TL072 op amp at the input, or to the darkened area around the pre-drivers Q105-Q108. There are some very funny measurements on those 100 series transistors.

I'm also having trouble explaining how all of this comes together, and then there are those odd resistors soldered atop the ballasts.

ideas? i'm wondering if this might have been a lightning strike.

thanks!

[Enzo]

Easy part. Either the relay contacts close or they don't. MAke sure you didn't measure resistance from output A to speaker terminal B and vice versa. Also, Measure continuity between the speaker terminals and the relay output side, and also measure resistance from the amplifier circuits - the ballasts perhaps - to the hot side of the relay. Lack of continutiy from amp to terminals is not necessarily the relay. Of course it could be the relay, and if it can be opened, burnish its contacts and verify freedom of motion inside.

The two 6k8 resistors are dropping resitors and I expect them to get hot. They drop the 74v rails down to the 15v zeners. I would not be surprised to see browning of the board normally.

What would cause them to OVERheat? SHorted zener or cap, but not likely both zeners are shorted, besides, you can measure them. If the IC burns up, it can short one rail to the other. Neither would short to ground - after all, the chip has no ground conection - but they can and do short together. And that would max the current. Pop the chip out and the rails would come back, assuming the 6k8s are OK .

What does the TL072 measure between pins 4 and 8? I mentioned elsewhere that the chip was suspect.

R212. Only thing can burn that up is excess current through Q207. Is 207 short? I'd replace it anyway, if enough juice flowed through it to burn the R. I think we covered how this whole circuit works last time, so refer to that of needed. Why failure? Remember, we don't know what was done to this amp, it may not be in as-failed condition. In any case, the current path is through the relay from the 40v rail, through Q207, and through R212. SInce the relay is designed for continuous on, that means it is OK for Q207 to stay on. Maybe not for the speakers, but Q207 has no trouble holding the relay on all day. So even if Q207 shorts, it is just the same as being turned on as far as the relay and resistor are concerned. SO even if it is bad, Q207 did no harm the resistor. That leaves the relay and the diode across it. Either of those appear to have been soldered recently? A shorted diode D203 means Q207 connects R212 across the 40v rail, right? And that would be 6 watts to dissipate.

Failure current would not have come through Q207 base, i would have had to come through R208, and that is 1k.

There is another possibility I discuss below.

3. IC601 is the meter driver. Two channels. Without looking it up, I would not be surprised if it is a headphone driver IC in real life. But it coulod be specifically made for VU drive. In any case R601,602 are current limit resistors - or voltage dropping if you want to think of it that way - from the +/-40v rails to power the IC. Both are 1k8. The 3k3 is R612. R612 is the dropping resistor that continues on to provide power to U301 with ZD602. There is a companion R613 - it is over further left - for the other U301 supply zener.

Looks to me like U601 shorted, and just like U101 could have shorted the two rails together. I kinda doubt the two meters themselves were involved, and there is no voltage coming out to the speaker terminals that the circuit was not designed to handle, after all, the peak voltage is the same as the rails. Look for U601 rails shorted together.

Here is a theory. SO far there are two failures that are independently related to 40v rails. WHat if somehow the 74v rail was shorted to the 40v rail? Now the various parts we have been discussing have an extra 34v to dissipate. How would that happen? SHorted isolation diodes D401-404. ANy one of those shorts, and whenever the 74v rail is commutated on, it sends 74v back into the 40v rail. Just a thought. And if I read it right, you'd never know until you cranked the amp high enough to invoke the higher rails.

That is some serious sleuthing if that turns out to be the case.

Funny ballests? Clear it out, and make it stock.

D201 .597 / .495
D202 .597 / .480
D203 .184 / .184 *
R218 10K (way off)

What are those, readings each direction?
D201 is reverse parallel Q205 BE, so naturally we get a junction drop each way. Same thig D202 and Q206. D203 has the relay coil across it with its low resistance. That masks any junction drop readings. It isn't shorted, that is what counts.


The burnt up U101 may not be related to the burn up 200 and 600 circuits. COuld be but don't assume it is.

Q301 .0005 +74.1 .0007
Q302 .0007 +74.1 .0008
Q303 .0007 -78.7 .0005
Q304 .0007 -78.4 0

Looks OK. These run the commutators. The collectors are refernced to the 74v rails and will ne th esource fo curren twhen they turn on. They only turn on when U301 tells them to in response to signal peaks.

Trans E C B
Q201 43.1 -0.59 43.1
Q202 .007 12.1 .014
Q203 12 .007 6.6
Q204 .014 6.5 .007
Q205 11.5 43.2 12.1
Q206 0 12.1 -0.584
Q207 10.2 18.9 10.8

We went through expectations here last time, refer to it. But if the relay clicks on after power up, then these things would seem to be working.

And all those Q1xx readings? Let's wait and see what the amp is doing. Both channels are reading the same and symmetrical as well. Fix the burnt stuff, fix U101. If the amp sits there politely at idle with no DC output skew, I'd say we are making progress.

[bob p]

Easy part. Either the relay contacts close or they don't. MAke sure you didn't measure resistance from output A to speaker terminal B and vice versa. Also, Measure continuity between the speaker terminals and the relay output side, and also measure resistance from the amplifier circuits - the ballasts perhaps - to the hot side of the relay. Lack of continutiy from amp to terminals is not necessarily the relay. Of course it could be the relay, and if it can be opened, burnish its contacts and verify freedom of motion inside.

I measured from the relay's inputs to the output solder points on the relay where the red and blue channel output wires take off to go to the speaker terminals. I'm fairly certain that I measured across the relay and nothing else. I'll double check that. Insofar as one of the channels is inverted WRT the other, maybe I need to check the blue wire on one channel and the black one on the other channel instead of the red wire. (or vice versa). Its very tough to get the relay package to open up. I stop out of fear as I pull on the relay's case when I hear the PCB flexing and creaking.

The two 6k8 resistors are dropping resitors and I expect them to get hot. They drop the 74v rails down to the 15v zeners. I would not be surprised to see browning of the board normally.

FWIW, the other M-500t that I am using for comparision has no discoloration whatsoever around these two resistors. It also does not have the plastic sheath covering C103 charred away, and it does not have the spaghetti that covers the leads of R179 charred away. To me, it looks like one amp has a blackened PCB and several burned parts, while one amp has no signs of excess heat whatsoever. My take on this is that the discoloration on the PCB and the charred parts aren't normal wear and tear, and they're indicative of a more serious current/heat problem. My suspicion is that these signs of heat are all attributable to the failure mode, and not to normal operation.

What would cause them to OVERheat? SHorted zener or cap, but not likely both zeners are shorted, besides, you can measure them.

Which zeners and caps are you referring to? And as a silly question -- how do you test a zener, compared to a regular diode? I don't think that my diode tester is going to take the zener diode to avalanche, so are we just looking for voltage drops and the absence of shorts and open circuits?

If the IC burns up, it can short one rail to the other. Neither would short to ground - after all, the chip has no ground conection - but they can and do short together. And that would max the current. Pop the chip out and the rails would come back, assuming the 6k8s are OK .

What does the TL072 measure between pins 4 and 8? I mentioned elsewhere that the chip was suspect.

measured Z across pins 4-8 starts off at several kohms and keeps on climbing with the passage of time. i stopped watching after it climbed to 15k.

R212. Only thing can burn that up is excess current through Q207. Is 207 short? I'd replace it anyway, if enough juice flowed through it to burn the R. I think we covered how this whole circuit works last time, so refer to that of needed.

no short in Q207. is it safe to replace Q207 and R212 now (before the meter and relay stuff gets fixed), or are we just going to submit the new Q207 and R212 to being blown-up?

[bob p]

Why failure? Remember, we don't know what was done to this amp, it may not be in as-failed condition. In any case, the current path is through the relay from the 40v rail, through Q207, and through R212.

i'm not following the failure path to the 40v rail. tracing backwards from ground, we have R212, Q207, and RL101+D203. Then I'm assuming that the power passed through those two thermal switches at the top of the diagram (points 5, 1, 2 and 38 on the schematic), passed by R613 and Q205, to a point labeled "39" on the schematic, to point 27 which meets the 40V rail. Is that the path you're thinking of?

If that's the case I'm wondering about if I have to worry about other parts like R613 and Q205 (which isn't labeled as Q205).

[bob p]

sorry about the multiple posts. my browser is acting up and i keep loosing text, so i have to commit the text to a post before i lose it.

Why failure? Remember, we don't know what was done to this amp, it may not be in as-failed condition. In any case, the current path is through the relay from the 40v rail, through Q207, and through R212. SInce the relay is designed for continuous on, that means it is OK for Q207 to stay on. Maybe not for the speakers, but Q207 has no trouble holding the relay on all day. So even if Q207 shorts, it is just the same as being turned on as far as the relay and resistor are concerned. SO even if it is bad, Q207 did no harm the resistor. That leaves the relay and the diode across it. Either of those appear to have been soldered recently? A shorted diode D203 means Q207 connects R212 across the 40v rail, right? And that would be 6 watts to dissipate.

D201 and D202 look like they have original solder joints with red marker dots on them. D203 (relay diode) has no dots, but could be original. D202 and D203 look like 4007. Don't remember if that is stock or not. since the relay appears to be functioning, would you bother lifting D203 to test it?

more to follow...

[bob p]

There is another possibility I discuss below.
...
Looks to me like U601 shorted, and just like U101 could have shorted the two rails together. I kinda doubt the two meters themselves were involved, and there is no voltage coming out to the speaker terminals that the circuit was not designed to handle, after all, the peak voltage is the same as the rails. Look for U601 rails shorted together.

I looked it up. The meter driver is a TA7318P, which is a dedicated VU meter driver, not an adapted headphone driver. I've attached the data sheet below.

for use with a bipolar power supply, the voltage rails are at pins 5 and 9. measuring the Z on them, it starts at about 1k, jumps to 10k and keeps climbing, like i'm charging a cap somewhere.

data sheet below:

[bob p]

Here is a theory. SO far there are two failures that are independently related to 40v rails. WHat if somehow the 74v rail was shorted to the 40v rail? Now the various parts we have been discussing have an extra 34v to dissipate. How would that happen? SHorted isolation diodes D401-404. ANy one of those shorts, and whenever the 74v rail is commutated on, it sends 74v back into the 40v rail. Just a thought. And if I read it right, you'd never know until you cranked the amp high enough to invoke the higher rails.
...
What are those, readings each direction?

i don't remember if i mentioned this before, but the voltages on all of the PSU caps are what they're supposed to be. that is to say, the caps that supply the different rails were't at the same potentials. i think that might rule-out our rail overvoltage hypothesis.

here are those measurements you asked for:

D401 and D402: 0.384 / climbs to open loop
D403 and D404: 0.380 / immediate open loop

The burnt up U101 may not be related to the burn up 200 and 600 circuits. COuld be but don't assume it is.

putting the 200 and 600 circuit faults together was easy enough. i have no way of explaining how they'd be related to the IC101 circuit faults.

[Enzo]

i don't remember if i mentioned this before, but the voltages on all of the PSU caps are what they're supposed to be. that is to say, the caps that supply the different rails were't at the same potentials. i think that might rule-out our rail overvoltage hypothesis.

Under my theory the voltage would look OK, but when the commutating transistor turns on, THEN and only then the wrong voltage would get places IF the isolation diode was shorted. FOr example if D401 were shorted, then when Q405 turns on, the 40 and 74v rails would be connected together. Then again that alone would be problem enough I guess.

If they are not shorted, they are not shorted.

I looked it up. The meter driver is a TA7318P, which is a dedicated VU meter driver, not an adapted headphone driver. I've attached the data sheet below.

for use with a bipolar power supply, the voltage rails are at pins 5 and 9. measuring the Z on them, it starts at about 1k, jumps to 10k and keeps climbing, like i'm charging a cap somewhere.

OK. Regardless, I seriously doubt the meters themselves caused any issues. Any faults in teh IC is more likely to burn the little meter movements out than anything the other way around.

And yes, you are indeed charging a cap - the power supply caps.

[QUOTE]I measured from the relay's inputs to the output solder points on the relay where the red and blue channel output wires take off to go to the speaker terminals. I'm%2y phase relationshup between channels will have no impact on the relay contacts.

My previous post was written stream of consciousness, so if the relay pulls in and drops out like it should, then the D203 is not shorted obviously. If the diode were open, Q207 would give up the ghost pretty quick.

You ZEN moment of the day:
Put a sigh in front of you that says "Someone else worked on this before me." Never forget that. This is not an amp that failed and now you have it in the as failed condition. Aside from whatever original problem it had, you also have the previous tech's additions to that. Clearly he could not fix it, and we do not know what he might have done to it while trying.

For example, my theory about the 40 and 74v rails getting shorted together by a bad diode was more related to an as failed amp, but it could easily have been the other guy shorting power rails together with a slipped probe or something. Leaves no physical evidence other than burnt up stuff.

The fact that several places in the circuit are BURNT, not merely failed, is too convenient for me. I have to think they are related. These are not things that burn up in a normal blown channel type situation. Nothing should harm the meter circuit. Except maybe power supply.

Well, one thing maybe... lightning. And I don't say that lightly. Lightning damage does not follow schematics, it follows physical proximity. Various things that have NO relation burn up because they are near each other on the board.

i'm not following the failure path to the 40v rail. tracing backwards from ground, we have R212, Q207, and RL101+D203. Then I'm assuming that the power passed through those two thermal switches at the top of the diagram (points 5, 1, 2 and 38 on the schematic), passed by R613 and Q205, to a point labeled "39" on the schematic, to point 27 which meets the 40V rail. Is that the path you're thinking of?

If that's the case I'm wondering about if I have to worry about other parts like R613 and Q205 (which isn't labeled as Q205).

Yes, point 27 and friends are the 40v I refer to.

The current through any circuit failure or otherwise has to come from somewhere. SO either it is the existing power rail, or it came from the outside world by way of something metal dropping on the board or some such. SO I follow the path you just described up to the relay, and the relay is connected straight to 40v. The fact that the drawing goes up through thermal cutouts and a wandering path does not alter that it is a direct connection to the 40v rail. If the 40v is OK, then other things connected to it are not threatened. SO no, I don't suspect Q205 and R613 since they are connected to 40v normally. SOme other circuit drawing excess current from 40v won't affect them.

My only potential worry there would be if some other voltage like 74v from a screwdriver short got on the 40v rail. Then they would face the same over voltage the other stuff did. ERmember that is just a theory. Remember that Q205 and the timing cap work together to pull the relay in, so if the relay waits a few seconds and pulls in, then those xstrs are working.

I believe the other amp does not show signs of heat. So is U101 original? The zeners I refered to are ZD101,102 above and below U101 on th drawing. The caps are the e-caps in paralle with each zener. If a zener or cap shorts, then the 6k8 feeding it will get hotter. Seems unlikely both side zeners would fail together though. A shorted IC would then move up my list.

Zeners are just diodes, so if they are shorted, they show it. They can get leaky, so a 15v zener can wind up with say 6v across it. But usually, if you just test them for forward drop as a normal diode, they are most of the time OK when in zene mode too. If they are not shorted, act more or less like a real diode frontwards, and result in 15v at teh IC, then they are OK.

no short in Q207. is it safe to replace Q207 and R212 now (before the meter and relay stuff gets fixed), or are we just going to submit the new Q207 and R212 to being blown-up?

Q207 and R212 ARE the relay stuff, and I don't think the meter circuit wil get in the way of anything - unless the IC is shorted inside. If you like, remove the meter IC. Now R601,602 no longer go anywhere, and the resistors dividing off the speaker line won't load anything down. Fix everything else, then reinstall the IC and see if it works. If R601 gets hot then, the IC is bad.

You had burnt traces before, ever determine what they were?

[bob p]

Under my theory the voltage would look OK, but when the commutating transistor turns on, THEN and only then the wrong voltage would get places IF the isolation diode was shorted. FOr example if D401 were shorted, then when Q405 turns on, the 40 and 74v rails would be connected together. Then again that alone would be problem enough I guess.

If they are not shorted, they are not shorted.

ah, I see your point now. that was pretty insightful. so insightful that i missed it the first time around!

OK. Regardless, I seriously doubt the meters themselves caused any issues. Any faults in teh IC is more likely to burn the little meter movements out than anything the other way around.

well, if the meters are dead, this amp might end up having a different fate than if they are not. in that case, given the lack of parts availability, i might consider sending the amp out to one of those guys who does flat rate repairs and just letting him fix everything.

[bob p]

And yes, you are indeed charging a cap - the power supply caps.

am i correct in interpreting the test to say that the observed charging of the PSU caps indicates the lack of a short across those pins, and that IC601 tests OK on that test? before pronouncing it OK, how else would you assess IC601?

The fact that several places in the circuit are BURNT, not merely failed, is too convenient for me. I have to think they are related. These are not things that burn up in a normal blown channel type situation. Nothing should harm the meter circuit. Except maybe power supply.

Well, one thing maybe... lightning. And I don't say that lightly. Lightning damage does not follow schematics, it follows physical proximity. Various things that have NO relation burn up because they are near each other on the board.

hmmm. lightning. that could open an entirely new can of worms. what surprises me about this is that the burned areas are on the front (input op amp), middle (relay and protection), and back (meter control) sections of the board. although they're kinda far apart in physical proximity, the middle and back sections (protection relay and meter IC) are close electrically. still trying to figure out the input opamp thing. maybe we had two failures and there's no point in trying to tie it all together.

I believe the other amp does not show signs of heat. So is U101 original? The zeners I refered to are ZD101,102 above and below U101 on th drawing. The caps are the e-caps in paralle with each zener. If a zener or cap shorts, then the 6k8 feeding it will get hotter. Seems unlikely both side zeners would fail together though. A shorted IC would then move up my list.

yes, the input TL072 opamp appears to be original. ZD101 and ZD102 aren't shorted:

ZD101: .650/.830
ZD102: .540/.838

still having broswer faults, so i'll break up my posts. sorry.

[bob p]

Q207 and R212 ARE the relay stuff, and I don't think the meter circuit wil get in the way of anything - unless the IC is shorted inside. If you like, remove the meter IC. Now R601,602 no longer go anywhere, and the resistors dividing off the speaker line won't load anything down. Fix everything else, then reinstall the IC and see if it works. If R601 gets hot then, the IC is bad.

You had burnt traces before, ever determine what they were?

the problem i had with PCB traces on that other amp were not actually burned traces from something going wrong in the amp. they were traces that got yanked from the board by some yayhoo with a soldering pencil.

What does the TL072 measure between pins 4 and 8? I mentioned elsewhere that the chip was suspect.

measured Z across pins 4-8 starts off at several kohms and keeps on climbing with the passage of time. i stopped watching after it climbed to 15k.

it looks like PSU charging again, and the absence of a short in IC101, at least across those pins. Same question as before: what do you think of the status of this IC?

OK, let me see if I follow your recommendations:

1. pull out those additional non-stock ballasts. who knows why they are there.
2. pull out IC601. then replace the stuff that's bad in the relay circuit, Q207 and R212.
3. pull out IC101 (input opamp), leave it out and recheck voltages on the Q100 series of transistors.

anything else to recommend at this stage of the game?

[Enzo]

Yes, replace the ballast mess, I assume the underneath ones are open? Make them all good.

I'd likely replace U101 on principle. They cost about a quarter. If you are concerned, install a socket, then you can replace burnt ones easy. I think it should be there. SS amps are HEAVILY feed back for stability. Without the IC, the output stage is free to drift around as it desires. Without that IC, even a good amp could wind up with serious DC offset. Generally the only thing that will damage that chip is something on the input line or real power problems. if the zeners are making 15v then the IC is not at risk.

Oh, the missing traces were a different amp. Sorry.

An IC with a dead short can measure that way, but plenty of dead transistors inside it could short the rails and not appear so from the outside. If you place a diode between ohm meter leads, it won't show a short, but apply it across a power supply and it sure will. Ohm meter tells you something is shorted, but can't tell you it is not.

I don't know about U601, but if you pull it, then it can no longer get in the way of the rest of the work. You can verify the meter movements easy enough without it.

Yes, fix Q207 etc.

Input problem and other problems may or not be related, but either way they have to get fixed. A lot of time the tie ins become apparent after all the repairs are done. You get an aha moment and realize this cause that to fail and so on.

Here is a scenario for lightning. If the lines actually were struck, the thing would be toast. But lightning is a huge curent flow nearby, and as such it induces current into conductors around it - as in wires. SO the cables to the input and the speaker wires from the output could have independently had voltage spikes induced in them by the same nearby strike. The connection may not have been inside the amp.

AS I recall, your amp would sit there powered up with reasonable currents, so there should be no problem taking hot measurements.

If U601 is already toast, powering for a few more minutes wont hurt it. DO R601,602 get hot? Is the voltage at the chip ends of R601,602 reasonable for that chip per the spec sheet.?

Same argument for U101. If it doesn't load down the 15v zener rails, then most everythig is OK, at least OK enough to work with.

I'd say get the amp stable, get it passing signal, get the relay working right, then get the meters going.

The TA7318 seems to be in stock for $3 at B&D.

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