2SB755 Transistor Data?

[Enzo]

No, I don't.

Just remember the relay normally waits a few seconds then clicks on to enable the speakers if it detects no problems.

I am used to following transistor logic, I learned relay logic a long time ago, and it is similar. The E-C acts like a switch, it is either shorted together or open. Pull the base voltage towards teh collector voltage and the thing turns on - the e-c path conducts. It is a little relay, it is a little switch, for all intents.

If you jumper across a power switch in an amp, it does not hurt the switch. After all the wire is a path for current AROUND the switch.

Q207 drives the relay coil. The right end of the coil is at +40v. When Q207 is on, the left end of the coil is connected through R212 to ground. Current flows, and the coil energizes, which pulls in the relay. (pulls in means the contacts close. the coil pulls the armature down to do this.) SO we could just short from E to C with a probe tip or something and the relay should pull in.

R211 holds the base of Q207 down - keeps the xstr off. Q205 has the job of turning Q207 on. Q205 collector is at +40, and the emotter connects to the base of Q207 - through a resistor. If something turns Q205 on - E to C flows - and thus the +40 finds its way to the base of Q207. Actually the current finds its way there, there will not be +40 at that base. So you could short E-C on Q205 and the relay should pull in.

I mentioned this already I think, but in case you are not sure what the relay is doing...
One end of the relay is at +40. If the relay is not energized, then that +40 will be at the other end of the coil as well. No current through the coil resistance means no voltage DROP. As soon as Q207 tirns on, the left end of that coil goes down to a low voltage. Now current flows through the coil resistance, and most of that 40v is across the coil. Can't get at the coil terminals? the diode is across the coil. Any voltage across the coil will be across the diode. Meter probes across the diode. If zero volts - and the diode is not shorted - the the relay is off. If you get 25-30 volts or something, then the relay is on.

Now some reverse logic. If we ignore the rest of the Q2xx xstrs, then we see R205 up there trying to pull the base of Q205 up to +40 - pulls the base towards the collector. SO absent those other parts, that resistor turns on Q205, which turns on Q207, which pulls in the relay. If we ground the base of Q205, it turns off Q205, and ...the relay drops out. 220uf C202 sits at the base of Q205 to ground. When you first apply power, that cap has to charge up through the 56k R205 before the base of Q205 can get to a high enough voltage to turn on. Thus C202 is the timing cap for that period of delay at power up. Remove that cap and ther eis no delay - instant on. Reduce the value of the cap and the delay is shorter. Or make the thing 1000uf and teh delay will be four times longer. Whatever, that is how it works.

Now the other four xstrs. They all connect more or less to the base of Q205, and to ground. Turn them on adn they short the base of Q205 to ground and turn it off. That drops the relay out. So all those xstrs serve to kill the speakers when trouble erupts.

The two channel outs are sampled through R201,202 and fed to the base of Q202. Positive peaks there could turn it on, and since it is connected to the base of Q205, when it turns on, Q205 wil be turned off. Note Q203 is opposite polarity but otherwise parallel with Q202. The same outputsample that went to base Q202 also goes to Q204. All that does is serve to invert things so negatove peaks will also turn off Q205. Trust me.

Q206? If it turns on, it also grounds the base of Q205...relay drops out. Normally a special little negative supply off C517 keeps the base of Q206 low. In the absence of that voltage, a couple resistors to +40 will pull that base up and Q206 will turn off Q205. If Q201 way up there turns on, that would bring +40 to the base of Q206, turning it on...relay drops out. Without getting into it, the base of Q201 is fed by lines direct from the limiter circuits of the channels. if teh channels go into limit, it drops out the relay.

I will post this and refer to your readings separately.

[Enzo]

In both reading groups - A and B for short - Q207 shows close to 40v on the collector, so the relay must not be energized. SInce the base is lower than the emitter, it can't be turned on. COnsistent.

If you look at Q205, it is an emitter follower - just like a cathode follower - so whatever is at the base will be more or less what is at the emitter.

Your readings varied, but note that Q202C and Q203E are tied together and always read the same as each other. Also they are tied to the base of Q205, so what is on them is also what is on the Q205B. COnsistent.

The Q202B and friends have zero volts, so there is no DC offset on the output busses to trip them. Good.

The voltage on Q204C is OK, since it is coming through Q203. However the Q203 reading don't look quite right to me. You can lift R204 - from the base of Q205 to the Q202,203 - and that will defeat the Q202-204 protection circuit, eliminating it from the contest for now.

The first group of readings is odd. For example Q205B and Q206C are wired together and should have the same thing on them, yet your reads show large difference.

The base of Q206 is at zero, so it is not being tripped by something either. You could pull Q206 and defeat teh tie in to the chanel limits for now.

It ;looks like you have inconsitency at teh base of Q205. Possible leaky Q206, leaky C202 - rememner C202 is just the timer. Make sure D201 is OK.

[bob p]

i replaced D201 with a 1N4007 today, just before taking those last two sets of measurements. its hard to imagine that its gone bad after only a few minutes on the job. even so, earlier this evening i did re-check it in-circuit, where it is connected in parallel to the relay.

IIRC measurements across the new diode in-circuit showed zero impedance (and zero voltage drop) in-circuit, which would suggest that it had either changed from a diode into a jumper wire (like the previous diode), or that the relay in parallel with it has no impedance. at the time i didn't bother to lift the new diode to check it to see if it had gone bad.

edit: on a re-check the voltage drop measurements across D201 in-circuit seemed normal.

[bob p]

well, the good news and the bad news is that there are some interesting new findings.

I lifted R204 to defeat the Q202-204 protection circuit. Then I tooks some new measurements with the amp at idle:

Trans    E        C        B
Q201    43        0       43
Q202     0        1.0      0 
Q203     1.0      0        1.2
Q204     0        0.82     0
Q205     4.2     43.5      4.7
Q206     0        4.7      0
Q207     3.9     36        3.3

As you suggested, I shorted Q207 E-C with a probe tip to pull the relay in. It worked. The relay pulled in and established continuity between the OTs and the speaker terminals.

Remembering that the drivers and OTs tested OK back on Page 1, I was brave and I hooked up my sine wave generator at minimum output to the Left channel input and a 15R power resistor to the Left channel output. I shorted Q207 E-C and a large clean sine wave appeared at the outputs. This was encouraging.

I repeated the test in the Right channel, and upon shorting Q207 E-C I noticed a large distorted signal on both the input and output traces, accompanied by a loud squeal/oscillation in the KHz range that presumably was coming out of the power resistor. I shut the amp off and disconnected the input signal. I powered it back on briefly with no input and the 15R output load. The squeal repeated, and by the time that I was able to shut the amp down R174 went up in smoke. You'll find it attached to the collector of the Right channel driver Q118. Its near the bottom in the right channel preamp section on the schematic.

The good news, I guess, is that the protection circuit was actually working. Tomorrow I'm going to re-connect R204. I guess I'll need to re-check the right channel drivers and outputs. Any ideas on the source of the oscillation and the burned R174?

[Enzo]

Oh dear. I might not have been so brave just yet.

I would recommend just making the relay circuit work before moving on to sending power around it. The voltages you have provided do not look like the amp is trying to be in protect, it LOOKS like - the big if - the relay circuit itself is damaged. That says nothing about the amplifier channels behind it.

Several thoughts.

First, this is solid state - you do not need a load. I always fire them up without a load first. If an amp goes to DC, without a load there is nothing to draw current from it. Add a load and now the poor amp has to do WORK with the DC. Without a load the output can do pretty much whatever it likes without overworking itself.

Once I know an amp functions without a load, then I can try with a load. In fact, an amp that makes a nice sine wave unloaded but the wave collapses under load is sending valuable clues.

Next, those Q2xx protections turn off the speakers. But the amp channels are functioning - or not - whether the speaker terminals are live or not. You can monitor them even if the relay stays off. Just scope at the channel ballasts, or an output emitter, or at the inductor in the output bus. If you see a unwanted waveform, no point in sending it to a speaker or other load.

I am not so sure the Q202-204 would have saved it. They didn't seem to be the cause of the relay staying off.

Looking at the R204 lifted numbers we find more or less nothing on those three xstrs, as we expect. But look at Q205B, 4.7v. SOmething is draggging that down. With Q202 etc out, then the Q206 path is all that is left. The base of Q206 is at zero, so nothing is turning it on. So looking at Q205B, I see these possibilities:
1. Q206 shorted
2. C202 shorted
3. D201 shorted
4. R205 open

Hey, I think I may have confused things with the diode. I been referring to D203 - across the relay coil - as D201 I think. D201 is a vanilla diode too, but need not be as high current. A 1N400x will be OK nonetheless.

Your resistor might have been screaming, but could it have been the PT - or magnetic resonance whatzit?

That burnt resistor is a bad sign - I'd immediately look for Q118 shot and check the outputs.

Just about Q202 are the Zobel networks for the channel outs. ANy of those resistors open? it is amazing how important those things are to SS amps. The caps rarely fail, but an amp screams and it can burn those resistors right out.


And overall, I hope to hell I am not leading you astray. It is a lot harder to work on something hundreds of miles away than one in front of you.

[bob p]

Hey, I think I may have confused things with the diode. I been referring to D203 - across the relay coil - as D201 I think. D201 is a vanilla diode too, but need not be as high current. A 1N400x will be OK nonetheless.

Whether or not you made the mistake of confusing the number for D201 and D203, the fact remains that the real D201 was found to be defective when I tested it, so it was replaced with a 1N4007. THEN the relay started working. So I guess we got lucky that D21 got checked out at all. I think we still need to consider what D201 is designed to do, and why it went bad in the first place.

I lifted the REAL D203 (the one across the relay) and measured it with the diode test on my DMM: Open loop in one direction, voltage drop of 0.465V in the other direction. Looks good to me.

Looking at the R204 lifted numbers we find more or less nothing on those three xstrs, as we expect. But look at Q205B, 4.7v. SOmething is draggging that down. With Q202 etc out, then the Q206 path is all that is left. The base of Q206 is at zero, so nothing is turning it on. So looking at Q205B, I see these possibilities:
1. Q206 shorted
2. C202 shorted
3. D201 shorted
4. R205 open

Now that I know that you've been confusing D201 with D203, does the third entry in your 4 member list, does number 3 really refer to D201, or to D203 across the relay? I just want to be sure that we are on the same page and that I understand what you're referring to.

Back to your list:

1. Q206. I have a replacement on-hand but I haven't installed it.

2. C202. I have no way to test it, but I do have some used replacements that I pulled out of an old dead TV. They may be no better than the original part.

3. D201. It was replaced yesterday. Do I need to lift it again?

4. R205. Lifted and measures 56K.

Just about Q202 are the Zobel networks for the channel outs. ANy of those resistors open? it is amazing how important those things are to SS amps. The caps rarely fail, but an amp screams and it can burn those resistors right out.

are you referring to R188-191? they look like 2W 10R resistors, and they all test at 10R-11R in-circuit with the amp turned off.

Your resistor might have been screaming, but could it have been the PT - or magnetic resonance whatzit?

I have to admit, now that the amp is screaming I'm not so sure that the problem is in the protection circuit's logic. Perhaps the funny values in the protection circuit are just what happens when part of the amp goes bad and I just don't understand it. I have to admit, this Carver amp is a major PITA.

That burnt resistor is a bad sign - I'd immediately look for Q118 shot and check the outputs.

that's on my to-do list.

[bob p]

R174 is visibly burned, though it still measures 10R.

Q124 and Q126 are shorted out. They're the 2SB755 power transistors in the Right channel.

Q116 is the driver for Q124 and Q126.
Q118 is the driver for Q120 and Q122.

Here are some test numbers. The first line for each transistor are voltage drops, the second line are Z measurements.

Trans   Pin 1-3   Pin 1-2   
Q115   .176v    .481v
       580R     >20k

Q116   .162v   0.9v -> OL
       536R     >20k

Q117   .174v    0.9v -> OL
       580R     >20k

Q118    OL       OL
        262R     2k

I'm not sure I understand the measurements for Q116 and Q117. When performing the diode test on Pins 1-2 the voltage slowly climbs upward to 0.9V and then changes to open loop.

I'm not sure why driver Q118 and R174 would be burned out, while its outputs Q120 and Q122 test OK.

I'm also not sure why driver Q115 tests OK while its outputs Q124 and Q126 are shorted.

I would expect to find bad drivers with the bad power resistors and good drivers with the good power resistors.

[Enzo]

#3 on my little list refers to the new correct D201, the one across Q205. SInce the point was to see what was turning Q205 off, a shorted D201 would be the same as a shorted Q205 shorted B-E.

If shorting across Q207 pulls the relay in, then D203 at the relay must not be shorted.

Back to your list:

1. Q206. I have a replacement on-hand but I haven't installed it.

2. C202. I have no way to test it, but I do have some used replacements that I pulled out of an old dead TV. They may be no better than the original part.

3. D201. It was replaced yesterday. Do I need to lift it again?

4. R205. Lifted and measures 56K.

OK, keep something in mind here. WITHOUT A LOAD, all the relay does is connect the amp circuit to the output terminals on the rear of the amp. The only additional circuits connected are the inputs to the meters. SO if the amp is not blowing fuses, you can have the relay on or off and it won't make a difference. If fuses blow, you won't get as far as the relay anway.

1. Q206, replace it or remove it. If yuo just pull it, then you will see whether or not it was holding Q205 off.

2. C202. DOn't need to replace it, if it measures short, then it is bad. In any event, you can just pull it. Remember, all it does it act as a timer. Without the cap, the relay should pull in right after power up, no 5 second delay. If it was bad, then we can replace it later, after we fix the blown up channel.

3. D201, no need to lift it. Does it measure shorted in circuit? if not, then it is fine. In-circuit measurements can be tricky, yes, but if something measures short, then lift it and determine if it is shorted or something else nearby. If it does not measure shorted, then it isn;t shorted. Nothing elsewhere in the circuit can mask it being shorted, right?


R188 etc, yes those. My typo should have read "just ABOVE Q202 are the ZObel..." Just checking, very important to making an amp stable.

Do you have a scope on the outputs?

If lifting R204 allows the relay to come on, then either Q202 et al are screwy, OR they are doing their job by responding to a huge signal on the bad channel. A huge oscillation will still average zero on a DC meter, so scope what is at R201,202 .


Off to the bad channel, screw the relay for now.

I'm not sure I understand the measurements for Q116 and Q117. When performing the diode test on Pins 1-2 the voltage slowly climbs upward to 0.9V and then changes to open loop.

Ever connect an ohm meter across an electrolytic cap? What you are seeing is your meter charging up the caps in the circuit. When the caps are empty they look like a low resistance to the meter, but as they charge, the resistance "increases."

Q118 looks to me like a 2SC2591, which is a TO220 - same package as the popular 78xx voltage regulators. Thes tab transistors are a common style. ALL are pinned the same. Looking at the front, tab to the rear, they are BCE.

If Q124,126 are short, then remove them, otherwise they will confuse further readings of the other parts.

REmember, Q115,118 are NPN, while Q116,117 are PNP, so for similar readings you must reverse you r meter leads when switching from opne type to the other.

Look at Q115. There is a resistance path from B to E like this: from B through R147 (56), R143 (470), R151 (68), and back to E. 56+470+68=594. That is pretty close to your 580 ohms. B-E is your pins 1-3.

Look at Q118. Same resistance path, but the resilt is half as much. ONly difference must be the xstr itself. PUlllit and test it out of circuit.

Out of circuit, the junction from B to either E or C will yield about .47 or whatever. In circuit, your B-E has that 580 ohms in parallel. SO instead of .471 you get that .176 or whatever. SO when a xstr reads low like that, always consider the parallel circuits. Of course, an xstr can be bad and measure low. SO one has to think about what he looks at. If all the drivers in all the channels read that way, then likely it is normal. Usually you can see the low resustance path on the prints.

Pins 1-2 is B-C. There is no very direct resustance path, so you are just charging the power supply with your meter. Note that if you reverse your meter leads, you will get a different story. The right polarity and you get the one drop - .47, but reverse them and you get an open, or a chargeup.

Q118 is clearly anomalous, replace it.

R174 may be OK, but I'd replace it if it smoked. OK for testing maybe, but for reliability, replace.

When this blew, the fault current followed a path. Clearly it was through R174 and Q118, and since Q124,126 are shot, it went through them. To gwt from one of those sections to the other, the current either ran through R170 and R150 - check them - Or through R170 on through Q122 and R156. (Also maybe R172, Q120, R154) SO check all those.

Pull Q118 so it doesn't confuse the meter and check junction drops on Q120,122 and compare to Q119, 121 (the still good channel)

Why would one driver and the OTHER outputs fail? It is all about the failure current path. A blown output CAN damage its driver, but often as not it doesn't. There si a + and a - power rail, and when things fail, current flows from one to the other in a path we don't want. All it has to do is find its way from one to the other.

A trick I like to do is make a Xerox of a blown channel schematic, then highlight all the burnt parts I find. Notice how Carver darkened the signal path through one of the channels to make it easuer to foillow? It is sorta like that but failure current uinstead of signal. Point is, that when I see a line of charcoaled parts, and it seems to pass through a part that is not bad, that part is suspect, because all that current passed throug it along with the others. And I replace it to be sure.

[bob p]

1. Q206, replace it or remove it. If yuo just pull it, then you will see whether or not it was holding Q205 off.

Q206 removed. Here are my voltage drop results:

Red Lead on Base:
E-B  0
C-B  .518

Black Lead on Base:
E-B  0
C-B  .706

Looks bad to me.

2. C202. DOn't need to replace it, if it measures short, then it is bad. In any event, you can just pull it. Remember, all it does it act as a timer. Without the cap, the relay should pull in right after power up, no 5 second delay. If it was bad, then we can replace it later, after we fix the blown up channel.

C202 removed. It was bulging at the bottom. This wasn't noticable until it was removed. Z measurement shows open loop.

3. D201, no need to lift it. Does it measure shorted in circuit? if not, then it is fine. In-circuit measurements can be tricky, yes, but if something measures short, then lift it and determine if it is shorted or something else nearby. If it does not measure shorted, then it isn;t shorted. Nothing elsewhere in the circuit can mask it being shorted, right?

Not Shorted.

Do you have a scope on the outputs?

I haven't turned the amp back on since its parts started smoking.

So here's where we stand with the amp right now:

1. D203 is now reconnected.
2. R205 remains lifted.
3. Q206 removed
4. C202 removed
5. Q124 removed
6. Q126 removed
7. Q118 removed

R174 tests OK but I need to order a replacement

If lifting R204 allows the relay to come on, then either Q202 et al are screwy, OR they are doing their job by responding to a huge signal on the bad channel. A huge oscillation will still average zero on a DC meter, so scope what is at R201,202 .

The last time that I turned the amp on, resistors started smoking and power transistors got fried. Are you suggesting that its safe to turn on now, with full AC?


more to follow...

[bob p]

Off to the bad channel, screw the relay for now.
...
Q118 looks to me like a 2SC2591, which is a TO220 - same package as the popular 78xx voltage regulators. Thes tab transistors are a common style. ALL are pinned the same. Looking at the front, tab to the rear, they are BCE.

Q118 tested out of circuit:

Red lead on base:
B-E  .145v
B-C  .412v

black lead on base:
B-E  .145v
B-C  .422v

looks bad to me.



Q115 tested in circuit:

Red lead on base:
B-E  .176v
B-C  .481v

black lead on base:
B-E  .176
B-C  charges PS

If Q124,126 are short, then remove them, otherwise they will confuse further readings of the other parts.

removed. here are their out of circuit test results:

Q124 out of circuit
red lead on base
B-E = 0v
B-C = 0v

black lead on base
B-E = 0v
B-C = 0v

looks bad to me.

Q126 out of circuit
red lead on base
B-E = OL
B-C = OL

black lead on base
B-E = .451v
B-C = .426v

looks ok to me.

Q120 and Q122 in circuit test
red lead on base
B-E = .024v
B-C = .426v

black lead on base
B-E = .024v
B-C = OL

?

Q119 and Q121 in circuit test
red lead on base
B-E = .024v
B-C = .432v

black lead on base
B-E = .024v
B-C = OL

R150 = 67.5R
R170 = 0.4R
all ballast resistors appear to be about 0.4R on my meter and non-shorted
R156 = 5.1R
R154 = 5.1R

[bob p]

wow. carver SS gear is a real PITA to fix. after this, I may just go back to Champs.

[Enzo]

The next time would be a lot easier, since you would not have to learn how it works, learn how transistor switching circuits work, and so on as part of the process. PLus, the next one might just have a bad volume control.


Q206, C202 - yep, bad.

1. D203 is now reconnected.
2. R205 remains lifted.
3. Q206 removed
4. C202 removed
5. Q124 removed
6. Q126 removed
7. Q118 removed

R174 tests OK but I need to order a replacement

D203 on relay. Without that, the relay coil inductance could damage Q207. Lift diode for testing, but don't operate it that way. If Q207 works, then it is OK.

R205 - if OK, then resolder.

Q206, C202 - replace at your leisure.

REmember until we KNOW the channels are working, we are NOT using a load. So whatever the speaker relay does doesn't matter. We need to fix this part, but it won't save the channel circuits in the slightest, so powering the amp at this point is moot. Fix the channels.

And removing the bad channel parts - yes.

The last time that I turned the amp on, resistors started smoking and power transistors got fried. Are you suggesting that its safe to turn on now, with full AC?

No I am not saying that. We need to at least find all the burnt out parts and replace them. You did have a load that time, though, and that stresses the amp a whole lot more than without.

[Enzo]

Now the channel.

Q118, yes fried.

You are showing both polarities with your meter. That is good, alwyas worth checking reverse. Q115 is NPN, so the red lead to base is the test. Black to base is reverse. if you are unsure, look at the symbol. B-E looks like a diode, so test in the same direction you would if it WAS a diode. The imaginary diode from B-C poinrs the same way as the E one. They both point away from or both towards teh base. PNP tests with black to base. Red to base is the reverse there.

The Q115 test is good. Remember there is that low 500 ohm path from B-E. That is what the meter is primarily seeing, so it shows the same both ways. Out of circ, the part would show .47 or whatever from B to both other terminals.

By the way, a good xstr tests about the same B-E and B-C. But a common failure mode is a short C-E. The two measures are then in parallel. So they can both look OK, exactly the same in fact, but the EC short is still there. Always check for EC shorts.

OK Q124,126. 124 looks shorted to me too. SOmething I do as policy: when I find a transistor shorted, when I remove it from the circuit, I either bend the legs over, or cut one off. That way I will not install a bad one by mistake.

Q126 looks OK. And that is normal. Two xstrs in parallel, if one shorts, then it conducts ALL the current, and the other one sees nothing. It is real common to see one bad xstr in a row of five in a large amp. That is for EC shorts only. A row of parallel power xstrs - if there is a BC short, it usually takes out the whole row.

Check R158 at the base leg of Q124. Think of it as checking the screen resistor after replacing a shorted EL34.

The good news is you can run the amp with only one of the two power xstrs, at least for test. The amp would only be able to prooduce half power, but without a load that is not an issue.

Q119-122 look OK. See there is a low resistabnce path B-E for those: Q122B, R156 4.7 ohm, R150 68 ohm, R170 0.22 (not 0.022) ohms. ABout 73 ohms. That explains the low voltage readings.

R150 = 67.5R
R170 = 0.4R
all ballast resistors appear to be about 0.4R on my meter and non-shorted
R156 = 5.1R
R154 = 5.1R

REmember, R170 and friends are dual resistors - center tapped if you like. From the center leg, 0.22 to each end. 0.44 across the whole thing.

Those ballasts won't short. They will be open or not, that's about it.

R152,150 should connect to the output bus I believe, otherwise they would have used one resistor. Is that the case?

Looks like the Q122 grouping is OK.

If it were me, I'd replace Q118, any burnt resistors, stick Q126 back in, and see what happens. NO LOAD. Monitor the channel out at the ballasts or the coils. SOmeplace convenient.

Since Q118 burnt, I'd check Q112, and compare it to Q111.

[bob p]

Q206, C202 - replace at your leisure.

well, replacing at my leisure means that i'll have to generate a list of parts that i need to buy, and order a batch of parts. to any extent that its possible, i'd like to troubleshoot the remaining parts in the amp before i place a parts order, since i'll need just about everything. i don't really have any SS parts on hand, so this amp will probably require multiple parts orders for repairs. if i can consolidate stuff on orders, that would help a lot.

REmember until we KNOW the channels are working, we are NOT using a load. So whatever the speaker relay does doesn't matter. We need to fix this part, but it won't save the channel circuits in the slightest, so powering the amp at this point is moot. Fix the channels.
...
No I am not saying that. We need to at least find all the burnt out parts and replace them. You did have a load that time, though, and that stresses the amp a whole lot more than without.

IIRC I had smoke with and without a load, but maybe that was with and without signal input. I can't remember for sure. would we be better off powering up this thing gingerly on a variac, instead of using full AC power? i'd like to avoid blowing up more stuff if i can.

thanks again for your help. i won't be able to check the things mentioned in your last message until later tonight.

[bob p]

any ideas where to find a genuine Toshiba 2SB755? Nobody seems to have them, and I'd rather buy one Toshiba to replace the bad one than buy two of something else.