Fault Tracing a Carver M-500t

[bob p]

Making some progress. I was getting inconsistent readings to ground on one of the input jacks. Sometimes one channel looked shorted, and sometimes it didn't. (IIRC it was the right but I'm not sure).

I had already ordered-in all of the small electrolytics needed to replace the 20+ year old 'lytics on the amp board as part of routine PM, so I swapped them all out. I also replaced some metallized film caps that were in the burned spots on the PCB. The short to ground problem on the inputs went away, so it was likely a bad cap somewhere. As we had discussed before, there were a lot of areas on the board that had visible heat damage. With the new lytic caps, a lot less bias current is required to meet the 6 mV spec, so some of them must have been leaky.

(As an aside, how easy is it to blow up an opamp? After doing the recapping, the signal generator mistakenly had maybe 5mV of +DC offset because of a knob that wasn't properly zeroed. When the amp turned on, the signal trace at the input looked OK for a short while, and then became very ugly and spikey -- kind of like a tangent function if you remember them from your geometry days. I pulled the IC and swapped in another.)

Now the Left channel produces a clean sine wave output at the speaker terminals into no load.

The right channel is causing problems though -- the amp will idle fine with no Right channel input, but the protection circuit keeps triggering when an AC test signal is fed to the R channel input and I scope the speaker terminals. The relay energizes after the 3 second turn-on delay, and on the scope I see a blip of what looks like a tall clipped waveform on the screen for a fleeting instant, then the protection relay triggers again, flattening out the scope trace. The relay continues to cycle in periodic fashion, and I intermittently see a fleeting blip of what sometimes look like either: a) a clipped AC waveform that's transposed upwards with a DC offset, or b) a tall square wave with rounded corners that looks like its all DC. After the waveform appears on the outputs for only an instant, the relay cycles into protection mode. Then the cycle repeats over and over again.

Now here's what's interesting -- the odd behavior I've mentioned above only appears at the speaker terminals. At the center-taps of the R channel ballast resistors, the sine wave outputs look completely normal throughout the amp's power range. I'm at a loss to explain why the DC appears on the outputs but not on the ballast resistors. The only thing between those two points is ... the relay. But if the DC is coming from the relay and appearing at the speaker terminals, I would think it should appear on the ballast resistors as well. Maybe its coming from somewhere else.

[bob p]

Well, I was wrong; My DMM tells me that there's 1.25VDC on the R channel ballast resistors at all times (none on the Left), but the DC only seems to trip the protection relay when the scope leads are on the speaker outputs. The relay doesn't seem to be tripping if the scope leads are on the ballasts. I can't explain that. I'm looking at the schem now to try to determine where that low voltage can be coming from.

[Enzo]

5mv of offset shouldn't harm the op amp.


What does that waveform do at the ballasts WHILE this happens, not before or after. Oh, and are you viewing the signal at the ballasts with the scope AC or DC coupled? We need DC for this. A DC offset won't appear on an AC coupled display.

When this happens, when the speaker relay goes OFF, is there any DC remaining on the speaker terminals? Thinking some odd trouble in U601. But with 82k in series, that seems unlikely.

How abuot this? You interrupt the speaker line while any current is flowing - either signal waveform (at any point other than zero crosssing) or a DC offset - and the inductance of the speaker voice coil will provide an inductive kick. Ever unplug a working speaker and seen the spark at the jack? MAybe this is what the scope sees on the outputs. Back on the amp side of the relay, all this would be invisible, since it was the disconnection FROM the amp that triggered the symptom on the speaker side.

Then the relay triggering would be a separate problem.

Is R202 OK? It samples the R channel out into Q202, part of the shut down circuit. SInce the L channel doesn't do this, we can assume Q202 itself and associated parts are OK.

SInce the R channel seeems to work internally, find out what is triggering the protect relay. WE know that the outputs are sampled through R201, 202 and fed to Q202, and also inverted through Q204 to feed Q203. That way the signal can go pos or neg and still trigger one or the other of Q202,203. SInce those two are in parallel, either one can trigger Q205 and shut it down.

Q206 can also shut things down via Q201 up there, and it in turn is driven by limiters in each channel - R185/D105 and R184/D106.

So by monitoring the collector of Q202 and the base of Q206, we might find which path is shutting the relay down, and from that have a clue.

[Enzo]

Ah, simulposts.

DC is enough to cause relay trouble.

Scope leads? Maybe the output doesn't like being grounded, which the scope probe ground lead would do?

Well, it is all DC coupled in the amp, and servoed back into itself, so offset anywhere shows up through the whole thing typically. I'd go through and look at the DC levels through the amp, like the test points shown on the L channel. Keep in mind they balance, so where it is supposed to be +1.4 and -1.4, if you find +2 and -0.8, at least we know the parts are still maintaining their separation properly and the whole thing is being pushed to the side.

And another point, sometimes if a voltage is too positive - like +1.3 instead of zero - it COULD be from something on the + side turned on too far, but it ALSO might be something on the neg side NOT turning on enough, or not conducting well enough. Say a weak xstr or a resistor that upped in value.

[bob p]

What does that waveform do at the ballasts WHILE this happens, not before or after. Oh, and are you viewing the signal at the ballasts with the scope AC or DC coupled? We need DC for this. A DC offset won't appear on an AC coupled display.

Its hard to say -- the problem where the relay cycles repeatedly in and out of protect mode seems to be related to dual-channel hookups to the scope. If I'm simultaneously comparing the output of the ballasts to the output at the speaker terminals, then the amp goes into the protect/reset/protect loop. OTOH, if I disconnect the scope probe that's monitoring the speaker terminals, then the repeated cycling of the relay doesn't seem to happen.

I never noticed this sort of behavior with my other M-500t, but then I don't think that I ever tried to simultaneously scope the signal at the ballasts with one channel of the scope while scoping the signal at the outputs with the other channel. Perhaps I had the polarity reversed between the probes, and the protection circuit didn't like the way things were grounded.

When this happens, when the speaker relay goes OFF, is there any DC remaining on the speaker terminals? Thinking some odd trouble in U601. But with 82k in series, that seems unlikely.

U601 is absent from the amp board. We removed it to simplify troubleshooting of the amp circuit. The original IC is still sitting in my parts box, and I haven't ordered a replacement yet. (Nobody seems to have it except BD, and I'm waiting until I know more to place and order with them.)

Then the relay triggering would be a separate problem.

Is R202 OK? It samples the R channel out into Q202, part of the shut down circuit. SInce the L channel doesn't do this, we can assume Q202 itself and associated parts are OK.

Now its gotten more interesting. I saw the L channel manifest the slow chatter of the relay as the amp entered the "protect/unprotect" loop. This happened when I tried to simultaneously scope the ballasts and the speaker terminals on the LEFT channel. I wonder if this could be a grounding/polarity issue between the scope and the protection circuit. The problem goes away immediately if I disconnect one of the scope channels from the amp. I also noticed -0.9VDC of offset on the L side ballasts, and 0 VDC on the right. The problem seems to be intermittent and/or migratory. :-/

Now here's the good news: All of these observations were made with NO LOAD other than the scope itself. It turns out that if I scope the output at at the ballasts, and use only one scope channel, I can hook up a DVM and an 8R dummy load to the outputs, and both channels will put out 43+ VAC RMS into the 8R load. That's 231 WRMS, which is pretty close to the amp's rated output of 250 Watts/channel.

It looks like the problem might be in the protection circuit, rather than in the audio section of the amp itself.

[bob p]

DC is enough to cause relay trouble.

Scope leads? Maybe the output doesn't like being grounded, which the scope probe ground lead would do?

a very reasonable hypothesis.

Well, it is all DC coupled in the amp, and servoed back into itself, so offset anywhere shows up through the whole thing typically. I'd go through and look at the DC levels through the amp, like the test points shown on the L channel. Keep in mind they balance, so where it is supposed to be +1.4 and -1.4, if you find +2 and -0.8, at least we know the parts are still maintaining their separation properly and the whole thing is being pushed to the side.

yes, the fact that the amp is entirely DC-coupled makes it harder to isolate the problem. with a tube amp that has coupling caps to isolate every stage, locating the stage that has the problem becomes much easier.

back to the subject of the relay, the Q207 emitter resistor R212 is a 270-ohm 1/2 watt type. i replaced the original resistor because it had become so hot that it fractured from the heat, and burned the board at both ends of its leads. the bad news is that the new Q207 emitter resistor is getting really HOT. hot enough that you can't hold your fingers on it. there's got to be WAY too much current going through that resistor. remember that I mentioned that the cover for the relay had become discolored from opaque white to brown in the area over the coil. it was obvious that way too much current was passing through the relay. I've changed both Q207 and its emitter, but I think I've still got excess current flow to contend with. I'm not sure why, as U601 has been removed.

[Enzo]

You beat me to it, I was going to suggest the same measurement technique - both scope channel - on the good channel to see how it reacts. You didn't mention before, I do not think, that you were using two scope channels. Little extra clues keep coming in.

Is your scope grounded to your signal generator? If not, you could be driving a ground loop through this amp. Get the test gear on the same ground connection. My scope has a three wire mains plug so it is earthed that way, but my signal jenny has to have a jumper in place for that to be the case. Ground them together from the front, don't rely on the mains plugs. MAke any difference?

ANd how exactly are you connecting the probes? Where is the ground clip attached for each?

I see the input stage grounds leave the board and go to chassis at point 14, just below the input jacks. The ground for the powr amp and power supply, including speaker outs, collects and leaves the board to chassis via point 3 - upper left. DO both these connections have integrity?

If I had to guess at the cycling, with all your clues, I'd be thinking that somehow the scope connection is grounding the output. The amp will not like that.

U601 - I forgot it was out. Well, I said it seemed unlikely.

U601 has zero to do with Q207 and its resistor. The only thing they have in common is that both circuits are powered off of +40. 601 of course also uses -40, but that doesn't matter. None of its current runs through either R212 or Q207.

So what is the voltage across R212? Offhand, since the rail is +40, and the relay wants about 24v, that leaves 16v to drop. Lose maybe a volt in the xstr and that leaves 15, but let's use the whole 16v. 16v across 270 ohms is 59ma. 16v @ 59ma = 0.95 watt. I'd want at least a 2W resistor there if that is the case. 59ma seems OK for Q207 though. I seem to recall your +40 was pretty close to 40. Are my theoretical figures at all close to your amp? I forget, is this the original relay? What is the resistance of that coil? Does the current seem more or less consistent with that resistance assuming about 24v? WHat indeed IS the voltage across the relay when on? Or what is at Q2307 collector and subtract that from the +40? Betwen relay coil resustance and V drop across it, and the resistor R212 having to sop up the rest, Ohm's Law should tell us what resistor wattage we need.

It would be darn rare, but conceivably there could be some shorted turns in the relay coil. And you already replaced D203, so it would not be some odd semi-leaky sort of thing. You got 24VDC in your shop, you could extract the relay and connect it straight to 24 and check the current draw. And you could see if leaving it on that way made it hot. A continuous duty relay left on will get warm, but should not get remotely melty or burnt.

You want to verify current? Those two thermal breakers at the top of the drawing are in series with the +40 and serve only the relay and U601. 601 is gone, so disconnect one of those thermals and connect an ammeter in its place. Fire up the amp and when the relay clicks on, voila - relay current reading. Does 59ma seem unreasonable for that relay?

[bob p]

Is your scope grounded to your signal generator? If not, you could be driving a ground loop through this amp. Get the test gear on the same ground connection. My scope has a three wire mains plug so it is earthed that way, but my signal jenny has to have a jumper in place for that to be the case. Ground them together from the front, don't rely on the mains plugs. MAke any difference?

I re-did some wiring to assure that the signal generator and the scope are on the same duplex outlet. Both have 3-prong cords. I don't have a jumper wire in place yet. Although the scope has a front panel grounding post, the signal generator doesn't offer one on the front or the back panel. The only option would be to run a jumper wire off of the ground side of the banana jack on the signal generator's output and take it to the scope front panel. I haven't done that yet. I did try floating the amp on an isolation transformer, which I had not done before. Didn't help much.

ANd how exactly are you connecting the probes? Where is the ground clip attached for each?

the ground clip for the probe on the ballast resistors is hooked up to the heatsink. the ground clip for the speakers is connected to the speaker terminal.

I see the input stage grounds leave the board and go to chassis at point 14, just below the input jacks. The ground for the powr amp and power supply, including speaker outs, collects and leaves the board to chassis via point 3 - upper left. DO both these connections have integrity?

i'm not sure -- i see those points on the schematic, but i haven't found them on the layout diagram yet.

So what is the voltage across R212? Offhand, since the rail is +40, and the relay wants about 24v, that leaves 16v to drop. Lose maybe a volt in the xstr and that leaves 15, but let's use the whole 16v. 16v across 270 ohms is 59ma. 16v @ 59ma = 0.95 watt. I'd want at least a 2W resistor there if that is the case. 59ma seems OK for Q207 though.

i still have the original relay in the amp. the relay coil energizes at about 10-12 VDC, and it creeps slowly up to 24 VDC. actually, the DC voltage creeps well beyond that. watching it for a few minutes, it climbs slowly, about 0.01V at a time, to reach 26 VDC over a few minutes. It keeps on climbing, and R212 keeps getting hotter and hotter. There is no doubt in my mind that if I let the amp continue to run, the DC voltage would just keep on climbing and that R212 would fail. this looks like the failure mode that we had going on before that caused R212 to fracture and burn the PCB. Too much voltage and current going through R212.

I measured the votlage across R212 at 10.25 VDC. At the spec'd value of 270-ohms, that's 38mA, and about 390mW. Way too much power for a 1/2 watt resistor.

The funny thing is that my other M-500t doesn't do this. although I haven't pulled it out of service to compare measurements, R212 NEVER got hot enough to cook and discolor the PCB, so there's a real problem here. Perhaps I should double check the value of the +40 VDC rail. Why would the voltage across the relayt keep climbing upward??? For this to happen the rail voltage has to be changing, or the relay resistance has to be dropping, or the Z of R212 has to be continually falling as it gets hotter and hotter. Interestingly, when I grab onto R212 to feel the heat, the voltage across the relay terminals immediately drops slightly. When I let go, it climbs again. Presumably this is heat related.

[bob p]

I seem to recall your +40 was pretty close to 40. Are my theoretical figures at all close to your amp? I forget, is this the original relay? What is the resistance of that coil? Does the current seem more or less consistent with that resistance assuming about 24v? WHat indeed IS the voltage across the relay when on? Or what is at Q2307 collector and subtract that from the +40? Betwen relay coil resustance and V drop across it, and the resistor R212 having to sop up the rest, Ohm's Law should tell us what resistor wattage we need.

DC-R on the relay coil is 575-ohms a few minutes after shutting it off.
at 26 VDC, that calculates to 45mA of current, more than the 38 mA calculated for R212.
Power dissipation in the relay: 0.045A * 26V = 1.17W.

It would be darn rare, but conceivably there could be some shorted turns in the relay coil. And you already replaced D203, so it would not be some odd semi-leaky sort of thing. You got 24VDC in your shop, you could extract the relay and connect it straight to 24 and check the current draw. And you could see if leaving it on that way made it hot. A continuous duty relay left on will get warm, but should not get remotely melty or burnt.

sorry, i don't have a 24 VDC supply. I'm only just starting to tinker with SS stuff, and I've never had any sort of need for that sort of thing as a tube guy.

at any rate, i think that at least one problem is definitely in this section of the amp. if nothing else, we're on the trail of why the old R212 broke and why the PCB is burned there. looking at the big picture, we still need to explain why the board is all burned around U601, and there's got to be a common tie holding all of this together. in regard to the burned area around the inputs, i imagine that one of the bad caps around the input/opamp was responsible for the votlage rail short and the resistors overheating. so i think the big question now is what's going on at the relay and how this ties into U601. i don't think U601 is a coincidental occurrence. its probably related.

You want to verify current? Those two thermal breakers at the top of the drawing are in series with the +40 and serve only the relay and U601. 601 is gone, so disconnect one of those thermals and connect an ammeter in its place. Fire up the amp and when the relay clicks on, voila - relay current reading. Does 59ma seem unreasonable for that relay?

you won't believe this either -- my shadetree technician's shop has no ammeter.

[Enzo]

OK. The signal jenny have a switch in the front mentioning ground? That is, is the thing set up so it can drive into a balanced line - 600 ohms being a common one. In fact a lot of them have a switch to connect an internal 600 ohm termination. Can't do that with one side grounded. If there is continuity between the ground side of the output and the earth ground, that ought to be good enough I guess.

I guess I don't know why the heatsink would be any less good than the chassis itself. How about if you connect one probe to the output terminals, and that ground clip to the black one should be sufficient for the scope. Then just connect the second probe only to the ballasts - no ground clip. DOn't know why it would matter, but something obviously matters.

Wire 14 - I don't know which way the amp sits for you. So with the input jacks right in front of you, move straight back a little to the IC, then angleoback 45 degrees to the left, and W14 should be just past C104 and just left of R183.

Wire 3 (or 4) - don't angle left at the IC, keep straight back. There should be a group of wires just before the "missing" relay. 3 adn 4 are ground.

AS far as I can tell, the two wires jump over to the corners, front left and right, where presumably screws will grounf them to the chassis.

To test integrity of the connectin I think I would measyre resustance between the ground leg of the input jacks and the ground speaker terminal. Youu get about zero ohms, then the grounds are intact.

I am probably wrong, but I get the feeling that grounds are somehow the issue with the scope symptom. And of course if you can scope the output by itself and into a load, perhaps all this scope mystery is a waste of time.

R212 - yep. comparing to the other unit might answer a lot of things. But left with only this amp, I still like pulling the relay and testing it on its own.

Watching the 40v is a great idea. Years ago we discussed the potential for some rail voltage problem like a leaky commutator xstr or something. SInce it is a simple rectifier supply, I don't think of any other way for that rail to climb. But if it stays at 40v, then so much for that notion.

You cuold still do that thermal breaker trick and see if the current stays more or less stable or if it ramps up. I can't see how Q207 would be involved, after all when it turns the relay on it acts like a switch anyway.

You laid out the possibilities pretty well, so:
See if the +40 rises
See if current rises or is stable
Hit R212 with freeze spray and see what happens to the voltage across the relay coil, and for that matter across R212.

If you need to keep freezit OFF some nearby parts - as in trying to determine which one is sensitive - yuo can make a little barrier with a piece of paper used as a shield. or roll it in a tube and set it over R212 - or whatever - and squirt down into the tube.

But sounds like the amplifier part is working now then?

[Enzo]

More simulposts at 5AM...sheesh.

Your meter doesn't have current function? OK, how would you handle it in a tube amp? You'd insert a 1 ohm or maybe a 10 ohm resistor and measure voltage drop across it. Don't see why that wouldn't work in place of an ammeter. Many years of field service made this boy resourceful, by god.

24VDC? could dial down the variac and rectify it, and slap a 470uf across that. Instant DC. Need to use an isolation before the variac of course. But maybe that is more work than it is worth.

And I think you are right, U601 burnt up for whatever reason eveything else did.

And I have the overall impression that something pretty nasty got connected to the input jacks at some point and ultimately is the cause of all of this.

[bob p]

thanks for pointing out the locations of those numbered wire points on the PCB. i wouldn't have found number 14, as i was looking in the wrong place. i'll check on those ground continuity questions tomorrow. its getting pretty late right now.

regarding the climbing rail -- i'll check that too. if the 40V rail is climbing, that would have to be a leaky commutator, wouldn't it?

is the amplifier part working? well, it looks like its working at 1 KHz into a dummy load. i haven't checked other frequencies, and I haven't been bold enough to hook it up to speakers.

Your meter doesn't have current function? OK, how would you handle it in a tube amp? You'd insert a 1 ohm or maybe a 10 ohm resistor and measure voltage drop across it. Don't see why that wouldn't work in place of an ammeter. Many years of field service made this boy resourceful, by god.

well, my meter does have a current function, i've just never used it. i don't have a clamp attachment. i guess that to use the meter, i'd have to pull one of those thermal switches and put the meter in its place so that the meter is actually in series in-circuit with 100% of the current passing through it. i've never done that. i've always measured drops on a resistor or a shunt, because as I recall the meter's current ratings are really low. essentially, shunt measurements is what I've been doing by measuring across R212, though the number 270R isn't very handy (like 10R would be) when it comes to calculations.

24VDC? could dial down the variac and rectify it, and slap a 470uf across that. Instant DC. Need to use an isolation before the variac of course. But maybe that is more work than it is worth.

believe it or not, kludge methods like that are my specialty. but right now the kitchen table is getting kinda crowded, so i've been trying to avoid the inconvenient tasks.

And I have the overall impression that something pretty nasty got connected to the input jacks at some point and ultimately is the cause of all of this.

seeing that both the inputs and outputs are effected, i'm thinking that there had to be a nearby lightning strike, and perhaps the interconnects and speaker cables picked up inductive currents. it doesn't look like the AC mains took the strike. its hard to imagine what other than lightning could have done something like this.

[Enzo]

If the 40v climbs, I would suspect that, but we would also want to check the other rails to see if they are climbong as well. REmember, this is not a straight transformer power supply - it is a magnetic resonance whatchamacallit. SO if the OT primary drive circuit is screwing up , that could increase voltages. These amps do not like having that triac shorted across. SO if +40 has its friends climbing, that's wher I'd look.

Ammeters. yes, insert the meter in series, that is how it is done. Your meter only have the 200ma scale or something? My fluke has 300ma and then 10 amps. Clamps are mainly for high currents. 50ma might be hard for one to detect.

[bob p]

If the 40v climbs, I would suspect that, but we would also want to check the other rails to see if they are climbong as well. REmember, this is not a straight transformer power supply - it is a magnetic resonance whatchamacallit. SO if the OT primary drive circuit is screwing up , that could increase voltages. These amps do not like having that triac shorted across. SO if +40 has its friends climbing, that's wher I'd look.

Protection Circuit

the ground->protection problems are gone now that I stopped trying to monitor the outputs in two places simultaneously on the scope. the protection circuit didn't like the scope grounding setup. (I didn't get around to trying using one probe ungrounded, though its an idea that i'll keep in mind for next time.)


Ammeters.

my DMMs: one meter is limited to 200mA. the other can do 10A, but only with a 30 second duty cycle every 15 minutes. in most cases its just easier to measure across a shunt so i don't have to worry about cooking something.


Rail Voltages

i looked at rail voltages and checked all of the test points on the PSU board. the voltage rails are all elevated by about 8%, due to yesteryear's 115 VAC equipment operating on today's 125VAC lines. none of the rail voltages seemed to climb.

i watched the 40 VDC rail with one DMM while I watched the voltage across the relay with another. the 40 VDC rail stayed steady at 43 VDC while the voltage across the relay gradually climbed and leveled off at 26V like it did before. so its not a rail problem. its a relay-resistor problem.


Relay / Resistor

i don't have freeze spray, but if i grab onto the hot resistor with my fingers to feel the temp, as my fingers absorb the heat I can see the voltage across the relay drop temporarily. when i let go of the resistor, i see the voltage start to climb again. presumably the Z of the resistor is dropping with thermal de-rating. i think that's a positive test result.

so the rail voltage is staying constant, and voltage across the relay is gradually climbing like it did before. i decided to get rid of that skimpy 1/2W resistor. as a temporary band-aid fix I replaced the 270R 1/2W resistor with a 220R-2W and a 47R-2W in series. the voltage gradually climbed and leveled off at 26.2 VDC and it stayed there for a couple of hours at idle. then i ran 25W of sine wave power through the amp for a couple of hours. no problems.

the new resistors were a little warm, but nowhere near uncomfortable to touch. dissipation is still under a watt, so a 270R-2W should be fine as a final replacement value. although the amp is working fine, i'm not sure whether I want to be satisfied with the way things are, or if I should replace the odd-footprint relay with a generic one. subbing a replacement wouldn't be too hard -- bolt a bracket to the chassis, bolt a relay onto the bracket, and re-scale the Q207 emitter resistor to keep the relay-resistor voltage divider balanced for 24 VDC.

i think that with the little 1/2 watt resistor, the total Z of the relay-R212 circuit was dropping as the components heated up, and current through the circuit was increasing. Although I haven't directly measured current, this seems to be what's going on. The funny thing is that in the other amp, this never happens. R212 never gets hot but in this one its REALLY hot -- so hot that a 1/2 W resistor just won't cut it. Its hard to blame the problem on a new 1/2W resistor, so i'm thinking the relay has to be the culprit. Its hard to explain why the circuit started going into thermal runaway. shorted relay coil? its hard to think of something else to blame it on. i'll have to pull that other amp down to take some measurements for comparison.

do you have any thoughts on this?

[bob p]

Testing

I ran the amp full tilt into a dummy load at 400 Hz, 1K, 10K and 30 KHz without any problems. right now the amp is playing music at moderate levels and sounds fine.


Meters

there's still the problem of the dead metering circuit. i've pulled the original U601 and I have no idea if its good or not. i'm not sure whether its worth spending any time on troubleshooting the original chip. if its bad, then trying to use it may just cause trouble and wasted time. the part isn't that expensive, so i'm more inclined to just buy a new one and start with it. its just too bad i can't think of anything else that i need to buy from B&D. it just bugs me to pay $7 to ship a $3 part that only weights a few grams.


Lamps

this amp is also going to get a lamp upgrade. i'm going to replace the 8V lamps with 6.3V bayonet mounted lamps, and use the 6.8R dropping resistor as we had discussed earlier. the LED circuit is a good idea, but its going to be hard to drill/tap the chassis to mount the TO-220 Vreg. there's just no room to get a drill or a tap in there without extensive disassembly, and the lamp switch would be very easy. so i think i'll take the easy way out for now.

thanks again.