Fault Tracing a Carver M-500t

[bob p]

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.

I was planning on a socket. They're cheap and I can get the socket and the chip locally at Ratshack. having the socket, if the IC blows up, easy to pop in another.

not sure what you mean by making 15V on the zeners. i suppose that you're talking about the potential difference across them when the amp is under power? my diode drop tests have been power off, of course.

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.

any tips for removing those 9-pin linear chips without burning them? the suction bulb method doesn't seem to get all of the solder off on either the first or second passes, there's no way to protect the chip with a clip-on heatsink, and unless you get all of those pins cleaned off, the chip just sits there with one pin holding the whole thing to the board, and you have to reheat it over and over.

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

yes

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.?

thanks. then that's the next step.

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.

not sure i understood what you meant about the changes to the zener rails. i suppose you're talking about votlages dropping below normal somewhere?

[bob p]

another question -- do you know where to get those big, square 5W dual-element ballast resistors like they use in the M-500t?

[Enzo]

Yes, I mean the zeners regulate the rails to about 15v when the amp is powered. And if U101 is internally screwy, it could load that voltage down. if the voltage sits there at about 15v, then the zeners are doing there job, and the IC is not trying to thwart them. If the zeners were open, then the voltage there could drift a lot higher than 15v and the IC would suffer. Nothing cosmic.

The inline ICs are to me the same as DIPs. I think of them as DIPs with only one side. I don't do anything different for them. I use a desoldering station, but if I were to do it by hand, I would use my desolder pump. I do not like the bulbs, I hat ehtem maybe less than I hate braid, but I don't like them. For manual solder exttraction I use one of those culinders where you cock it and press the button and the spring loaded plunger pops back and makes a suction pulse. They come in plastic or metal, and I like the metal ones. They can be had cheap.

I used to buy them in bulk at $5 each. You could spend a lot more on a name brand one, and buy replacement tips for them, but the replacement tips cost about as much as a whole cheap surplus sucker. So I just buy the surplus suckers to begin with.

The spring loaded thing can make a stronger vacuum pulse than a bulb.

When desoldering, if the hole does not clear, don't reheat the hole and try again to get the rest. Resolder the hole and try again with a fully soldered hole. The melting solder transfers heat to the rest of the solder better than a half filled hole can. And make sure to hold heat on it long enough for the hole to thoroughly melt all the way through.

But your board is single sided, yes? SO there should be no issues with plate through holes retaining solder.

Rock the IC back and forth a little as you desolder, to try to unstick the pins from the edge of the hole. If they stic, I sometimes get my needle nose in there, grasp the end of the pin, and wiggle it around to crack free of the edge solder.

But a desoldering station is really the way to go. The iron tip is a hollow tube you put over the pin or end of componenet lead. When teh solder melts, I move the tip in a small circle to move the pin away from all the sides of the hole and then hit the suction.

SOmeplace like MCM will have dual resistors. 0.22 ohm 5w duals would be like MCM # 28-0540. They don't look like yours but would work.
www.mcminone.com

But unless I was ordering already, I would just take two single 0.22 5w cement resistors, set them side by side, and at one end, bend one lead over to the other and wrap it around and solder right next to the body. Now that common wire goes in the center hole, and the two loose ends go into either end hole. Voila.

I am sure other places that sell parts for consumer electronics repair would have them too. Places like MAT electronics or Electronix come to mind. I don't buy from them, but they send me catalogs all the time.

[bob p]

The inline ICs are to me the same as DIPs. I think of them as DIPs with only one side. I don't do anything different for them. I use a desoldering station, but if I were to do it by hand, I would use my desolder pump. I do not like the bulbs, I hat ehtem maybe less than I hate braid, but I don't like them.

well, all that i have is a couple of non-adjustable soldering pencils and a couple of bulbs. that's all i've ever needed for "macroscopic" projects with PTP wiring of vacuum tube circuits. i'm just starting to do "microscopic" SS work and i don't have a temp controlled soldering station, a desoldering station, rework station, etc. just a cheap soldering pencil and a bulb, as that's gotten me along ok with tube stuff for a loooooong time.

For manual solder exttraction I use one of those culinders where you cock it and press the button and the spring loaded plunger pops back and makes a suction pulse. They come in plastic or metal, and I like the metal ones. They can be had cheap.

i think i've seen these things before, but i'll have to look to try to find them.

When desoldering, if the hole does not clear, don't reheat the hole and try again to get the rest. Resolder the hole and try again with a fully soldered hole. The melting solder transfers heat to the rest of the solder better than a half filled hole can. And make sure to hold heat on it long enough for the hole to thoroughly melt all the way through.

thanks for the tip. hadn't thought aobut that.

But a desoldering station is really the way to go. The iron tip is a hollow tube you put over the pin or end of componenet lead. When teh solder melts, I move the tip in a small circle to move the pin away from all the sides of the hole and then hit the suction.

what exactly is a desoldering station, and what does it do to make it worth several hundred dollars to buy one?

SOmeplace like MCM will have dual resistors. 0.22 ohm 5w duals would be like MCM # 28-0540. They don't look like yours but would work.
www.mcminone.com

those resistors at MCM look like an inline package:



the resistors that i'm looking for are rectangle shaped (close to square) and they have the 3 leads for the 2 resistors mounted at the corners of an isosceles triangle. not that i couldn't get those to work, but since i have to order anyway...

But unless I was ordering already, I would just take two single 0.22 5w cement resistors, set them side by side, and at one end, bend one lead over to the other and wrap it around and solder right next to the body. Now that common wire goes in the center hole, and the two loose ends go into either end hole. Voila.

since i have to order a bunch of stuff anyway, i might as well find the right part and place an order with whoever has it. any other ideas on that? i guess if all else fails i could just roll my own with two individual cement resistors, though i'd prefer a cleaner look if i can get it.

[bob p]

now this is odd -- as i was removing IC101, Q207 and its emitter resistor, I noticed that there are some "extra" resistors soldered to the green side of the PCB. somebody has added 4.7R 1/4W resistors in parallel to each of R174, R175, R176, R177, which are 10R each. those are on the collectors of the Q115-Q118 drivers.

???

it almost looks like they were trying to increase the rail voltage on the drivers, instead of addressing a sagging rail problem.

[Enzo]

a cheap soldering pencil and a bulb, as that's gotten me along ok with tube stuff for a loooooong time.

Sure. We use what we have. You are finding that it is not quite as effective clearing small holes on a pc board though. And this is a one sided board. That means the solder is on the sirface. Two sided boards have plate through holes so the solder flows from one side through to the other. This is even more dificult to clear easily.

Some guys like bulbs, and more power to them. A LOT of guys like braid, though I can't see why, but apparently it works well for them, and you might try a roll of it. If you hate it you are only out a couple of bucks. I like the pumps because they apply more powerful suction to the melted joint.

MCM 21-8235 is one like I use. Six bucks. CEDist sells a similar one with the Weller name on it for like $20 - I have found the cheapo ones work just fine. I have found them at surplus for under $3.


Or $4.95 at Electronix stock SD1044


For chassis work like old Fenders, I often get out my old gun. I have used it for over 50 years now. It sits next to me on the tool cart.



Who says low tech doesn't work?

Just wait until you encounter the "joy" of surface mount crap... I mean components.

what exactly is a desoldering station, and what does it do to make it worth several hundred dollars to buy one?

A desoldering station is a desoldering iron with a base unit, like a soldering station versus a soldering iron. In the case of desoldering, though, there is usually a vacuum pump of some wort, and that base unit houses that. And a desoldering iron, is what I mentioned, a soldering iron with a hollow tip, and a suction pump connectd to it through a hose. Apply the iron to a soldered joint, and when it melts, hit the vacuum and it sucks the liquid solder out of the joint like water through a straw. SLick. Would I buy one for two ICs a year? No. And when I did field service, I did not carry it with me, I used the hand pump. Pulling an 8 leg IC or a three leg transistor off a board by hand is no big deal.

But when you sit at a bench working on pc boards all day, the economy of it starts to pay off. It is so much faster and easier than by hand. Especially when I have to change out a 64 leg IC. Two rows of 32. That is a lot more than 9.

And when you place the bulb on the joint, you have to hold it over the tip of your iron, so the iron is in the way of a good air seal around the bulb tip and the work. The iron is like a foot in your door. The desolder iron IS the bulb. The tip itself covers the work, and the IC leg or the end of a resistor lead, or whatever is sticking through the pc board can fit right up the hole in the iron tip. The solder sucks up around it.

desolder iron tips:


You would consider buying one when the time saved and convenience make it worth while. My desolder station sits uner my solder station on the corner of the bench. They are both on while I work, so when something needs unsoldering, I reach for that iron handpiece, and when soldering I reach for the other handpiece. Desoldering a resistor or diode from a board is as quick as vwoop vwoop. (Sound effect of vacuum pump.) It leaves a neat clean hole, the unsoldered part usually falls right out of the hole, and the hole is left clean and ready for a new part.

I know it is your money I am spending. Unless you were extremely serious, I don't think the average hobbyist would need one. But in a commercial bench, I will never be without it by choice. I use a very basic Pace unit, and I like it well enough. I have not used other brands. Apparently the Hakko is popular. I found my Pace worth the $600. But there are less expensive models. And some are not so fancy.

MCM house brand for $289 72-6340


Hakko hand held self contained MCM 94-410 $219


ANd the lowest of tech, MCM house brand desolder iron $16.49. Stock 21-8240. it is a hand held iron combined with my hand pump.


Such tools may not make sense in your shop, I don't know what sort of volume you do in the sort of things what would need this, but pc boards won't be going away soon. On the other hand, surface mount is becoming more and more common, and I just am on the edge of needing it enough to invest in some new gear. But not quite. A few hundred bucks for a basic hot air rework, and I am not quite there. Likewise you may not quite be there for a desolderer.

I'll stop selling it now.

Yep, I know those MCM dual resistors are not laid out the same, you'd have to bend the legs into a tripod, but electrically they are the same, and the inline ones like that are a lot more common than the flat ones like yours these days. I don't know who would have the older type.

I don't know what the parallel resistors were for. if they look like a serious mod as opposed to the last tech dicking around, leave them. I don't think it was for voltage so much, those resistors maily limit current I believe. The lower resistance would make larger currents available to the drivers. ALthough maybe that is the same thing, lower resistance means less sag under higher current draw.

[bob p]

I don't know what the parallel resistors were for. if they look like a serious mod as opposed to the last tech dicking around, leave them. I don't think it was for voltage so much, those resistors maily limit current I believe. The lower resistance would make larger currents available to the drivers. ALthough maybe that is the same thing, lower resistance means less sag under higher current draw.

too late to save them, they're long gone. i don't think that they were an official Carver mod, i think that they were someone else's doing. the amp had the standard 10R resistors mounted on long leads standing off of the board for cooling, and it looked like somebody just tacked 4.7R resistors across the joints on the solder side of the board as an afterthought.

when i took them apart, i expected that the 4.7R were just tacked on the underside and would be easy to remove. it turned out that somebody had actually removed the original 10R resistors and replaced them with new 10R resistors on top of the board, and twisted the leads around the leads of those 4.7R resistors on the underside of the board. then the twist was covered with a solder joint. i would think that an OEM mod would have replaced the 10R with a lower value rather than strapping something across it. this definitely looked like a hack fix when I was trying to get everything apart, and AFAIK there's no TSB for this mod. there were also a couple of places where i wasn't confident that the revised solder joints only touched the desired components, so i just pulled everything to return to stock.

I pulled all of the 4.7R resistors out and got rid of the bent-up 10R resistors and replaced them with new 10R resistors so the amp is back to stock like the other one. perhaps the lower Z does allow more current for the drivers. i'll keep that in mind for later upgrades/improvements, but for the time being i think its a good idea to take the amp all of the way back to stock to get it up and running. i can always reassess the "mods" situation once the amp works properly. i just want to get rid of all of the things that look like amateur band-aid fixes.

[Enzo]

Is it just my browser, or is this place really SLOOOOW tonoght? Posts are taking forever to load.

If they were not on your other M500t, we would suspect the original design works. SO stock is good on the 10 ohms.

While you are at it, if that U101 is waiting, ANY dual op amp you have will work there - 4558, 4560, 4565, 4580, 5532, 2068, TL082, what else yu got? 5532 and 2068 are low noisers, not that TL072 is noisy.. The 072 is a general purpose type. And if you plan to socket it anyway, the sub would work well enough for you to continue on the rest of it.

[bob p]

Yep, I know those MCM dual resistors are not laid out the same, you'd have to bend the legs into a tripod, but electrically they are the same, and the inline ones like that are a lot more common than the flat ones like yours these days. I don't know who would have the older type.

thanks. i pulled off those extra 0.47R resistors that were sitting atop the ballasts. the ballasts measure a small resistance of less than an ohm across each element with my DMM, so it looks to me like the original ballasts are okay. i have no idea why in the heck somebody added .47R 2W resistors in parallel with the .22R 5W ballasts. like I said before, this amp has a lot of strange things going on.

i finally took that relay apart. on the back side of the coil the plastic cover was so hot that it had discolored from opaque white to orange, and there was a radial fracture in the case. it had obviously become very hot during that episode that caused Q207 and its emitter resistor to fail. (we're thinking that's related to failure in IC601.)

i took the relay apart, and measured across the leads, and i was getting incosistent resistance readings from the OT emitters to the speaker terminals, anywhere from low Z to 500k depending upon the relay armature position. using all of the magnficiation i could muster, i thought i saw some charring on the contacts. i used an emory board to clean things up (or to ruin the contacts, depending upon how you look at it), and now the Z across the contacts has dropped by 500k to less than a half-ohm. bad relay.



so here's where we stand:

1. relay bad, could stand to be replaced. contacts roughed up with an emory board as a band-aid fix.

2. TL072 input op amp removed. plan to replace with another TL072 in a socket. i tested the Z across those 2W dropping resistors to the opamp, and it took forever to get up to their 6k8 reading. obviously charging a cap somewhere, but the final results were within tolerance.

3. IC601 for the meters removed.

4. Q207 for the relay and its emitter resistor removed.




we're going to be at a pause for a while, as i need to order parts. just as you'd expect Murphy's Law to prevail, i'm missing a few key pieces and i'll have to order them.

I'm thinking about replacing some additional components in the areas of PCB discoloration, just to be on the safe side. like C103, C104, C105, C111 and R179 that are located close to R182 and R183 and may have suffered heat damage. anything else in that zone on the board that you would worry about, and order just to be on the safe side? zeners?

when I was removing IC601, the pin 1 solder joint with R615 was cracked. heat i guess. i'm wondering if there are any other parts in the area that would be partricularly vulnerable to heat damage, like C601 or C602. R601, R602, R612 and R613 obviously took a lot of heat, but being metal oxides, they should be able to take it. I have also thought about replacing C605, C606 and ZD601, ZD602 just to ward off the evil spirits. There is a lot of PCB discoloration around IC601 and its better to be safe than sorry.

if there are any other parts that you think would be suspects, needing possible replacement because of their location on the board or in the circuit, please do let me know.

thanks.

[bob p]

Is it just my browser, or is this place really SLOOOOW tonoght? Posts are taking forever to load.

If they were not on your other M500t, we would suspect the original design works. SO stock is good on the 10 ohms.

While you are at it, if that U101 is waiting, ANY dual op amp you have will work there - 4558, 4560, 4565, 4580, 5532, 2068, TL082, what else yu got? 5532 and 2068 are low noisers, not that TL072 is noisy.. The 072 is a general purpose type. And if you plan to socket it anyway, the sub would work well enough for you to continue on the rest of it.

REALLY SLOW tonight.

parts on hand? unfortuantely no. i haven't done much SS work, so I'm starting off with no appreciable parts on-hand. i have to order just about evertyhing when it comes to opamps, transistors, low voltage radial caps, etc.

regarding opamps, is there a lower-noise replacement for the 072 that's pin compatible? if i've got to order something, it might be worth ordering something that's better than plain vanilla.

[bob p]

back to the subject of the desoldering gear -- thanks so much for your detailed explanation. my volume for this sort of stuff is so low right now that i can't justify a reworking station that i'll only use ocasionally. SMD? forget about it! but i really do like the idea of your suction-pen replacement for the bulb. i'll give that a try for sure!

[bob p]

on the subject of ordering a replacement relay: the carver relay doesn't have a particularly helpul part number on it, it just says its a 24V audio relay. It looks like a DPST.

are all of these sorts of relays designed so that they have a similar footprint and they'll be interchangeable? I'm just trying to get familiar with how i need to spec a replacement. fwiw, here's a photo from an existing thread, though admittedly its not a great one:

[Enzo]

Your overall failure mode is still a mystery. What if someone plugged the amp into 240VAC somehow? Oh, we may never know.

Something like the relay burning up just shouldn't happen. It is designed to be on continuously, so even if Q207 shorts, it would be the same to the relay as if it were simply on. The only thing I can think of keeps coming back to somehow the voltage went way high on the 40v rails. R212 does serve to drop voltage so the 24v relay can run off the 40v rail, but resistors don't short as a rule, they burn open. And the common current through the relay would not burn the resistor normally, and a shorted Q207 would no more affect the resistor than it would the relay.

The relay did have bad contacts. Emery board is a good first step. Then a smoothree nail file or board. Then even smoother, fold a piece of emery cloth or wet/dry paper in half rough out and rub it up and down between the closed contacts. Press on the armature with your finger to apply a little pressure. Maybe 400 grit,and follow with 800 grit if you have it. I spent many years servicing pinball machines in the field, and prior to 1976 they were all relays inside. believe me I have a lot of experience servicing relay contacts. I have a burnisher I use on CLiff type jacks a lot, but I originally got it for relays.

Look up MCM 22-1235, the burnisher looks like a fine file, but it feels smooth. But it really is coated with fine diamond dust or other hard material so it is a super fine file, so it burnishes (polishes) contacts smooth.


Relays of that shape often come on a standard frmat, but these are a little different. I have the layout with trace patterns. These are DPST. I know you hate hacks, but in the absence of the real part and need a new one - assuming you cannot restore this one - one method would be to glue a new common type relay to the board upside down, and run wires down to the pads.

The TL072 is a common general purpose chip. ALL the dual op amps in 8-leg DIPs have the same pinout, they all will work in each others place. I routinely swap 4558/4560/TL072 and others. The TL072 is not a low noise part, but it is also not noisy. I don't think Carver went cheap to save 10 cents on in IC. 5532 is a low noise IC, 2068 is a low noiser too. Oh I think it is NE5532 and NJM2068. The 5532 draw a few more milliamps than the TL072, but since ther is just one chip, it won't matter. Where it does matter is where someone gets the bright idea to low noise his PA mixer so he can "record" with it and decides to change out 50 or 100 chips. NOW all those extra ma-s add up to a ton more current and the power supply can't handle it. No such trouble with one chip here.

Op amps are a commodity item, and they are the same in all the stuff we service. 4558 was real common through the 70s 80s. Now we see 4560, 4565, 4580 - just later chips in the 4558 series. 2068 and TL072 are also stil common. And TL074 is pretty much stil the universal quad op amp - 14 pin IC. It is the same as two TL072s in one chip.

I have pretty much discussed what I think might be bad so far. On the other hand, I don't have to order every part I need, I am used to having a pretty extensive parts inventory to draw from. I am not so concerned for nearby parts, but if in your view they look scorched, go ahead and replace them. Little caps and resistors cost little, and in the case where one really was bad, you don't have to go through another order cycle.

Many time I encounter charcoal situation, but often as not the damage is more superficial than anything. Nearby components are covered in soot. They may not have really been hot, they just got a lot of smoke deposited on them. A Qtip ful of alcohol wipes most of it off. I worry about caps more than resistors. Most resistors don't care how hot their neighbors get unless flames come out.

I think if I were gong to start a parts kit for general SS repair, I'd stock a few op amps - pick one type - and certainly 15v 1w zeners are common enough. The rectifiers you might use in tube amps will be good enough - 1N4007, a few 1N4148 for the little signal diode spots, higher curent diodes I'd just wait until I needed them, there are many kinds. It is likely premature to duscuss transistor selections and such.

When I first set up this shop years ago, I already had the Pace desolder station, but I had a partner who had none, so we found a used one online. That is an alternative to consider.

A quick tour of Mouser did not reveal a relay with your foot[print.

[bob p]

I got some parts in the mail today, so I replaced the following components:

removed IC101 (a TL072) and replaced it with a socket and a new TL072
replaced Q207 and its 270-ohm 1/2 watt emitter resistor
replaced R179, C103, C104 due to visible external heat damage

I powered on the amp and the relay is working. After 3 seconds the impedance between the ballasts and the speaker terminals drops to zero. and stays at zero. so far so good.

i tried injecting a signal into the amp, while monitoring the output of the signal generator on one of my scope channels. the signal disappears as soon as it is connected to the amp's inputs. it looks like the test signal is getting shorted to ground somewhere inside of the amp. The protection circuit doesn't activate.

I checked the voltages on IC101 and compared them to the reference voltages on the schematic. then I pulled the IC and measured voltages on the socket:

Pin#   Reference  In-circuit   Removed
1       0           -56mV        6.3V
2       0             0          26.9v
3       0             0           0.3mV
4       -14.7v      -14.6v      -14.8v
5       0             0           0.4mV
6       0            -2.0 mV     26.2v
7       0           -61.5mV       1.29v
8      +14.7v        14.78v      14.98v

I haven't had a chance to lift any components to see why the input signal is disappearing. Its late, and I have to put this off until tomorrow.

Do you find any of the voltage measurements on the IC to be helpful? I haven't yet determined why there are negative potentials on IC pins 1, 6 and 7, or why the voltages are what they are on the empty socket. I'll have to sleep on it.

[Enzo]

First off, as a rule you cannot measure signal at the input pins of an op amp. Something about them being at "virtual ground" which you might look up. I thhink the National Semiconductor web site has tutorials on op amps - among many other topics. If you are at all interested, you can go through their Analog U. AS in university - training materials for analog electronics. And anything written by Bob Pease there is great stuff. He's my hero.

If you pull the IC from the socket, you should be able to see signal at the pins to verify it gets there. The main way I test op amps it to look at the output pins. if signal is there, it is a safe bet it was at the input pins too.

With the IC removed, there is nothing to keep the amp circuit centered on zero. The output bus feeds right back to the input through R113 to pin 2 of the IC. Without the IC, that output feedback is more or less connected to the drive throughR192, R111. And the whole thing will drift to wherever it feels comfortable.

In circuit, the 072 has readings within millivolts of zero, and it looks OK to me. Rails are fine. Is there signal at pins 1,7? There is no ground connection in the chip, so even if C101 shorted, there would still be a minimum of 1k across the input due to R103. A few millivolts will have one polarity or the other, but it is millivolts, and not many of them.

Without the chip the amp will drift to whatever, and there will be no op amp to correct for it.

Since it is socketed, remove the IC and make sure signal gets there.


And measure resistance to ground across the input sockets just to verify they are not somehow shorted.