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Messages - DJPhil

The Newcomer's Forum / Re: Overheating Little Gem
March 01, 2011, 03:33:18 PM
My first LM386 build went like this. I had a bad salvaged capacitor that'd failed short in the output filter, and it worked out just like Joe said and gave the chip a viking funeral. This taught me to always invest in chip sockets. :)

Definitely start with the speaker and speaker connections as per Joe's instructions so you're sure there isn't a short in there.

With the battery and speaker disconnected (to be safe) you should see a very high resistance or open circuit between pins 4 [gnd] and 5 [out] (either side of the arse end of the chip from the dot at pin 1). If you don't see at least 1MΩ then it may be toast. If you're measuring with a meter that has a multi-megaohm range you might get some odd readings, especially if the chip's in circuit or you're wearing a static strap. That's normal, as high impedance measurements are difficult to do well. If the chip failed as a short (hard to tell without removing it) it might hamper your ability to find the short elsewhere in the circuit.

The big output cap (220µF) and the low pass filter cap (47nF) block the output from having a DC path to ground. If either of those caps goes short, or if there's any way for the output to just dump current to ground, it'll be likely to toast the output stage of the chip.

On these chips too much gain won't hurt anything, it just sounds horrible. You can stick any resistor you like between pins 1 and 8 to change the gain, and for super duper gain you can stick a 10μF cap in there, but that's only really useful for ultra low signals (RF mixers and magnetic tape heads, etc.). I've never seen an LM386 with open gain pins, so if it were me I'd try 10k in there just to have something to make me feel better. The volume pot's also optional, as it just provides an adjustable parallel load to go with the speaker, so it's absence won't really contribute to your problem.

Hope that helps some. :)
The Newcomer's Forum / Re: Transformers & Caps
February 28, 2011, 10:38:55 AM
MJL21193 is spot on. I thought of a couple of things to add that may be useful.

Your question got me thinking, and off the top of my head I can't think of anything that would present a shock hazard in disconnected and unpowered equipment except capacitors. If anyone can think of something else, please let me know. Certainly there are hot components, sharp edges, and a few nasty chemicals if you like eating or breathing ground up parts, but nothing related to electrocution. :)

There's an excellent article on electrical safety on the ESP site (absolutely amazing site by the way, further exploration is highly recommended).

Capacitors store charge in an electric field. An ideal capacitor retains it's charge indefinitely, but real components have internal resistance and other imperfections that cause the charge to dissipate over time. There's also another odd phenomenon called 'dielectric soakage' where a completely discharged capacitor in a shorted state, after removing the short, charges back up to a percentage of it's original charge. You can see this effect most easily with larger electrolytics. Hook up a 10uF or so electrolytic to a voltmeter and charge it up to say 10V. Put a 500Ω or so resistor across the terminals and you can watch it discharge on the meter. Then remove the resistor and you can see the voltage bounce back up to 3V or more. It's a well known problem in some analog to digital circuits that use a capacitor for sample and hold, as it goons up the sampling on large input changes. All capacitors do this to some degree, but it can be reduced greatly by using certain materials. Just something to watch out for, especially on tube equipment where hundreds (or thousands!) of volts can bounce back up to potentially dangerous levels. Increasing the time the cap is grounded can help minimize soakage.

You can test a cap to see if it's holding a charge with a voltmeter. The important thing to remember is to be careful of the voltage rating of the meter! This is a big issue for CRT and microwave cap discharging (special HV probes are mandatory), an important issue for tube amps (check the ratings on the meter and the probes), and almost a non-issue for all but the highest power SS amps. Almost every meter out there can easily handle 300VDC without trouble, but I'd be wary of using a $7-on-sale multimeter for more than 50VDC. Many do just fine with them, but I'm paranoid as I've seen a cheap meter fail dramatically even though it was used properly and within rating.

If you test a cap for charge, be very careful that you don't short the probe tips while they're across the cap, or it'll happily discharge through your probes! Caps discharging over a low resistance (any metallic conductor) will happily push as much energy out as their internal resistance allows, and this could easily be tens or hundreds of amps. This leads to melted probe tips, vaporized metal, and occasionally flying chunks of screwdriver, and it'll scare the hell out of you if you don't see it coming.

Hope that helps some, and thanks for letting me indulge in babbling on and avoiding my studies for a while. :D
Amplifier Discussion / Re: TIS-98
February 27, 2011, 06:07:07 AM
I second the BC series. They're made by Fairchild and I usually get them from Mouser.

The model numbers in that set differ for Vce ratings and noise. All the other parameters are the same, at least for what's listed on the datasheet. (Link) The Micro Electronics TIS98, Fairchild TIS98, and Fairchild BC546-550 series are all listed with the same pinout, but it's always good to double check.

I dug up the schematic for the Rg80, and it looks like you're replacing the differential pair in the final. The BC546 should work great, and it's what I'd go with, but if you feel like tinkering you could try a BC549/550 and see if there's a difference. It's slightly less noisy but underrated for the Vce voltage. Odds are it'd still work fine and you might be able to cut down on noise (assuming it wasn't coming from elsewhere, like the ancient FETs, hehe), but I wouldn't give it to a stranger that way as I'd feel horrible if it farted out on them.

Hope that helps a bit. :)
It's hard to say what happened without further investigation, but your hunch may be right.

First I'd check the cab and see what resistance you get across it on an ohmmeter. It'd have to be a decent meter, or one that allows a relative measurement so you can cancel the resistance in the leads, because you're dealing with small amounts of resistance. What you're looking for is a short, or less than an ohm of difference between shorting the leads together and running them across the cab inputs. It can get touchy with a four ohm cab, as the difference is so small. If you get a really low reading (or a really high one, like over fifty ohms) you may have a dead speaker or a bad connection. You'll have to test the wiring and the speakers and look for loose connections or fried chunks of speaker brains.

Then there's the Marshall cab. The schematic is available at Enzo's (awesome dude that one), so that'll help a great deal. If you're confident you can pop the case on it and we can help you find which part let the smoke out (though it might be obvious) and what to replace. Odds are decent that you can get the Marshall up and running again with a little tinkering.

The more experienced folks here might be able to bullseye your problem, but that might get you started.

Hope that helps. :)
The Newcomer's Forum / Re: reverb tank?
February 19, 2011, 08:54:07 PM
I did some poking around too and info is scarce to absent. It looks like Marlboro amps were rebadged imports, so finding a schematic would be unlikely. Fair warning, from what little I could find online there weren't a whole lot of folks who were impressed with the reverb on Marlboro amps. A new tank is more likely to sound different than the original than the same though, so it's really a guess and check operation to replace it unless you happen across an original replacement.

If you're motivated I'd suggest posting good pictures of both sides of the circuit board and the chassis where the tank mounts. We might be able to come close to figuring the expected input/output impedances for a tank, as well as the physical criteria with good measurements of the mounting holes.

Using the wrong tank can be a problem. Some tanks are designed with one end grounded to chassis on input, output, or both where others have the signal isolated all the way through. If the driver is expecting an isolated input on the tank and finds a grounded one it could see it as a short, or near zero input impedance, and the amp can be damaged. Other than that an impedance mismatch will most likely just sound horrible or silent. If it's possible to reverse engineer the drive and return circuitry that should give a reasonable idea of the specs you'd need to go shopping for an equivalent at Accutronics.

Hope that helps some, sorry this old amp is such a tough nut to crack. :)
Preamps and Effects / Re: Compressor Mods
February 17, 2011, 07:29:18 PM
Quote from: J M Fahey on February 17, 2011, 06:30:02 PM
Looks good, but I was dismayed when asked for LM3080's.
Looks like they are not made any more; NOS omes command from $15 to $30 each !!!!!!!!!!!!!  :duh :loco
Which is the modern replacement? (there must be some, although the ones I found so far were miniature SOIC and the like)
I poked around a bit and it seems there's not many OTAs made anymore. You might consider the Japan Radio Corporation's version of the LM13700 (Mouser link and datasheet). It's a dual, but it's only a dollar (here in the US anyhow). At that price you could leave the second half of the chip idle and still win out. I haven't done a detailed peek at the specs, but the original 13700 was used in synths, compressors, and LFOs for phasers for many years.
JRC has saved my bacon a few times, as they make more than a few older chips that aren't really easy to find. I'm a big fan of their LM386 clone (NJM386) as it comes in DIP, SOIC, and SIL (!).
Hope that helps. :)

Edit: Fixed a typo
The Newcomer's Forum / Re: Cleaning pots,etc.
February 16, 2011, 09:09:43 AM
Greetings and welcome!

  In truth it's likely you'll need to replace the pots as opposed to simply cleaning them, but it won't hurt to try. Probably the best writeup on pots I've seen is 'The Secret Life of Pots'. That's got a bit on disassembly and a bunch more on the electrical aspects. You'll probably want to desolder and resolder each one as you go to prevent mix ups, and be sure to write/mark/photo (aren't cellphones great?) the wiring so nothing gets swapped around. Cleaning them out can be done without opening the pot up, but all that really involves is finding a hole and jetting in some DeOxit or other pro cleaner. Disassembly followed by some careful swabbing with qtips (be careful to avoid leaving threads of cotton behind) and rubbing alcohol (90% or better) will knock most of the dust out but the worn out track is still going to be worn. If you find grease on the shaft try to preserve it or you'll lose all the resistance in the knob travel (unless you like zero resistance travel). You can sometimes bring an old pot back to life this way, and you'll learn a few things if you've never done it before, but it's not a guaranteed success.

  I've got these pliers that I absolutely love that are perfect for the tabs on pots. They're from a cheap all purpose pliers set, and they're a small version of a pair of lineman's. They're like needle nose except that the ridges in the tip are much larger, as though the pliers were shut and drilled out with a 1mm bit. They easily dig under folded metal like the tabs on pots and make it very easy to grip the tab from the sides or the end to pry it up. Much more control, and I'm less likely to stab my thumb with a jeweler's screwdriver I'm using incorrectly.  :duh Full size needle nose pliers are good for flattening down the tabs after reassembly in order to get everything flat again.

Hope that helps, and be sure to let us know how things go.

P.S. I couldn't resist attaching a picture of my favorite pliers.  <3)
The Newcomer's Forum / Re: Best home made DIY PCB method?
February 09, 2011, 07:37:14 AM
It took me half a dozen boards to get fairly good at making them at home, and I continue to experiment with different things to see if I can get better results. I'm using an old laser fax machine that sometimes prints when it wants to, magazine paper (mostly), a laminator for heat transfer (I wasn't talented enough to get consistent results with an iron), and either FeCl from Radio Shack (in winter) or air regenerated copper chloride (in summer) for etching. I'm lucky enough to have access to a drill press that'll hold just about any size bit, so I use that for drilling, but I could put my $8US pin drill to the task and get the job done if I didn't mind sore fingers.

The most important single thing I can think of is keep trying! Don't let bad results deter you, just keep at it and you'll get it. Being able to knock out a custom PCB in a few hours is incredibly helpful when you're tinkering with electronics. You can also take your time and make a masterpiece. It's a lot like baking, except that you can plug your guitar into it when it's done and it'll last a few decades longer. :)

I'd always intended to do a good, comprehensive writeup on home PCB manufacture. I must admit that I don't have a whole lot new to add to the sum total of what's on the internet, but if there's any interest I'd be happy to try to consolidate my findings in a guide for the forum. It's an odd passion of mine, as it's one of the first victories I achieved after returning to electronics.
I remember finding out a little about those when trying to troubleshoot a Backstage II. The older ones like the 110 are actually really nice, simple amps from what I've seen. Peavey's usually really nice about emailing you schematics if you ask nicely, though sometimes it takes a couple of tries. It's pretty likely that you'll be able to fix it up and have a great amp for the price.

Congratulations on the find! Keep us posted and we can help if you hit any snags. :)
Quote from: J M Fahey on February 07, 2011, 03:38:21 AM
Quoteim planning to build one too but need to find PCB Layout  :D
5) go to sleep, next day with a fresh mind start compacting or rearranging it, until happy with results.
This part is very important.
It took me half a dozen boards before I discovered that things look very different after a night's sleep. I'd go from pulling my hair out to sorting out the two or three bugs that were hanging me up.
Also a very useful technique for troubleshooting that freshly built pedal that thinks it's supposed to be an AM radio.  xP
Tubes and Hybrids / Re: Old tube guitar amp upgrade
February 03, 2011, 02:55:41 AM
Quote from: frank2 on February 02, 2011, 11:11:42 PMwhat do you guys think?
At those prices I'd leave the old one there, disconnected, and wire in some brand spanking new caps. It'll look the same unless you open up the guts. Folks here can give you better advice than I can about what exactly to use. Assuming they're all used in the filter their capacitance value isn't really important, just the working voltage (WV). You can pick up a 47uF (nearest standard value nowadays) 450WV electrolytic cap for one or two dollars US.

Definitely trace out a schematic before removing anything, or you'll regret it later. Mark the wires and make good diagrams. If you do swap out the caps for modern electrolytics, make absolutely sure you don't hook them up backwards. They'll only take maybe five or ten volts if hooked up backwards before they split their casing and vent capacitor blood all over the place, sometimes with great violence. I always turn away when applying power to a new creation for the first time as I was once nearly blinded by boiling capacitor electrolyte.

Regarding books, I think the best one I've read about valve amps was Jack Darr's "Electric Guitar Amplifier Handbook". I don't know if you can even buy it anymore, but I know it's out there on the internet somewhere in pdf form. It's sort of aimed at the electronics shop repairman as a guide to guitar amps.

Just my opinions, hope it helps some. :)
Tubes and Hybrids / Re: Old tube guitar amp upgrade
February 02, 2011, 05:32:06 AM
Interesting find!

I'd definitely consider changing out the main filter cap. Have you worked with multi-element caps before like the one in the big aluminum tube? The first time I worked on older tube gear that one threw me for a loop! I know I've seen a site somewhere where they discuss restuffing them with modern electrolytics to retain the same look. Pots, jacks, and tubes aside, the rest is probably just fine.

There is one thing that caught my eye, attached below. Did someone hack together a fix for the fuse? That looks extremely unsafe! If it were me I'd go through and insulate all the flying component leads so they had no chance of shorting out on something, though I'm a bit paranoid.

Keep us posted on what you decide to do. :)
Amplifier Discussion / Re: Noisy Cricket problems!
February 02, 2011, 04:20:29 AM
I'm not entirely sure what your trouble is, but I have something you can try. I made a Ruby (similar 386 amp) for a friend, and he found that it sounded horrible if his guitar's volume wasn't turned down a good bit. He's got an axe with an (my opinion) absurd amount of pickup options on it, and it ran a bit hot for the Ruby.

The Ruby's volume and gain controls are very, very interactive and require a lot of tweaking. In the Ruby we'd often turn the volume up, the gain down, and slowly bring up the gain until it started breaking up at about 1/3. With the volume at half the gain would go to about the same. Exceeding that brings the clipping on hard and the sound gets horrible.

If you dink with it a while and have no luck you might consider trying to post a sample of the audio, as it'd be really useful if it turns out to be a bugger.

Hope that helps. :)
This didn't sound quite right to me, so I went and fiddled with the numbers. I apologize if you know all this already, I just had to give in to my inner science teacher. :)

With a typical TO220 you could use the listed value on the site of 3°C/W, but I guessed that the worst case for an chip like the TDA2040 would be different. They have to thermally bond two big transistor elements (the output push/pull pair) as well as other circuitry, so often they have a bit higher junction to case resistance.  I looked up the datasheet and they list a max of 4, though they messed up and gave the unit as volts. If you run the calculator with 4°C/W you'll get a case temp at 85°C, but the datasheet lists 16W for 85°C at the case, so they're assuming a shade better than worst case.

So far things are looking good on the calculator, 25°C ambient, 150°C max junction temp, Rth jc at 4° = 85°C junction at 15W output. By not adding anything else we'd be assuming a perfect, ideal case. To make it more realistic for a case with no heatsink we'd have to add a case to ambient resistance. It's not listed on the datasheet, but for something like this you can get a good idea from a voltage regulator like the LM317. It's datasheet lists junction to ambient resistance at 50°C/W for the TO220 package, and it's junction to case is the same at 4°C/W, so we can add 46°C/W to the calculator to give us a good idea what would happen if we used no heatsink. Yikes! By solving for wattage, we peak out the junction at 2.5W.

The next thing to consider would be the heatsink interface material. Delete the 46 and add, for simplicity's sake, a worst case of 0.3°C/W. Odds are you can do better, and a bad job would be worse, but it takes a lot of power to make the grease matter due to it's low thermal resistance.

Last we add the heatsink at 9°C/W. Finally! Now with a bit of tweaking you'll find that you're maxed out at about 9W. By upgrading the heatsink to 4°C/W you'll just barely max out at 15W. Heatsink packages for 3-4°C/W or so are larger beasts, like this series from Ohmite, which ranges up to about $2US per at Mouser. With the largest one there at 3°C/W, our calculations give us a junction temp of 134.5°C at 15W.

Ok, so this looks grim. There's good news though, the chip doesn't dissipate 15W at an output of 15W. Figure 9 on the datasheet shows the curve for dissipated power vs. output power at 4Ω, and it peaks at about 11W output. Keep in mind as well that this assumes a running voltage of 14.4V out of a max of 18V. The reason the curve looks like a hump is that class AB amplifiers work the hardest at half power, and the shallow downward slope at higher power levels might indicate the chip having difficulty dissipating heat (ideally it'd be symmetrical). Ok, so we only need to dissipate 11W, maybe a bit more, at least for a 4Ω load. Plug in the numbers with the massive heatsink and beep, bwoop, a 105°C junction temp. Much better, plenty of margin, longer lasting semiconductor, and we can even use a smaller heatsink if we like.

If you bump it all the way back to the 9°C/W of the original heatsink though it climbs back to 171.3°C. Solution? Run an 8Ω load, and you'll only have to dissipate half as much power! Checking the curve in figure 10 to be safe (again for 14.4V) we get a max dissipation of 5.5W. Plug in the original 9°C/W heatsink and you get a junction temperature of a bit over 98°C. Excellent!

There is a bit of wiggle with the numbers, as always. Bump up the Tamb to 65°C to simulate stage conditions (hot lights and a pile of hot gear) and things change a lot. Double the wattage and half the heatsink rating and see what happens. Poke around online for a 1°C/W heatsink (heavy metal).

I hope all that babble made some sense, I've had a lot of coffee tonight. I'm pretty sure I got the math right . . .
Hope that helps.  :D

Edit: I just realized I forgot to comment on the freaking heatsink itself after all that crap. How rude!
I couldn't locate it at diystompboxes' store, but I found it with google. The 9°C/W counts the 'TIM' (Thermal Interface Material), so our 0.3°C/W for paste was unnecessary. These are kind of neat really, and the built in sticky pad makes them easy to use. I'm not sure I'd trust the adhesive to much banging and jarring, but if you treat your gear like you would a computer it'd probably hold up a while. I'm not used to seeing forged copper pin style heatsinks, and I imagine they do a bit better in lightly convecting air (like near a video card fan) than most types would. I will file these away in the mental library, I'm sure I'll think of a use for them before too long. Good call. :)
The Newcomer's Forum / Re: Forum Navigation:
January 21, 2011, 09:36:38 PM
Also, if it helps, you can edit a quote and chop it up into sections if it makes for easier reading. Just copy and paste the bracketed chunks as needed. The bits in brackets are called BBCode, and can be used to work all sorts of wizardry. It's deliberately kept very simple and accessible, so it's a breeze to use.

Hope that helps. :)