Ludwig Phase II - resistor on fire - need debug help

[robotmonster]

Firstly thanks to people who made this project possible, and congrats to all who completed the build!

After populating both boards and wiring everything, I powered the effect first time (with 48VDC supply) only to see R22 (100ohm) resistor (connected to Q6 collector) dying in a fire instantaneously. I removed burned resistor, powered up again and I saw the switches toggling all the LEDs fine, though no sound coming out yet (kind of expected in this case), so I got stuck there. Q6 is Fairchild BD677AS, pretty sure I got ECB orientation right per schematic/datasheet. Also sure PSU is correctly connected. Any ideas why is this happening?

I tried a another 1/4W resistor in place and had the same outcome. Was searching the forum and could not find a similar situation, except that someone has used a jumper in place of R22?? I have the measuring tools and solid build experience, but no EE background, so diagnosing problems of this sort can be sometimes challenging. Any help to narrow it down would be greatly appreciated!

p.s. Having this problem fixed, what would be the best way to start tuning all the trimmers? Should I look at the voltage table posted in the PDF document and tweak neighboring trimmers, or is there a better approach?

[antonis]

It's more than possible that you have some short circuit after R22 (I can't presume anything without circuit schematic)..

If it's right after R22, there are 480mA flowing through it and the power dissipated is 23W...!!!
(assuming that one leg of R22 is conected to 48VDC...)

But again, we need a schematic..

[digi2t]

It's more than possible that you have some short circuit after R22 (I can't presume anything without circuit schematic)..

If it's right after R22, there are 480mA flowing through it and the power dissipated is 23W...!!!
(assuming that one leg of R22 is connected to 48VDC...)

But again, we need a schematic..

Build doc, including schematic are here;
http://www.geofex.com/Article_Folders/Ludwig_P2/Ludwig%20Phase%20II%20Clone.pdf

Would some pictures of the board, especially around Q6 / R22 area, both sides, be possible?

[robotmonster]

Thanks for the helpful replies! I am away from the effect until tomorrow when I will be able to post some photos and hope I will solve the problem. Still a bit intimidated by all the trimmers, but I did find some info on the forum thanks to Dino (digi2t), so if I fix this issue I will read through that first and have some play with DMM and scope.

Meanwhile, here is the part of the schematic with R22 and Q6 for anyone interested.  I was also suspecting a short, but when this resistor is removed from the circuit, the rest appears to be running fine (LEDs, switches and the rest) which got me slightly confused (I was thinking about bad transistor or other neighbor component). I will do more research in that direction though. Does that mean that short is likely somewhere between point marked +35 (emitter of Q6) and ground, and can be anywhere in the circuit following 35V trace, while 48V power line is separate and effectively short free and that is the reason why the LEDs and the rest appear to be working fine?



Thanks for the link to the schematic. I printed PCB originally per document you posted. Though I have borrowed couple of ideas (RF26 distortion trimmer and similar) from the later updated document:

http://www.geofex.com/Article_Folders/Ludwig_P2/TotalLudwig-2nd_run.pdf

[R.G.]

That resistor will only overheat if something in the circuit is eating too much current, as the previous posters said. A reversed electro filter cap, a wrong-way semiconductor, or a solder short might do something like this.

You might want to temporarily insert a 1K resistor there instead of 100R. This will cut the current going through by a factor of 10, and will let you do measurements without burning up successive resistors. When you find the error, change it back to 100 for testing.

[digi2t]

R.G., would the Zener installed backwards create that problem?

[antonis]

I think not...

If the zener was reverse installed it should act as as simple diode, connecting R26 to GND..
(actually to 650mV higher than GND)

Q6 base should be biased to a diode voltage drop plus voltage drop through R23..
(so Q6 should be at cutoff region or create a negative voltage on it's emmiter...)

I didn't follow the complete schematic so I don't know what's next to +35V..

[R.G.]

No, if the zener was backwards, it would burn up the 1.8K resistor.   

However, if the Q6 transistor was installed incorrectly, it would burn up the 100R resistor, OK.

[pinkjimiphoton]

definitely check for bridges. but i'm thinking RG already told you your answer.

one caveat to replacing the 100r with 1k, couldn't that cause more issues by letting more current thru? kinda like wrapping a fuse in tinfoil?

if everything else seems good, that's awesome. it was too herculean for me to do, my brother dino built mine for me, for which i am forever indebted for his kindness.

keep at it. the results at the end will make it all worth while the first time your guitar yoys at you.

[robotmonster]

Again, thanks all for kind and motivating replies. I am getting very positive and anxious to look at it very soon!

According to my brain I must have checked orientation of all semi's fine, and many times in many stages during the build. It is a small number of pol. caps and diodes. Transistors are in large numbers, but super-easy to orient as they all face one direction per PCB.

All the components and wires are soldered on my PCB, except from 2N2646 which has a socket! Ouch!! I could have left that one out!! Could that be the cause!? Not having this transistor in socket on power up?? From earlier Repeater project I learned these can be a variable bunch and wanted to try few in this unit - for yet unknown reason...? Is there even a point doing it in this circuit?

1K vs fuse analogy makes sense - whatever happens with 1K in place would likely be a step forward in any case and very awesome case if I can start measuring things. R.G. thanks for the practical suggestion there! Build doc. already made me think a lot about Q6 orientation, so I doubt that is the case. The one I use has 3 grooved dots on one side only, and was easy to align with schem and traces, per datasheet below.

With all above said, I really hope its a hidden bridge of some sort! If not as easy as having 2N2646 properly in place. Will post what I find soon, hopefully some good news!

[PRR]

As a worst-case, if the 1K goes POOF! (or just fffssst), use 4.7K 1/2W. This will survive 48V for days and days.

The actual voltage at "+35V" will tell if you have a dead-short, a part-short, and the Ohms of a part-short, which may lead to a prime suspect. (Between Q6, many e-caps, and wire/PCB errors, there's too many suspects to sift; we need another clue.)

> replacing the 100r with 1k, couldn't that cause more issues by letting more current thru?

I=V/R. If you make R bigger, I gets smaller.

If we assumed dead-short on the end of R22 100r, then 48V/100r is 480mA (0.48A), which times 48V is almost 24 Watts. Which will indeed smoke a 1/2W part real quick.

With 1K the current is 48mA, the power is 2.4 Watts. Much less violence, though a 1/2W part still smokes within some seconds.

With 4.7K we get a bit over 10mA, 0.49 Watts. While a 1/2W part can't sit at 0.49W for decades, it will hold-up for thousands of hours. We only need it to last long enough for a few checks; the circuit sure won't work-right with this strangulation, we just to find why it is dead-short or near-short so's we can fix it.

[antonis]

I'm a little confused, but...

If Q6 is placed in socket, you just have to remove it and power the circuit with a R22 of 100R - if the resistor is on fire then it's for sure that there is a bridge/short circuit between Q6 collector and R22 left side..

If Q6 is solered on PCB, you'll have to desolder it for check - and it will be a good chance for placing a socket..

[digi2t]

I'm a little confused, but...

If Q6 is placed in socket, you just have to remove it and power the circuit with a R22 of 100R - if the resistor is on fire then it's for sure that there is a bridge/short circuit between Q6 collector and R22 left side..

If Q6 is solered on PCB, you'll have to desolder it for check - and it will be a good chance for placing a socket..

I don't think he means that Q6 is socketed. I believe that he only socketed the 2N2646 flip/flop transistor so that he may test for good ones. I think that's what he means. 

I can't see how not having the 2646 on the board would fry the 100r resistor though. I'm still thinking there's a short somewhere.

[wavley]

definitely check for bridges. but i'm thinking RG already told you your answer.

one caveat to replacing the 100r with 1k, couldn't that cause more issues by letting more current thru? kinda like wrapping a fuse in tinfoil?

You don't know how many times when I was in audio repair (which I have just opened my own shop with a storefront and all because there isn't one for 50 miles to the east and 100 miles in any other direction as a second job) that I had to say "Well, it WAS just this shorted component and that's why your fuse blew, but you had to go and wrap foil around the fuse and now you need a power transformer."

But PRR and RG have already explained why this approach is OK.

if everything else seems good, that's awesome. it was too herculean for me to do, my brother dino built mine for me, for which i am forever indebted for his kindness.

keep at it. the results at the end will make it all worth while the first time your guitar yoys at you.

And yes, it's a tough but rewarding build that is totally worth it once you hear that yoy!

Make sure that you hit the Ludwig debugging thread for info about the zener diode fix for the switch that works really great and personally I put a blend pot between my clean stereo output and the effect out so that I can turn the clean on and off with the stereo stomp (in mono) and tweaked my input amp values to play nicer with my baritone guitar.  All of which is documented in that thread with stupid questions from me and smart answers from the nice folks around here.

[digi2t]

Make sure that you hit the Ludwig debugging thread for info about the zener diode fix for the switch that works really great and personally I put a blend pot between my clean stereo output and the effect out so that I can turn the clean on and off with the stereo stomp (in mono) and tweaked my input amp values to play nicer with my baritone guitar.  All of which is documented in that thread with stupid questions from me and smart answers from the nice folks around here.

I wish those tweak and mods were cataloged, maybe incorporated into the build doc. That troubleshooting thread is pretty heavy to wade through.

[robotmonster]

Close up photos helped me see I can do better in terms of avoiding any potential bridges. Was surprised how much easier is to look at the magnified photo compared to the actual board under magnifier, which I almost always used to do. I learned something there! 

Basically, I re-flowed solder joints close to traces, trimmed all the leads as much as possible, brushed everything with toothbrush/alcohol and 100R was back in place and this time without fire!  I ordered these basic boards locally, immediately after PDF was first published, so no thru hole vias to help with excess solder. Bridges of invisible lines of solder could have been anywhere in the spots I had redone. Thanks for pointing me to that, because I had the rest of the circuit powered, was sure I got orientation right, so I was worried it is something else. I posted some photos of my boards (see link below) after this work was done, so you can see few spots around wire connection points on both boards, where I have "over-separated" the joints from traces, so most likely issue was around there, but I can not be sure, just happy that the sound is coming out of the circuit and that I am making progress!

http://imgur.com/a/0y4sR

First impression: Still plenty to do, and I noted only following working (this is with no exp pedal connected):

- Bypass sw.
- Bypass pot.
- Fuzz sw.
- Fuzz pot.
- Fuzz/Voice Fuzz seems to do something to the sound
- I could hear fuzz going through some fixed filter frequency.

Is it mandatory for the foot pedal to be connected for the rest of the functions to work? Could this (20K pot) pedal work or be easily modified to work (with a tapering resistor or similar)? At least to test basic functions? Pedal also has a polarity switch, and it is the only one I have around:

http://www.bespeco.it/images_prodotto/1847_foto_2.jpg

I could hear a faint ticking sound from 2N2646 (frequency changing with rate pot), but I could not hear any modulation whatsoever apart from some kind of weird and loud feedback loop modulation on certain trim pot settings when I toggle slow/fast start. This switch could also cause a one-off loud frequency sweep. Also FFM/Fuzz Rpt toggle did not seem to do anything. Was poking around with oscilloscope (looking at the schematic in search of some modulation) and could see sawtooths and squares in certain spots but nothing pointing me to the bad/wrong component (probably due to my lack of understanding). More systematic approach was needed, and I started measuring transistors on filter board per document, and did find some values way off very soon. I guess this is where the fun begins, but I am grateful for being finally at this stage.

Next for me is to redo all the wiring with separate wires going directly to the board, in favor of shielded wires and terminals I used originally to prepare things for testing. I guess it is also time to learn more about actual circuit and have a detailed sweep through all the recommended forum threads. In conclusion, pedal is making some sound now, and thanks to you all helpful people here and information you posted earlier I am confident I can get it working. I wanted this effect ever since the first time I heard it on Sonic Youth's Washing Machine in '95. Almost unbelievable I will soon have one of these super-rare museum pieces running on my pedalboard. Will keep you posted, again many thanks!

[pinkjimiphoton]

without the expression pedal, the filters may not sweep correctly. 20k should be fine... it wants to see 5k, but 10k is more reasonable for a range. i suspect you have some other issues tho, as it should pass signal and the filters should work if the ffm switches are on. if it doesn't go yoy yoy yoy all by itself, keep troubleshooting. but yes, the treadle is an integral part to the function of the pedal, so i would try plugging it in first to see if it works. you may wanna add a trimmer inside the pedal, and you may find you may need to either reverse the connections to the volume pedal's pot internally too... i would imagine a c or reverse taper linear would work well here.
fwiw, i run my clone with a "universal" expression pedal or a roland ev5, which is kinda hip cuz you can sweep past the formants with the filter, it's kinda like the end of welcome to the machine by pink floyd, when the engineer wipes everything below 1k. you can swell in and out of the effect... but some exp pedals the throw of the expression may be backwards from what you expect.
also, on the first revision, if memory serves, at least one of the switches is actually labeled backwards. so it may take a little experimenting... and a hell of a lot of reading.... keeping up on this project is a heady task not meant for the feint of heart.

[Keppy]

Make sure that you hit the Ludwig debugging thread for info about the zener diode fix for the switch that works really great and personally I put a blend pot between my clean stereo output and the effect out so that I can turn the clean on and off with the stereo stomp (in mono) and tweaked my input amp values to play nicer with my baritone guitar.  All of which is documented in that thread with stupid questions from me and smart answers from the nice folks around here.

I wish those tweak and mods were cataloged, maybe incorporated into the build doc. That troubleshooting thread is pretty heavy to wade through.

http://www.diystompboxes.com/smfforum/index.php?topic=95132.msg1001444#msg1001444
This link goes to the summary of the bugs that were found last year. The zener diode is a likely fix for the toggle problem mentioned in robotmonster's last post.

[digi2t]

Thanks Kep!! 

[Keppy]

All right, now that I've got a little more time...

First impression: Still plenty to do, and I noted only following working (this is with no exp pedal connected):

- Bypass sw.
- Bypass pot.
- Fuzz sw.
- Fuzz pot.
- Fuzz/Voice Fuzz seems to do something to the sound
- I could hear fuzz going through some fixed filter frequency.

Is it mandatory for the foot pedal to be connected for the rest of the functions to work?

The expression pedal feeds a control voltage to the filters. You might get some filter movement without it, but don't count on it. At least wire in a pot manually (and safely, as there's potential for considerable voltage on this pot).

Could this (20K pot) pedal work or be easily modified to work (with a tapering resistor or similar)?

Yes. Tack a 10k 20k resistor between the 2 outer lugs and you have a 10k pot suitable for this purpose.

I could hear a faint ticking sound from 2N2646 (frequency changing with rate pot), but I could not hear any modulation whatsoever...

Don't expect much without the rocker pot connected. The filters are biased all wrong without it.

Also FFM/Fuzz Rpt toggle did not seem to do anything.

See previous posts about adding a zener diode. As shown in the documentation, neither mode turns completely off at any switch setting. The zener is a necessary fix.

There are EXTENSIVE trimming procedures for this unit. Once you get it working, it will squeal like mad until you've tuned it up. Hit us up again when it works but sounds horrible.

Edit: modified bad math of last night's post.