How do I get better at problem determination?

[Regis]

I've been tinkering with amps for some years now and while I'm not on most of you guys's level, I can read a schematic pretty good and have done mods and such for most of my amps.

Something I've never been very good at is figuring out what is wrong when an amp acts up. This week I was tinkering with my VOM champ, some of you might remember me saying I rebuilt this thing. It was working ok and I was experimenting with swapping bypass caps and I killed it. I was getting no sound at all. I rigged up a poor mans signal injector with an old guitar tuner that had tones and traced it down to the preamp tube where I was working but I'm damned if I could see where the problem was. I must have shorted something out but couldn't see where. I ended up removing all the parts from the tube socket and remounting them and that fixed it.

Some of you may have read about that Quad sale I did and was asking questions about. The problem on that was a broken resistor lead under the cap can. I never would have been able to trace that down.

I've read R.G's debugging page, and have used those tips. I've never had any formal electronics training, I guess that would help.

Any other tips on how to get better at this?

[stokes]

Gerald Webers last book has some good points on trouble shooting techniques."Tube Guitar Amp Essentials" is the title.All of his books are pretty good as well,but if you only get one of them get the title named above.

[Enzo]

I've not read that bok from Gerald, but I do have the Desktop Reference and Tube Talk, and my one main complaint is that he never explains anything. He puts up a picture of the eyelet board and says move this here and change that to this. if you do it, you wind up with a modded amp. Might sound great, but you have no idea what you just did. Me personally? I understand the amps and would prefer he included a schematic - that speaks louder to me than the layout.

ANd the reason I get upset is that whatever you do to a Fender such and such model to increase the gain, decrease the flabby bottom, make it less bright, whatever is exactly what you would do to any make and model if you understood WHAT you were doing. Weber's method lets you know just what to do if it is a Fender Princeton say, but leaves you clueless if it is a Gibson something.

Regis, what you had was a solder bridge or a failed solder joint. All it takes is a little tiny bead of loose solder wedged between two socket pins. But you narrowed it down to that stage, didn't you? That means your effort was effective.

RULE #1. Any time an amp is working and it stops working after yuo worked in it, it is almost certain that the work you did killed it. THAT is where to start looking.

I had a local friend who I was mentoring a little on an amp he built. He came to me with it one day and was baffled it made no sound. A couple minutes later, I pointed to the blob of solder between the hot and ground lugs on the speaker out jack. He still feels guilty, but he shouldn't. I bet he always looks there in future silent amp cases now.

Point is if you are not really used to looking at it, things do not as easily look out of place. For that matter, how many Fenders have I "fixed" by moving the speaker plug out of the EXT SPKR jack and into the MAIN SPKR jack where it belonged? Quite a few.

Experience is a great teacher, keep at it. After 50 some years of it, I still learn a lot every day. It accumulates, and each new bit of knowledge stands on the shoulders of your prior learning.

There are two things here. One is learning how tube circuits work, but the other is learning troubleshooting skills. The essence of troubleshooting is to isolate the problem. We think analytically and we keep trying to narrow down where the trouble is. There are so many tricks and techniques. But they all boil down to isolating the problem. It is a systematic approach.

ANy system is a process, a string of steps that takes what you have at the start and makes it onto what you want at the end. Somewhere that process breaks down, and you have to find out where.

You are already here, that is a great resource. Read as many repair oriented threads as possible, and really think about HOW the problem was solved. Repair advice I give usually - I try - includes a why we did it that way. If not, it is there for the asking. But as yu look through the threads, watch where the give and take leads and why it went there.

[Don Moose]

My number one resource is R.G.Keen's Tube Amp Debug Page

[slidincharlie (Carlo P)]

About Gerald Weber's books

I agree to a large extent with Enzo's comments about Weber's books. Yet his last book gives precious advice especially about troubleshooting, helping the reader to isolate the problem (like Enzo said). Also the sections about mods that give the amp a different voice are very good (tweaking with caps and resistors in the preamp section). I think it addresses problems in a general way, and it is a big step forward compared to his previous books.
Yet other books on the general functioning of each section of an amp would help a lot. And of course, reading the posts of the many competent people (often pro techs) who write here and in other forums really gives lots of precious information.
I have created an excel database with all the topics found in books and forums that I have found useful, and it is a very precious source to me.

[Regis]

Enzo, thanks for that reply, there is some good advice there. About that Champ build, I figured it was something like that but couldn't see where it was. I used my solder sucker to clean all the old solder out before redoing everything.

About the why's of how a mod works, I'm still learning. I still have a lot to learn. And you are right, knowing how and why a circuit works helps a lot.

regis

[Alex R]

There's also a question of the attitude you take to the process. Something I still have to tell myself all the time is to trust problems to be solvable, and to trust myself to be able to find them, because they aren't mystic or otherwise beyond human understanding, just faults in a system that will work again when you find and fix the faults, which are real things with real symptoms. But you do have to be persistent, and that gets tiring and difficult, and sometimes you need to take a rest and then come back to it fresh and there is the problem looking right at you. Some personalities are better suited to this than others. You learn the meaning of patience all right.

Another thing is to go one step at a time and not rush around the amp doing stuff without testing it. This can be tiresome because it takes time to test the thing you just did. Maybe you have to put the board back in to do it and it takes half an hour. But if you don't do that (or check it some other way) every time you make a change you will very likely make it worse.

And another other thing is to be really sure that the results you're observing on a meter or a scope aren't wrong because you're not applying the measurement right. Today I was trying to refurbish a simple power supply that gives 28vDC to work relays in an old jukebox. I kept measuring the DC output of the bridge rectifier and it was 82, 84 volts. What? I thought. I looked at the AC input and it was 30, 32 volts. That's when the mystic thing kicks in, the sense that something you really don't understand that may well be totally independent of the laws of physics has begun to happen. I confess that it took me ten minutes staring at the thing, and 30 further minutes drinking tea to stop myself staring at the thing, before I realised that with nothing plugged in the supply had no resistive load to work on apart from my voltmeter's internal impedance, producing wild-looking results totally consonant with V=IR (which is the one formula you can't do without).

So yeah it's a learning process. This forum really is a great place to do the learning, as I've discovered over the last month or two.

[stokes]

Enzo,I agree with you about the way he presents the mods to some degree,but I think he is reaching out to a wider audience than the more tech oriented.I am sure you already knew the mods Gerald offers in his books.I just liked the way he lays out the trouble shooting techniques and think it would be a good place for someone to start to understand the procedures.It surely is an easier road than we had back in the days before the internet or the many books available today.Still have your "Navpers 10087 Basic Elecronics" handbook?Everything you need to know about tube amp circuits.

[Enzo]

Yeah, I'm not picking on Gerald, I just mean that reading his early books won't add to your insight into circuit function and troubleshooting skill. If you want to know how to do a lot of mods without learning what the circuit is doing, then Gerald's books are great. And I think that was his intended audience.

Alex makes a good point. The young audiophile that worked for me for a while looked at things like a religion. Ther was always some sort of supernatural influence. I once showed him something an amp was doing, and his response was, "That's impossible, it can't happen." Yet there it was in front of his face doing it.

ANother junior tech was trying to fix a cassetter recorder once - Yamaha four channel. Only would record on 2 channels. So I helped him. We determined ther was no bias osc at those two heads. Trace back to find the enable line for those two osc was off. A couple more steps to find a sensor was not activating. The sensor was on the deck surface behind the cassette, a photocel and an LED. Turns out to get a four channel recording, you needed to have a reflective label stuck on the cassette to tell the system it was not a common stereo cassette. (The unit used these peel off foil labels) The deck was working as it should, and we had just troublesooted (troubleshot?) the problem all the way through the unit and back out the other end. We were not aware of the label thing.

The tech said, "Oh man, we just wasted an hour." I responded, "No, we just verified our troubleshooting methods work, and we both learned something.


Here is why you learn HOW things work. When I get an amp that powers up but doesn't pass signal, one early thing I do is check plate voltages down the row of tubes. When I find a 12AX7 with 350v on the plate instead of 200, I can already guess the cathode will have zero on it. Why? Because the current through the tube and plate resistor is responsible for the voltage drop through that resistotr. No current, no drop. And no current up top means no current at the cathode either - so no cathode voltage develops. Tells me the tube is not conducting, and that means either a bad tube, a loss of heater power , or some serious negative voltage on the grid.

So understanding the tube stage lets me instantly put that voltage reading in perspective.

[oldog]

Jack Darr Electric Guitar Amplifier Handbook

This book is very helpful in learning common sense methods to follow when troubleshooting. It was out of print for a long time but a 4th edition is now available on Amazon.com.Part of it has been in Aspen Pittmans Tube Amp Book, to teach troubleshooting. You can see a sample there to see if you think it would help you.

[stokes]

This book is very helpful in learning common sense methods to follow when troubleshooting. It was out of print for a long time but a 4th edition is now available on Amazon.com.Part of it has been in Aspen Pittmans Tube Amp Book, to teach troubleshooting. You can see a sample there to see if you think it would help you.

http://www.pacificrecone.com/JackDarrBook.html You can down load the whole book here.

[Regis]

Thanks, Stokes, I downloaded that Jack Darr book and took it to work and printed it. I went through most of it and it does have some good troubleshooting techniques in it.

I appreciate everyones comments. It's become apparent that I need a better understanding of how tube amps work and why. That would help me in my PD skills.

I've been thinking about getting TUT1, I understand he explains a lot in that book. The problem with kevin is those damn handwritten schematics and his prices are pretty high.