SVT 4 Pro DI issue

[lowell]

So I called Ampeg. We've narrowed it down to revision 0. This was the first version of the SVT4 Pro. They do not have a schem in the database so they're diggin around in the hard files to see if they can find one. Keeping my fingers crossed. Again, nice to have one, but I may need to just do it by feel w/ the scope.

[Jazz P Bass]

Or draw it

[Enzo]

Oh my lord...

[lowell]

Honestly I get intimidated when it comes to tracing things out and drawing them. I guess the more I do it the faster ill get. Do I pass the time/expense on to the customer? How do you handle that Enzo?

[Enzo]

I don;t generally find myself tracing out circuit boards. If I look at something like a power tube board, the trace patterns sort of resolve themselves in my head, so I am seeing screen connections or signal to grid or whatever. I am sure I could trace out a Tube Screamer if I wanted to, but I just don't. At this point in my career, this stuff is almost a zen thing. SO ther are a lot of things I do in my head. If I see groups of parts, they may suggest to me what their function is, and I can often doa lot of testing without knowing the circuit at all. I realize that doesn;t help you at all.

To answer your question, I once rebuilt an old Jukebox amp, from a 1930s jukebox. It had tubes like 2A3 and other tubes with two-digit numbers and only four or five pins. The caps and resistors look nothing like they do today, though I knew what they were. This was not a pair of 6L6s. SO I took the time to trace all this thing out. I charged the customer for my time.

On the other hand, I have a little CHampster in here, so I traced it out - more of less a 5F1 circuit - to see how it was wired. SImple amp, took maybe 10 minutes. NO charge, they'll pay for the repair.

I charge a certain amount minimum for repair work. I reserve the right to be flexible when I feel like it. I think an hour labor at the shop rate is a pretty common minimum charge in many shops. Now on a lot of repairs, I can get the work done in less time than that. But every now and then something turns into a challenge, and against my better instincts I keep at it, telling myself, there's a lot of time I'll never get paid for.

REally not bragging here, but I am just so familiar with the Peavey solid state amps of the 1980s - the 400BH boards for example - I can about do it in my sleep. One night it was 15 minutes before the store closed. I had finished everything else for the day. There was a blown amp - XR600 maybe. I decided to just open it up and see what it was going to need, thinking I'd come in tomorrow and do the work. 11 minutes later, I had replaced the bad power transistors, and had the amp on the burn in bench. Personal record I suppose. I knew what to do, what to check, how to take it apart and reassemble it, basically ther were no mysteries to solve. Not that I was so sharp, it was just so routine. SO there I got an hours labor billed for 11 minutes work. Not typical, but that compensates for some times when I get bogged down.

I mean really, How long would it take to change out a input jack on some small Crate amp? Nothing to figure out, just do it. Wham, instant bench charge.

The first time I had to get the board out of a Fender Hot Rod Deville, it was a real pain inthe ass, took way too long. Now I have seen tons of them. BAck cover screws, stick them on speaker magnet. KNobs off, nuts off, board screws out, ground wire off, snip tie wraps off. ANgle board, push down clear the pot shafts, and flop 'er down and solder side is up. Takes just a few minutes. Just a matter of familiarity.


Didn;t pay off for me, but for your situation, I l;ooked through other models for a solid state, or at least transformerless DI circuit. CHances are if they already had a circuit, they wouldn;t waste time inventing the same thing again, and perhaps the schematic for the other product would help. Alas, I found none. But that trick might pay off next time.

[lowell]

Okay so here's the circuit for anyone that needs it. The IC was bad. It's all workin' now. Thanks for encouraging me to grow a pair and draw it out.

[Jazz P Bass]

That is awesome, Lowell.
You know, I have found that the hands communicate differently with the brain, than the eyes do.
And differently also, when you combine the two.
Drawing a circuit is very rewarding.
And the layout, too.

[Enzo]

I think, and like to say, "It isn;t the parts, it is the circuit that matters." You can look at all the parts in the world, but looking at the circuit puts them all in a context. It allows you to consider signal flow.

[km6xz]

This is an important concept that is not understood by beginning techs or hobbyists. Spending lots of time checking components would be like a doctor checking each cell or organ for function to see if one is bad when forming a diagnosis. In complex units that could be a very time consuming and non-informing exercise.
A complex system is actually just a collection of very simple sub-sections which all can be reduced in your head to a network that can be analyzed as one. It has input, output and some transfer function. For what goes in, what do you expect to come out? This allows grouping sub-sections to be treated as a network also, input is known, so is the output fitting what you expected the output to be knowing the general transfer function of the whole is.

Old timers generally spend VERY little time on diagnosis, a few well chosen tests selected by the symptoms, can isolate the problem very quickly. If an accurate diagnosis is not arrived at in 30 minutes, 2 hours is no more likely to generate one. The delay is seldom due to complexity but to jumping to an erroneous conclusion earlier one or not understanding the operation of the unit well enough. Stepping back and coming at it again later with a fresh mind will often show that 25 of the first attempt's 30 minute session was wasted by following a red herring.

The main reason for this ability is not just length of time in the profession, it is eras. At one time, to understand electronics the start was fundamentals, the math, the basic circuit concepts. Knowing this fundamental grounding of all electronics allows any sort of circuit to make sense and approachable as a network, even if never seen before. That was a different era in education and hobby self education that seems not to be the path taken now by people getting into it, who tend to focus on technique or application instead of fundamentals.
You can tell that most people asking questions for short cuts or technique tips assume that is how the old guys have all this knowledge, by knowing lots of technique. Well, a little, but most is a different philosophy of how to approach a problem which works just as well with or without prior experience with that particular unit.
The same people asking for short cuts how to accomplish their task almost never took the time to read the basic electronics books in the library. That makes a very bigg difference in their tech'ing effectiveness and is something I dare say, all the good techs did when kids. That is a generational difference. A lot of kids in the 40s and 50s had serious hobbies for which studying the basics of that hobby was just part of the hobby as much as doing the hobby.
It was not length of time in the profession but era in which they grew up made the difference. Maybe it was because we in a simpler era when stimulus did not need to be intense for us to get a kick out of it. How many kids would have the patience today to quietly sit in the corner and read and reread about fundamentals that by themselves did not do much. For example I bet every older tech here actually spent hours toying with charging and discharging a capacitor, timing it and comparing to the formulas. Millions of kids in the 50s were perfectly fascinated with very simple but absolutely essential basic experiments whose lessons never left them. That is just not an option now, a kid will get bored if their DVD player takes 2 seconds to load. How many would be content with experimenting with a razor-blade radio or home made buzzer? By missing out on the basics, the understanding of how something really works will always be a fog. To fix something replacing suspected parts until the symptom goes away is about all there is to fall back on. To most on this forum, even some who earn their living from repairs, shotgunning is the normal repair technique.

[Enzo]

Amen.

How many times have I banged my head against the wall for too long on some problem, only to come in fresh the next day and within minutes say to myself "Geez, why didn;t I see THAT yesterday?"


I cringe inside every time I read someone's "HAs anyone seen this before?" question. It is looking for a symptom response chart, a chart that doesn;t exist.