Recommend a minimalist oscilloscope specifically for guitar amp testing?

[Steve Conner]

The DSO Nano is one of the products from Seeed Studio that I mentioned earlier.

And, again as I said earlier, the input voltage limitation isn't an issue, because you're supposed to use a scope with a X10 or X100 probe anyway. You just have to make sure that what you're buying has the standard input impedance of 1 meg in parallel with about 20pF, and any scope probe will work with it. (another reason NOT to buy the Iphone scope app, IMO, the audio input has an impedance of about 1k)

I had trouble with this when I bought my Tek 222A, not only does it have non-standard input connectors, but the input impedance is 330k / 66pF and the gain is 3 times what the readout says it is. I managed to build an adaptor for standard probes.

If you want the kind of build quality Fluke has, you have to pay for it, that's the bottom line. The Scopemeters start at a couple of grand. I've seen the old 50MS/s ones sell used for a few hundred, but the batteries are likely to be shot.

[Enzo]

In my view, making your spec for the scope you buy all depend upon one repair is not the best way to go about it.

I was in field service for a long time, I trained other field service techs. My hand tools are still half stored in my toolbox that flew with me all over the country. And my scope? I didn;t have a tiny scope, I had a plain old vanilla 5" CRT scope like sits on benches all over the world. The one I carried was a B&K 35MHz type with a fake leather cover for the front. It fit under the seat in an airliner.

Just get a real scope. Is it REALLY that big a deal to grab the handle on a regular old scope and carry it to your friends place?

Field service is more than taking a scope someplace. You'll have a meter too of course. Gonna take your hand tools, I hope? And a work light, or is that covered?

How about parts? Gonna have any of those with you? Simple things like resistors, caps, and fuses are a serious pain in the ass if you have to drive back across town for each one. Jacks. A selection of tubes? Will you be needing any semiconductors? Diodes, transistors, ICs?

A word about specs... don;t worry about them. The most minimal bandwidth is more than enough for the audiio work we do, so there is never a question of "is 20MHz enough?" On the other hand, you don;t need a 250MHz scope at all, but if you find one at a ham fest for $50, there is no point in spending $75 for a 50MHz one instead, just because it is "closer to your needs."


And really, don't cheap yourself into a corner. You don;t need a fancy scope, but get something real you will find useful in your shop for the rest of time. I can watch the nice clear bright trace on my 5" screen, I don;t have some ipod app that I have to hold the screen just so to see anything, and the CRT display is in real time, I don;t have to wait for some clunky digital screen refresh. And I can reach up and turn a knob to speed up the trace or change the vertical gain, I don;t have to punch up some menu and reset a parameter and then back out to rejoin my pictures.

[dchang0]

Hi, Enzo--

Yep, I hear ya loud and clear. I service computers in the field as my day job and used to fly all over the country like you did. (Thankfully, I don't have to do that any more!) All those other things we have to carry are precisely the reason I favor ultra portability in my scope (or in any tool for that matter).

My current rule is: if it don't all fit in my car-trunk tool box, then I don't need it anyway.

Of course, this means that there are repairs that are beyond my capabilities at the time, at which time I recommend that they allow me to take the amp/computer with me OR return another day.

I used to carry a very fine full set of Wiha screwdrivers. They were truly a beauty to use. But now, I carry a single modular screwdriver/wrench with dozens of tiny bits. They're clunky and can be very hard to use, especially in tight spaces. I recognize the trade-off, though, and for the million times I benefit from the added portability, there might be one time I find that they are not enough for the job.

It also has to do with overall tool "hoarding." I'm sure many of you have workshops stuffed to the rafters with tools and gadgets. I know, I've been there. One day, my home was burglarized, and I came face to face with the reality that I was sitting on thousands of dollars of tools, most of which I used once and never used again. Stuff that I afterwards had to defend with a dog and an alarm system.

And a lot of it was useless stuff. I had an expensive wrench made specifically for removing Honda axles, etc., etc., etc. So, I went through my tools and picked out only the stuff I actually regularly use. Everything else I might need, I swore to rent if truly needed. And I began selling off all the rarely-used stuff on ebay or to friends. That money got me through the recession, and I'm finally down to the barebones setup.

Nowadays, I won't buy something unless I've needed it three times in the past (this amp isn't the only time I've needed a scope) and only if the proceeds from a job pays for it. It's a great rule, but even then, there are things I've picked up that turned out to be used only that one time after I bought it. Thank God for ebay, huh? If you buy something, use it for a while, and then sell it again, it's like long-term rental, but with a far lower rental cost than the tool rental sources.

[Enzo]

Fair enough, and no I sure don;t want to go back to flying around either.

I was under the impression you were, other than for this repair, setting up to do this sort of work regularly. One can't really do serious amp work without a scope.

[dchang0]

Thanks, Enzo-- eventually, I will get a full-featured scope, per your advice.

[Ken Moon]

You don't need to spend much to get a decent scope - I've had good luck over the years with BK Precision scopes - they don't have the brand name of Tektronix, HP or Fluke, but they are solid, and there is usually a good selection available on eBay for less than $100 - I paid $30 for the dual-trace Model 1471B that I use all the time:



I like to look for old farts (like me) who are selling them, or if it's sold by an industrial surplus seller (lots are available this way), I look for listings that at least show a trace on the screen, and state that the unit pictured is the one you get, as well as having a 100% seller rating.

[dchang0]

Wow, that's a fantastic deal, and it looks far more compact than the ol' Tek "tanks." Thanks for the tip--I will keep an eye out for BK Precision instead.

[km6xz]

Unless there is a very compelling reason, of which none have been stated so far, don't repair amp in the field. By doing it at home you are doing it where you will be in your comfort zone with the things needed to actually repair the unit. Doing it at your friend's convenience, in their home, will result in it not getting done in many cases due to lack of parts or the right tool, without a suitable work surface, dependably lighting, your heavy variac, signal generator etc. If the friend is too busy or inconvenienced to come over with his easy to carry box, he is too busy to get it fixed. Setups, tracing to the defective unit in a system, and after repair performance evaluation is about all that can be accomplished in pro audio in the field. When my shop was doing a lot of studio service, I had field techs who went out and did the type of work intended for field work like tape deck level and bias setup, but any defective unit was identified as such and brought back to the shop where more skilled bench techs would do the repair using lab type gear, extensive service info and a large parts inventory.
If you can't fix it at the friend's home you are the goat, if you do not have the right parts with you, you are the goat, only if you happen to have the perfet combination of parts in your pocket and the right defect that would respond to the limited resources in your pocket are you then the hero. Why risk disappointing them by doing it half-assed in the field?
You will need just a few test instruments for 85% of repairs, and a hundred thousand dollars worth for 5%(the 5 % which the owner is told it is not worth fixing solely because the tech has no clue what is wrong or how to fix it) Most just equipment is still good, unless physically badly damaged but received a "death sentence" by a shop because the tech did not know how to repair it.
At the minimum for analog audio gear repair.
A scope.....a lab type from a couple decades ago is cheap and will outlast new cheapies by many years. Get one of the portable Tektronix like a 465, 475, 2215, etc
A good low distortion audio generator. One with calibrated output level and frequency accuracy really helps speed things. A real cheapie that is the bench tech's dram generator is an old HeathKit IG-5218. It has low distortion(.008% THD at 1 khz), constant level output(very important, switch frequencies and you want the level to stay the same or else a freq. sweep is useless, 1% frequency accuracy with decades range switches, and accurate step attenuator to -80dbm range and accurate level and db meter. I have generators costing several thousand dollars but still use those Heath generators for general bench work. Ebay has a history of them in the $50 range. Grab one.
A decent digital multimeter for general static voltage and resistance measurements. They respond too slowly for detecting trends but they are accurate and quite reliable even the cheapies for $50
An analog meter for adjusting anything where you can see trends visually instantly. The solid state classic is the HP 410C for about $100-150 and the tube version, 410B, for about $50. You can't kill it or overload it. It is chopper stabilized and fast responding and very fast in settling to the ultimate needle position.
For AC only analog meters, the HP 400 series, with the 400B is desirable for audio or the later 400FL or GL
Dummy loads. You absolutely need dummy loads for tube amps and desirable for solid state. Surplus electronics stores will likely have old high wattage wire wound resistors, many have a sliding tap or two for adjusting to values less than full value in resistance. Most are slightly inductive which can lead an badly designed amp to instability...just like a speaker does. There are special non-inductive power resistors for loads but they do not simulate a speak as much, so give a more passive pure resistance. You will need 2, 4, 8 and 16 ohms for tube amps so a couple with tapes adjust correctly would allow matching the Z of an amp to the load. New, they are expensive, $80+ each for 250 watt. You can find them surplus or make your own from Nickel-Copper alloy wire for about $10
A variac. These are expensive as metered bench instruments but you can make one yourself from easy to find autotransformers and putting it in a case and meters. You will want both AC volt and AC current meters. Most of the cased types sold have AC Volt meters but not current meters. Current is really important as a performance verification tool, diagnostic and a safety tool. No power amp should be plugged into full mains until proven to be working without defect. If you plug in an amp into straight AC mains and damage it it is your responsibilty, even if your friend knew there was a problem to start.
You will also need a good test speaker. Only use it after proving that the amp is working perfectly, otherwise you will be replacing a lot of expensive drivers.

Later when you get more into it, you will probably want a good distortion analyzer and spectrum analyzer with tracking generator. A tube and semiconductor curve tracer will come in very handy if you can find one surplus. I have a Fairchild curve tracer that plots transistors and diodes over wide ranges of current and voltages...up to 1200volts and 3 amps at lower voltage ranges. A lot of Hfe testers fail to spot transistors which are bad only at operating levels. A curve tracer does. A tube curve tracer, like my Tektronix 570 is too rare to even hope to find but keep an eye out.
A bench power supply would come in handy later on. For tube stuff, at least a isolated supply that covers bias voltage levels
Besides a soldering iron, a good hi-power soldering gun is needed on older amps for some chassis connections, and other uses.
A magnifier light is worth its weight in gold. An electric toothbrush is handy for finding vibration sensitive solder cracks, trace intermittents etc.
That is a start. A lot of basic repairs can be done for this set up and cost only a few hundred dollars, paid for with a half dozen serious repairs

[defaced]

Re Ken's photo, I'm glad I'm not the only one who has to find random plastic box shaped things to prop up chassis ends. Very very annoying.

[dchang0]

Man, you guys are awesome--thanks for all the info, especially km6xz's detailed list of must-have tools for any serious repair shop, WITH specific recommendations. There's a lot of stuff on that list that I know I need but haven't gotten yet. I'll print it out and start acquiring.

I do get you about the "being the goat" and "home turf advantage." No worries. Having been a computer field tech for so many years, I'm comfortable in highly stressful situations. Once, I was out repairing a major server computer between 1-5am where the company was literally losing $10K/hr because 3rd shift couldn't work. I think everyone and their brother was there looking over my shoulder until I got it fixed! (Obviously, one can't pick up a quarter-of-a-million-dollar-server and throw it in the back of a truck and drive it back to the workshop--they are delivered on pallets with forklifts and assembled on the spot. So I've been trained into a "whereever I am right now IS my workshop" attitude.)

[defaced]

You can find them surplus or make your own from Nickel-Copper alloy wire for about $10

Nicrhome wire would be another good choice. That's what the heating elements in your toaster/oven/whatever are made of and is often used for large load banks.

[Chriscolt]

I am new to this forum and I realize this post is 8 years old - but to add my "2 cents", I agree 100% with KM6XZ - perfect explanation - considering my OCD when it comes to explanations and amp diagnosis (or any electronics troubleshooting) techniques I would have added a "current limiter" as a "must have" piece of equipment for troubleshooting amps especially tube amps. You can buy one for approx $40 to 75 or you can build one for under $10 - using parts from a Home Depot type hardware store - a single gang electrical outlet box - plastic - duplex grounded standard 120V outlet - remove
brass buss strap on both sides (Nuetral and Hot) to isolate each outlet, a standard incandescent 100Watt bulb, a 3 to 6 foot 3 prong 120V electrical cord with male (Plug) on one end and 3 wires on the other. Connect black wire to top outlet on hot side (brass screw), connect a piece of wire of the same gauge as the cord (strip the cord about 6 inches and cut off 3 inches of all 3 conductors discard the white and green, use the 3 inch piece of black insulated wire to connect the neutral (silver) side of the top outlet to the hot side of the second (bottom) outlet, finally connect the white wire from the cord to the neutral side of the bottom outlet - connect ground (green) wire of cord to the ground strap on the back of the duplex outlet. In essence, you wired the top outlet in series with the hot side of the cord (electrical feed) and the bottom outlet being fed from the series connection in the top outlet - so if top outlet was shorted - it would complete the circuit in the bottom outlet and it would work as a standard electrical outlet - the "short" is the 100 Watt desk lamp - it acts as a current limiting switch device - just plug the lamp into the top outlet and the device needing current limiting (the Tube amp) into the bottom outlet - with the AMP turned off, plug the cord feeding the new duplex
box into a standard wall 120V outlet and turn on amp - Viola - if the light bulb is dim there are no shorts in the amp (low current draw) if the bulb glows bright after 10 seconds - shut it down because that indicates a high current draw / possible short circuit. The bulb prevents all that current inrush from entering the amp and possibly causing extensive damage to the tubes, transformers etc...

You can also use a Variac in between the current limiter and the source power (wall outlet) so that you can slowly raise the voltage (start at 0 volts) and note the bulb slow - if it gets brighter as you raise the voltage after a few seconds (waiting for filaments and rectifier to warm up)

I also use a DE DER 5000 LCR - excellent handheld LCR for testing all passive components - caps, resistors, inductors, chokes, transformers, also measures impedance and tests components e.g. capacitors at 5 different internally generated frequencies - 100hz, 120, 1K, 10K, 100Khz - tolerances, ranges are excellent and easily competes with LAB grade analyzers costing thousands of $$$ more - cost under $150 USD - also for semi-conductors - testing, matching engineering -- is the Peak Atlas DCA Pro75 - designed and engineered in UK, they make a whole line of quality hand held (smaller than a pack of cigarettes) analyzers. The Pro75 is for semi-conductors, diodes (all types), transistors, JFETS, MOSFETs, SCRs just to name a few - The analyzer provides all the specification details of a semi-conductor same sa you would find on a manufactures data sheet displayed on internal LCD screen, by scrolling you will get a couple to 9 or 10 lines of information depending on the device - the 3 test leads are color coded but are universal - IOW u don't have to know what leads go where, u dont even have to know what u are testing - the meter can be connected in any combination of ways it will determine what the device is, its specs and will tell u what leads are connected to what component legs - e.g. the green lead is connected to a NPN BJT transistor base lead, red is connected to emitter and blue to collector. When optionally connected to a computer via USB cable, the information is provided in written and graphical format including user defined test /performance curves - used to log various components and store results - results can then be overlaid and used for comparison, diagnosis or matching - e.g. matching hfe of 2 dozen Germanium transistors for a 3 transistor fuzz pedal build.

Also carry several analog and digital multimeters and have an old HP Capacitor analyzer that I use to test tube amp caps for leakage at actual operating voltages - can run tests up to 600 VDC.

I highly recommend Gerald Webers Tube Amp books and videos for further instruction, there are 5 or 6 volumes and they cover basic electrical engineering concepts and advanced diagnosis of tube related amplification circuits using actual popular brand schematics, examples, actual customer service request and questions and real life examples of amp mods, repairs, re-engineering of factory errors - etc...

Gerald Weber of Kendrick Amplifiers based out of TExas, holds a weekly (Tuesdays @ 8PM ET) 1 hour webinar geared towards Tube amp and accessories - repair, design, engineering, overview and discussion - using actual forum member Questions, issues brought to the meeting (sent in via email prior to meeting start) - where the entire team (25-40 members in attendance, USA and overseas) participates in the content discussed and problem, design whatever questions answered - schematics shared on screen, voice communication via LAPTOP built in microphone via internet connection or separate dial up conference bridge.

First few meetings "Free" Trial - includes one on one support from Gerald 7 days a week - for problems that cant wait until Webinar - Many of the forum members own Amp Repair business throughout USA - many are musicians, hobbiests, amp / music lovers who enjoy restoring, repairing, or designing and building their own Tube or solid state amps


Chriscolt

[Dave H]

[bob p]

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LOL