How to signal trace Fender Champ guitar amp Oscilloscope stage test D-lab
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- เผยแพร่เมื่อ 28 พ.ค. 2017
- As requested, here is a basic signal trace method via oscilloscope + audio generator. This is the way I normally go thru an amp looking for loss of signal. A general purpose oscilloscope will do the job just fine. Don't spend the big bucks on one. I recommend Iwatsu or Leader units. If you follow this plus utilize the amps schematic, you cant go wrong. Hope this is of value. TD
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I love the simplicity of these amps - and they still sound great!
i'm a total noob trying to make my own audio amplifier with many tubes. i wasn't sure where to measure signal and am glad you did this video. thanks!
Thank you so much Terry! I am one of those subscribers who asked you to help us how to troubleshoot a Fender Champ. I really didn't expect from you a reply as many other people might do the same, but you answered my question through this video! Thank you so much, you are a great man! Thank of you many tube guitar amps will survive all around the world. All the best for your new projects in the future!!
Another excellent video, always clear & concise and great to watch.
You still have ground reference on your neutral wire. The device under test should be isolated away from the ground and neutral via an isolation transformer.
Thank you Terry, can't wait to try this on my 57 champ build kit.
Thanks again Terry....great video and great explanation, as always!
As a 40 year veteran of electronics instrument repair, I have to disagree with your recommendation to float your scope ground. You may influence inexperienced techs to cut the earth ground off their test equipment cords. Back in the day, my company provided each tech with an isolation transformer for the device under test. Then we could ground a scope probe to the chassis. One other thing: when you state signal voltages, ensure you're clear if it's AC peak to peak, or DC. Otherwise, your videos are very helpful.
As a recovering methamphetamine addict, I concur.
I can't give enough thumbs up for this!!!! so awesome!!!
Thank you so much for us with no amp tech within a 5 hour drive
Great tutorial Terry!
Thanks for this tutorial. I may try a similar run through on my champ clone.
I very much enjoyed this.
Cool I love the AA764 champ.
Great educational video!!!
I am going to chime in the lifting of the ground on the scope. Maybe you meant the isolation transformer? If you have lifted the ground off the scope and the chassis is hot, hooking a scope ground to the hot chassis will make the scope chassis hot - at power outlet voltage.
Many thanks for this. I looked through all your videos but didn't quite find what I was hoping to find. One of these days, could you do one showing how to find the source of noise with a scope? I've been chasing down some hum in a preamp and I can't determine the source of it, but I haven't tried with a scope yet.
Informative and useful! Very important tip about isolating the scope gnd. Thank you!
I wish you were my next door neighbor!
Great! Thank you!
Thank you for this!!!!!
Excellent video. I'm a huge champ fan myself. Any suggestions to minimize hum from this circuit?
I am writing to inquire about an issue of my Markbass CMD151P Jeff Berlin amplifier.
I turned the amp on and practised for about 5-10 minutes. The sound was good, bright, full, nice. After that, the sound started to be weak, volume is little bit down, feeling is hard to play and not responsive. So, I stopped to play and wait for about 5 minutes (wait with turn the amp off or wait without turn the amp off), the sound became full and rich again but only for 2-3 minutes. Then again the same issueis appeared.
Even though the offical service in my country (Turkey) couldn't reilize an issue, they accepted to replace my amplifier with a new one. The issuse pursued in the new amp and I decided to change other elemets; my guitar has also replaced other related parts such as socket etc. However, the issue is still exist. I started to belive the issue can be from a specific batch production.
I would appeciate if you help me to solve this problem or give technical advice and check procedure that I can apply as an Electronic Technicihan.
Excellent, easy to follow video.
Regarding the ground isolation, could you also connect the amp (or perhaps the oscilloscope) to an Isolation Transformer? Would that help do the same thing?
Not the scope! Just the amp!
I've got a 76' twin reverb that has been modified to have a hi gain channel but recently I was hearing microphonics from one of the pre amp tubes so I swapped the tube and still same problem. I tried both the new and old tubes in different sockets and they were fine. Is this a bad tube socket?
Do you have schematic 'walkthrough' video?
To isolate the scope from the ground seems a good idea to avoid short circuit and protect the amp and scope but a very bad idea for safety because in this case
the target of the short-circuit will be YOU !
Great video. Simple and to the point. I have a simple question for you. I was always told that we need to connect our circuit in test through an isolation transformer for the reason on not burning the oscilloscope down. So, by cutting off the ground pin from the oscilloscope plug we have the same effect? Thanks in advance.
NO!! Not at all!! You still have ground reference through the neutral.
Have you had any issues with the trigger on the IWATSU ? Regards Chris
Thank you Terry for the excellent tutorials. So while your ‘scope is not grounded, just for clarification sake, your audio generator and Champ amplifier in this video are in fact grounded, correct?
Are you testing before or after the caps! Does it matter or depends on the CRO you're using?. Is that why you isolate the CRO?
Hey Terry! I have a 60's Ampeg GS-12 Reverberocket 2 that just developed a loud hum after a short warm up. Have wine, oscope, signal generator and schematic......can you or anyone on the channel give me some words of wisdom on a troubleshooting plan to locate the failed component? I recapped and retubed this amp a couple years ago and I less than 20 hours play time on it.......and it's been sitting unused until I fired it up to use with a pedal steel guitar I procured recently.
I need to get test leads that clip on like that
Hello, for this test what type of probe is that is it simply a 100x?
Thanks! Specially the "Ground" advice!
Very intresting, So you use a power cord with removed ground lug for the scoop. Why not using such a power cord for the amp you are testing?
do you have a video on how to scope these amps---trying to find a hum or buzz issue ?
I’m new to O’sopes but I have been playing with tube amps since high school as a hobby, I’m 67 now.
I bought a ChiCom 100mhz digital scope and it has a normal IEC power cord with ground. Do these need a ground lift too, (plastic case)? The manual doesn’t mention any cautions.
Terry--thank you so much for your videos! I've learned so much from you. Just for clarification, the last connection you make on the 6V6 is to the OT blue wire. You say "pin 5" in the video, but isn't that actually pin 3? Don't mean to nit pick--I'm trying to locate the source of an annoying buzz in a 5e3, and using your vid to figure out the checkpoints.
Yep, I may have mis-spoke, may have been induced by too much wine!
so pin 5 for the control grid was what you tested 1st THEN you went to pin 3 for the plate, and that was the 'wonky' looking was with no reference to ground............
Well spotted, Jim ! I had noticed this too, was a bit confusing at first! But chapeau for Terry !!!
do all scopes need to be 'ground isolated' even for modern tube amps with the three prong connector?
You don't ever lift the ground on the scope. Only the device under test, by using an isolation transformer.
Im currently working on restaurating an old Hohne MH18 amp. Are there any dangers of damaging my scope when probing?
My scope is an HP 54111D with some basic Keysight probes.
So, ungrounded scope? Hmm, well, so what happens if your grounded amp makes connect to your ungrounded scope which for some reason now has voltage on it? Same sparks, right? I would say leave your scope along and if you have an amp that doesn’t have a 3 way ground plug, set it up, plug it in, carefully turn it on and take your volt meter and read from chassis to another grounded equipment ( scope) looking for AC and DC. I recapped a 50w Marshall JCM800 2204 the other night. Well the 50 uf / 50 uf audio cap had a ground coming from audio ground on the circuit board. Turns out I didn’t realize another short metal wire going from the cap ground lug to chassis ground had broke….no ground! So now the grounded cap is floating 242 Vdc back to the audio jacks and circuits there. I got a nice buzzing shock on my finger tip touching my audio generator when I connected it. Luckily I was taught years ago the one had rule when probing around high voltages and didn’t complete the circuit by touching the grounded chassis with my other hand, Now there is a good safety tip, one hand rule! Great presentation otherwise.
Can you see any distortion/saturation on a scope?
Mr. Terry:
I have heard from different - yet reliable sources - to NOT lift the ground from the o-scope. Told it was SOP to lift the ground from the DUT.
Can you perhaps make a viddy to make all of this clear?
I mean, safety first - right?
Thanks!
RW
I'm going to defer to the professor but, I will share my experience with working on television circuits from more than 30yrs ago, we used to "float" the scope by using a ground lifter because when you're probing the high voltage circuit, if scope is grounded, it's easy to cause an undesired arcing/Sparks across the probe. So when you use a Gnd lifter your probe is safe to poke around the circuit without causing unnecessary excitement. I hope this helps otherwise the professor will hopefully provide a better explanation.
I greet you very much, I am a beginner who would like to learn with an oscilloscope, but I am really afraid to connect anywhere about the earth cable, something I read that be very careful, so the solution is, in my case, to cancel the ground with plugs regards, thank you very much for your comments
Hi, Terry I have now been successful getting a sine wave on my analog scope from my function generator through my amp with a dummy load and the amp powered by the variac. But I have searched for days to find a simple answer to a question: If I work only on amps with transformers, do I need to plug the amp into an isolation transformer? Would this type of amp be safer to work on plugged straight into a GFCI outlet?
Hello, an isolation Xfmr wont do any good, However, a Variac would be the best choice.
TD
What ac signal level should be seen at the 6v6 plate with 150mv input?
good question
Great video. If you were hearing cycle hum, where would you see that in the scope
Right across the speaker output jack.
Thx, helped me locate a blown grid leak resistor!
Cool deal man, Great find!
Signal tracing a simple tube amp is okay, but solid state ones?! At the moment I'm lost in a quasi-complementer type old SS Peavey poweramp. It's a pain in the arse for a beginner. Nice video as usually by the way!
Ah yes,the old quasi-complimentary stacked NPN T03 power stages.
They are tricky to set up without destroying the O/Ps. I've had a bit of
experience with those in the old RCA transistor manual circuit (Brute 70watt
rms) quasi-comp amp Biasing them is the biggest trick with the PNP/NPN
driver stage and the biasing diodes.
Unlike tube circuits, if the bias is wrong or one of the driver transistors fails,
all of a sudden you have a bunch of blown power transistors.
Hi Terry, do you ever use a digital oscilloscope? whats your thoughts on those? thanks
A guitar amp is basicly simple, but sometimes I get lost in receivers and hifi stuff, is the signal lost, or me? lol
Cool video! I have a question about the modified power plug on the scope. I've read several sources that recommend AGAINST using isolation transformers, or any grounding defeat on the scope. Would it be better to have the amp (in this case) plugged into an isolation transformer? I know tektronix advises against using a "floating" scope. EDIT: some further research indicates the amp should be isolated rather than the scope being "defeated." Not that I'm trying to make any criticisms. I just want to understand the Do's and Don't's so that I don't fry my scope (!), my vintage amp (!!), or me (!!!). Thanks again for the video. Cheers!
Hello, Isolating the ground lug on the amp would produce noise, since the hum balance circuit is referenced to chassis. The reason for isolating the AC cord ground lug is to protect the scope. There are instances of improperly grounded devices drawing current thru a grounded piece of test equipment. The test equipment turns into a fuse, in other words probes melt, components burn up. I have used my isolated ground scope method for 40 years, never an issue. The articles you are reading may be referring to other test situations. However on a tube amp, I would recommend the o'scope ground isolation. Hope this helps.
@@d-labelectronics So you're saying that a device under test that's connected to an isolation transformer with no physical connection to the mains and ground can still draw current through the oscilloscope when the scope is grounded?
How do i avoid blowing up my oscilloscope doing this with the high volts? Do i set it to AC coupling well as ground isolate the lead.
Use a X10 probe, Run your scope on a ground isolator
Thanks so much for that and your tutorials rock. Subscription complete, cheers
Hey, I'm not sure if you could help me out or not. I have a Fluke Scopemeter 105b II. I use it solely for the purpose of looking at analog audio signals. When using it to test a low voltage signal coming from the head unit from 0VAC to 5VAC the Scopemeter displays the sine wave perfectly. When I use it to look at the signal coming out of the amplifier at 10VAC to 50VAC the sine wave looks crazy and adjusting the settings doesn't help at all. Do I have to change something on the oscilloscope to look at higher voltage analog sine waves with out the waveform looking like it's having an aneurysm?
don't float your scope
The contents of this video is great and very informative, but de audio quality is not that great to listen to.
The audio quality of the intro is very hollow, and after the intro, the audio is off-balanced to the right.
For a audio technician, is this a little bit disappointing.
YOU ARE LUCKY YOU DIDNT BLOW THE SCOPE UP. POOR PRACTICE NOT ISOLATING THE CIRCUIT FROM THE SAME GROUND REFERENCE AS THE SCOPE.