How to troubleshoot weird oscillation Fender BandMaster tube guitar amp

A great video adventure. Troubleshooting a strange preamp oscillation, generated within the stock circuit. Utilizing my oscilloscope, I was able to track it down. Although not totally eliminated, I wonder, is this present in all vintage Fender amps? Has it been overlooked? I will explore this in future repair videos. Yes, I always try to keep my videos fun, however there is sometimes a serious side of D-Lab

[@elwrongo]

Bandmaster, fab! "And its not the tremolo!", nearly fell off my chair as had just finished saying, "might be the tremolo."

[@johnmac8084]

Spooky, it's alive, it has a heartbeat 😄

[@johnsampson1096]

A few glasses of wine, you won't notice the oscillation! Great diagnosis as these are tough problems to find................... I once had a Realistic reel to reel Tape Recorder made in 1965 that had static in the audio. It was o.k if listening to but in record, it was there forever. An old TV guy fixed it for less than 10 bucks. It was a ceramic and noisy resistor!

[@andyleibrook6012]

Just now was able to sit back and watch this one without distraction. Absolutely worth the wait. Thanks for your videos.

[@rciancia]

Terry, that was very interesting indeed. I've seen those Ceramic Disc caps with hairline cracks so I thought you nailed it. You've proven that poking around with a stick is always the best first step. Nice job my friend

[@mikecamps7226]

All we get to see is the view of the tray of the chassis which shows all original parts..........which would imply that the power rail caps are also probably original. The pulse suggests the power rail. The caps are functional as there is no apparent hum but have to be completely dried out

[@ThePianoforever]

A very nice and clean Blackface amp. We love the work you do and look forward to seeing your wonderful results on the future projects you will be helping us on for the studio.

[@JamesE707]

The theories behind electrical/electronic engineering don't cater for quirks like we saw in your video. Good video and find!

[@DeathCapAmplification]

That was awesome. Your videos on the topic of oscillations have helped me tremendously in several cases. Thanks so much for this Terry. Really cool stuff. And I love the methods you used to track it down. Very cool.

[@LitesLAB]

That was great Terry! Neat problem and cool to see how you handled it. I can honestly say I learned something new, awesome! Thank you :)

[@preiter20]

Great troubleshooting Terry! The trace looked like my last EKG...😳 Another excellent investigation!

[@johnnymcquinn6025]

Really good one Dlab! Hard to find things some times. Been gone a while, i am in school for electrical engineering. Thanks to the inspiration from you, I wanted to know more about circuits. Now I will design and do many things...thanks!

[@sidbob169]

Great repair, fun one to watch! Ahhh, a new chassis stand, huh?...looks nice!

[@bijouxbijouxbijoux]

Well done on another great video Terry, fun and entertaining, And we all learn a lot at the same time ☺️

[@richysradioroom]

I really enjoy watching things like this....I dont know why? I love a mystery that is resolved.

[@wadereynoldsgm]

This was interesting. Thanks for sharing!

[@belguitars7102]

Brilliant stuff, keep these videos coming for us geeks!!!!

[@theguitaramptech]

That was a weird one, Terry. I did a blackface Bandmaster recently, but I certainly didn't notice anything like that....but, did I even think of looking for it. Well solved as usual, mate.

[@johnsweda2999]

Still got issue probably another resistor, I would have thought you'd have a non-contact probe to probe the components for breakdown, carlson's lab make a version you can build

[@Davidjb37721]

Thanks for your time

[@Deadeye901]

Just love a mystery

[@faultlessguitarsandamps1116]

Great to hear you are still learning too . That makes me feel better . Hahaha .

[@EngineeringVignettes]

My mind was saying "noisy carbon comp" as you were describing this... they can be easy to find with a signal tracer or Carlson super probe while tapping on them. Looked like some low frequency _motor boating_ feedback noise happening there, induced by vol change or a mechanically induced noise? Gratz on finding it :D Cheers,

[@douro20]

Noisy resistor syndrome...this is why signal tracers used to have a "noise" function to detect internal arcing in old carbon resistors.

[@eugenepohjola258]

Howdy. You are a great narrator ... Regards.

[@Evie_Kowalski]

I have learned a lot from you and that is why i continue to watch.. Why this one seems to still be a problem? Is there more that should be accomplished to solve the current oscillation?

[@ProphetNikoz]

Great troubleshooting! Could be oscillation on the power rail. You usually got a big cap on the B+, feeding the output tubes and a smaller cap after a resistor, feeding preamp tubes. The resistor drops down some voltage and also makes and RC circuit with the mentioned smaller capacitor. But still the circuit CAN oscillate on sub-hearing frequency. Saw in on one of the amps. Could it be it?

[@alphabeets]

Would it help to try to clip on a new cap one at a time to each of the caps to see if that affects the oscillation? It might give you a clue as to what component is causing this.

[@Fixologist1]

Wish I had a dollar for every time I successfully troubleshot with a chopstick.

[@Ricobass0]

The 'scope trace you showed at around 7m10s still has the burst of oscillation every few seconds. I can't see that you fixed anything, sorry. Did you check the cap and 1M resistor you took out? Were they faulty?

[@Tonetwisters]

Seems like it was still dancing pretty good to me ... but I'm just a guitar player ...

[@donaldfilbert4832]

My thoughts exactly !!!

[@sting1111]

This seems a bit inconclusive.

[@martyyoung598]

Why did you have the trace going so slow this time while watching for noise? You usually have it scanning fast looking for prickly spikes while you tap?

[@d-labelectronics]

A follow up video will be shot and posted tomorrow. The amp has never sounded better

[@bobblum5973]

Just to "recap" ( no pun intended, honest! ): What Terry said at the end was that he thought he'd cured the noise problem reported by the owner, but that the "blip" on the scope trace might be normal, he didn't know since he doesn't typically look at that point in the circuit with a scope. So rather than trying to identify and cure something that may not be causing problems, he just verified if the amp is operating normally.

[@mikeh2520]

What kind of time base sweep on the scope do you use in this video to show the oscillation?

[@nathanwatrous1519]

Just so you guys are aware I have also discovered that having a digital volt meters hooked up can also cause a parasitic oscillation to happen too and it is because of this video and having those scope hooked up that I was able to figure out that this condition was going on.

[@fenderay6strings425]

I having a problem with my early 60 Blond Bassman Just oscillating???

[@christopherfasulo1318]

Sorry to ask here, but I have a question on a White's metal detector search coil. Do you have an email address I could send my question to? Thanks for all of your great videos.

[@bemi-ton]

what frequency are the pulses? Is it something on the AC? Soldering iron thermostate ...

[@Starcrunch72]

I thought that too...but Terry normally has things plugged into the isolated power supply....but...I've seen wierd rupple on AC before too

[@dougakers7974]

Nice

[@donnyboy911]

Hi Terry...can not find how to donate ...post link ? Ok?

[@kornami8678]

Did you monitor the high voltage with the scope? Could be a bad power supply filter cap.

[@d-labelectronics]

I monitored the plate voltage on the 12AX7 with the scope, no oscillation, it was after the .047 cap

[@DoingMoreKustoms]

Another Day in Heaven.... Let'$ Get iT 🌴💀👍

[@andrewdenine1685]

Was wondering if you could give me a clue as to what you thin my problem could be ? It's a traynor ycv80 2 12 playing on either channel the sound cuts out completely. If I ever so lightly put my finger on ch. 1 volume knob or the gain knob the sound will cut back in. I changed preamp tubes 5 weeks ago but now this is happening. Tubes preamp are 3 12ax7 tung sol reissue power tubes are 2 5881 Svetlana and 2 6l6gc tung sol. I put the old 12ax7 back in they are the old sovtek 12ax7wa . Amp worked after putting old preamp tubes back in but then it happened again and again the lightest touch and the sound cut back in. Would preamp tubes cause this or could it be a bad solder

[@victorayotte5351]

Just a couple suggestions: clean your pots and tighten your tube sockets. Sockets can loosen over time.

[@everythingtakin]

Hello. Ive been in the possession of a very old fender head . I believe a 1962 concert. Made in fullton CA. FENDER electrical instruments on front panel. Im afraid to bring it for service. Who do I trust in boston area. Id love to use it. It works I just know its beyter to not run them without service first. I wish you could work on it. You seemvery honest and trustworthy. Any suggestions for techs in boston mass area. Someone youd recommend.

[@d-labelectronics]

Sorry, I do not know anyone in your area, seems amp tech are getting scarce.

[@waynegram8907]

Why did you set the Oscope timebase to a very slow sweep frequency/moving Dot on the Oscope to see the Oscillations frequency?

[@d-labelectronics]

To be able to see the trace

[@waynegram8907]

But can't you see the oscillation frequency on a faster Oscope Timebase setting? That very slow sweep frequency/moving dot on the oscope are ment to view very low frequencies. The oscillation is at a higher frequency since the customer can hear it which means the frequency of the oscillation is a higher frequency

[@DeadKoby]

I'm working on a SilverFace Bandmaster now.... it's not as well cared for as the one you're working on.

[@johnsimms3957]

I wonder if Leo encountered things like this?

[@bobr8565]

What, no wine?

[@MarderExplorer]

I have a '66 Bandmaster but no scope, only a decent DMM. I'd need a scope to check it out, right?

[@Joetechlincolns]

Keep an eye out for a 20 MHz scope, if on a budget. A good rule a thumb on used analog O-scopes is a dollar a MHz. So, 20 to 30 bucks. A signal tracer is a nice piece of test gear to have and there are plans to make one of your own if you can't fork out the inflated prices for old vintage one's. You can do quite a bit with a DVOM and an old Analog VOM as well.

[@Starcrunch72]

@@Joetechlincolns Agreed, a scope is a nice added tool for the bench, however it isn't a necessity until you get really deep into a specific oscillation issue. I've been half-heartedly looking for a scope to add to my bench, but haven't had the funds. If you have 2 decent meteres, a variac and a simple light bulb current limiter, the rest of the tools are gravy.

[@coyotecirclestudio9611]

Starcrunch72 I used to repair amps for years without a scope (so I acknowledge, yes, it can be done), but now that I have been using one for the last four or five years, I wouldn't want to work without it. It is useful for so much more than just oscillations. I can go front to back on an amp and visually see the wave forms, and after getting used to using one for a while, it shows you things a DMM (digital multi meter) could never catch. There's actually a lot of measurements you can do with a scope that can't be done reliably with a DMM. A VTVM (vacuum tube volt meter) can catch or measure some, but a scope is best. (try measuring the evenness/offest of vibrato pulses with a DMM. Won't happen. Can't see crossover distortion on a meter, digital or analog. You can measure max output before distortion with a scope. You can see microphonic tubes on a scope. Etc, etc.... There are just so many easy yet insightful functions and measurements that can be performed with a scope that just can't be done with a meter alone.) At this point in my career, I consider the scope an essential tool to save time and avoid unnecessary guess work. Any competent tech should at least be familiar with their use, and really should be in the habit of using one as a first stop diagnostic tool.

[@matthewf1979]

No, they aren’t like that normally.

[@gustavohernandez8731]

porque no separastes el preamp del amp principal podrias de esta manera acotar el problema

[@sting1111]

I really need an oscilloscope to trace guitar amp noise. I am tired of guessing. I don’t have the experience to choose one. Can anyone offer advice? Thank you for your time.

[@martyyoung598]

Terry has done some nice videos about choosing a scope. It’s been a while back, maybe a couple of years. He talked about some of the lower priced off brands like Leader very good for this type of work.

[@sting1111]

Marty Young thank you Marty!

[@coyotecirclestudio9611]

Luckily, for audio signals in an amp, you don't need a terribly powerful or fast scope, you aren't measuring microwaves. My mentor always insisted analog scopes are better for audio tracing than digital scopes, but he had several of both, so I don't think that's a hard and fast rule. I purchased a Tenma 72-6800 20MHz dual channel scope new for maybe $400-ish, and it has served me very well as my main o-scope for the last four or five years. (I have no connection to the company, just a satisfied customer) No complaints. You could go cheaper, but this was the best "big boy" scope I could find at that price point at that time. You want to get at least a 20MHz scope, after that the choices are up to you. I really like having two channels to compare different wave forms, or to to be able to flip between two separate monitoring points, but to be honest, 90% of my work I only need a single channel. I found this article pretty helpful, but I'll admit it is a bit of a slog to read through. Very tech and spec heavy, so take your time, but it does explain a lot of what you want to look for from an amp tech's perspective. tone-lizard.com/oscilloscopes/

[@bobblum5973]

@@coyotecirclestudio9611 I tend to agree with your comment about digital versus analog scopes. Modern digital scopes should be fine for audio work, older digital scopes needed to be higher end models to handle higher frequencies but the technology has improved greatly over time. On a related note, there are some things that an analog meter make easy that are difficult on one with a digital readout. I've used a few digital ones with a bar graph as well which have been pretty decent. *Sting:* Good luck with your scope search!

[@1974UTuber]

So the amp had a regular timed spike and fall on the display. In a hospital tgey call that a heartbeat. Then you and the owner decided it needed to die and you wanted to make tge poor thing flat line. lol Poor thing will now never evolve to its true potential