Now this is a great idea and doing it in this manner will be invaluable to so many people who just want to learn and have an interest in this. Forums are great but seeing and watching makes all the difference. I'll be tuning in
When I heard the sound I knew what it was, DC across the pot. Once you have heard that sound you know. Then you need to track down the source of the DC and how it is getting there. I had a stepped type treble and bass controls on a Magnavox console that would pop with every step change due to leaking caps. Replaced the coupling electrolytic caps in the preamp and amp and no more pops.
I think what you are doing is a great idea! Reminds me of what Marlin Perkins did with Mutual of Omaha's Wild Kingdom show back in the 60's through the 80's. There was a disclaimer at the beginning of the show that all events may be staged, but depict actual animal behavior in the wild. A great learning tool that recreates events which would be hard to come across by chance in the wild and catch on film.
I think I should urgently replace my capacitors on the Orange OR120. That's probably where my problems are. Many thanks, and best regards from Germany, Kay
WTG Terry, thanks again for sharing your valuable knoeledge. Those dang leaky caps strikes again! Knowing which stage or section of the amplifier to troubleshoot comes from understsnding the circuits of each staage, power supply, preamp, amp, or speaker. The preamp cam be devided into sections, tone, fx, or controls and their tubes. I've learned from you a lot aboit tubes and their purposes or multipurposes, how to break down each stage by their tubes, PS, preamp, and PA. One thing that I do first is troubleshoot for bad connections, then old caps. Way back, we tested tubes first at the Rad Shak becsuse the caps and pots were not old yet. Then maybe a blown speaker from abuse? Sometimes we got lucky, and bought a ""Lifetime"" tube or cheapo speaker from Radidio Shak!
PS: btw, way back, I had a Rad Shak MIJ guitar amplifier that had two twelves as I recall. It had four tubes, I guess, since it had reverb and tremolo. It was either a 6aq5oir 6bq5 amp. Lafayette sold a simular guitar amp that was supposedly a rebadged Univox. It serms that you favor "made in the usa" amps. The MIJ copy cat amps were built cheap, and at best were good for home practice. I'd like to see you do a vid about an old cheapo made in Japan copy cat amp, to see a diagnostic 50+ years later. . Rad Shak amps are quite rare, but you might find a tubie Univox for funage.
Congrats on the new series, Terry! xraytonyb is doing a similar series on his channel. Building an integrated stereo tube amp kit and walking though EVERY aspect of it. Tony even tossed the circus board in favor of point-to-point wiring! The circuit is SO simple, it's comical seeing it laid out on a PC board!
David Perkins, I love Terry and Tony, both great teachers of how to do point to point wiring, and do it right. It is an art, and takes that old school knowledge which is becoming a lost art. I tip my hat to a couple of great teachers here on the tube.
There can be leakage on the board itself too, and changing the caps is just a temp fix and they aren't really leaky. Should desolder one end first and check if there is any voltage to confirm suspicions. If there is none, start measuring the board, they attract crap from smokers and moisture, also that wax that CBS started adding later can become conductive and it needs to be removed in some cases.
nice idea for the series Terry - thanks for all the great content. you mention the problem here being a leaky "tone cap" rather than identifying it as a leaky "coupling cap" feeding the tone stack. thought it might be helpful to point that out since "tone cap" is the standard term referring to a cap between a tone pot and ground responsible for altering tone and using that term to describe these frequency tuned DC-filtering couple caps could be a little confusing for some audience members. cheers.
just started fixing amps love this and i have seen the resistor thing years ago when i first got into tube amps as a young teen tried to help him but told him you have messed with this thing to much
Terry Nice video when I was in high school our Electronics teacher had us built this RCA AA5 chassis but he did not tell us he included either wrong value parts or defect parts !
It’s always such a joy to work on old hand wired boards and chassis mounted sockets and pots. Today’s typical tube amps are ugly multiple double sided circuit boards with surface mount components and a rats nests of ribbon cables. You’ve got to have an electron tunneling microscope to see what you’re doing. You literally have to rebuild an amp just to change a blown hissing resistor. Good look on your new full time work. Always enjoy your videos!
I would not have thought .3 or .8 of a volt would cause that much of an issue on the other side of the Cap. Thanks Terry for the 'awakening' of such things.
Hi Love all your videos. Maybe my hearing is not so good, but I could not hear the rumble. I think a seperate mic for the amps speaker would help. Dub in the audio track. Various sounds amps make when sick are important for diagnostics. Looking forward for more Dlab videos. Bob
Great idea Terry. I always look forward to your videos. I don't know if you would want to give away your parts supplier information, but if you do, showing where and how to find good parts would be a good video too. Of course, that may overload and deplete those vendors' supplies!
Howdy. Great. I had a persistent problem with my DIY amp. The driver stage of one channel sounded awful and the DC levels were nuts. I disconnected the input cap. No change in levels. Then disconnected the stage neg. feedback cap. Still no change. Finally I disconnected the global neg. feedback cap. Voila ! Unmounted the cap showed a resistance of about 800 ohm. Brand new cap and faulty ! After replacing and reconnecting everything the stage worked fine. Since then I have encountered another leaky cap in the bag of 50 items I bought. Lesson. Sacrifice the time to resistance check caps before soldering them in. Will save one a lot of headache in the long run. Regards.
Yeah... Longtime subscriber and I do enjoy seeing these type of issues and the solution. Perhaps slightly more explanation. Okay DC on a pot but why does that make it scratchy? My guess is DC discharge causing tiny sparks, almost microscopic but enough to change the audio signal from flat nothing to a series of point discharges as the pot surface is rotated. Oscillations, hum loops, conductive boards, NFB resistor sizes verses the tap on size of NFB added or removed, Hum in general, plate and cathode wires to close to grid, raised heater...why? and How do you make one? Wrong size pot what happens...symptom, Linear when should be audio tapered, X-Cap bias feed circuit, crackle from carbon comp resistor fail, why dual bias circuit could be useful in these times with hard to find tubes. Some ideas. Love the format, induce a problem guess the issue and why the fix is the fix. Great info channel, keep up the great work.🙂
Thanks Terry! Does the signal ride on the DC and that is why you’re blocking DC…can you provide a little more explanation there ? Also, it would be helpful to know how to wire jacks and pots and provide explanation on how the signal passes ..
I'll bet there are lots of folks who would love to see one of these done with an inoperable Fender tremolo circuit. I just spent a few hours fixing one. I tried everything. Once I took the LDR leads out and tested the resistance out of the circuit, it was obvious the resistor had gone bad. I assumed since the light was blinking that the opticoupler was fine. I wound up making my own new "roach" with an LDR and high-brightness LED. Trem works better than ever now. Anyway, I'm sure you have a much better approach to it than I did and I could have saved a lot of time. I make tons of RU-vid videos. I'm a Realtor so nobody watches mine, but I know good video production and this format is great! Short, to-the-point and packed with good info.
For future videos perhaps include a method for comparing the bad component with the replacement, say using ESR / LCR meter etc. Many viewers may not have such a meter but it's the diagnosis / confirmation procedure which should be of interest.
So simple ,with just using a voltmeter to find the errant culprit or culprits. After hearing the pot was replaced and had the same issue one must go a little deeper to find the problem. Thanks Terry.
I sent my Champ in to have it serviced, the volume pot was noisy and it started popping and making static. It came back with a new rectifier tube and was still noisy at the volume pot. I replaced the pot myself and the popping came back. Grrrr..the original Fender tube was probably still good. I noticed the power tube and rectifier tube were kinda loose. I replaced the tube sockets myself. All is good. I will be watching the videos.
This video and the next ones are a vital look at how to repair your own amp. Many of us, your loyal subscribers would have known the answer to the 3 choices you presented because you have explained it before. I knew. By the by, I normally watch RU-vid videos on the iPad speakers but on this one I used headphones and the opening music was great! I found out what I’ve been missing! Love from spring in the Rocky Mnts. Thanxz
Wow!, that is incredible, & I would not have thought about that being the issue. I have thusly been educated! I was under the old, (& flawed) impression of if it's not broke, don't fix it. I like to think that after much confusion, & striking out, I would surely thought to check the caps, but that definitely made it a conscious potential issue for me. Thank you