Oscillating amplifier - Tips to fix

[@jp-um2fr]

I was getting a little worried, mind you, I do understand keeping videos new must be a challenge. As long as you are OK, that's all that matters. Not many 'old school' channels left on YT now.

[@peterlarkin762]

Too true, channels like this are a rare beast in the thik tok era. Dave on 12VoltVids is still going strong at least.

[@JohnAudioTech]

I do have a back log of videos to produce. Work has been keeping me busy and this "fixer-upper" house is needing my attention as well.

[@peterlarkin762]

@@JohnAudioTech Just keep on doing want you need to do. Houses are way more stressful and time consuming to repair than amplifiers that's for sure!

[@Bluelagoonstudios]

Last week I had to repair my own Technics SU-X102, first notice a resistor that got warm, and the transistor too on the supply board. When the amp went on, there was a peak current to the outputs. That almost blow my speakers. So put the amp on, and spray some cold on the transistor. And jup that was the suspect. Changed the resistor and transistor. Checked the usual things before closing down. Good repair and can do a few more years in my living room. All tough, I don't like those STK modules. But I got this one for free, so not mocking too much 😊

[@TrevorsBench]

This is gold, thanks John

[@copernicofelinis]

Nice to see you back to the bench, John.

[@alexarif2835]

Great explanation

[@davidahmad6090]

Cheers John, very informative as usual.

[@haraldh.9354]

thanks to go in detail with this topic

[@VIPINSAINI_20]

NICE TO SEE YOU

[@jessejames5081]

Just discovered your channel and thought I would ask a complicated question haha! I have taken it upon myself to do a project that I am not able to do, and I can't find any one locally that works on electronics and circuitry. I want to use an Altec Lansing Hydra shock Bluetooth speaker to play my guitar through because of the unique features that it has. The problem is that it does not have an auxiliary input. And I believe that even if it does all of all texts speakers seem to have latency issues when you are able to plug into an auxiliary port. So I need to install an auxiliary port and fix the latency issues. While somehow keeping the light show features. I know this is a long shot but do you have any advice you can give me or a direction you can point me in? I truly would appreciate it! Thank you

[@JohnAudioTech]

Not much help I can offer on that not knowing this device. Some class D power amp chips don't even have analog inputs as they use a digital front end. Latency is annoying for instrument amplification. I'd look elsewhere for your amplification.

[@jlev505]

I wish I would have seen this a week ago. I bought a damaged (non working power supply) Soundstream D100. I had never seen an amp like this! All discrete components. Not a single IC anywhere. I started tracing back from the base of the PS transistors. Led me to a MPSA42, MPSA92, and 2 TIP31. The collector of the 42 was fed by a to-92 that had C13F on its face. Couldn’t find any data sheet anywhere. It tests as a PNP. I found a GE SCR with the C13F? Anyway, the collector on the 42 had 6.8V. I had touched my probe to the back of the tip31 while probing the collector of the 42 and all the sudden I have rail voltage and a working amp!! After further investigation I noticed a resistor that was reading 94.8 Ω. It was a 33 Ω resistor. It was tied to the tip31 emitter the tip31 was paired with the 42s emitter as well. Replaced resistor and instantly had a working power supply and rail voltage. I learned a lot. All due to an accident. I also learned it is a multivibrator circuit. Very interesting stuff. Apparently Nelson Pass designed the amp I purchased. It sounds amazing for being almost 40 years old and 50x2 @4 Ω. I’ve repaired many class AB and class D amps that were filled with ICs. They were much easier to work on. Mainly because I had done it so many times and spent so much time troubleshooting. Discrete amps definitely sound better than IC amps. Thanks for all your videos. You have helped me a learn a lot when I first started repairing amps.

[@florentinmarica4450]

Thank you

[@Uraim]

yeah, i got a few years ago a Pioneer SA7800 from my uncle and it originally had 2sc2525 and 2sa1075 big mt200 transistors, and like i had to substitute one of that transistor for two 2sc3519 and 2sa1386, okay i tried to find a 85Mhz power transistor but i didnt seen it anywhere, so i opted for those which can do 50Mhz, which is still very good, i would say that 2sc5200 and its pair is known very well, but i feel like 30-35Mhz would been too slow for that NSA type of amplifier that i have. I actually did try it with slower transistors, i had some like 15Mhz transistors, and that amplifier didn't sound right, the frequency cutoff were too bad and the amplifier barely played any high frequencies like above 18-20khz, but with those 50Mhz transistors it can play above 50khz, and i can actually hear the difference between like the 18-20khz limit and the 50khz. Its so much nicer. Also recently i wanted to buy more transistors for my amplifiers to cover more power tdp, but while i wanted to buy more of those 50mhz transistors, they became absent, so i got some from other website, but they look a bit fake in my opinion, so i didnt put in in my amplifier for upgrade purpose. Now at least i have a SA7800 that sound something like the original amplifier, the bad thing with this amplifier that its too good, if i walk somewhere, meet someone and i hear music from their equipment, their stereo sounds so bad, while my amp sounds so detailed. Its like i always hear those cheapskate 2.1 speakers most of the time and people say oowwww its good, just hear it, the bass owww. In my equipment, i also recapped all eletrolyt capacitors, and i dumped most of the transistors due they were noisy, my uncle got the equipment in 1980, and the manufactured time is 1979 march. My amplifier passed 45th year 2months ago. :)))) It got a new life, i also done everything right, and i can dare to leave the amplifier turned on, its really realible in its new hearth! Thank you for the infos in the video, liked it!

[@SheikhN-bible-syndrome]

What would make the sine wave be round on the bottom and more of a point on the top of the sine wave?

[@johnyang799]

That is distortion. Namely 2nd order harmonic distortion. This is usually caused by nonlinearity of active components like transistors, mosfets.

[@SheikhN-bible-syndrome]

@@johnyang799 how do you fix it?

[@johnyang799]

@@SheikhN-bible-syndrome Depends on what you are asking. Assuming it's an amplifier then I need to know the circuit schematic. If it's a diy kit then I guess you can get a better one? Some with the name like blameless or Douglas would be good. Not sure if JohnAudioTech sell those JAT501 kits. If it's a circuit you designed yourself then It's more complicated. First what's the input stage look like? Is it single ended input or differential pair? Do you have global negative feedback? or it's a single transistor amplifier of some sort?

[@johnyang799]

@@SheikhN-bible-syndrome Recommendation is to adopt a blameless amplifier design from Douglas Self and start from there.

[@peterlarkin762]

Excellent advise above. I'll just add, it's a commercial amplifier start checking transistor and resistor specs and check if it's over-biased or if anything is overheating.

[@마루-r9x]

문해력의 출발 1,방대한 양의 활자에 노출에 노출되라 2,문장을 많이 구사해보는것말고 답이없다 원고지의 양에 비례함

[@austayo]

Its probably because its on a breadboard, but electrolytic caps for decoupling make my amp go hehe have very high frequency oscillation and have it destroy your ears.

[@JonDeth]

I would start with scoping the power supply rails, and then if it wasn't obviously sourced from there, pursuing other active stages to find the base frequency being experienced at the output, or a strong harmonic thereof to locate the cause.

[@b0xjoint]

I am hoping you get a chance to work on the JAT-801. I will be building that one next 🙂