Thanks Teemu, beautiful cheesy amp, from the beginnings of Japan´s musical industry. I guess it must be *very* clean. The Fet preamp, up to and including the volume pot, should drive nicely an LM386 or any TDA20xx. PS: the Suomi labels were very interesting too. Obviously I can´t read them but I *love* that tongue´s sound. How do I know? Because I love Finnish cinema.

IF you are using a series lamp fixture, the lamp glowing brightly means you have a problem, most probably shorted (burnt) output transistors, not excluding some other nasty possibilities. Anyway, the series lamp is protecting it from getting worse, and letting you measure something. Try to post the schematic or a link pointing to it, so we all speak about the same thing. Try to read what the output transistors are, just to begin with something.
Bye.

Dear Armstrom. There´s something weird here. C1 is meant to NOT affect gain, at least within the audio band; it will be "flat" from a certain "cutoff frequency" upwards.
The cutoff frequency for an RC network 47k/47uF is around 0.068 Hz. The 47K comes from considering both 100K biasing resistors in parallel for audio purposes.
C1´s value would affect gain if you punched in , say, a 48 PICO farad instead of 47 MICRO farad.
Please check your simulation.
Bye.

Dear THChrist.
Your posting was very opportune for me: a client came to buy one of my 100 W Bass heads, and asked me to repair his MG15 DFX (the same as yours but with digital effects) , which had an unusable line out !!!!!!!!!!!!!!!!!!!!!!!!!!!!! (Incredible). I talked him about you, he was very surprised and thought I was joking. He kept repeating as a Mantra: "It can´t be a coincidence" ..."It can´t be a coincidence" ....... etc. etc.
The sad fact is, nothing can be done there: that In/Out is part of a very poor passive (resistive) mixer. In practical terms, touching anything there will mean losing the reverb . My last ditch solution was to drill a neat hole in the back panel and install a jack that sent out the speaker signal, attenuated 10/1 by a 10k resistor from the "hot" speaker terminal (I just soldered it to the red speaker cable) to the jack´s hot terminal and a 1 k resistor from the jack´s hot to ground, plus a black cable from the jack´s ground to the power supply ground. I used a plastic insulated jack, similar to the other ones; it worked like a charm.
I liked the amp very much, it was loud for its 15 watts, the speaker was very good, and the distortion became wall-shaking when the new line-out went into the line-in of my 100 W amp, which was driving a 2x12" cabinet.
If it´s not clear to you, I may post a simple drawing.
Pictures: impossible, my client run home with everything to disturb HIS neighbours.
Bye.
JM.

Hi BigC. I do like very much those Yamahas, specially for their excellent clean sound . I´ve been looking at the schematics (thanks Teemu for the links, thanks Enzo for posting them). They´re very good, if somewhate dated (obviously, they come originally from the late 70´s), they were state of the art for that date.
Specially interesting is the use of Fets emulating tubes, unfortunately only on the input circuit. The distortion is not bad,it´s comparable to an MXR Dist+ or similar, but definitely not "tubey".
There´s not much you can do, I think that your best bet would be to build some good pedal and stick it in your setup.
Changing OpAmps would do very little, if anything, to change the sound.
The power amp is *very* interesting, *very* HiFi, surely ripped, from some of their Hi Fi line, bur not "rocker" at all.
Your best bet would be a good pedal or pedalboard and use it only as a faithful amplifier (or use the combo very loud, allowing the output stage distortion to smooth the somewhat fizzy preamp dist.) ... but your neighbours will hate you.
Bye.

Thanks, Joecool, it´s very kind of you.
Good idea about the dual (or multi) posting, anyway the "lingua franca" here is English.
I´ll stick to it unless I see our "partner" has some difficulty with it.
Thanks.
JM.

Hi rockman, beautiful amp !! One of the best SS amps ever made , probably the best built.
It was called "the Twin Killer" and used by BBKing, somewhat like getting a Nobel Prize.
You have a thermal problem (you already know that), your amp is sick but not dead, try to avoid killing it, it would be a pity. To begin with, ONLY plug it into a series lamp (Teemu posted the schematic, search for it) .
You'll have to make some measurements to guess what the actual fault is. You have also a non-sounding but not dead channel, that´ll be seen later. Post your schematic or link to a published one so we all talk about the same one, or we´ll quickly become crazy.
Bye,
JM.

Hi GPW. Can somehow scan or trace and post the schematic ? Thanks.

Hi RDV. Thanks, excellent translation. I had second thoughts about my post, but since it was adressed specifically to my Chilean neighbour, and he already was referring us to a Spanish written page, I just wrote it in Spanish. I´ve already seen that 99,9% of projects here refer to the same 2 or 3 chips: LM3886, TDA20xx and specially the ubiquitous and very easy to use LM386, which is a good idea. I just wanted to make clear to our friend that it was somewhat unlikely that someone else would "debug" a new PCB design for a chip not easily available.
Thanks.

Hola DSMNOISEM. Lo que veo con los STK es que aparentemente son hechos "a pedido" para los grandes fabricantes orientales, en cantidades de 100K a 1M, según sus especificaciones, las que, obvio, son Hi Fi, stereo, etc., y lo que aparece en el mercado debe ser un "sobrante de producción". El problema para un constructor aficionado como nosotros, es que debemos comprar "lo que hay", y eso varía mucho. En cambio, el LM3876 original, fue luego reemplazado por el LM3886, manteniendo el pinout (bueno, 1 pin quedó sin usar), lo que permite, si diseñás una plaqueta, que sea útil para muchos,durante  mucho tiempo. Fijate lo que pasó con el TDA1514, usado por Marshall y otros: hoy cuesta muchísimo conseguirlo; con un STK eso habría pasado muchas veces. En cambio, los TDA20xx son abundantes y todos los mono tienen el mismo pinout. Los STK son típicos de Sansui, Pioneer, etc. y creo que alguien como Yamaha los usó, pero si hoy te aparece uno quemado, tenés que tirarlo (o, con suerte, adaptarlo). Saludos desde Buenos Aires.

Beatiful piece of equipment. Is it a single 240W power amp or two 120 W units? In any case, you´ll find on the net the schematic of a Traynor 100 or 120 W power amplifier, with two MJ802 devices; it should help you repair the one you have, being basically the same.If you can´t, I´ll search it into my poorly organized files .

Basically, yes, but don´t do everything at the same time.
I suggested *different* options to be explored one by one.
Of course the series lamp will be used in every one of them, until eventually you succeed.
Now I´m closing shop , very tired, but I´ll try to write for you some kind of "road map" to guide you in your testing.
I see a very high probability of success.
Bye.
J M Fahey.

Hi. I just found the schematic you posted.
In decreasing order of probability:
1)Output transistors shorted/burnt
2)Output transistors improperly mounted (bad mica, etc.) Measure the insulation from any of their pins or body to ground.
3)Ooooops!, maybe you miswired them , check the coloured cables, pinout, etc.
4)Waaay too much bias (which would turn them hard on), measure the voltage across R14.
5)Bad, shorted power supply capacitors (not very likely but not impossible)
6)Bad Tr3 or Tr6
7)While testing, remove short protection , Tr4 and Tr5 or just remove D1 and D2. Anyway the series lamp is now your protector.
8)Don´t plug the amp straight into the regular power outlet until it´s finally fully fixed, the series lamp will let you play up to 20 or 30 W which is plenty for testing.
9)The amp, plugged into the series lamp but without output transistors *should* power "almost normally" and let you measure, mainly supply and bias voltages and D.C. voltage on the output jack.
You have already checked the power transformer and power diodes, so far so good.
Pray, it helps.
Good luck.
J M Fahey.

Dear Jack.
By what you describe, you originally had shorted output transistors.
Now, although you replaced them, the fuse keeps blowing, fast!.
Probably  the original dead MosFets took some other components with them, which keep the failure alive.
First of all, please scan and post here the schematic (try to make it as readable as possible, specially small text)
2) Build a series lamp fixture, basically an extension power cord where the "hot" wire is cut and a socket and an an incandescent lamp (60/100W) are inserted in series, so that current has to go through it.
You plug it in the wall mains socket and the suspect amplifier on the extension socket.
If all is well, the lamp will blink and then glow barely orange; if the amp is shorted, it will glow brightly, limiting current but not blowing like a fuse would, then you can measure *something* on the amp, and start troubleshooting.
Don´t touch the preamp, reverb, etc. , this is a power board problem.
Keep in touch.
J M Fahey

Your Yamaha really sounds good, so, don´t mess with it unnecessarily.