Yamaha A-1 Repair and restore: beyond Deoxit

Can you take a picture of this glue on the transistors, and a close up showing the hot parts / signs of burns?
Is the bias fluctuation on just one channel or same behavior on both channels?

You can clean off any glue with some acetone and a cotton bud / q-tip (apply it only to glue, need to hold it there for a little while to soften it).
 
only sign of burn that i see is the blue reistor right in front of the bag cap, it has clear sign on burn but measures 10 ohm so i think it is just the glue. the glue is very dry but also very hardened and difficult to scrape off. i dont have acetone. so im not tryingto scrape it off because it will breal the pcb before coming off.
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i am as certain as i can be that the pcb that the manual calls the power circuit board board is perfectly fine even though i thought it was smoking. no signs of burn, all resistors measure fine and the bias is a perfect 35 votls and all caps show ni signs of bulding. even though i have all the caps to recap i am going to hold off on that until i find the problem.
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the problem has to be in the big board (picture in previous post which is what the manual calls the electrolytic circuit board as far as i can tell) that has the be where the smoke comes from, it is where the bias readings are crazy and it where it gets so ht that it cant be normal. but like i said i repaired the burned trace, THE RESISTORS ARE Measuring fine and i dont see any signs of burn other than on the resistor that measures fine.

the pictures in the service manual are useless. cant see anything in them. i have the black and white crap version if someone has the color version id appreciate a copy.
 
Did you ever measure the rail voltage / +/- 51 vdc at the big caps ?
 
no i did not. do you mean the voltage across the positive and negative legs of the big caps?
 
when i say all resistors measure correctly i dont actually know what they are supposed to measure, what i man is that they dont show OL
 
ok i have something new. i turned the amp on, connected the voltmeter to the TP1-TP3 which is the left channel bias point on the power amp circuit and is supposed to read 7.4mV. i turned up the volume and kept my finger on the volume knob. sent the voltage to 200-300mV and started smoking. this time instead of turning it off i kept my finger there until i could see the source of the smoke. it is the blue resistor that i said above measures 10 ohm and ruled it out as the source of burning., well now it is toasted.
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it took 10 seconds from turning the volume up and putting my finger on the volume knob for it to turn red hot and smoke thick white smoke.
 
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it is both of them. for both channels. it is just that the left one smoked first and more. but both got hot.
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now the one for left channel measures 6.6 ohm and the one for right measures 1.63 MEGA OHMS. obviously my measuremtns dont mean asnything. that fusible resistor or the emitter resistor could have been bad as well. my measurement does not mean anything
 
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its not gonna be as easy as replacing those resistors, i need to find the reason they fry
 
FR101 and 102 are probably bad too because apparently measuring does not mean anything

RFoGg0o.jpg
 
that business with touching the volume, sounds so out of whack to me.
 
when i say all resistors measure correctly i dont actually know what they are supposed to measure, what i man is that they dont show OL
i forgot to mention that witohut touching the volume knob the right channel would play music for long times with no smoking or overheating and the right channel would be very quiet but some sound would come out of it whith some noise, it is touching the volume knob with my finger that makes it smoke like that with audible hum coming out of the speakers.
 
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unless someone has an idea. it going in the trash can. dumb of me to think that i can do this.
 
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Sorry but I don't understand why it is so complicate to refix ....

Why do you talk about " touching the volume knob " ???
Nothing can happen if you " touch " the knob
Do you turn it ?
If yes , what for ?
Volume control should stay at zero position (minimum) always when you work on unit adjustments
I think that it is time to stop your investigations and ship it to someone who knows what he is doing
you can also ship me your big board with the long capacitor board if you know how to delete it properly (don't touch any wrapping wires) and very well packed to avoid any dammage during shipping
 
1 - now we know that R119 and R120 are fried. service manual says: metal oxide resistor 10 ohm and then it says 2P. im assuming 2 watt. R121 and R122 measure 0.2 ohm, they should be 4.7 ohm.


2 - C107 C108, C109, C110 all show discoloration. could be due to the heat from the two resistors. Mylar cap 0.056 micro farad. should i replace them too?

all 4 emitter resistors measure 0.8-0,7 ohm, when measured from the back (after i fixed the burned trace). i bought 10 of them. i can replace them but i dont know how much good it will do.

yes now i know how to discharge the caps and i have been doing it everytime i want to touch anything.
It is between figuring it out myself and the trash can. i dont think that it is worth shipping to a repairman or woman.
 
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the problem is here on this board and it is not even that busy, if i cant fix it, i should just not deal with electronics:
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here is a high resolution picture of it:

click for hi rez: https://imgur.com/E2gmu2E


E2gmu2El.jpg
 
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