The continuing saga. So I replaced the filter caps and bias caps, fired it up on the limiter and we have sound - out of one channel. I tapped all the tubes and the 12AX7s were all very microphonic so I put in a new set and that problem went away. I swapped the two 6AN8s, problem stayed with the channel. Swapped the EL84s around, problem stayed with the channel.
Ok, the next step is taking some voltages. As you can see on the schematic pins 6 and 2 of the 6AN8s are tied together and should have around 75v. One side does, the other has 148-151v on channel B so I must be looking at either a cap or resistor in the chain since swapping the tubes didn't make the problem move.
Pioneer SM-83 stereo amp
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gui_tarzan
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Pioneer SM-83 stereo amp
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--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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gui_tarzan
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Re: Pioneer SM-83 stereo amp
Voltages on Ch. B 6AN8:
1 - 239
2 - 151
3 - 156
6 - 150.8
7 - 28
8 - -5
9 - .5
Ch. A
1 - 284
2 - 81.7
3 - 95
6 - 81
7 - 37.4
8 - -279 (and a loud buzz when I measure it)
9 - .75
1 - 239
2 - 151
3 - 156
6 - 150.8
7 - 28
8 - -5
9 - .5
Ch. A
1 - 284
2 - 81.7
3 - 95
6 - 81
7 - 37.4
8 - -279 (and a loud buzz when I measure it)
9 - .75
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Leo_Gnardo
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Re: Pioneer SM-83 stereo amp
Have you probed all the R's around those 6AN8's with your ohmmeter? You know how old R's like to drift, and the higher the value more likely to drift way off or even go open. See if you don't snag a drifter or three.
Then it's suspect leaky caps. Sub em out one after t'other. Good thing there aren't many.
Then it's suspect leaky caps. Sub em out one after t'other. Good thing there aren't many.
down technical blind alleys . . .
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Stevem
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Re: Pioneer SM-83 stereo amp
Pump signal thru the good channel and follow it down stream with a meter set for AC volts, then do the same with the dead channel,if it's coming into a grid and not out the coupling cap on the plate, then there Ya go!
The buzz you got on that testing is good,as that means that all of the down stream circuit from that point on is working!
The buzz you got on that testing is good,as that means that all of the down stream circuit from that point on is working!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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gui_tarzan
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Re: Pioneer SM-83 stereo amp
Yep, that's what I thought too. I did put my meter on the caps set on DC to make sure none were leaking and on the bad channel I'm getting less than 1v or close to zero volts on all of them. That's with a signal going into the amp. I will check AC and the resistors (there are a bunch of them!) today and see what I get.
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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gui_tarzan
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Re: Pioneer SM-83 stereo amp
R70 and R32 are 47k resistors on pins 1 of the 6AN8s that are supposed to drop B+ to 205v from 360v. They are both connected by the same wire from B+ so I get 358.8 on that side of each 47k resistor.
Ch. B R70 measured 33k and the voltage was 239v at pin 1 of the 6AN8.
Ch. A R32 measured 46.7k and the voltage is 284v at pin 1 of the 6AN8.
I replaced R70 with a new 47K (46.6k) and I now have 192v there but still have 284v+ on Ch. A.
Ch. B is nearer to the correct voltage now but still no sound (so it's got to be further down the chain) and Ch. A has almost 100v too much and I get sound out of it. WTH?
Ch. B R70 measured 33k and the voltage was 239v at pin 1 of the 6AN8.
Ch. A R32 measured 46.7k and the voltage is 284v at pin 1 of the 6AN8.
I replaced R70 with a new 47K (46.6k) and I now have 192v there but still have 284v+ on Ch. A.
Ch. B is nearer to the correct voltage now but still no sound (so it's got to be further down the chain) and Ch. A has almost 100v too much and I get sound out of it. WTH?
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
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Re: Pioneer SM-83 stereo amp
A high plate voltage at one stage is caused by three possible things.
1) that tube or section of that tube is very weak and pulling little current.
2) the Cathode or grid resistor is screwed.
3) the grid is getting fed dc by a leaky up stream cap and throwing off the gain stages bias.
And yes , some times a leaky cap will not make for much hum so you do not think that the cap is a issue!
It's been said in books that anything more than 1/4 volt of leakage is a issue, but I say that that is far too much!
1) that tube or section of that tube is very weak and pulling little current.
2) the Cathode or grid resistor is screwed.
3) the grid is getting fed dc by a leaky up stream cap and throwing off the gain stages bias.
And yes , some times a leaky cap will not make for much hum so you do not think that the cap is a issue!
It's been said in books that anything more than 1/4 volt of leakage is a issue, but I say that that is far too much!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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gui_tarzan
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- Joined: Thu Jan 16, 2014 3:10 am
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Re: Pioneer SM-83 stereo amp
Updated voltages and resistor values. I replaced the Ch. B resistors and caps circled in red. Still no output. I measured the AC voltage going into the grids on the power tubes and all four measure the same (I forgot to put it on the schem) and the outputs to the OTs all measure the same AC voltage.
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--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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gui_tarzan
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- Joined: Thu Jan 16, 2014 3:10 am
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Re: Pioneer SM-83 stereo amp
I also swapped the 6AN8s again to make sure one wasn't weaker than the other. Same readings no matter which socket they're in.
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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gui_tarzan
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- Joined: Thu Jan 16, 2014 3:10 am
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Re: Pioneer SM-83 stereo amp
Well you know what they say, the last place you look is where you'll find the problem (no output on Ch. B). The problem with no output was the 8/16 ohm switch on the back. Everything checked out for signal from front to back so it had to be something else. Man, that was a journey!
Now I still need to figure out my voltage differences and replace a few more out of spec resistors but this one is almost done.
Now I still need to figure out my voltage differences and replace a few more out of spec resistors but this one is almost done.
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
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Re: Pioneer SM-83 stereo amp
It was getting to the point where the main issue had to be something simple!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
gui_tarzan
- Posts: 607
- Joined: Thu Jan 16, 2014 3:10 am
- Location: The 26th State
Re: Pioneer SM-83 stereo amp
The last of the bad parts (caps & resistors) are replaced, voltages are normal, oh my GOSH does this sound incredible! A friend gave me some RTR speakers to use and the sound is amazing. I've never had a tube stereo to listen to before and I know this isn't the top of the line but man oh man does it sound good. I can hear instrument clarity I've never heard before.
I can see some tube hifi amps in my future. Maybe with some 2A3s I've got in a box of misc. tubes. Hm...
I can see some tube hifi amps in my future. Maybe with some 2A3s I've got in a box of misc. tubes. Hm...
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
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Re: Pioneer SM-83 stereo amp
If you ever get the chance to hear two Mc 60s pumping into a Altec 606, or two big Klipsch speakers tucked into a room your Jaw will drop all over again
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!