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Yamaha CA-1010..no way to adjust offset?

Phototone

Phototone

Super Member
Just looking at the schematic and service manual for a Yamaha CA-1010 over at HiFi engine website.

The service manual has adjustment procedures for 3 things.... 1. Bias, 2. Balance of the moving coil phono stage, and 3. Adjustment of the output meters to show accurate power output...and that is IT.

Do some solid state amps -that are NOT cap coupled-not require a DC balance (offset) adjustment?
 
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Some DC offset circuits are "fixed" or self aligning ( DC servos).The CA-1010 and most of the M-series with a few exceptions. What kind of offset readings are you experiencing with your 1010 ?
 
avionic said:
Some DC offset circuits are "fixed" or self aligning ( DC servos).The CA-1010 and most of the M-series with a few exceptions. What kind of offset readings are you experiencing with your 1010 ?
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Haven't checked yet. This is theoretical question based on service manual.
 
Nope, no adjustment for offset. Bias only. AFAIK, a proper set of matched output transistors is needed to get offset within proper range. I may be wrong.
I did notice that when my bias was adjusted properly (it was WAY off), my offset went down, too.
Tom
 
If you have a large offset, the first step is to replace the transistors in the differential stage (TR301 and TR302) with an hFE matched pair.

If you still have a large offset with matched input transistors, you can correct the offset by trimming R344.
 
ecluser said:
If you have a large offset, the first step is to replace the transistors in the differential stage (TR301 and TR302) with an hFE matched pair.

If you still have a large offset with matched input transistors, you can correct the offset by trimming R344.
Click to expand...

Where is R344...I only see resistors up to number R343 on schematic. Main Circuit Board. Nevermind, its above TR301 & TR302.
 
ecluser said:
If you have a large offset, the first step is to replace the transistors in the differential stage (TR301 and TR302) with an hFE matched pair.

If you still have a large offset with matched input transistors, you can correct the offset by trimming R344.
Click to expand...

Does this mean if I replace R344 by a nice trimpot, I could get the offset exactly 0? :scratch2::D
 
I'm getting -19mv one channel and +20mv the other. No I did not invert my tester leads. Positive to red speaker terminal both times. This was after I checked and adjusted the bias for both channels for Normal and Class A to factory specs. I would rather get closer to zero.
 
Phototone said:
I'm getting -19mv one channel and +20mv the other. No I did not invert my tester leads. Positive to red speaker terminal both times. This was after I checked and adjusted the bias for both channels for Normal and Class A to factory specs. I would rather get closer to zero.
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Pay no attention to the + / - indication on your multimeter on the offset reading.20 mv is really not that bad. Your offset swing could very well be toward the positive or the negative-whats important is "how much"..:thmbsp:
 
I wouldn't worry about this small offset.

The problem comes from the fact that R308 (27k) is not equal to R315 (12k), and the collector current in TR301 is very close to the collector current of TR302 since the collectors are loaded by a current mirror circuit (the base current in TR309 is very small, ~0.1mA). If R344 was not present, there would be a positive offset of the order of 60mV at the output, assuming a hFE of 500 for TR301 and TR302.

The fact that you have a negative offset in one channel, and a positive offset in the other channel is an indication that your transistors are not perfectly matched, or they are matched in each channel but not the same value in the left vs right channel. You should pull out the transistors, noting their original position, and measure their hFE. Make the best pairs you can.

Yes, a trim pot for R344 would make the adjustment easy. I would use a 175k fixed resistor in series with a 100k trim pot.
 
Well, I tried that, and I guess I fried some transistors, because now it won't come out of protection, even when I replaced the original value resistors. I am so frustrated, I don't know how to test transistors. No fuses blown. I guess my only choice is to order all new transistors for the whole unit and shotgun replace them all? I'm so good with tube amps, I get so frustrated I don't know solid state.
 
I would rather get closer to zero.
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Sounds like your a little closer to zero now.:rolleyes:Should be able to measure DC offset while in protect mode.Probably why its in protect, excessively high offset.:scratch2:
 
avionic said:
Sounds like your a little closer to zero now.:rolleyes:Should be able to measure DC offset while in protect mode.Probably why its in protect, excessively high offset.:scratch2:
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Sorry, the speaker terminals won't engage. I put back the correct value (as per schematic) resistors...no sound either channel now...relay does not click. Its not now showing any offset on speaker terminals.
 
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Phototone said:
Sorry, the speaker terminals won't engage. I put back the correct value (as per schematic) resistors...no sound either channel now...relay does not click.
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You have to use test points before the relay..
 
Phototone said:
It worked before I did my little experiment, since I put everything back original, if nothing was fried, it should work again. hence, I think I fried some silicon.
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Most probably...Start by checking for solder bridges or cold solder connections. If your soldering looks good. Start pulling and checking your output transistors with a multimeter. And work your way back to the drivers and pre-drivers.
 
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Phototone said:
Well, I tried that, and I guess I fried some transistors, because now it won't come out of protection, even when I replaced the original value resistors. I am so frustrated, I don't know how to test transistors.
Click to expand...

I'm sorry for this misadventure but I really don't see that a small change in R344 may cause this failure. Did you preset the fixed resistor and trim pot combo to 220k prior to powering the amp? Maybe my choice for the fixed resistor and trim pot offered a too large variation on the offset. If the trim pot was all the way to one end.

With the original resistors, measure the DC voltages at the base, collector and emitter, with reference to ground, for TR306, TR308 and TR309.

For transistor testing see these threads:

http://www.audiokarma.org/forums/showthread.php?t=43186

http://www.audiokarma.org/forums/showthread.php?p=2362176#post2362176
 
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stoutblock said:
I assume you have your speaker swith on? Been there....
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Oh yes, and I did pre-set the substitute resistor/pot combo to 220k. for each channel. Like I said, I'm fairly good with electronics, except all my experience is with tube amps..(odd I know), and not solid state.
 
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