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Setting Voltages on Sony TA-1120A

petehall347 said:
am thinking the +b3 voltage is too low for some reason .
Click to expand...

I am still learning to read schematics, but I now see where each of the voltages are marked for the b3+ voltage and the emitter/base/collector voltages for the four output transistors X116, X117, X118, X119. I'll try to get these voltages read at the driver board connections and see if anything looks out of the ordinary.
 
i take it you are checking the voltages for setting centre voltages at the output transistors cases ?
one of each channel should be at rail voltage . the other roughly half of that .
 
petehall347 said:
i take it you are checking the voltages for setting centre voltages at the output transistors cases ?
one of each channel should be at rail voltage . the other roughly half of that .
Click to expand...

Yes, I am trying to center the voltages. I have been taking the reading at the output capacitors (C320 for left and C420 for right) as was suggested and adjusting R310 (and R410). I've been getting around 23.5V on each channel which is about 1/4 of the rail voltage. That is the measurement that has been concerning me as I have been looking for half the rail voltage at those output capacitors.
 
check at the transistor cases .
p.s i made a mistake . there are 4 outputs each channel so 2 should be rail voltages and 2 half of that .
cases are collectors by the way .
 
petehall347 said:
check at the transistor cases .
p.s i made a mistake . there are 4 outputs each channel so 2 should be rail voltages and 2 half of that .
cases are collectors by the way .
Click to expand...

I gotcha. I'll get in there and take those voltages as soon as I can this week.
 
petehall347 said:
check at the transistor cases .
p.s i made a mistake . there are 4 outputs each channel so 2 should be rail voltages and 2 half of that .
cases are collectors by the way .
Click to expand...
So I ended up getting home today early enough to take the case voltages on the output transistors. For both the left and right channel, I get one pair of transistors with case voltages of approx 89V, which is the rail voltage I'm reading from the transformer. However, the other pair for both left and right channel are only reading around 23V.

I also took several other voltage readings from the driver boards where all of the different wires connect. I am only getting around half of the spec'd voltages from the schematics on almost all of those connections. Readings are the same (effectively) on left and right channels. Here's what I am seeing broken out:

Location - Reading - Spec'd
X118 emitter - 22.8v - 42v
X116 emitter - 22.8v - 42v
X116 collector - 87.5v - 92v (rail voltage, so this reading should be okay)
Protection b3+? - 41.4v - 92v
C312 (output cap) - 22.8v - 42v
X116 base - 22.8v - 42v
X117 collector - 22.8v - 42v
X119 emitter - .025v - .025v (these last two I believe I've adjusted with VR314 to approx 25mv already)
X117 emitter - .025v - .025v
 
would like to see all voltages for x114
x304 voltages will also be interesting to see .
 
Last edited:
Voltages for x114 on left amp
E - 23.7V
C - 88.2V
B - 24.2V

Voltages for x304 on protection circuit
E - 42V
C - 89V
B - 87.6V
 
quick look is saying x304 is running rather hot ... or it is not working properly .
its late here and i am falling asleep . will go over this when i am awake .
its a bit odd how you can get bias dialled in ok .
i know these cap coupled amps can be a bit odd setting up .
try setting biases a bit lower and see if centre voltages rise up some .
last one i did i used 2 multi-meters and traded voltages against each other until i got it right .
 
I really appreciate your help. I'll see about biasing lower to see if that affects the voltages at all. It is strange that the bias holds well and the amp functions perfectly despite the low voltages.
 
petehall347 said:
quick look is saying x304 is running rather hot ... or it is not working properly .
its late here and i am falling asleep . will go over this when i am awake .
its a bit odd how you can get bias dialled in ok .
i know these cap coupled amps can be a bit odd setting up .
try setting biases a bit lower and see if centre voltages rise up some .
last one i did i used 2 multi-meters and traded voltages against each other until i got it right .
Click to expand...

I tried adjusting the bias voltage and monitored the affects it had on the case voltages of the output transistors. Changing the bias had a slight affect on the voltages of the pairs of transistors reading at the rail voltage, plus or minus a few volts as I changed the bias. However, it had no noticeable affect on the voltages of the pairs of transistors with low voltages. They remained effectively constant, unaffected by turning the bias pots.

Still wondering if my issue has something to do with x304 in the protection circuit?
 
It seems to be pointing that way. It's a 2SC756. I did a quick search but couldn't find any info on direct replacements for this transistor. Could you recommend a good modern replacement? I see some Sony 2SC756's listed as NOS on ebay, but I'm a little wary of these. May be my best option, however.

Also, just so I learn as I go, what makes you suspect this transistor when looking at all of the voltages? Looking at the schematic, I see the b3+ voltage coming from the protection board to the driver boards, but just wondering what specifically points to this transistor so I can know what to look for in the future.
 
Thanks. I'll check those readings again before replacing. It's hard to make out on the schematic from hifi engine, but looks like the emitter voltage should be closer to the rail voltage of 89V, like the base and collector voltages are showing. If the emitter voltage isn't high enough, then it would follow that it isn't supplying enough voltage to the driver boards, correct?

I'll probably try a NOS Sony transistor from the auction site. Thanks again for your help, and I'll report back once it's been replaced. Still strange to me that the amp was working and sounding fine.
 
Very happy to report that replacing X304 in the protection circuit has fixed my problem. The old 2SC756 had no base to emitter reading on my tester.

I can center the voltage correctly now, and readings on output transistors and output caps are consistent with schematic. I had to lower the bias after replacing the transistor, but it's now steady at 25 mv. Getting only +- 5 or so mv DC at speaker terminals. I'd call it a success. Thanks petehall347 for your help.
 
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