Setting Voltages on Sony TA-1120A

[bberkom]

I am in the process of restoring a very nice Sony TA-1120A integrated amp, and I have questions about the procedure for setting the AC bias current. I'm comfortable setting bias on amps where values and test points are clearly stated. However, the instructions in the service manual require the use of an attenuator and an oscilloscope, both of which I have access to at a friend's shop (it's just a pain to get out there). I am wondering if there is a way to measure the bias current using my DMM just to make sure it's holding steady and within reason after I replace the old electrolytics. I think I need to measure across the emitter resistors, but I am not the best at schematics and would love some help and to learn in the process.

Here is a diagram of the driver boards.

Thanks.

 

[Moving Ahead]

I thought the manual is easy. Maybe you're over complicating it.
Bias 25mV across R331 (.5ohm top left of board) -use mini grabbers. Vr314 (10K)
Center voltage 42.3 V VR310 (50K)
There are separate boards for each channel.
easy peasy

 

[bberkom]

I thought the manual is easy. Maybe you're over complicating it.
Bias 25mV across R331 (.5ohm top left of board) -use mini grabbers. Vr314 (10K)
Center voltage 42.3 V VR310 (50K)
There are separate boards for each channel.
easy peasy

Ah sorry I should have specified the AC-bias adjustment. My bad. The DC bias as it's referred to in the manual is really straightforward as you describe it. At least I think you are describing the DC bias adjustment. The AC bias is what requires the attenuator and oscope. Edited post to include AC-bias in description. Maybe I am just missing something though.

 

[bberkom]

I thought the manual is easy. Maybe you're over complicating it.
Bias 25mV across R331 (.5ohm top left of board) -use mini grabbers. Vr314 (10K)
Center voltage 42.3 V VR310 (50K)
There are separate boards for each channel.
easy peasy

Actually, the part I may be missing is where you say "Center voltage 42.3 V VR310 (50K)". I don't see that in the manual anywhere, but that would be the VR to adjust for the AC. That may be where I need more clarification.

 

[dshoaf]

What Sony calls AC Balance is more commonly known as DC Offset these days. The technique is intended to be sure that the center point is zero and that both the push and pull side of the amp are working equally.

My suggestions if you don't have the gear or time is to set the DC level to 1/2 of what the power supply rail is and you'll be darn close.

Cheers,

David

 

[bberkom]

What Sony calls AC Balance is more commonly known as DC Offset these days. The technique is intended to be sure that the center point is zero and that both the push and pull side of the amp are working equally.

My suggestions if you don't have the gear or time is to set the DC level to 1/2 of what the power supply rail is and you'll be darn close.

Cheers,

David

Thanks for the advice and clarification. I am still unsure of where my test points are for my DMM when I am adjusting VR310 and VR410 on the driver boards. I successfully adjusted VR314 and VR414 when measuring across R331 and R441 to 25 mV. Just not sure where to measure when adjusting the other pots. I'm still pretty new to this level of work, so I want to be sure I'm in the right spot.

 

[dshoaf]

Ok, so I found the service manual on Hi Fi Engine. To get into the ballpark with AC Balance, measure the DC voltage here - in yellow.



An easy point to measure at is that big capacitor at C320. It stands free in the chassis. There will be another for the other channel close by. You want to measure on the side that shows DC voltage. The voltage rail calls for about 93 volts so 1/2 of that would be 46.5 or so volts. Adjust the VR310 to get close to that.

Cheers,

David

 

[bberkom]

Ok, so I found the service manual on Hi Fi Engine. To get into the ballpark with AC Balance, measure the DC voltage here - in yellow.

View attachment 847251

An easy point to measure at is that big capacitor at C320. It stands free in the chassis. There will be another for the other channel close by. You want to measure on the side that shows DC voltage. The voltage rail calls for about 93 volts so 1/2 of that would be 46.5 or so volts. Adjust the VR310 to get close to that.

Cheers,

David

Thank you that is exactly what I needed to know. Thanks for providing such detail. I knew it was possible to adjust this using a DMM, just couldn't put it all together.

 

[bberkom]

Got it all back together and running the big Sony SS-3300 speakers I got with it. Match made in heaven. Or Japan. Sounds great. The amp was running very hot before the recap, but now I can't feel any heat around the heat sinks even after running hard for an hour. Thanks again for the help.

 

[Moving Ahead]

well done

 

[bberkom]

So I am reviving this thread because I am getting some strange voltage readings on this amp. I had it back out to replace the indicator lamp, and I figured I'd make sure everything was holding stable. The amp sounds great and functions perfectly, but I am still concerned.

I read 89 VDC at the power supply, measured at C508, the large 4400uf power supply cap (schematic calls for 93 V). I am able to set the DC bias per the service manual to approx. 25mv reading across R331 and adjusting VR314. There is only plus or minus 1 mv of DC at each of the speaker terminals as well after it warms up for a few seconds. So far so good I think.

However, when I measure the DC voltages for each channel either at R317 on each driver board or at the output capacitors (C320, 3000uf), I am only getting around 23.5 VDC. Even when maxing out VR310, the most I can get is around 27 volts. I understand that this needs to be around half of the rail voltage (schematic calls for 42.3 V at these points). So I am only getting one fourth of the rail voltage on each channel.

What am I missing that could be causing such a disparity? I am getting the same readings from both channels, so to me it seems like a power supply issue. Again, the amp functions perfectly and sounds great. The only other issue I can think of is a slight mechanical transformer hum that usually goes away once warmed up.

 

[Binkman]

Might just be at low level or idle.. then when you crank up the amp a tad or more the ps. kicks into a resulting higher rail voltage, probably a power ic in the amp and a 5--10vdc switching ic. possible. but I'm sure if at 11a.m. on the vol. and the fat lady is singing? you're fine. pretty sure its an AB amp deal with sony.

 

[bberkom]

I walked away and ran some errands to clear my head and that was going to be my next test, with it actually doing some work, not just idling. I've had the volume nearly to 3:00 hooked up to my big Sony speakers. It about makes me nauseous it gets so loud but still doesn't distort even at that level. Seems to be working fine, but I'll probably take some more readings just for kicks.

 

[bberkom]

Might just be at low level or idle.. then when you crank up the amp a tad or more the ps. kicks into a resulting higher rail voltage, probably a power ic in the amp and a 5--10vdc switching ic. possible. but I'm sure if at 11a.m. on the vol. and the fat lady is singing? you're fine. pretty sure its an AB amp deal with sony.

Voltages stay around 23-24v at output caps even with music playing fairly loud. I'm still wondering what is going on with these measurements. I'm not too worried since the amp is working so well but don't want to be doing long term damage anywhere if I keep running it.

 

[Binkman]

if the basics like bias are good.. no pops or crackles or whatever.. it's not a wall breaker amp okay? you'll burn up the outputs easy. else get a wall breaker if you want loud.

btw.. ear clipping.. you've driven the amp to make you sick... ???

 

[bberkom]

Haha no not quite that loud, just making the point that it is stable even when pushed hard. I rarely have volume past 11 or 12 o'clock.

 

[petehall347]

measure voltages at output transistors . p.s cases are collectors . get base and emitter voltages too
is it both channels with odd readings ?

 

[bberkom]

I'll have to get additional readings later for the output transistors. It is both channels showing low voltage at the outputs.

 

[petehall347]

might be a supply issue then .. but saying that you said supply ok . . collector voltages are maybe low for some reason .

 

[bberkom]

might be a supply issue then .. but saying that you said supply ok . . collector voltages are maybe low for some reason .

Yes the power supply is checking out fine even if a little lower than the schematic at 89V measured at the large cap near the transformer. I probably won't be able to get any more voltages from the output transistors until this weekend, but I still can't find a single actual symptom that the low voltages might be causing as far as operation of the amp.

I've been looking at the schematic in the meantime, and the only other part common to both channels may be the Regulated Power Supply board pictured below. I replaced the 50u 100v electrolytic cap on this board already, but perhaps something on this board is altering the voltages? I wish I was better at diagnosing the issue from here. Any help is appreciated.


Edit: The only other changes I made to the power supply section was upping C510 from 1000uf to 2200uf and C509 from 2000uf to 3300uf. Do not think this would affect these voltages, however.