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Need Realistic STA-200 service manual

B

Bozomask

AK Subscriber
Subscriber
Hello fellow members!

I have a an STA-200 that I need a service manual for. I have downloaded the schematic here which is a help. Someone has changed the output transistors to some other types and has made other modifications to the circuits. There is a service manual for this unit at HIFI Engine but though I have checked daily for months they are not taking in new members.

Would someone who has access to HIFI Engine please be so kind as to download this manual for me? Thank you for your consideration!

Any help or suggestions on this unit welcomed gladly!
 
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Thank you too ecluser! Any hints or suggestions? Sound is low and extremely distorted on both channels on headphones. Have not hooked up speakers.
 
Don't connect speakers until you have good sound on headphones. Did you clean the switches and controls with electronic contact cleaner? What components were replaced? By what?
 
The part changes that immediately caught my eye from previous ownership of one of these new in 1976 was the output transistors. The originals in my new unit were one 2SA680 2SC1080 pair for each channel. This unit had on one channel a 2SB655B and 2SD675B, the other channel had a 2SB655B and D339. The emitter resistors of one channel had been changed from .25 at 2 watts to .47 at 5 watts.
 

not sure of the wattage you need, i don't see the units total wattage listed in the service manual. i'd guess 60 watt would be safe enough to start but you might need 100watt bulb. i think i'd replace all the outputs with mj21193/4g and get the right emitter resistors in it.

let ecluser guide you, i'd guess he is familiar with the unit.
 
I jumped the gun and bought some Toshiba outputs..there are rated at 100 watts and the unit is rated at 40 watts into eight ohms. Those MJ units look good at 200 watts and they are specifically made for audio output use so I think I will go that route. How do I calculate the emitter resistor value? I have heard it should be as low as possible, but I have no idea if I should differ from the schematic and if so how to select the value.
 
I run my DBT with a 40W bulb most of the time. For the emitter resistors I would use 0.22 ohm/2W. This is close to the original. A larger resistance would make the protection circuit more sensitive. Did you clean the controls and switches?
 
Not yet. The two 5 watt .47 ohm emitter resistors were both split open and had dumped gritty sand or something into the unit. I thought it best to get that mess cleaned up first, that's when I noticed the outputs were a different number from what I had remembered and that the split resistors were a different value than the other channel, so I removed the driver boards from the heat sinks and came to the experts here to ask for help.
 
To split open a 5W emitter resistor, there must be shorted power transistors. What test equipment do you have?

As a side note, I am not familiar with this model. It just happens that I bought more than 100 original Realistic service manuals from a former Radio-Shack service tech and I saw a request on hifiengine for that manual some time ago. So, I scanned the manual I have and uploaded it to hifiengine. I will do my best to help you.
 
I have a couple of DMMs, a Chinese capacitance meter that sorta works I think, a DMM with a scope function, an RF signal generator with a frequency counter, a Sencore mighty mite tube tester, an output meter, an old kit built VTVM, a Radio Shack FET analog meter that has never seemed to work correctly. I guess that is about it. I am willing to buy anything that is truly useful or necessary. I do appreciate your help!
 
Should be enough. There is an error in the schematic of the power amplifier. The junction between R15 and R16 is connected to the output bus, but the schematic does not show this connection. Make a correction to your schematic.

This is very interresting because it will allow you to operate the amplifier without output transistors. If the stages composed of Q1, Q2, Q3 and Q4 are good, you will have good sound on headphones. Check Q7 for possible short between collector and emitter, that would cut off the power supply to the first stage.
 
My schematic shows that juncture connecting to one side of VR1 that is used to set the bias if I read it correctly then goes through chokes to output coupling capacitors C11 and C12 then directly to the speaker switch, there apparently is no speaker protection. Where else should this juncture terminate? Thanks!
 
There is a convention in schematics when two lines are crossing. If there is an electrical connection between those lines, there is a "dot". For example, the connection between R24, R25 and the output bus. But the schematic doesn't show a dot at the junction of R15, R16, and the output bus. There should be a dot on the right side of VR1 at that point.

I found another error on R18. The "upper" schematic shows a diode (D7) in series with R18. In fact, this is a 33V Zener diode but the schematic shows a regular diode. That Zener diode is omited in the "lower" part of the schematic.

VR1 is a mid point (symetrical clipping) adjustment. There is no bias adjustment in this amplifier stage. Page 6 of the service manual refers to the "bias" adjustment but it is not the bias (current in the output transistors) as we are used to see. Since you have the driver boards out of the receiver, I suggest that you replace R12 by a 250 Ohm trim pot, to be able to adjust the bias.
 
You have good vision! With my old, blurry eyes there appears to be a dot, though smaller than the others, at that point. I know DC offset should be 0 volts, what bias voltage would I be shooting for after replacing the R12s with trimpots, and where would I measure it. It seems bias generally runs about 30mv?
 
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