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Realistic System Eleven STA-111 DC Offset w/wo SW On?

JamVal

JamVal

Addicted Member
I purchased a Realistic System Eleven STA-111 that passed the DBT but DC offset is very high - over 200mV on both channels. So I set it aside, looking for manuals which I haven't found.

Looking through some threads today, I came across something I hadn't noticed. There is a switch on the back just to the left of the speaker jacks, labeled SW with ON and OFF positions. The switch was in the ON position since I bought it. I flipped the switch to OFF and now DC offset is at an acceptable level <25mV. But when you place the SW to the ON position, the DC offset goes way up.

I read that this SW switch enhances the bass. Does anyone know if this effect on DC offset is normal for this receiver? The unit looks almost new and was very clean inside and out when I bought it.

I also just read that this receiver matches up very nicely with the Minimus 7W speakers. I had a set prior to buying the STA-111 but sold them.
 
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I bought a STA-111 and a STA-11 and had similar readings for bias on both units. I went as far as replacing the outputs on the 111 and still had the same bias. I paired the 11 with a pair of minimus 11 and it sounded great!
 
craigsc said:
I bought a STA-111 and a STA-11 and had similar readings for bias on both units. I went as far as replacing the outputs on the 111 and still had the same bias. I paired the 11 with a pair of minimus 11 and it sounded great!
Click to expand...
Thanks for confirming that. It does sound good for a small 30 watt receiver. With the DC offset that high I thought I would be getting some distortion, but none that I can hear. The 1982-1983 Radio Shack Catalogs listed the STA-111 at $359 - that's about $860 today.

I could kick myself. I had a nice set of the Minimus 7W's that I sold. Now I read that those were a great match with the STA-111.

Also I want to ask what outputs did you put in? I think the stock IC was an STK1030, which I understand is hard to find. I've read that an STK1050 will work.
 
I have 2 STA 111. I really like the looks and build quality. One is working fine and is currently being used in garage setup driving pair of Rectilinear X's. The second unit is in great condition but there is audible distortion in left channel. I have checked for obvious issue on circuit boards - no leaking capacitors or burnt resistors. Everything looks ok. Could there be a problem with one of the output IC amps? Hoping to identify issue before I send off for repair. Any feedback is welcome.
 
2chanfan said:
I have 2 STA 111. I really like the looks and build quality. One is working fine and is currently being used in garage setup driving pair of Rectilinear X's. The second unit is in great condition but there is audible distortion in left channel. I have checked for obvious issue on circuit boards - no leaking capacitors or burnt resistors. Everything looks ok. Could there be a problem with one of the output IC amps? Hoping to identify issue before I send off for repair. Any feedback is welcome.
Click to expand...

There is a switch on the back of the unit labeled 'SW'. It had a similar effect as a "loundness" switch, but when it is switched to the on position it gave a big increase in DC offset. High DC offset can result in distortion. If that 'SW' switch is in the on position, try turning it off. Check to see what position that SW switch is in on your good unit.

I made sure the SW switch on mine was turned off. In the off position check the DC offset.
This thread explains how to do it and what levels are acceptable and where things can get problematic for your speakers.: http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/
 
JamVal - Thanks for the link. Measured DC offset and found following. This is with switch set to off.

R Channel - 38 mv
L Channel - 246 mv

Confirms what I am hearing. Now to try and figure out why. I always hook up equipment on a bench with throwaway speakers before putting it in service. Will add DC offset to process.
Any hints on what could cause such high reading on left channel?
 
I'm far from an expert, but from reading the various threads on this topic, it can be caused by one or both of the input transistors going out of spec; ie they are mismatched.
I no longer have my STA-111, and my readings were in the acceptable range with the SW switch off. Reading through the posts on this thread I recall that the STA-111 does not have discrete transistors for input and output.
IC chips were used - see my post #3 above. The unit came with STK1030 IC chips. But from my recollection, they are difficult to find. A substitute is an STK1050 which CraigSC (post 4 above) said he used successfully. Ask for some advice in this solid state forum under a new thread about reliable vendors for IC chips. From what I have read there are some bad vendors selling junk. You will want to replace both, even though one channel is OK.

But, first, on an outside chance that your SW switch has dirty contacts, and is not shutting that one channel off even though the switch is in the off position, try taking the DC offset readings with the switch on. If left channel is still at exactly 246mV maybe the switch needs to be cleaned. Open the case up, spray deoxit into the switch contacts, and toggle the switch back and forth several times, spray again to flush it out.

I doubt that is the issue, but it's worth a try, a lot easier than replacing the IC Chips.
 
JamVal's post #8 has a good point about checking the rear EQ switch. The one on my System 7 STA 7 receiver is loose and must be toggled a few times occasionally to get sound from the matching Minimus 7 speakers.
 
I checked with both switch on and off. Here are readings on and off for both channels.

RC 38mv off - 199mv on. I understand this is a little high but not abnormal for age.
LC 246mv off - 269mv on. Abnormal.

I started looking for Power packs but will wait until I can do more research as to cause before starting to replace parts.

Thanks for your help.
 
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