REALISTIC STA-78 Blowing Main Fuse
- Thread starter JB8
- Start date
JB8
Well-Known Member
Unfortunately no, seems they were removed. But, if I scavenge from the other board I can just pull whatever I need plus have an example of what it should look like when I'm done. The other unit is beautiful cosmetically but someone really did a number on the main amp and power supply, lots of oops jumpers and such, so I don't think I'll ever feel compelled to rebuild it. I'd love to get this unit back to it's "normal" configuration so I should likely just get after it. I'll keep this thread updated, and the other 3 active threads I have going, ha! What can I say, lots of projects, no hurry on any of them.
Lavane
Lunatic Member
JB8
Well-Known Member
Great looking little amp there, Lavane, I'll be it sounds fantastic! Love to plug in an ES335 and let it sing.
And yessir, that's likely the plan - build the one then sit on what's left. I've had close to ten of these and am always on the hunt for more, so I'm sure there will be more in the future. I'll keep you posted, good luck on your projects, I'm thankful we all have units to work on that we truly enjoy!
And yessir, that's likely the plan - build the one then sit on what's left. I've had close to ten of these and am always on the hunt for more, so I'm sure there will be more in the future. I'll keep you posted, good luck on your projects, I'm thankful we all have units to work on that we truly enjoy!
JB8
Well-Known Member
Last update for a while, got some great words of wisdom from markthefixer regarding an SX-440 I'm also working on, his sage advice was "and when units get tough, multiple opponent round robin chess is not the path to success.
this one fits in that category." I've got too many unit's being worked on and need to hunker down and finish them one by one, so this STA-78 is has been relegated to the #3 spot on the list.
Specific to next steps once I jump back in, I'm going forward with ecluser's advice to swap in the 2K ohm resistor at R706 to protect D705 and call it good. The reason I chose this path was because when I pulled the donor unit I found the phono pre-amp had been mucked with, none of the leads were connected as they should be and there was an ugly jumper on C315, yada yada yada, I have zero faith in that unit considering how shoddy the previous owners work was. Atrocious to say the least. I have a couple vintage Realistic stand-alone phone preamps on the way, I'll rebuild one and sell it with this unit when the time comes - if the new owner wants to use the two line level inputs, super, if they want to attach a turntable, I'll have that covered as well.
As always, totally appreciate all the support on this forum, exceptionally talented and generous people here, many thanks! I'll update this thread when she's dressed up and ready to show off, or when I have more questions, which ever comes first.
this one fits in that category." I've got too many unit's being worked on and need to hunker down and finish them one by one, so this STA-78 is has been relegated to the #3 spot on the list.
Specific to next steps once I jump back in, I'm going forward with ecluser's advice to swap in the 2K ohm resistor at R706 to protect D705 and call it good. The reason I chose this path was because when I pulled the donor unit I found the phono pre-amp had been mucked with, none of the leads were connected as they should be and there was an ugly jumper on C315, yada yada yada, I have zero faith in that unit considering how shoddy the previous owners work was. Atrocious to say the least. I have a couple vintage Realistic stand-alone phone preamps on the way, I'll rebuild one and sell it with this unit when the time comes - if the new owner wants to use the two line level inputs, super, if they want to attach a turntable, I'll have that covered as well.
As always, totally appreciate all the support on this forum, exceptionally talented and generous people here, many thanks! I'll update this thread when she's dressed up and ready to show off, or when I have more questions, which ever comes first.
Ed240
Active Member
The STA-78 was made by Tandy Electronics in Massan, Korea.
Lavane
Lunatic Member
I never liked the use of fixed resistors for setting bias and or DC offset.
All of the RS lower watt receivers I've come across, including a few integrated amps just had a voltage adjustment pot for each channel, no adjustments for bias or offset. The one RS odd duck I have from the mid 70's uses transformers instead of differential pairs. That was kind of old tech by then I would have thought. I've never bought a single receiver, but they tend to appear from time to time. I have a few in the garage collecting dust.
Nobody wants one around here. 
JB8
Well-Known Member
Quick update here, the SX-440 project is now on hold, very difficult problem to fix, so I tucked it away for a bit and have been concentrating on a few other units, and this is one of them.
After replacing R706 with the 2K ohm resistor all seems well, very happy with the sound and performance of the unit, in spite of its strangeness. I am, however, back to addressing the mono/stereo switch problems - yes, turns out the phantom, placebo stereo effect I thought I was hearing was just in my head, so I'm back to sleuthing. Ecluser, I'm tracing the wires as you noted, (lots of white leads) after having cleaned the switch again. I can't hear any difference between mono and stereo switch positions unless I engage the loudness button, then I get not only a fuller low end but the hard left/right details stand out as they should, but I can also make it a mono/loudness option as well. The only thing I don't have is just standard stereo without the loudness button engaged.
I'm also studying the schematic to better understand how the signal flows, still perplexed on how the mono/loudness switches interact, I'm sure it's fixable!
After replacing R706 with the 2K ohm resistor all seems well, very happy with the sound and performance of the unit, in spite of its strangeness. I am, however, back to addressing the mono/stereo switch problems - yes, turns out the phantom, placebo stereo effect I thought I was hearing was just in my head, so I'm back to sleuthing. Ecluser, I'm tracing the wires as you noted, (lots of white leads) after having cleaned the switch again. I can't hear any difference between mono and stereo switch positions unless I engage the loudness button, then I get not only a fuller low end but the hard left/right details stand out as they should, but I can also make it a mono/loudness option as well. The only thing I don't have is just standard stereo without the loudness button engaged.
I'm also studying the schematic to better understand how the signal flows, still perplexed on how the mono/loudness switches interact, I'm sure it's fixable!
JB8
Well-Known Member
This morning I started thinking the issue may be associated with the loudness switch, difficult to get to it without pulling the gang, so I did just that. I didn't completely remove it but got it to a position where I could get some cleaner in the switch and then exercised it for a bit. (Side note - my wife always appreciates this aspect of receiver repair, the exercising of push buttons must look pretty kooky - in/out, in/out, in/out, click/click, click/click...)
At any rate, the cleaning didn't help, but, and I may have mentioned this, when the loudness button is engaged you can switch it to mono with great effect, which leads me to believe it's not the mono switch. That said, if the loudness switch is wonky, could it possibly be effecting the stereo/mono output? Such a fiddly issue here but since I know how the unit should sound I'm still convinced this is worth addressing.
At any rate, the cleaning didn't help, but, and I may have mentioned this, when the loudness button is engaged you can switch it to mono with great effect, which leads me to believe it's not the mono switch. That said, if the loudness switch is wonky, could it possibly be effecting the stereo/mono output? Such a fiddly issue here but since I know how the unit should sound I'm still convinced this is worth addressing.
JB8
Well-Known Member
In checking continuity I noticed the mono switch has one post that's not soldered, is this an oversight from the build or is it intended? The pad still has the green coating and doesn't appear to connect to any other section of the PCB so perhaps it was never meant to soldered?
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JB8
Well-Known Member
Thank you, Ecluser, I appreciate the confirmation.
To help test, I found a youtube clip for testing left/right issues which helped determine the extent of the issue. I ran my phone into the AUX ports for testing.
With none of the switches engaged, I do get some left/right separation, but it's more like center left and center right. Engaging the Mono switch fully centers the signal. With Loudness engaged I get more of an expected hard L/R pan, a significantly wider spread. With Loudness engaged AND Mono, the signal is summed and fully center.
I'm sure this is a dumb question, but here goes - is this all about a bad switch or are there other components that should be considered? It's a stumper to me because it seems like the switches are working to an extent but not as they should, just not sure if partial stereo can be tied back to a switch failure, and if so, based on the conditions, is it more likely to be the Loudness switch, Mono, or both?
Any advice is much appreciated!
To help test, I found a youtube clip for testing left/right issues which helped determine the extent of the issue. I ran my phone into the AUX ports for testing.
With none of the switches engaged, I do get some left/right separation, but it's more like center left and center right. Engaging the Mono switch fully centers the signal. With Loudness engaged I get more of an expected hard L/R pan, a significantly wider spread. With Loudness engaged AND Mono, the signal is summed and fully center.
I'm sure this is a dumb question, but here goes - is this all about a bad switch or are there other components that should be considered? It's a stumper to me because it seems like the switches are working to an extent but not as they should, just not sure if partial stereo can be tied back to a switch failure, and if so, based on the conditions, is it more likely to be the Loudness switch, Mono, or both?
Any advice is much appreciated!
ecluser
Super Member
Since the mono switch centers the signal, this switch is not in cause. I wonder if the previous worker didn't make a bridge on the "dub-out" jacks, to create a common signal for a sub-woofer for example. Check this because if the L and R Dub-outs are connected together it will reduce the separation but not to the point of making a mono signal at the volume control.
JB8
Well-Known Member
If we feel like the mono switch is likely fine, and the wiring doesn't seem altered, could the loudness switch be faulty or since there's a slight stereo effect should it be ruled out? Also, is there a way to bypass the loudness switch to give us a better idea of how the loudness switch is impacting the stereo field?
JB8
Well-Known Member
Such an easy test, this is why I come here, learning from this logic path!
With only the left AUX channel being fed with a test tone from my phone the signal split the uprights, right down the middle. I could pan left and right and the signal stayed the same, centered, engaging the mono switch didn't change anything spatially but the signal got louder. If I engaged the loudness switch the signal moved to the left channel only.
With only the left AUX channel being fed with a test tone from my phone the signal split the uprights, right down the middle. I could pan left and right and the signal stayed the same, centered, engaging the mono switch didn't change anything spatially but the signal got louder. If I engaged the loudness switch the signal moved to the left channel only.
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JB8
Well-Known Member
I've checked the diagram along with continuity, seems all is correct based on the wiring schematic, 20 and 23 appear correct.
I also did some additional testing with the loudness switch engaged. If I send a tone through the left channel only I do get some bleed into the right channel. In other words, even though the loudness is providing what seems to be a healthier stereo signal it's still likely not pure stereeo separation. Baffled!
I also did some additional testing with the loudness switch engaged. If I send a tone through the left channel only I do get some bleed into the right channel. In other words, even though the loudness is providing what seems to be a healthier stereo signal it's still likely not pure stereeo separation. Baffled!
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