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REALISTIC STA-78 Blowing Main Fuse

JB8

JB8

Well-Known Member
Greetings!

I have an STA-78 in good cosmetic condition, seems to have come from non-smoking environment. Previous owner made some cap upgrades including bumping up the main filter caps from 6800 uF to 10K uF, and from 35V to 63V. Several of the PS caps were also replaced including replacing all 470 uF were swapped out with 1000 uF values. Can't remember having an STA-78 without the phono board, looks weird to have that void. At any rate, it blows the main fuse at power up and I'd appreciate some help!

Here's where we're at:
  • I've got it running through the DBT and I'm seeing a quick burst of light at power up, then fades to almost 100% out, but the light is still shining dimly. Tested again without DBT and it blew the fuse at power up.
  • I changed several of the PS board caps back to the original uF's - removed caps all tested not too far off from spec
  • Sounds fantastic, very quiet with volume at zero, nice stereo separation, loudness has significant impact on sound field, more expansive than average
  • Output transistors testing fine in-chassis
  • When using the DMM on the Diode setting, I'm getting an OL when measuring across D702 (meter reads 0.025V with red light and alarm sound even after replacement), C702 and C708 (cap testing setting, you wait, then it fires and OL) when installed - all of which have either been pulled and replaced or thoroughly tested and working
I've included a couple pics for context, thanks again for any direction you can provide, always appreciated!

IMG_0723.jpg IMG_0724.jpg IMG_0725.jpg
 
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Did you test the bridge rectifier? I wonder why he went up from 470uf to 1000uf? That's a bit drastic. Was the phono stage removed? Check all the solder points on the power supply board with a magnifying glass. From the picture, one in the middle looks like the solder pad is missing. If that is a regulator I see on the power supply board, maybe check that too?
 
Yes, very strange "upgrades", I've never seen anything quite like it. The work was actually well done, nice solders, no missing pads, kind of a cool copper bridge tying the ground legs together on the main filter caps, interesting. The cap brands and types used were all over the map but were all quality components. I also can't remember seeing one with this bright orange and red wiring and I can't tell if the phono board was ever installed and or how it was removed and modified.

By the way, the test point voltages on the PS were all within tolerances, some off a bit but nothing dramatic. Is bumping up the filter caps up from 6800uf t0 10000uf something the unit will tolerate well or could it lead to ancillary issues?

I'll pull the bridge rectifier to test out of the chassis today, thanks for the advice, hoping it's a simple fix!
 
Upping those big caps may not be a problem. I've upped them before. I just made sure the rectifiers were beefy enough. One amplifier I went from 6,500uf to 10,000uf without issue. The power supply diodes and other components in the power supply the factory used were over kill to begin with. I used bigger main filter, coupling, and a few others in a little Marantz integrated, but I replaced the diodes also to make sure they could handle the inrush.
 
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I just looked at some other nudes of this receiver. They all have that blank spot where you would expect a phono board t9 sit. So I'm guessing the phono stage is incorporated on one of the other boards, like maybe the tone board.
 
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Thanks for the advice here, I'm used to seeing that phono board but I'm sure you're correct in that it's incorporated elsewhere.

I'll see about upping the fuse value as well, great suggestion. It's just the single main fuse located on the back of the unit, a 3A, so snagging a 3.5A should be pretty easy. When I first tested this unit for power a few weeks ago I had it running through a different power strip with a surge protector, so I'm assuming that's why it initially fired up fine and didn't fry the fuse.

I'll let you know how the bridge rectifier tests out and likely replace the other diode on the PS board (D705) since I've already changed out D702. I used a 1N4007 on D702, do we think there's a need for a beefier diode here due to the big-boy filter caps. I have a pair of 6800uf 50V caps I could easily swap in, but I'll resist the urge to do so until figuring out the PS action, ha!

IMG_0138.jpg
 
JB8 said:
  • I've got it running through the DBT and I'm seeing a quick burst of light at power up, then fades to almost 100% out, but the light is still shining dimly. Tested again without DBT and it blew the fuse at power up.
Click to expand...
There is no need to test the main bridge rectifiers. If one rectifier was shorted your DBT would light all the time. The main fuse has a too low amperage for the new 10000uF capacitors. You saw by yourself that the inrush current is much higher than the idle current. As said by cademan, increase the amperage of the main fuse. The original fuse, for the 120VAC units, was a 3A fast blow. You might try a 3.5A fast blow fuse. Personally, I would use a lower amperage (say 1.5A) slow blow fuse.

The 470uF capacitors that were replaced by 1000uF ones are clearly not the cause of the blowing main fuse because these capacitors were preceded by 22 Ohm resistors. The inrush current in these capacitors is low.

It is obvious to me that the phono board was removed. Follow the wires from the phono jacks. Do they go to a phono preamplifier stage somewhere, or do they go to the input selector switch?
 
If only there was a mic-drop emoji! Love it!

I'll try the fuse value change, sounds like it may have been the only issue the entire time and maybe the 1000uF caps were fine to maintain, we shall see.

I'll take some pics of the chassis and wiring as well specific to the phone preamp question, can't be that much of a mystery.

This is such a great forum, thanks, all, I'll keep you posted!
 
ecluser said:
There is no need to test the main bridge rectifiers. If one rectifier was shorted your DBT would light all the time. The main fuse has a too low amperage for the new 10000uF capacitors. You saw by yourself that the inrush current is much higher than the idle current. As said by cademan, increase the amperage of the main fuse. The original fuse, for the 120VAC units, was a 3A fast blow. You might try a 3.5A fast blow fuse. Personally, I would use a lower amperage (say 1.5A) slow blow fuse.

The 470uF capacitors that were replaced by 1000uF ones are clearly not the cause of the blowing main fuse because these capacitors were preceded by 22 Ohm resistors. The inrush current in these capacitors is low.

It is obvious to me that the phono board was removed. Follow the wires from the phono jacks. Do they go to a phono preamplifier stage somewhere, or do they go to the input selector switch?
Click to expand...
Looking at pictures of other STA78's I found, they look the same as his. An older Realistic integrated amp I had the preamp and phono sections on the same board.
 
No Sir, definitely no phono eq amp to be found. You're absolutely correct, @ecluser . I've had ~10 of these so should have known better...

When in the phono function position and a turntable running it does flow through and is clear, but super duper low volume. I have a donor STA-78 I think I'll pull the phono amp from, and may swap in some 6800uF filter caps, the 10000 are not necessary.
 
The two inputs, phono and AUX, sound the same, very low volume when the turntable is plugged in and playing. AUX input sounds fine with digital audio source (phone) playing music, expected volume.
 
I just looked at the pictures I found. My bad I didn't notice the black board mounted vertically. I assumed it would be horizontal like the others.
It looks like someone removed it and just soldered the phono board output wires right to the phono RCA jacks then. They must have used a stand alone phono preamp, or didn't have a TT and wanted another line level input.
 
The phono preamp is visible on the photo of post #9, but this is not the same specimen (same model though) as the receiver on post #1. On post #1, there is a large white band (-) on the large 10000uF capacitors, not on post #9
 
Lavane, I was thinking the same thing regarding the stand alone phono preamp. I've always appreciated the phono amps on these guys, to my ears they sound great, so not sure why the previous owner would remove it and still beef up the PS, etc. Interesting indeed!

And now I'm the one apologizing, I should have been clearer regarding the pic in post #9, it's an example of one of my previous units with the phono amp intact.
 
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