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REALISTIC STA-78 Blowing Main Fuse

I'm betting he just didn't care about having a turntable and ditched it for another line level input. I guess you can troll eBay for a STA-78 being parted out.
 
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Quick update, I swapped in some 6800uf 50V main filter caps and all is working fine except for the stereo function - even though the light is coming on for stations in stereo the only way to get it to go from mono to stereo is to engage the loudness button, then I get a nice wide spread. When not in loudness mode engaging the mono button has no effect on the sound, and all the switches and pots have been thoroughly cleaned. Good times, ha! One more thing to track down, but no more blowing fuses. I'll save those big caps for another unit, this guy didn't need them in first place.
 
It sounds like your getting closer. The loudness button isn't effecting stereo separation with other sources is it?
 
Hey, Lavane, yessir, getting there! And great question. I'm currently doing some testing, seems the mono button isn't working across the AUX function either, so it's loudness with a heaping portion of stereo or a very mono version of both FM and AUX content.

Considering the extent of other unique conditions I'm finding on this unit - crazy cap combo plus no phono preamp - I wonder if he somehow decided to bypass the mono button/switch and just insist on loudness as the only stereo option, would not put it past him. I'll check out the wiring tomorrow, too busy enjoying the music and tone of this receiver, it's incredibly strong, I can barely turn it up 25% without it overfilling the room. Listening to everything from Pink Floyd to Dave Brubeck, wowzers, amazing sound!

Btw, checked and set the idle, all good at ~5 mV. Also decided not to borrow the phono preamp from my donor STA-78 and instead I found a reasonably priced Realistic 42-2930 Phono Preamp on that auction site that I'll likely rebuild and include with this unit if I end up selling it. I think advertising it with the two line level inputs is actually not a bad thing especially if you have a phono option if needed via the separate pre-amp. Still need to clean this guy up, get some cool blue LED's installed with a dropper resistor, clean and polish the "glass", remove the faceplate and knobs for cleaning and get the back of the unit tidied up as well. Lastly, the wood case it came with has some deep scars, so I have an extra case I may use instead. A light sanding and some Odie's Oil will get it showroom ready, I'll post some pics when it's done.

Really appreciate your support here, learning a ton on every thread!
 
Ya, you might want to take a good look at the mono and loudness switches like you said. Maybe see if the soldering and wiring looks messed with. That is odd. You do at leased have a donor to compare. Those Realistic phono preamps aren't horrible. I may get one in the future and play with it. Heck, I use a cheap Pyle phono preamp at my bench hooked up to a U-Turn Orbit Custom TT. It was well worth the $15 new.
 
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All of the wiring appears to be intact, no shenanigans, so now I'm just staring at the pushbuttons and wondering how the magic happens, specifically:
  • If the mono switch is not working, how is it that by engaging the loudness button I'm getting stereo?
  • Is it safe to assume that it has to be an issue with the mono switch itself since it doesn't work across any of the functions?
  • I'm seeing the stereo beacon light up as it should, stereo separation sounds great when in loudness mode - what's the best way to test the mono switch? Does it need to be pulled in order to properly check it? Continuity check?
While I'm a fan of the loudness feature I'd still like to get this sorted and be able to listen to a source without it engaged, any help here is much appreciated!

IMG_0728.jpg
 
It doesn't appear to be messed with. Is the stereo/mono switch before or after the loudness tap? If you can determin if one end or the middle is the input on that switch, you may be able to put a meter across those unused solder lugs to see if you get continuity.
 
You can not see if the mono/stereo switch has been messed from the top of the board. You must look at the underside of the switch PCB. One pole of that switch connects the L and R input terminals of the volume control together. It does affect all the sources. The second pole of that switch is related to the stereo beacon on the FM band. There are many wires of the same colour, the white ones particularly. It is a possibility that the original position of the wires was changed by a previous worker, at one end of the wires (the switch board), or the other end (tone board). The layout of the switch board is very detailed in the service manual. Make sure the mono/stereo switch is cleaned with contact cleaner. If it still doesn't work, follow the wires from that switch to confirm that it is connected as it should be.
 
ecluser said:
Make sure the mono/stereo switch is cleaned with contact cleaner.
Click to expand...

I pulled the gang of switches, continuity was good so I cleaned the loudness and mono switches again, this time with a little F5. I'd already used the D5 but must have missed spot, the mono switch is working great now across the functions, whew!

Ok, on to cleaning, thanks again, Lavane and ecluser, much appreciated!
 
Good work. At leased it was something simple in the end. I ran into a mono/stereo switch on the back of a Sony power amp that had built up carbon.
One channel was weaker than the other. I dought it was ever used in mono, or
bridged mode. It just built up carbon sitting in one position over the years.
 
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The saga continues!

I paused on the cleaning to make sure everything is buttoned up properly. After installing the LED's the unit blew another 3A fuse, so I went with the 1.5A slow blow as an alternative and haven't had any issues since. That said, I was and am concerned that something still isn't quite right. I then started to test voltages and most seem within tolerances. Here are a few from the PS:

Pin 12: 12.94V DC (should be 13.8V)
Pin 13: 33.45V (33V)
Pin 14 29.70V (29.4V)

B+: 27.17V (29V)
B-: -27.17V (29.5V)

With the idle current set at ~5mV I'm getting 43mV DC (should be less than 25mV) at the right speaker terminal, 9mV on the left. I've replaced all of the transistors on the main amp, both channels, except for TR601, 602, 603, 604.

This is definitely a weak spot in my troubleshooting. I've spent some time reading through other threads that discussed things like differential pairs, bias transistors, etc., but again, chasing this down is something I could use some assistance with.


IMG_0735.jpg
 
Thats right, a lot of these Foster made amps and receivers only have the one voltage balance adjustment per channel. Instead of trimmers to adjust DC offset and Bias like other brands did.
 
Your problem is that TR601 and TR603 (left channel) are mismatched. They should have the same hFE, within 5%. There is no need to match them to 1% since all the resistors in that amplifier circuit have 5% tolerance. Further offset tuning can be done by adjusting R607. If your offset is positive, you must reduce R607. If your offset is negative, you must increase R607. Are C603 and C609 new? Leakage in these capacitors will affect the offset.
 
Thanks, ecluser, I'm on it!

C603 and C609 are not new, I'll replace those straight away along with their counterparts on the right channel.

Do you prefer the 1815's or 1845's as the replacement for the 1570's at TR601 and TR603?
 
Thank you again, ecluser, always appreciate your experience and guidance. That totally did the trick, I now have ~10-12 mV on both channels at the speaker terminals. The previous pairs hFE was off by roughly 10%, new pairs are matched within 2-3%, went with the KSC1845's, worked perfectly.

One more question and hopefully I'm good to go. When the previous owner removed the phono preamp he also removed the 33V power source that lead from pin 13 on the PS. The PS, by design, anticipates delivering that voltage to the pre-amp, but since it's no longer being tapped for the 33V are there any issues with running the unit as it is? I'm still thinking about the fuse situation and speculating that if the phono preamp isn't pulling the expected voltage maybe there's an imbalance contributing to the fuse issue? May not be a big deal but would love your thoughts here.

Thanks again, excited to wrap this project up!
 
It could be problematic that you have no load on pin 13 because there will be too much current in the Zener diode D705. The WZ-320 is rated at 500mW but without load on pin 13 you have more than this. I suggest that you replace R706 by 2kOhm
 
Excellent, thank you for the direction. I'll need to place an order for some 2kOhm 1/2 W resistors, so once they arrive I'll get one installed and finish things up.

If, before the resistors arrive, I decide to pull the phono board from my parts unit, I'll need to run some new wire from the PS to the phono pre-amp board. Is there a preferred wire to install? I'm assuming solid core but curious about gage in this scenario. I could pull the wiring from the other unit but I'd like to use new stock if I end up doing the surgery.

Thanks again!
 
The phono board draws approximately 5mA. Any small wire will do the job for the power supply. If you intend to reinstall a phono board, keep R706 bellow 1kOhm or the Zener will not provide good regulation. In a Zener regulator, it is important to consider the load current. Without the phono preamplifier, the load current is the base current of TR701 (approximately 0.1mA) which is very small.
 
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