realistic sta-2700 going into protection mode

[jsdspif1]

I've been using a new to me sta-2700 for a couple of months now. At times I've run it pretty hard. The other night running it "pretty hard" it shut itself off and then back on , I then cranked it way down but it again went into that protection clicking off and then back on. I thought maybe it was the speakers so I removed them and installed some other speakers (I'm just running a single set of 2 8ohm speakers, "speakers A"). At that point I just turned the receiver off. Now last night while using it at quite a low volume after about a half hour it shut itself off and then turned itself back on and at that point I just shut it off because I have a problem somewhere. I didn't really feel any warmth coming off it so I don't think it's heat related.


I bought a service manual for the 2380 because I know they are similar, thinking that the troubleshooting would maybe give me a hint but it doesn't have anything about what to look for if it's going into this protection mode. I'm hoping someone or multiple people will have an idea what to look for as to why it's happening. Thanks in advance.

 

[jsdspif1]

I got a tip on this that the power supply board can have problems with bad caps and/ or bad solder joints so I'm going to look that over. He also stated if dc offset is high replace the two transistors at the input stage. I found this old post from the dc offset topic (pic below) and I have pretty close to those readings. My right channel is 3 millivolts but my right channel is 100 millivolts. I'd like to replace the transistors in mine but I don't know which two are at the input stage/don't know which two to replace. I also am using the service manual and schematic from the realistic sta-2380 because that's all I could find. So if anyone could help me out with which transistors I need to replace along with where I could obtain the service manual for the sta-2700 I'd appreciate it. Thanks.

 

[Squlliam100]

I got a tip on this that the power supply board can have problems with bad caps and/ or bad solder joints so I'm going to look that over. He also stated if dc offset is high replace the two transistors at the input stage. I found this old post from the dc offset topic (pic below) and I have pretty close to those readings. My right channel is 3 millivolts but my right channel is 100 millivolts. I'd like to replace the transistors in mine but I don't know which two are at the input stage/don't know which two to replace. I also am using the service manual and schematic from the realistic sta-2380 because that's all I could find. So if anyone could help me out with which transistors I need to replace along with where I could obtain the service manual for the sta-2700 I'd appreciate it. Thanks.View attachment 1263746

I have two 2380s and one 2700 all three do this. Recapped the power supply in all of them and it's the same thing. Around half valum they shut off speakers. Thinking about recapping the protection circuit just haven't got around to it.

 

[jsdspif1]

A little off topic but same problem concerning the realistic 2380. It was also randomly shutting down and I recapped the power supply, changed the dc offset /bias transistors on one channel that showed about 80 millivolts or so but other channel had only 7 or something and found quite a few solder joints that looked "cold". The unit worked good for a number of months at all different volume levels and inputs,but a few days ago it started shutting down again. No particular pattern and no changes or moving of anything. One thing I do notice is when it shuts down I sometimes can turn it right back on but other times it won't turn on until maybe hours later. Again,no specific. The other morning I had just turned it on very low volume watching tv for about 10 minutes and it shut off. A few nights before I had been running it pretty hard for 2 or 3 hours with a fan blowing on it and it shut down so I just turned it back on at low volume and all was fine. When it does shut off if it doesn't happen to want to turn back on right away I do notice a small red light on the receiver acknowledging that I'm pressing a button on the remote control so I'm assuming that portion of it is still receiving power. My thought was maybe a relay has got dirty contacts so occasionally power is interrupted enough to shut down the receiver? actually I'm going to need to double check all of this because now I'm mixed up as to whether I repaired the 2700 I have or if I did the 2380?? As for the 2700 I was using it in my barn and when I fired it up the other day it made some fizzling sounds from inside the receiver and had no output but did not shut itself off so I shut it off and turned it back on and it started to smell as if something was getting hot inside the unit so it's back to the drawing board. I now have a onkyo 8255 (I think 8255) that I will try in the house to replace the 2380.
Anyway as far as the relays go, do you think a relay with dirty contacts could be a cause of the shutting down problem?

 

[BrokenHill]

There is a youtube video that discuses the shut down problem with the STA-2700 &
STA-2380 and how to fix this problem.
youtube link >

I never did have this problem with my STA-2700 and I have owned it since I bought
it new in 1989, what I did notice is that over time it did not sound as good as I
remembered in the first 10 years of use, about 5 months ago I took it apart and was
looking around inside and noticed that the capacitors were a brand I never seen before
and yes there was some of that awful glue on some of the power supply capacitors,
I replaced the power supply capacitors with Nichicons and used Elna Silmic ii elsewhere,
cleaned off the glue, soldered all the wire wrap posts on the circuit boards.

I was very pleasantly surprised at how good this amp sounds now and I think they
are worth fixing or rebuilding. (real sleepers)
I have checked the DC offset and I get -38.1 mV on the right channel and -1.2 mV on the
left channel, I did read the post in Amplifier Distortion, DC-Offset, and You!

Realistic STA-2700

Pre Repair:
R ch: 7.2mV
L ch: 77mv

POST Repair:
R ch: 7.2mV
L ch: 2.8mV :thmbsp:

A pair of matched transistors for the input stage was the remedy. I match them in house using the Peak Atlas DCA Pro.

I would like to get mine closer then it is now, I finally found a schematic for the Realsitic
STA-2700 uploaded it to imgur.com for full size jpegs.

I don't know which transistors at the input stage control the DC offset for this amp,
I could use some help if someone has more experience and can spot which transistors
in the schematic controls DC offset so I can replace them with a closer matched set.

 

[avionic]

I don't know which transistors at the input stage control the DC offset for this amp,

TR601a,b and 602 a,b long tailed pairs(differential amp pairs)

 

[audiodummy]

Weird... wonder if I spotted a schematic problem... I'm confused. Is it easier to trigger protection on left or right channel? Meaning, if you set your balance all left versus all right, which one would trigger protection first?

If it's one channel before the other... suspicious...very suspicious...

 

[avionic]

which one would trigger protection first?

The channel with the problem will trip the protection.....

 

[avionic]

If it's one channel before the other... suspicious...very suspicious...

No..........................

 

[Goldie99]

If it's one channel before the other... suspicious...very suspicious...

@audiodummy - I assume you're referring to the schematic values for R624a & b ? if so, I suspect it's probably just a misprint, but it should be easy to check.

 

[audiodummy]

I sure hope it's a misprint. Would be extremely hard for me to check as I don't own one of these.

Anyway that's where I'd hack the circuit to truly fix the problem instead of cutting leads...

 

[Bill Ferris]

Watching this thread with interest.
No problems yet with my STA 2700, even though it has been in continuous 24/7 use since purchase new in 1992, and delivering clean clear music as I type from an adjacent spare bedroom driving two Minimus 7`s w/a Optimus 12" subwoofer.

Never have needed to pop the cover after initial bench check out before placing in service.

I`m convinced that using quality power surge/spike protection on all my electrical/electronic devices/equipment has kept me from having to service them, along with good ventilation to minimize thermal stress.
I`m on the lazy side, so this seems to keep me from having to work on my own electronics.

Good luck on getting that good receiver fixed, and performing as it should, sorry to not have been helpful.

 

[BrokenHill]

Back in the 90s I rented a shop in a industrial complex for 10 years and I was really hard
on my STA-2700, it's a real work horse receiver/amp.

Avionic, thank you very much for your help I will order a bunch of those transistors and
match up a couple sets for replacements.