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Thread: Realistic STA-225 - outputs sound for a second or two, then disappears

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    Realistic STA-225 - outputs sound for a second or two, then disappears

    I'm trying to figure out what's wrong with a Realistic STA-225 receiver/amplifier that I picked up at a swap meet. When I first powered it on, I got no sound at all, so I opened it up and checked the amp board. Their were several burnt resistors, so I changed those out, then I checked the output transistors (there's 4 of them, 2 for each channel) and sure enough, two of them were bad. So I ordered replacements, made sure the biasing was correct for each pair, tried the thing again and... still nothing. I put my scope on it and found that the signal was reaching the amp board, but not leaving it. I'm somewhat inexperienced and didn't really know where to go next with it, so I took the completely logical next step of completely and literally stripping the entire amp board and testing each individual component. Yeah.... So I did find a couple more bad components, a diode and another resistor. So I changed those out, put it all back together and.. still doesn't work, BUT it does now output sound clearly for about a second or two, then disappears. I've spent waaay more time on this than I would have liked, but I'm now determined to figure this out one way or the other. My intuition is telling me it's either something to do with the output transistors still, although both channels are behaving the exact same way... or perhaps it's something to do with the relay. The only thing about that is I've inspected the entire unit and I've looked at the schematic extensively and I don't see a relay anywhere... So any help on where to go with this next is much appreciated!

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    I would be looking for open resistors starting with R602, check voltage across D601 16V zener.

    https://music-electronics-forum.com/...1&d=1584976269
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    Supporting Member Jazz P Bass's Avatar
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    I am curious where you found replacement output transistors.

    Wonderful schematic posted.
    All of the static Vdc voltages you need to troubleshoot the unit.

    It's a shame that current literature is entirely lacking in this respect.

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    Old Timer olddawg's Avatar
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    If you stripped the board make sure you didn’t install an electrolytic cap backwards? No DC offset? Do you have some freeze to “tickle” some components. Your schematic gives you some voltages. Use it. You should be able to check your signal at the diff pair.

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    Thanks for all the replies folks. I've made a little bit of progress and have come to a bit of a revelation, but first let me answer and comment back. dmeek, there are NO bad resistors or diodes anywhere on the amp board, when I stripped the board entirely, I check EVERY component and they are ALL working. Jazz P Bass, I got the output transistors off ebay, they are a matched pair, and were a "match" to the originals, but not quite exactly the same. But my logic is that you have two matched output transistors for each channel, left and right, so as long as the two for each channel are matched to each other, they don't necessarily need to match the pair on the other channel exactly, at least in order to get sound out of it. olddawg, nope, I definitely didn't install anything backwards. I AM getting DC voltage on the speaker outs though. About -4V on the right channel. So that's a problem. Left channel DC seems OK. I can try freezing some components. Anyway, let me try and describe the "revelation" I've come to.

    So the way this amplifier powers on is through a combined power/speaker selector knob, it's pretty self explanatory, turn it all the way left to power off, turn one click right to power on speakers A, turn another two clicks right to switch to speaker B, turn another click for A+B, etc., I've got a 1khz test tone sine wave coming in, and when I put my scope on the amp board, I'm getting the signal at the input of the amp board on both the left and right channel, and, if NEITHER the A nor B speakers are selected from the front panel knob, I do get an amplified signal at the outputs of the amp board... so that's good.. Now, as soon as I switch the selector knob to either the A OR B OR A+B position, the sine wave sound will come through the speaker for about a second then disappear, like I mentioned, however I'm now seeing on the scope that the signal is disappearing at the output on the amp board. So basically, the amplified signal is present at the amp output UNTIL i engage the speaker selector switch, at which point the signal disappears from the amp output...So I think I'm going to start inspecting around the speaker selector knob area, I see from the schematic there's a couple resistors in that area..or maybe there's bad contact being made in that region.

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    Supporting Member The Dude's Avatar
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    You shouldn't be hooking up a speaker if you have -4V coming out of the amp. You could do more damage to both amp and speaker. My guess is that the amp is going into protect when it sees DC on the output, but it's hard to say since we don't have a complete schematic. Do you have one you could upload? It seems you have one side working, so you should be able to compare sides and find the issue.

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    Old Timer olddawg's Avatar
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    Another trick is to float the output transistors on the “bad” channel and see if the other channel works. If you have one working channel then you can compare the two circuits. Yes.. do not hook the bad amp channel to a load with a DC offset. Btw.. checking caps especially with a meter does not necessarily mean they are good under load. It’s a relatively simple amp. I can’t tell from the piece of the schematic if it has a protection circuit. Older stereos used relays.

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    Quote Originally Posted by StormLord11 View Post
    Jazz P Bass, I got the output transistors off ebay, they are a matched pair, and were a "match" to the originals, but not quite exactly the same.
    I think what Jazz was getting at, if those transistors are obsolete, and you got ebay replacements that came from China, they may be fakes.

    What did you mean by " 'match' to the originals", same part number?

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    Supporting Member Jazz P Bass's Avatar
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    OK!!! So I pretty much got this thing working... When I first started taking a look at it a while back, I was pretty inexperienced and I'm a bit embarrassed to say I never checked the voltages on the power supply board... well I started poking around there..turns out there was a broken trace going to an output rail... So I patched that up and... SOUND! The only issue I'm having now is a pretty bad ground loop 120hz hum/buzz in just the right channel. There's some pretty erratic DC voltage on that speaker output as well, going rapidly between about 0-50 mV. This seems like a pretty common problem that I can probably figur eout, but I wont turn down any advice anyone might have as to where to start looking.

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    Without seeing it I would still tend to think your hum is a DC offset. Now that you have 2 working channels you can compare voltage measurements from the good channel to the bad channel. It’s should be easy to isolate at this point.

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    Thanks! Yeah it definitely seems to be something with the DC biasing on the output transistor, I've all but got rid of the hum/buzz completely now by tweaking the variable resistor at the base of the transistor... I think it's fixed.. thank you!

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