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Audio distortion from Phono Stage

HotRodStereo

HotRodStereo

New Member
Hi!

I am still learning the dark arts of vintage audio repair, and I am looking for some sage wisdom on a unit I am trying to fix. Here is the story -

Its a Sony STR 2800, I cleaned it up and it was working great, got everything all balanced and checked and listened for a while and then sold it before Christmas to a nice guy who wanted to give it to his daughter. He came and I did a demo of the unit for him and at that time, everything worked great. Not long after christmas he contacted me and said they had set it up at home and played a few records, then turned the unit off and put a new record on, turned it back on and then there was a horrible audio distortion. Since i warranty everything i sell, i took it back and am looking into it to see if i can get to the bottom of it.

The distortion is on the right side channel, ONLY on the phono setting, AM, FM, and Tape monitor all play clearly but even with no phono plugged in and no signal going through the unit, the distortion is present. It sounds almost exactly like when you rub a microphone with a peice of cloth, its not a whine or scream like a bad capacitor usually gives. I went through and tested all 4 transistors in the phono stage and the voltages present were all alright. (i tried to attach the service guide but it says its too large) The caps are obviously old, and since i had some laying around that fit the bill I figured i would shotgun them in there (only 8 of em) and its still making the same sound, right channel only and again, only on the phono stage.

When i send a signal through the unit with my signal generator I it comes through clearly with the distortion in the background. I checked all of the RCA ports and while they do make some noize to the touch, it doesn't seem to be the source. Checked and reflowed the ground as well.

Additionally, When the unit is cold and started up there is no distortion, but after about... 15 seconds it comes on. So heat seems to be a factor.

Diagnostically, I have an O scope, Heathkit signal tracer, signal generators, multimeter, and a rather suspicious heathkit capacitester.

I don't have anything specifically to test transistors (on the docket for sometime this year), am I on the right track here or am I missing something thinking it has to be Q551 or Q552?
 
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Clean again the input selector. If it doesn't fix, interchange Q501 with Q551, or Q502 with Q552. If one transistor is faulty the trouble will shift to the left channel.
 
Not looking at the schematic but if those transistors are the ones in the phono stage, agree that flip-flopping them would move the problem if they are the problem. Is that specific type one of the known to be problematic ones? There is a list floating around on here somewhere of known to suck vintage transistors. One of the fail methods is getting noisy, usually they sound like wind noise or popcorn. Phono stage with heavy EQ and high gain it might be more like you describe.
 
gadget73 said:
Not looking at the schematic but if those transistors are the ones in the phono stage, agree that flip-flopping them would move the problem if they are the problem. Is that specific type one of the known to be problematic ones? There is a list floating around on here somewhere of known to suck vintage transistors. One of the fail methods is getting noisy, usually they sound like wind noise or popcorn. Phono stage with heavy EQ and high gain it might be more like you describe.
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One of the transistors is right on the "Known to fail, noisy" list, 2SC632. I didn't have time to crack into it last night but great tips all around, I will swap them tonight and see if the noise follows.
 
I remember reading some place that the ksc1845's wouldn't work in a phono stage, but can't remember which brand.

They said to use ksc1815's instead.

Best to compare gain.
 
If so, they aren't really complimentary pairs then, and I'd also replace the A705 with KSA992.

It usually depends on the Ic of the transistor you're replacing. If the originals had an Ic of 100mA, then yes, the KSC 1815 would be the better replacement if you can't confirm what current the circuit will be see. The 1845/992 pair only has 50mA Ic.
 
kwayde said:
I remember reading some place that the ksc1845's wouldn't work in a phono stage, but can't remember which brand.

They said to use ksc1815's instead.

Best to compare gain.
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Great catch! Looks like this thread talks about it, but joy of joys somebody did a test and reported back on it, looks like it should be alright...

Since i would have to order either from mouser since i dont have them on hand, I will grab plenty of both to play it safe. I belive the 1815s have a lower HFE and may just be a better replacement. I need some 1845s on hand in any case.
 
The transistors an that stage don't need to be complementary. There is 80uA in Q501 and 2.4mA in Q502.
 
I'm away from easy access to data sheets, and you're undoubtedly correct about that, but I usually see them use complimentary pairs IF they use one Q that has a compliment.
 
It's not a push-pull circuit, this is a two stages amplifier. I know that we should use complementary transistors in a push-pull circuit, but this a completely different circuit.
 
I wasn't doubting you, and like I said I'm away from data sheets and schematics, so don't even know IF the original transistors. WERE a complimentary pair by accident or design. Just giving my observations that, if the one is bad, and they were complimentary, that I would also replace the other with the compliment one. C1845/A992 or c1815/A1015 (IIRC) and base it on the current specs of the OEM Qs.

You're much more knowledgeable about circuits than I am. Just trying to help the OP.
 
just as a side note, I’m now looking at the SM for the 2800L (downloaded from electrotanya). The scat lists the c1362 for Q501,551, but if you look at the page that shows all the different transistors, it lists the 2SC632A for Q501,551 with the 2SC1362 in parentheses( I.e. as a second choice). When googling around to try and find datasheets, web bcs listed 2SC632A as a compliment of the 2SA705. Interestingly, both of those are listed as having an Ic of 200mA and Vce Vcb of 50/50V, whereas it looks like the 1362 has an Ic of 0.2 uA (micro amps). Somewhere there must be a typo, but it isn’t the typical “mA”, looks more like a Greek “u” to me. Soooo (said in the voice of Steve Martin) by spec sheets alone, neither of the listed subs (C1845, C1815) are good unless the current in the circuit is checked. They likely ARE fine, but this shows how confusing some of the data we have to work with can be.
 
This is a long shot, but may be worth investigating. I saw a video a while back by someone who fixed noise in the phono stage of a Sony receiver from that era (STR-7065, if I recall correctly.)

The negative contact of the RCA jack is just a press fit into the ring that includes the connector to the internal wiring. He suggested that enough oxidation forms over time to disrupt the very weak signal from a phono cartridge. His solution was to make a little solder bridge on the back of the jack, between the outer ring and the back of the negative contact (similar to where the red dots are in this photo.)
1705736383784.png
From the pictures I've seen, the STR-2800 appears to use the same jacks.
 
Whether it's the source of the problem or not, if they look anything like that I'd be tempted to first use a dremel with a wire wheel and at low speed go over all those wire-wraps, add some flux, and give them a touch of solder too. And it wouldn't hurt to do the same dots of solder on the lower pair of rings that the negative wires are attached to.
 
jheu02 said:
Whether it's the source of the problem or not, if they look anything like that I'd be tempted to first use a dremel with a wire wheel and at low speed go over all those wire-wraps, add some flux, and give them a touch of solder too. And it wouldn't hurt to do the same dots of solder on the lower pair of rings that the negative wires are attached to.
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Yes, I only marked the top two for the sake of simplicity, but absolutely, all four should be soldered.

Good suggestion, too, to clean up the wire wrap connections, as well.

I'd be curious to learn whether this possibly solves the issue.
 
It's certainly a quick and easy try before digging further. That said, with known noisy transistors, they should eventually be replaced anyway. But always best to solve the problem first and then go improving things.
 
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