I’m rebuilding a Sony STR-6055 that’s a bit of a problem child. I’ve started on the amplifier driver board first and I’ve already found two transistors with a short. So far the replacements made are all of the 2SA706 and 2SC1124 with TTA004b and TTA004b. The rest of the transistors are to92 packages, 2SC632A, 2SC634A, 2SA678 which happen to be higher gain transistors with high-ish current spec for a to92.
I pulled a couple 2SC634 from the board and they measured with a gain of 404 and 421 on a Peak DCA75. Ultimately the KSC1845 matches perfectly with the exception of the 50 mA current rating vs the original 200 mA.
So being circuit dependent of course I’m trying to figure out where they would work.
I’m guessing that here in the phono preamp and tone amp that the current would be low enough that the 1845s would work well here.
My concern mostly is in the amplifier.
The differential pair is another place that has pretty low current demand on the devices correct? So 1845s there?
What about Q704, Q705 and Q706? They have Q705 listed as a 2SC1124, but it’s actually a 2SC634A in circuit.
Are my thoughts correct on all of the previous transistors (preamp and differential pair on amp board)? If so I guess my question pertains to these last three then.
Another option I have is BC550C and BC560C. Though some don’t consider those low noise.
Thank you,
Dan.
I pulled a couple 2SC634 from the board and they measured with a gain of 404 and 421 on a Peak DCA75. Ultimately the KSC1845 matches perfectly with the exception of the 50 mA current rating vs the original 200 mA.
So being circuit dependent of course I’m trying to figure out where they would work.
I’m guessing that here in the phono preamp and tone amp that the current would be low enough that the 1845s would work well here.
My concern mostly is in the amplifier.
The differential pair is another place that has pretty low current demand on the devices correct? So 1845s there?
What about Q704, Q705 and Q706? They have Q705 listed as a 2SC1124, but it’s actually a 2SC634A in circuit.
Are my thoughts correct on all of the previous transistors (preamp and differential pair on amp board)? If so I guess my question pertains to these last three then.
Another option I have is BC550C and BC560C. Though some don’t consider those low noise.
Thank you,
Dan.
my 2 cents- The 1845 in the phono and diff pair should be fine as the current is likely not to be a problem (this is an "estimate" not math)
Q705/706 could be 2SC2690/2SA1220
if Q704 is replaced then Q703 should be as well- however I do not have any subs for these
hope this helps
Q705/706 could be 2SC2690/2SA1220
if Q704 is replaced then Q703 should be as well- however I do not have any subs for these
hope this helps
Sorry to derail a little this post... But since I also have a nice SONY STR-6055 that suffers one issue...
Description of the symptoms :
1 - I power on the unit and tune it to a clear FM station, meter come to center and red stereo pilot lights, everything is fine.
2 - I power off the unit, without touching anything else.
3 - I power on the unit again and the tuning is now off, the needle of the meter is full at right, stereo light is off, I have to retune to the FM station.
I have the SONY STR-6055 service documentation and schematics.
Any idea ? 😕
Thanks !
T
Description of the symptoms :
1 - I power on the unit and tune it to a clear FM station, meter come to center and red stereo pilot lights, everything is fine.
2 - I power off the unit, without touching anything else.
3 - I power on the unit again and the tuning is now off, the needle of the meter is full at right, stereo light is off, I have to retune to the FM station.
I have the SONY STR-6055 service documentation and schematics.
Any idea ? 😕
Thanks !
T
@saabracer23
Aside from the differential input pair Q701 and Q702, I wouldn't worry about noise performance of other transistors in the amp. Noise performance in the PA is dominated by noise of the input pair and negative feedback absorbs noise contributions from the other devices, in the same manner as feedback helps suppress transistor distortion.
Aside from the differential input pair Q701 and Q702, I wouldn't worry about noise performance of other transistors in the amp. Noise performance in the PA is dominated by noise of the input pair and negative feedback absorbs noise contributions from the other devices, in the same manner as feedback helps suppress transistor distortion.
Sorry to derail a little this post... But since I also have a nice SONY STR-6055 that suffers one issue...
View attachment 1374154
Description of the symptoms :
1 - I power on the unit and tune it to a clear FM station, meter come to center and red stereo pilot lights, everything is fine.
2 - I power off the unit, without touching anything else.
3 - I power on the unit again and the tuning is now off, the needle of the meter is full at right, stereo light is off, I have to retune to the FM station.
I have the SONY STR-6055 service documentation and schematics.
Any idea ? 😕
Thanks !
T
How long do you wait between powering off and on? Has it always behaved like this?
It could be drift with temperature of the local oscillator. That could be perfectly normal.
Why would that help against temperature drift?
I could understand that advice if the frequencies on the frequency scale didn't match the tuned frequency, or if the preselection filter didn't track properly. That's not applicable.
I could understand that advice if the frequencies on the frequency scale didn't match the tuned frequency, or if the preselection filter didn't track properly. That's not applicable.
Regarding the original question, noise-wise the KSC1845 should be fine for the phono preamplifier input stage.
No, IIRC, My STR-6055 not behaved like that before. When I had it in 2000s, it only had the transistor driving the Stereo pilot that was dead (lamp was always lit) and I replaced it, no issue.
No need to wait more than a few seconds, nearly the time to power it off and repower it on.
I was primarily thinking about a drift occasioned by some leak somewhere - like a coupling or decoupling cap, or supply voltage - given that offset tuning... But where is the question ? Or would there be another cause coming from a faulty "frequency locking" circuit of some sort? Or possibly both causes ?
Yes. I did it some years ago, with the service manual in hands, and the suitable meters, IF / AM / FM and stereo generators : my STR-6055 was just in need of light readjustments, nothing further. And as @MarcelvdG stated it, I think that it's probably not where lies the cause of that offset tuning...
When tuned, my STR-6055 doesn't show any instability or drift around the FM tuned station, nor the Stereo function : it works as it should. moreover, the offset is always in the same direction : tuning meter is always full right.
T
It still is in the frequency locking circuit (AFC). DC offset and drift, almost certainly a leaky capacitor. You tune it into range, and it tends to stay there if the drift rate is low enough and didn’t initially exceed the threshold for locking. Turn it off and everything gets re-set. It’s now out of locking range and doesn’t even try - it just tells you that you are off tune.
It’s one place I wish they wouldn’t use electrolytic caps. Even class 2 ceramics need to be used with care since the value changes with voltage (use one at least 10X the applied DC).
It’s one place I wish they wouldn’t use electrolytic caps. Even class 2 ceramics need to be used with care since the value changes with voltage (use one at least 10X the applied DC).
Just for your info......
A loooooongt time ago I tried an experiment. A friend of mine brought an old Sony Cassette deck. Can´t remember the model,
but it was one of these lying down and the back with the VU´s angled towards your eyes.
He complained about noise. It was clear, specially with the playback amp, which didn´t mute, when the deck was paused.
This was before BC550C/560C, so I replaced all transistors in the playback amp with BC414/416.
Sound was excellent. Playback amp noise was gone (lowered to way better than just acceptable level) 😉
I assumed there was no automatic frequency control because I saw no knob to turn it off on the picture of the front. I forgot that some radios have AFC that you can't turn off, my parents actually had one of those. It was annoying, because it always locked to the wrong station at turn-on. I modified it so they could turn it off.
If there is an AFC, the issue can very well be related to it.
I like Marcel's theory.
When you provoke the mistuning by cycling the power, is the FM station audio still present but perhaps distorted, with tuning meter deflected?
I surmise you have to break the "stuck" meter indication with the tuning knob? But when tuning is once again correct, is the tuning dial back in the exact same position?
As an additional similar experiment, try shorting the antenna terminals in an attempt to kill signal strength, or disconnect antenna if necessary. Is tuning meter back near center, as in when no station is present? When you restore the antenna, does normal operation return without having to touch the tuning knob?
If these tests are as I predict, I think I can explain what's happening.
When you provoke the mistuning by cycling the power, is the FM station audio still present but perhaps distorted, with tuning meter deflected?
I surmise you have to break the "stuck" meter indication with the tuning knob? But when tuning is once again correct, is the tuning dial back in the exact same position?
As an additional similar experiment, try shorting the antenna terminals in an attempt to kill signal strength, or disconnect antenna if necessary. Is tuning meter back near center, as in when no station is present? When you restore the antenna, does normal operation return without having to touch the tuning knob?
If these tests are as I predict, I think I can explain what's happening.
Thank you @wg_ski , @MarcelvdG and BSST for your advices and tips !
I read my SONY STR-6055 Service Manual, and found that there's effectively an AFC circuit, consisting of D101, D102 and C120, occording to the circuit description in the manual and the reproduced front-end section schematic below (see lower right) :
Without deeper investigation, just looking at that schematic, I would first suspect C126 1µF/50V to be possibly defective, as it is the sole electrolytic capacitor in the circuit... (@wg_ski hint)
@BSST : I note the test procedure you suggest. I'll plug my stored STR-6055 to check it accordingly and come back with the results.
Wait and See... Again, my apologies for AF-to-RF hijacking your subject @saabracer23, but who knows ?
T
I read my SONY STR-6055 Service Manual, and found that there's effectively an AFC circuit, consisting of D101, D102 and C120, occording to the circuit description in the manual and the reproduced front-end section schematic below (see lower right) :
Without deeper investigation, just looking at that schematic, I would first suspect C126 1µF/50V to be possibly defective, as it is the sole electrolytic capacitor in the circuit... (@wg_ski hint)
@BSST : I note the test procedure you suggest. I'll plug my stored STR-6055 to check it accordingly and come back with the results.
Wait and See... Again, my apologies for AF-to-RF hijacking your subject @saabracer23, but who knows ?
T
I'm guessing a bit, but I think this behavior may be entirely normal. If the suggested experiments in post 14 behave as imagined, I'll offer up a description of what's likely happening. This to advise that you may be trying to find a defect that's not really there.
No worries at all, good luck!
For Q704 I ended up using some BC337 with higher gain, I have some in the 500ish area.
What are the purpose of Q705 and Q706? It looks like the output emitters drive into their bases and their emitters connect to the output signal. I’m curious if it would be beneficial to go with low noise there and what kind of current they will see.
Again, the originals on the board are 2SA678 and 2SC634A. I’m considering using the BC337 and BC327 here again. I’ve read their noise figures are pretty decent, they have high gain, and spec out for current and such vs the originals.
I have many other options, but I’m trying to keep things inline like Ft and capacitance. I have ZTX694/795 and 2N5401/5551 which easily meet the current rating, but the 2N devices are rather lower gain. Oddly the NPNs measured around 500 while the PNP was only about 100 on each of them.
Dan
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Q706 is a so-called bias spreader that establishes the voltage between the bases of Q707 and Q708; it's a temperature sensing shunt regulator that attempts to keep the bias current through R733 and R734 stable despite varying transistor temperatures. Notice that it's shunted by C704, so relatively little dynamic audio current flows through Q706. Noise isn't much of an issue.
Q705 is the Voltage Amplification Stage (VAS) and has rail-to-rail audio swing that provides driving voltage to the output stage. But its noise performance isn't very demanding since it's within the amp's global feedback network, as asserted in post 4.
Annotated voltages indicate there's 2.4V across R711, so operating current through Q705 and Q706 is 2.4V/330 ohms = ~7.3mA.
Q705 is the Voltage Amplification Stage (VAS) and has rail-to-rail audio swing that provides driving voltage to the output stage. But its noise performance isn't very demanding since it's within the amp's global feedback network, as asserted in post 4.
Annotated voltages indicate there's 2.4V across R711, so operating current through Q705 and Q706 is 2.4V/330 ohms = ~7.3mA.
Q704 is the bias spreader.
Q705 and 6 are the short circuit protection. High gain and good quasi-saturation characteristic are desirable here (BC237, 337 fit the bill nicely, get C rank). Noise is a don’t care there or in the bias spreader for that matter.
Q705 and 6 are the short circuit protection. High gain and good quasi-saturation characteristic are desirable here (BC237, 337 fit the bill nicely, get C rank). Noise is a don’t care there or in the bias spreader for that matter.
Hi @BSST, I followed your test procedure 👍, and here are the results :
Let's do some power cyclings :
Then we have :
* when the MUTING circuit of the STR-6055 is ON, the meter needle is deflected to the right, and there's no FM radio station audible, perfect silence.
* when the MUTING circuit of the STR-6055 is OFF, the meter needle is deflected to the right, and a radio is normally audible, undistorted, but this radio (Ka-Boom FM 103.3MHz) has taken the place of the original one (France-Info FM 103.5MHz).
In other words, a lower frequency drift has occured, and it is always in the lower frequency direction.
Yes, that's it.
I have to "re-hook and drag back" the original lost station (France-Info FM 103.5MHz) by turning the dial knob slightly to the right, and then once the station is hooked again, I turn back the dial knob to its original position, meeting exactly the initial dial position of the lost station (France-Info FM 103.5MHz), in order to have the meter needle centered.
OK, let's proceed :
During the procedure, I do not have to touch the tuning knob, and the needle meter stays always centered, it never moves.
I hope I am clear in my description ! 😕 So what can you conclude, then ?
--------------------------
As I have now my STR-6055 on my bench, I will locate C126 1µF/50V Electrolytic, and replace it by a small Polyester or Polypropylen 1µF/63V. Anyway, it can't be a bad thing, and possibly the cure, who knows ? 😉
Wait and See...
T
Let's do some power cyclings :
- I set the tuning at 103.5MHz FM (France-Info), needle meter is centered, the FM station is well-received.
- Then power off the STR-6055, wait for 5 to 10 seconds.
- And re-power on.
Then we have :
* when the MUTING circuit of the STR-6055 is ON, the meter needle is deflected to the right, and there's no FM radio station audible, perfect silence.
* when the MUTING circuit of the STR-6055 is OFF, the meter needle is deflected to the right, and a radio is normally audible, undistorted, but this radio (Ka-Boom FM 103.3MHz) has taken the place of the original one (France-Info FM 103.5MHz).
In other words, a lower frequency drift has occured, and it is always in the lower frequency direction.
Yes, that's it.
I have to "re-hook and drag back" the original lost station (France-Info FM 103.5MHz) by turning the dial knob slightly to the right, and then once the station is hooked again, I turn back the dial knob to its original position, meeting exactly the initial dial position of the lost station (France-Info FM 103.5MHz), in order to have the meter needle centered.
OK, let's proceed :
- I tune to 103.5MHz FM : the needle meter is well-centered, the radio station is cleanly received.
- I disconnect the antenna : I obtain hiss or mute, depending if the muting circuit is activated or not. The needle meter is still centered, no movement.
- I reconnect the antenna : the needle meter stays well-centered, the radio station is cleanly received again, initial situation.
During the procedure, I do not have to touch the tuning knob, and the needle meter stays always centered, it never moves.
I hope I am clear in my description ! 😕 So what can you conclude, then ?
--------------------------
As I have now my STR-6055 on my bench, I will locate C126 1µF/50V Electrolytic, and replace it by a small Polyester or Polypropylen 1µF/63V. Anyway, it can't be a bad thing, and possibly the cure, who knows ? 😉
Wait and See...
T