Well, I've come this far, I might as well experiment a bit further! I probably don't have any small capacitors to use to create the feedback network, so I'd have to order some parts. I've got to do a few other things like clean the potentiometers anyway - the balance and tone controls are a little scratchy, so I might as well persist.
You could try without the cap. The reason to use one is simply to keep the DC offset at the chip output low but try it without. If the offset causes C214 (the output coupling cap) to be reverse biased then simply reverse the cap polarity for the test.
It is uncharted territory but worth trying.
It is uncharted territory but worth trying.
One thing that occurred to me is that I may be able to examine the feedback loop and tone controls on the channel that has the opamp removed, as removing the opamp breaks the circuit and should mean that more components are measurable in situ. I don't know what I would learn from that, but I did some brief reading on passive vs active tone controls and I'm starting to understand a little more of what I've got myself into! It's not like I'm without music while I'm playing around with this, so I can take the time to get it right as it's a very nice looking piece of kit and just needs the sound quality to match.
It does make it easier to check although I have doubts you will find any faulty or out of spec parts tbh. It would be very unusual because it would mean both channels have the same 'problem' if that were the case.
I wish we could find details on that opamp though but I haven't been able to turn anything up yet.
Right.... that's it for tonight 🙂
I think it’s the same as a Hitachi opamp, there’s a link to its datasheet in post 39. I have no idea if it is the same, I just found a list of equivalent ICs somewhere and this was a match in the list. The provenance of the list is unknown; the circuitry in the test circuit in the datasheet is similar to that for the preamp so it is plausible that they are the same.
I remember the Hitachi chip from years ago and it was most often used for phono preamps. It had a very good reputation back then for its performance. The pin outs do seem to fit. Interesting.
Two more interesting data points: the CX-550 opamp is also used for the phono amp in this unit, which fits; and I've spotted some Hitachi-branded wiring on the accompanying power amp, which means that Sony were definitely sourcing parts from Hitachi at the time. I'm tempted to buy a couple of the Hitachi opamps, since they seem to still be available from what appear to be legitimate sellers.
If you think you can use them then maybe no bad thing to get them while you can 🙂 I get the feeling you won't be happy anyway until you have swapped them 😀 😀 😀 They are an excellent chip.
Very true! For now I've invested in some sockets for the opamps, to facilitate further diagnosis without needing to get the soldering iron out again - that will enable me to try out the Hitachi chips easily enough when they arrive too.
I don't know if you can, but a socket for a DIL package makes two nice SIL sockets with the careful application of a craft knife 😉 Fortunately, on the higher-quality sockets that have round receptacles for each pin, you can neatly push out the CX-500's "missing" pin with a pair of pliers, thus making a perfect socket for the opamp.
It's crossed my mind that, whilst the capacitors I can measure still have the right number of uF capacity, they may not be as alright as all that. A number of them - many of the 10uF ones in the signal path, and the 100uF power stabilising capacitors near the opamps, are sitting a bit wonky. It's not the usual capacitor failure sign, but that's made me wonder whether it's worth swapping them out. I mean, it's not a huge cost and it can't do much harm...
It's crossed my mind that, whilst the capacitors I can measure still have the right number of uF capacity, they may not be as alright as all that. A number of them - many of the 10uF ones in the signal path, and the 100uF power stabilising capacitors near the opamps, are sitting a bit wonky. It's not the usual capacitor failure sign, but that's made me wonder whether it's worth swapping them out. I mean, it's not a huge cost and it can't do much harm...
Ah, I did wonder but thought 'nah' you won't be doing that 😀 But you are 🙂
That is always a possibility and to be honest checking the value alone doesn't tell us much at all. Capacitors often fail by developing a high E.S.R. which is equivalent series resistance. It means a 100uF cap is still a 100uF but effectively in series with a highish resistance that makes the cap ineffective, particularly in applications where the cap is in a low impedance part of a circuit like a PSU.
There is no way of knowing without measurement although a scope is useful and should show no AC voltage across the cap (such as high ripple in a PSU.
The age of the caps is not necessarily a guide, I've tested caps out of equipment as old or older than this and they measure as well as new parts. That doesn't mean they don't fail and age but it doesn't mean they automatically do so either.
Yep, I'm not one for blindly believing that recap will make everything better - I saw far too many people using the word "soundstage" when describing the effect of their recap, which puts my on my guard a bit 🙂 and I guess that the likelihood of any given capacitor to fail or age is in part dependent on where it is in the circuit, as that'll determine the kind of life it's had. Having seen what big capacitors from the "capacitor plague" era look like when they fail by bulging spectacularly, it feels like them being a little wonky shouldn't be the end of the world. Then again, I've also seen an obviously failed old capacitor that looks like it took a leak down one of its legs, so... 😉
That is is it exactly
you can often second guess which caps are the ones that are going to be stressed and hich will be the ones that have deteriorated. Usually the ones that see high ripple current and/or high temperatures from surrounding parts.
Well, that's one more thing eliminated: I swapped out all the electrolytic capacitors for shiny Nichicon gold ones, and there's no difference. I did get some good soldering practice though! (I didn't bother swapping the caps on the phono stage, but I did also try disconnecting the power lines to the phono stage and that made no difference, so that can also be eliminated as a source of the noise.) Guess I'll have to wait for the replacement opamps to arrive and see what that does...
It's so difficult to say without hearing it for real but hiss generally comes down to basic circuit design. I don't think you tried altering the feedback loop to make it low impedance. That would be one thing to experiment with.
Yes - now I have one opamp in a socket and a bunch of spare capacitors, it'll be a lot easier to experiment with that.
I was idly thinking about this, and I think I might be starting to make sense of things.
The voltage regulator is working correctly (at least, the positive rail works correctly and the negative is within 0.3V). The voltage on the phono section is also correct (again, the negative within 0.3V). But the voltage on the EQ section is not. Since the voltage drop across R311 is 0.6V instead of the expected 1.3V (22.5-21.2) and similar is seen on the corresponding negative rail resistor R312, that means that the EQ circuit has a higher effective resistance than specified, which means that something in the EQ circuit is not right. Assuming that the voltages in the service manual are correct and that Sony's designers knew what they were doing, that means that one of the components in the EQ circuit is not behaving as expected.
The spec for the HA12017 (which we're assuming is the same as the CX550) does state the formula for the bias resistor based on the input voltages, and if the resistance is incorrect it will increase noise. The voltages being incorrect means the voltages and resistance don't match, which causes noise - but is not the cause of the voltages being out of line.
Aside from a capacitor to ground, the regulated voltage rails are connected only to the opamps - but the behaviour of the opamp is also determined by the nineteen components around it, so the opamp isn't necessarily the cause - but I measured all the resistors and electrolytic capacitors and they're within spec; that just leaves the opamp and the ceramic capacitors. I took the opamp out of its socket and measured C207 and C208 and they read roughly double what they should, but I don't know if my capacitor tester is accurate for a 47pF capacitor. The HA12017 says that these having higher values than recommended would increase THD, so that just leaves the feedback loop and the opamp itself as potential causes.
Which is to say: of course your suggestion to try a simpler feedback loop was a good one all along - now I understand why it's a good suggestion. 🙂
The voltage regulator is working correctly (at least, the positive rail works correctly and the negative is within 0.3V). The voltage on the phono section is also correct (again, the negative within 0.3V). But the voltage on the EQ section is not. Since the voltage drop across R311 is 0.6V instead of the expected 1.3V (22.5-21.2) and similar is seen on the corresponding negative rail resistor R312, that means that the EQ circuit has a higher effective resistance than specified, which means that something in the EQ circuit is not right. Assuming that the voltages in the service manual are correct and that Sony's designers knew what they were doing, that means that one of the components in the EQ circuit is not behaving as expected.
The spec for the HA12017 (which we're assuming is the same as the CX550) does state the formula for the bias resistor based on the input voltages, and if the resistance is incorrect it will increase noise. The voltages being incorrect means the voltages and resistance don't match, which causes noise - but is not the cause of the voltages being out of line.
Aside from a capacitor to ground, the regulated voltage rails are connected only to the opamps - but the behaviour of the opamp is also determined by the nineteen components around it, so the opamp isn't necessarily the cause - but I measured all the resistors and electrolytic capacitors and they're within spec; that just leaves the opamp and the ceramic capacitors. I took the opamp out of its socket and measured C207 and C208 and they read roughly double what they should, but I don't know if my capacitor tester is accurate for a 47pF capacitor. The HA12017 says that these having higher values than recommended would increase THD, so that just leaves the feedback loop and the opamp itself as potential causes.
Which is to say: of course your suggestion to try a simpler feedback loop was a good one all along - now I understand why it's a good suggestion. 🙂
Unfortunately that is one thing you can not rely on and errors occur all the time. They shouldn't but they do. Sometimes voltages can be obviously incorrect in service manuals and other times it is just the details that are wrong.
If you look at the emitter of Q303 (the regulated negative rail) and follow the line to the opamps you will see it starts at -22.8 volts and ends up at -21.2 volts. So 1.6 volts lost along a piece of wire 🙂
The phono stage also has the two transistor differential stage in front of the opamp and that looks to draw around 3.6 milliamps. So that will make the phono section appear to draw more current than the other opamp.
You must check caps out of circuit and yes it is possible the meter loses accuracy at those low values. Also don't 100% assume that 47pF is what is fitted without actually looking at the markings on the cap... that's another area where manuals are not always guaranteed to be correct.
You're quite right - I had spotted the errors around the regulated positive and negative voltage. The emitter from Q303 should join between R310 (marked R301 in the diagram) and R312 instead of right of R312, and the positive and negative regulated connections should only measure +/-21.2V after R309/R310/R311/R312. The measurement points are shown correctly on the PCB layout (where R311 is slightly down and right of where it should be - it should join on to the wire marked 15; the bridges from wires 14 and 15 to the right do not exist; the gaps in the traces where those bridges land also do not exist).
I guess, given that there are those kinds of errors in the diagrams, it's quite plausible that they could have made an error on all the calculations for IC201 and transposed them incorrectly to all the places in the diagram 🙂
I also think (fortunately) that that doesn't change anything about the cause of the issue or the next steps in my investigation.
I guess, given that there are those kinds of errors in the diagrams, it's quite plausible that they could have made an error on all the calculations for IC201 and transposed them incorrectly to all the places in the diagram 🙂
I also think (fortunately) that that doesn't change anything about the cause of the issue or the next steps in my investigation.