The 4558 is a bipolar op amp and needs a DC return path for the bias current from its -ve input to the O/P. This is provided through the slider of the bass control (a bad design implementation) and if the slider is open the bias conditions for the op amp are upset and the O/P will eventually saturate against one of the rails. The first op amp provides none of the tone control functions and is purely a gain stage with a gain of about 8 and also providing a low impedance O/P to drive the tone control network. The second op amp provides all of the tone control functions. However, with the second op amp disabled there is still an audio path through the the various Rs and Cs of the tone control network. The relatively low values of the capacitors CE10, CE18, CE14 provide a significant degree of bass cut when driving into the input impedance of the following stage of the amplifier, which is why you are seeing a dip in the gain at low frequencies with the frequency response measurements. With the second op amp operating correctly it will provide a high input impedance so as not to load the tone control network and a low output impedance drive to the following stage. This will eliminate the bass cut phenomenon you are seeing. Sorry about the long-winded comment but I hope this helps the understanding of the circuit operation.
@SoddingaboutSi
6 жыл бұрын
Stephen Bell Don't apologise Steven thanks for the verification. I was thinking of putting a high impedance device to the input of the opamp to prevent this happening again if the pot goes open. Would you expect to see some intermodulation or phase distortion as both op amps are passing signal in parallel but one via the r c network?
@stephenbell9257
6 жыл бұрын
I am not sure if putting in a high impedance device like a FET input op amp would totally fix the problem if the pot slider goes open circuit. Even a FET input op amp requires a DC return path from each input to allow the input leakage current somewhere to go. The DC gain of the op amp also needs to be controlled by some degree of negative feedback. If the input goes open circuit there is no feedback and the op amp will amplify any input voltage by the op amp's inherent high gain, typically 100,000. So even a fraction of a mV on the input results in the op amp's output going hard against one supply rail or the other. Without a wholesale redesign of the circuit the best I can suggest is that you put a high value resistor, say 1Megohm, from each end of the bass control pot to the slider. In the event that the slider goes open again there will still be a DC path from the op amp input to its output and the op amp will remain relatively close to its normal operating point and will pass signals without gross distortion. The boost and cut range of the bass control will be slightly degraded with this mod but otherwise it shouldn't affect the circuit operation too much. Hope this helps.
@SoddingaboutSi
6 жыл бұрын
Thank you Stephen.
@mohsentabouna6401
2 жыл бұрын
@@stephenbell9257 I'm an engineer in mechatronics and I have zero knowledge of what stephen is saying.. I was fixing my marantz and it had the same issue and the bass slider was bent also. I should I had your knowledge
@jeromeclarke4141
7 ай бұрын
From Suriname 🇸🇷 January 2024 I enjoyed listening and reading the comments of others nevertheless as an amateur electronic technician I just got a Marsntz PM 350 some parts were missing/luckily I cane across your video and realized that the 4560 pre amp was missing/used 4558 instead/? Now i can correct that / further the fuses tends to burn upon powering up the Amp /need some advice relevant to thus issue " compliments from Suriname 🇸🇷 2024 January
@johnsweda2999
6 жыл бұрын
What about changing the op-amps for different types this is more than likely could improve the quality of the amplifier. Worth experimenting and see what you get interesting to see the video on this? Why are the two capacitors so high off the board can't see any interference form anything that would affect them Do one channel at a time and compare the difference with modern op-amps that are more effective
@SoddingaboutSi
6 жыл бұрын
John sweda Tried a few. No change really. Seems good with this one which is well respected.
@johnsweda2999
6 жыл бұрын
Simon Spiers I would have thought you'd have to change both stages simultaneously the first op amp is more crucial work isn't it? it controls everything from a kilohertz up but this has a relation on the second one so both need to be changed for different types then just changing the one and capacitors as well would have a bearing. Have you ever thought about designing your own amp? What about making a good quality amp based on an expensive one in the market for a reasonable price. Just an idea with a solid stage output and a valve preamp built in I think nakamichi did something similar in the 90s I was wondering if you had a solid state amplifier and you fed it into Transformers like a valve amplifie does it have any bearing on quality and Performance is it any better I've not seen it done. I'm assuming that would smooth out the transistors and let the transistors run at a lot lower output power causing less stress and less distortion just relying on the Transformers to boost them back up again on the output??
@HomeroGraco
6 жыл бұрын
Is it me, or the service manual has the wrong labels for bass and treble? There it says RE27 is bass and RE25 treble, but in the board itself it is inverted. Btw, my 4558D had -13V on pin 1, I got a replacement, same thing. Replaced the cap (56pF) immediately after it, the same. The resistance is fine. It seems my treble control is the culprit? How did you open it?
@SoddingaboutSi
6 жыл бұрын
Very carfully. All the tone pots were intermittent or open.
@HomeroGraco
6 жыл бұрын
Simon Spiers managed to open one of them in the meantime. It looks a bit out of place. Also, the track has some sort of grease on it, which I guess shouldn't be helpful. I'll clean them all now
@SoddingaboutSi
6 жыл бұрын
Yes mine had grease too. Bend the contacts slightly but you may find the plastic is fragile.
@HomeroGraco
6 жыл бұрын
F#$+$-#&&@ pots! They were all bent and dirty, I cleaned them all, put them back and now the Marantz is alive! I'm so happy! Thanks a lot for helping me!
@angelguillen862
3 жыл бұрын
buena tengo un amplificador de ese modelo sera que me puedes el diagrama que tiene una falla y no la puedo encontrar
@williamchow1624
6 жыл бұрын
The part2 did seem abrupt.
@SoddingaboutSi
6 жыл бұрын
Yes. When I watched it online I realised the balls up!
@peterjmcgee4680
5 жыл бұрын
if I were you I wouldn't go crazy with that Marantz amplifier anymore it's not worth it because Marantz is no longer in the top of the oil field Laneige changed over the way they made their equipment I got so cheap and the way that they built it the quality of the sound just went to hell I wouldn't even bother those little units like that Jose amplifiers weren't worth the time of day repair them if you're not any real resale value on them unless you're repairing it for somebody who owns it then it's alright the people like it repaired it for them but if you're going to repair anything for resale stick with the Vintage early 70s pioneer Marantz Sansui Kenwood Brands like that early 70s to very early 80s have a good resale collectors out there that like to buy them at but when you get into the late 80s into the 90s that equipment is really terribly built the quality of the work and the craftsmanship just went to hell I mean you just look at the stuff that they make today we live in a breakdown throwaway Society if it breaks down they take it they throw it away and they buy a new one when most of the stuff when it breaks down is a minor repair like flat screen TVs nine times out of 10 it's just a capacitor in the power supply that's gone bad and all you have to do is change it and the TV is up and running again nobody has the time and nobody wants to do this anymore the day of the repairman is almost dead you're a few of us out there old timers that are still going to do it because I like it keeps my brain occupied and I'm always reading up on the new technology because they say what's wrong with you learn something new everyday you'll never be stupid
@SoddingaboutSi
5 жыл бұрын
I agree with a lot of what you say but not all. When this amp was made, Marantz still had a reasonable reputation of making quality gear, although not as good as the stuff from the 70's. This model had a good reputation of being a nice sounding amp for the price. It is nothing special, but as it was in super condition it was worth spending some time on. As you say, i hate this throw away society we live in, and that is partly why i started the channel so encourage others to repair their stuff themselves. And its a great feeling afterwards too. Thanks for posting.
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