EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: stefon on December 31, 2022, 04:38:20 pm
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This is output of synthesizer. How this suppose to work? Is this some kind of weird mute/noise gate? but U147 is cmos inverter? but power supply is connected to - of output opamp? When I remove opamp and 4049 there is analog signal on left side on 4.7K resistor, but if I put chips in, signal is gone. (opamp - tl062 and 4049 are 100% working). Germanium diodes looks ok on multimeter, 0.2V drop.
(https://i.imgur.com/JFgFJUe.png)
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Can you tell us the make or model or a link to the manual so I can look that snippet of circuit up please?
There's other connections made to that '4049 that's not shown on your circuit snippet!
Chris Williams
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This is a rather unusual circuit design where a digital IC ( ‘4049 ) is being used in an analog circuit. Evidently the ‘4049 is mixing several analog signals at the input of the inverting op-amp output stage.
There is one version of ‘4049 known as “unbuffered” which can pass analog signals. I believe the part number is something like CD4049UBE. The more common version (‘4049 without letter U in part number) has an internal buffer which will generally only output a logic low (0V) or logic high (5V).
If the ‘4049 in your unit has no U in the part number then I suggest trying a ‘4049 with the U in the part number. Someone might have replaced the original IC with the buffered version.
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Is this some kind of weird mute/noise gate?
Basically, yes.
The earlier CMOS inverters were implemented very simply as a pair of complementary mosfets. See the diagram.[attach=1]
Only the 'n' mosfet is being used in your design. When the input labelled as C is high - enable audio - this n-mosfet will turn on and connect the audio signal to the input of the op-amp, via the inverter's \$\overline{C}\$ output to -Vss. When the audio is disabled, input D is active - this connects the op-amp output back to its input via a 1k resistor - presumably to reduce noise.
Note that the audio signal is operating into the virtual ground at the op-amp input, so there should be very llittle signal voltage present. This is important to prevent the gates protection diodes and the p-mosfet from impinging on circuit operation. This is the reason for the limiting diodes D114, D115, D116 and D117(?) on the inputs to the chip.
Edit: As elecdonia has noted, there are later (post 1980?) versions of this chip that may have more sophisticated protection mechanisms - these might interfere with the operation of your synth circuit.
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Hi!
Members ElecDania and Andy Watson have cracked it – a more careful look at that snippet reveals the protection diodes are Ge, so they will clamp over–voltage s to a value well below that at which the silicon diodes in the i.c. starts to conduct!
You can be caught out because the snippet just states "4049" with no prefix or suffix letters quoted, so anybody glancing at that without access to the whole book with a parts list, etc., might put a '4049BE or '4049BP original and then be scratching their head for evermore, wondering why there's no audio!
The reason the OP couldn't fathom it out is precisely because whoever draughted that circuit designed it to use a specific characteristic of the internals, without drawing the circuit to highlight this – it should have been drawn showing the internal MOSFETs separately with a special note added!
Would a '4007 work in this position?
Chris Williams
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Anyway it is Sequential Six-trak synth output.
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Anyway it is Sequential Six-trak synth output.
Back in the 1970's it wasn't unusual for a circuit designer to utilize "unknown" features of a given IC chip. IIRC the original CD- series CMOS chips first made by RCA all had -A suffixes in their part numbers.
The exact replacement for the original factory part would be CD4049A
The later unbuffered version, CD4049UBE, may or may not work in this application. Given that the post by Andy Watson contains a schematic for one section of the original '4049, a replacement could be constructed on a small perf board.
By the 1980's the CD4016 and CD4066 analog switch parts (4 channels per package) became the go-to device for switching analog signals in this type of application. In fact the CD4066 is still a popular part today.
Small world: I happen to own a Sequential Circuits Pro One self-contained analog synth. It's on my list of "items to restore." Actually it was mostly functional the last time I checked it out. Biggest issue is the plastic shaft of one front panel control pot is broken off flush with the front panel. I have exact replacement knobs but I haven't discovered a source for the pots themselves.