Cyrus III amplifier - output stage failure.

[Radio Wrangler]

Rdson is shorthand for the resistance an FET acts like when it's doing analogue switch service and it's turned fully on.

The circuit diagram snippet is too small to see what the various input signals are, and to see the bigger picture which might explain why two chips are used when each is a triple two-ways switch. Also, how does the ground pin of IC101 get to ground? there are necessary connections missing.

You need the datasheet of the switch device before you can begin to decode what it is up to.

TG means transmission gate..... Texas instruments speak for something which is an analogue switch, though it can also be used to switch the routing of logic signals.

This is one of those circuits where you'll likely have to sketch your own versions before you can understand it. The schematic you have ought to show all the connections, but the way it's drawn, and those ICs as black boxes make it lousy for reading what's going on or explaining it.

Let's flip to the opamp and look at things from its viewpoint.

It sees a feedback path from its output to it's negative input. It needs this to control the gain andoutput bias voltage.

The feedback is controlled by two resistances. One is obvious in the form of the network between the pins. This is the series element of what acts as a simple potential divider. We also need a shunt element to make up the potential divider for the feedback to give any gain, and this element is what is connected to the opamp - input via those switches.

Excess gain says the shunt element (complicated as it may be) is looking overall too low resistance on the channel and input selection where you find excessive gain.

Without seeing more of the diagram and spending time re-drawing it, I can't go any deeper.

David

[dave cox]

To me, it looks like the inverting amplifier, with CD selected, has an input resistor of 22K1 and TWO feedback resistors - the 84K5 in parallel with a 30K1. This gives a gain pretty close to 1. If you take the 30K1 out of circuit the gain will got up to ~ x3.8. A similar argument holds for the VI input but it would need a different resistor to be open circuit - seems a bit unlikely unless there is physical damage.

I don't see how shorted 'switches' can increase the gain. Open switched would be easy to diagnose and I'm sure you would have already spotted that.

dc

edit : the schematic is available from cyrus -hooray !

[samjmann]

Thanks Rw for going to all this trouble. I do understand this rather better now.
On first looking at the diagram, I was lost somewhat.

DC. Your gain calculations are in line with what I found with a scope:
Pin1 of IC107 3.8vPP signal. Pin1 of IC108 1vPP (good channel)

So a gain factor of 3.8 as calculated.

Will report back again soon.

P.S. DC I can't find a schematic on the Cyrus site, there is a user manual however which does explain how to set up offsets on the inputs.... The CD input is fixed and seems to act as a reference.

Regards, SJM.

[dave cox]

here

[samjmann]

Finally fitted the 1206 size 30.1K replacement. The nearest I could find was 33K. A little less feedback, not noticeable on speakers as any level difference between channels. No doubt a scope and test signal would be different. For some reason they never fitted a headphone socket to the unit...

I'll fit the 4A fuses now the output stages seem stable, for starting up I fitted 2A just in case something else was wrong. When you first open up one of these amplifiers they do look rather engineer unfriendly. I ended up taking everything out at which point you can run it, but without any heatsinks.

Would I like to look at another? Well er maybe...

SJM