Ross phaser (with Tonepad schematic) problem

[bizzle]

Please let me know if I should post this to another thread....

I have a Ross Phaser which is the later version with three LM13600N chips and one RC4558N chip. At a glance, I believe it uses the same schematic as the Tonepad phaser (http://www.tonepad.com/getFile.asp?id=99).  When switched on, I can hear the phased output, but it is very quiet. I would imagine there's a problem with the output amplifier circuit, but I don't really know how to troubleshoot it.  Can anyone provide some insight as to how to go about troubleshooting the problem? I'm armed with my pocket multimeter...

Thanks,
Bill

[Mark Hammer]

Not sure what you mean by "phased output".  Do you mean the volume of the effect while in effect mode is low, but bypass volume is fine?

[bizzle]

Yes, bypass volume is fine but "effected" output is very low.

[bizzle]

Anyone?

[Mark Hammer]

One very simple test to perform is to examine whether power is actually getting to the chips.  So, if you have a meter, set it to a range that will measure DC voltage,, touch one probe to the chassis, and with the other probe check for the voltage on pin 8 of the dual op-amp, and pin 11 on each of the LM13600 chips.  It should measure something in the vicinity of 9vdc (pos or neg, depending on which probe touches where).  If not, then the reason why the volume is so low may have its origins in the circuit not actually being powered, and passing signal only in passive manner.  Of course, you should not be able to hear a modulated phasing-like signal under those circumstances.

It is also possible that the footswitch has seen better days.  You can reroute (if length permits) the in and out wires from the circuit board to the in and out jacks, just to see if the volume is adequate when the switch is not part of the equation.

That's a start.  Hope I haven't pitched it at too low a level.  I prefer not to insult people whenever possible.

[bizzle]

Thanks for the suggestions.  There is appropriate voltage at the V+ pin of each IC and bypassing the switch made no difference.  Any other ideas?

[George Giblet]

Measure the DC voltages at pins 1, 3, 5, 6, 7.  They should all be about half the supply voltage.  Other options are a dodgy 1uF output cap or a dodgy switch - the later can be checked by shorting the terminals on the switch when in effects mode (try both in and out contact sets).

[bizzle]

Assuming you were referring to the RC4558 chip, voltage at pin 1 was about 1/4 of the supply voltage.  Does that mean the chip is not functioning correctly?

[George Giblet]

> referring to the RC4558 chip,
Yes - sorry about that.

> voltage at pin 1 was about 1/4 of the supply voltage.  Does that mean the chip is not functioning correctly?

If pin 3 was at 1/2 supply and pin 1 was at 1/4 supply I would be starting to think the RC4558 is dead.  If pin 3 was also at 1/4 supply then the IC might be OK, then I would check the circuit that creates vb (10k+10k+10u) is at 1/2 supply.  If that was also at 1/4 supply I would suspect the 10uF cap is dead.

[bizzle]

Voltage at pins 3, 5, 6 and 7 were all half of Vs, so perhaps your hypothesis about the dead chip is correct.  Is anything else necessary to confirm this?

[Mark Hammer]

If the switch isn't bad, and you are getting a FULL reduction in amplitude in effect mode, AND the chips do not at least show superficial signs of being bad, then the next logical place to look is the 1uf electrolytic output cap.  The pedal is not THAT old that one would necessarily treat all the electrolytics as suspect, but that 1uf near the output IS the final common pathway.  Since you are getting a modulation effect from all indications, we can safely assume that the cap is the most likely candidate for having strayed off-spec.

[bizzle]

So are you saying that the failure mode of the RC4558  chip would not simply be reduced output, i.e. 1/2 of what it should be?  In failure would it most likely have no output?  Would the best way to check if the capacitor is at fault be to simply replace it?

[gez]

Using the linked-to schematic as a guide for chip/pin numbers, take measurements for the outputs of the OTAs* (pins 8 & 9 of each chip).  If one stage has failed, it would account for your problem and would screw the bias for whatever followed (output mixer).

* Edit - the LM13600N chips, that is.

[bizzle]

IC nos. 2 and 3 (numbered per the schematic) both show 1/2 Vs at pins 8 and 9.  Values for IC #4  vary periodically, I'm guessing because it is part of the Rate circuitry.  I suppose I didn't need to check #4 anyway, since I can hear that the Rate control works.

If the 1uF cap near the output weren't working (and was the only issue) and I shorted it, would I be able to hear normal output? 

[George Giblet]

> Voltage at pins 3, 5, 6 and 7 were all half of Vs, so perhaps your hypothesis about the dead chip is correct.  Is anything else necessary to confirm this?

It's verging on the inevitable that the Opamp is dead.  It is quite hard to get the situation you describe with a working chip!   
As far as the circuit goes it does not make sense to have 1/4 Vcc out the output with a working chip - a working buffer will/*has to* produce 1/2 Vcc (with the biasing as given for that circuit).

The only other way you can get this is by loading down the output heavily.  If you look at the circuit there's 27k resistors connecting to pin 1, they aren't not going to load the chip by any stretch of the imagination.  The only other option is a short on the 0.0033uF cap and even then it might be hard to create the situation you descibe.

It's time to bite the bullet.

[bizzle]

I'm perfectly happy replacing the chip if it's reasonable to assume that it's the source of the malfunction.  It would save me the $80/hr that I would be charged if I took it to and electronics repair shop.  And since the chip itself costs less than a dollar...

I'll give it a shot.  Thanks for all of your help.  I will report back when I've gotten some results.

[Mark Hammer]

If the 1uF cap near the output weren't working (and was the only issue) and I shorted it, would I be able to hear normal output? 

Yes and no.  Keep in mind the cap is there to block DC going to the next device in line.  That device will probably also have a DC blocking cap at its input for protection against the previous device lacking one, but if it doesn't you may encounter problems.

[Valoosj]

Sorry to hijack this topic, but what kind of switch would you need for the extra stages board? It says it can add 4 stages?
I downloaded both files at tonepad, but there are only wires connecting the two pads, no switch.

Is it a 2pdt likes this?


     1-4
  X 2  5 Y
  N 3  6 O

[Mark Hammer]

Think of the path to the extra stages as being just like a pedal where you have a wire in and a wire out.  The toggle for selecting 4/8 stages has its own in and out wires that come from the main board.  Those wires are either directly connected to each other by the toggle OR connected with the daughterboard's in and out, just like what you'd do with a stompswitch.

[Valoosj]

Ok, so I was right in my previous post. The other wires just confused me at first.