Musicman 2100 RD channel switching problem or confusion?

[pontiacpete]

This is the 110 'one hundred' 2100 RD. I hope someone here can explain how the channel switching with the foot pedal is suppose to work.
It seems that the pedal is the only way to activate the distortion channel.
There are 2 volume pots and 2 gain pots. Two of the Vol/Gain controls have written above the knobs on the faceplate that they are controlled by the foot switch. When the Foot Sw is pressed for distortion the LED lights and these 2, volume and gain, are activated and seem to work. When it is turned off the other volume pots is activated but the other gain pots is not. Instead the gain pot from the other channel works with the second volume pot.
Nothing seems to activate the first channel gain pot. Once in while that pot will come on but then drop out.
But it's hard to get it to do this at will.
The switch is working in the pedal. The gain pot that's not working is measuring ~100K and sweeps down to a few ohms.
The schematic I attached is closest to this amp but there are two 16 pin ICs, 14053 and 14016. These are not in the schematic. Am I using the wrong schematic? Is there a more accurate schematic out there?

Also, Does anyone have any experience with this type of problem?

[pontiacpete]

The schematic is correct, i finally located the 4053 and 4069.
The question is, are they working?

4016 not 4069

[Jazz P Bass]

What is the correct number of the second IC?
CD4016 or CD4069.

The 4016 is a quad pack bilateral switch & the 4069 is an inverter.

http://www.fairchildsemi.com/ds/CD/CD4016BC.pdf
http://www.fairchildsemi.com/ds/CD/CD4069UBC.pdf

The CD4053 is shown at the top right of the schematic.
That is a multiplexer IC.
See this for how it works:http://www.geofex.com/article_folders/cd4053/cd4053.htm

[pontiacpete]

Thanks Jazz, that's 4016.
Mistake, the 4069 is what I have in stock here. Had it on my mind.

So the 2 are 4053 and 4016

[g1]

See page 4 of attached pdf:

[pontiacpete]

Forgot to mention this is the GD-2A, with the 6L6s.

[52 Bill]

The 4053 selects the gain control and distortion circuit, the 4016 selects the master volume control. So if only one of the gain controls is working, I'd check to see that the 4053 is getting the correct switching voltage, is securely in the socket, etc. Then if all else tests okay, change it out.

[Enzo]

Look at the schematic for the gain controls, upper right. Your 4016 and 4053 are really just different versions of the same thing. For our purposes we can think of them as a bunch of JFETs in one part. 4016 is a collection of switches. For each switch, the control pin turns it on or off. The 4053 pairs them up, so a single control pin turns one on as it turns another off.

Now the circuit. IC3 pin 1 is where the results of all this leave to continue through the amp. Signal enters that through C23. Now just to the left, those square boxes with switches in them. Those are the switches in the ICs. Your JFETs.

Your two "channels" are the upper and lower path. Each path ends in a pair of those switches and then they join at C23. In each case there is a series switch to turn the channel on or off to C23. And just left of each is a second switch to ground. so when a channel is on, the series switch closes and the shunt to ground opens. And when the OTHER channel is selected, the series switch opens and the ground shunt closes.

Look closer at the switches, not the upper channel that both boxes have a pin 4 connected, and the lower channel has two boxes each with a pin 15. That is just their way of drawing the fact that it is a 4053 with its linked pairs of switches. The volume controls are switched the same way, but they used the 4016 sso the pairs are not already inside the IC.

The two channels are then selected by two sets of control voltages. The footswitch makes Y and Z control signals , one being the opposite of the other. In the logic circuit world we refer to those control voltages as high and low, or 1 and 0. But on and off works too.

SO if I read you, your volume controls are switching OK, but the gain controls are not. That tells me your 4053 circuit is not working. So either the IC itself cannot switch the one channel off, or the control voltage is not making it to the IC. Over at the volume controls the Y and Z signals are clearly marked as to which switches they control. Upper right at the 4053, it is less clear. They are all being controlled by Y, note down by the footswitch circuit it says by Y "to 4053 ABC select". If the volumes all switch, then we know the Y and Z both work, so we go to the 4053, find which pins are ABC selects and see if the voltage there toggles with the footswitch. I have no idea which pins they are, look on the data sheet for the IC. Of the control voltage is not getting there, find out why. If it does, then change the IC.

[pontiacpete]

Nice explanation Enzo, thanks. On the datasheet ABC are pins 9, 10 and 11.
THese are getting -7v when the switch is selected LED 'on' distortion on. When turning it off it goes to +7.
On pins 6,7, and 8 there's -8v on each LED 'on', when turning it off the voltages stays negative but drops ~1v.
All other pins have no voltage.
Voltage is getting to the IC, and the switch is toggling the voltage from negative to positive but it's not working. Is it suppose to
toggle + to - ?

Or is that a +/- next to the 8v at Y? Looks like the chip is bad.

[Enzo]

You have a +/-8 by Y and Z, meaning it toggles to one or the other. When Y is +8, then Z will be -8, and vice versa. 7v is close enough.

So you have control voltages, but the IC apparently won;t switch. I'd try a new IC. But you have an ohm meter, use it. Pin 15 on the drawing switches from pin 1 to pin 2. So does 15 to 1 go on and off, and does 15 to 2 go on and off as you stomp the switch? And does pin 4 switch to either pin 3 or 5 as you stomp. If not, the IC is bad. If you do get high and low resistances, then the signal path is not getting there for some reason. I suspect your IC is bad.

[pontiacpete]

Clipping to pin 15 and to 1 the resistance is 125K when I stomp on the switch it goes open then back to 125K.
Same thing with from 15 to 2 except there is 65k.

Pin 4 to 5 there's 99 ohms, when switched it's 110 R
4 to 3 there's 58K and nothing happens when switching.

[Enzo]

Pin 4 to 3: I see in the circuit that switch is parallel to a 4.7k resistor, a 100k pot and a 47k. with the pot at one end we'd see about 51.7k. with the pot the other way we'd see about 151.7k. Try turning that gain control to see if it affectrs the reading.


But that is just curiosity. Your control voltages are there, but the IC doesn't switch. Sounds like a bad IC to me.


4053 is a fairly common IC, and I;d wager most reasonably well stocked repair shops would have them. Consider that against shipping costs if you order.

[pontiacpete]

Yes the dual gang pot when turned goes from about 50k to 150K

I have some 4053s on order

[pontiacpete]

Replaced the 4053 and all is well.
Thanks to everyone for the help!