Channel switching - Blues Deluxe

[Enzo]

Op amps are three for a dollar. If you think one might be bad, replace it and find out.

[MarkusBass]

Enzo is right that you can test the opamp by replacing it but this is true only if there is a socket for the IC. Otherwise, (if desoldering and soldering is involved) I woudn't recommend doing so.
I such cases I make sure that I know a reason of the problem before I start soldering. And in this case there is a lot of options to test the opamp without soldering. For example you can measure some voltages; on pin 3 of U3 there should be always +0.6V. On pin 2 there should be either 0V or +2.4V (depending on which channel is selected with "drive select" or the footswitch). On pin 1 (TP31) the voltage should change from +14V to -14V when channels are switched. On this pin the voltage is also visualised by LD1 LED so you can verify even without a DMM whether channel switching works.
If you get correct voltages on pin 1, you should test voltages directly on RY1 and RY2 relays, which actually perform the channel switching.
So I would change the U3 opamp only if I get correct voltages on the inputs and incorrect voltages on the output.
Other symptoms, which you describe (only noise and no signal) suggest that the problem is not related to U3. It may be relater to e.g. Q1 switching transistor but this again can be easily verified by measuring voltages in the circuit. Can you give us some more details of what is currently working and what is not working? What voltages you get on relays? Does the voltage on pin 4 of U4 changes when you switch channels?
PS: some of the symptoms that you describe may suggest that relays do not get correct signals. So it may be quite inportant to check voltages on them.

Mark

[lowell]

Ok

U3 pin 3 is +.74v, pin 2 is +70mv or 3.5v.

RY1 pin 16 is -15v, pin 1 is -14v or +9v when switched

RY2 pin 16 is -15v, pin 1 is -14v or +9v when switched

U4 pin 4 is -15.75 and -15.85 when switched

[MarkusBass]

Ok
U3 pin 3 is +.74v, pin 2 is +70mv or 3.5v.
RY1 pin 16 is -15v, pin 1 is -14v or +9v when switched
RY2 pin 16 is -15v, pin 1 is -14v or +9v when switched
U4 pin 4 is -15.75 and -15.85 when switched

As you can see, the U3 gets correct voltages and it works correctly (as well as the relays). I don't see a reason to replace it with a new one. Regarding U4 - I'm sorry - I wrote pin 4 but this is just power supply pin. I ment pin 7 - the output pin. The pin (together with Q1 FET) causes that the power amp input is muted (for a moment) when channels are being switched. I'm not sure what are other problems with the amp, but if it is no signal on output, I would check the amp with an osciloscope. On the schematic you can see that the signal goes from the preamp through J3 to the power amp. So if the jack contacts are dirty or Q1 has failed, you may get such symptoms. But again, this can be easily verified with an osciloscope.

Mark

[lowell]

Markus,
Ok pin 7 of U4 is -14.56 on both channels. It does fluctuate for 1/2 a second when switched. Is that normal correct?
Otherwise the amp sounds great. It's only when the amp is not being played that this noise is audible. Channel switching works too. This noise started after I modded the +-16v power supply. I'm thinking of undoing the mod and seeing if that helps. Could a bad voltage regulator cause this? Meantime I'll poke around w/ o-scope.

[MarkusBass]

Markus,
Ok pin 7 of U4 is -14.56 on both channels. It does fluctuate for 1/2 a second when switched. Is that normal correct?
Otherwise the amp sounds great. It's only when the amp is not being played that this noise is audible. Channel switching works too. This noise started after I modded the +-16v power supply. I'm thinking of undoing the mod and seeing if that helps. Could a bad voltage regulator cause this? Meantime I'll poke around w/ o-scope.

I wasn't sure what was exactly the problem. You have two options; either revert the changes, or make sure that 7815 and 7915 ICs are working correctly - you choose. Regarding 7815 and 7915 I hope that you know that they are not symetrical. 7815 has pins: IN, GND, OUT, wheras 7915, depending on version, has pins: GND, IN, OUT or IN, GND, OUT so it's easy to make a mistake when using 7915. I also hope that you haven't left R85 and R86 resistors in the curcuit - they should be removed or at least have decreased value. If you already have the ICs, I suggest that you check whether they are connected correctly.
I personally, in similar cases, leave resistors (as it was in original circuit). I just make sure that air can easily flow around them.

Mark

[lowell]

Markus,
You quite possibly fixed the problem w/ that suggestion. There has also been an annoying very high pitch noise since I put these regulators in, with R85/R86 out that noise is gone. I'm leaving the amp on to see if the other problem is still there.

How did you know those resistors should not be there? From experience? Regulated supplies do not need dropping resistors?

[MarkusBass]

Markus,
You quite possibly fixed the problem w/ that suggestion. There has also been an annoying very high pitch noise since I put these regulators in, with R85/R86 out that noise is gone. I'm leaving the amp on to see if the other problem is still there.
How did you know those resistors should not be there? From experience? Regulated supplies do not need dropping resistors?

The resistors' values were calculated for use with Zener diodes. Without Zener diodes the voltages would be just slightly higher than required. With 7815 IC it's different. The IC requires a higher voltage on the input to operate correctly. But be careful; as far as I remember the maximum input voltage for 7815 is 35V. It is recommended to add a resistor so that the input voltage is 20-22V (after the resistor). Otherwise the curcuit may get very hot or even fail due to termal problems. You may check 100-220 Ohms resistors (see what voltage you get on input).
Since you did not remove nor changed the input resistors, the input voltage was to low for the IC to work correctly - hence oscilation or noise you were getting.

Mark

[lowell]

Markus,
I'm slightly confused. Do I want to lower the value of R85/R86 (330ohms) or raise it? Right now I put 100ohms 5w cement ones in there and the input voltage to the regulators are +-53v. I'm guessing I need to increase the 330ohms... not decrease it. If I increase it then I can bring the input voltage down to +-22v right?

On another note I'm not getting correct voltages after CR20/CR21. The schem says +-32.8vdc but I'm getting +-51.5vdc.???

[MarkusBass]

Lowell,

English is not my native language . So I'm sorry if something I said was not precise. I wanted to decrease the resistors so instead of 330 Ohms I would put e.g. 220 or 100 Ohms.
Lest's recap what voltages should be in the power supply, and what actually are there:
--> Should be (original schematic): on CP16 and CP17 connectors from the power transformer there should be 28 VAC. After CR20 and CR21 diodes there should be +/-32.8VDC. After R85 and R86 resistors (and on CR22 and CR23 Zener diodes) there should be +/-16V.
--> After the modification (you replaced Zener diodes with 7815 and 7915 ICs) you should still have +/-32.8V after CR20 and CR21 diodes, then there should be two resistors (rather decreased than incresed, so let's say 220 or 100 Ohms instead of 330 Ohms), on the inputs of 7815 and 7915 you should have voltage like +/-22V (this is "safe" voltage for the ICs).
But...
You say than after CR20 and CR21 diodes you have +/-53V. This should be clarified because it seems to be completely wrong. Either you are measuring the voltages incorrectly (please note the difference between VAC and VDC in the explanations above), or there is a problem with the power transformer, or the voltages on the schematic are incorrect.
I would start with a measurement of VAC voltage on CP16 and CP17 connectors (from the transformer). Please note that CP17 connector is connected to the ground, so you need to measure the voltage between CP16 and the ground.
If this is not 28VAC, it should be clarified first. This can be clarified either by calling Fender support or for example by simulating this power supply in LTSpice.

Mark

[Mars Amp Repair]

Hello folks,
I agree it would be the ultimate solution to replace the brute force zener supply with the reg IC's. you wouldn't have all that wasted power dissipation. Being that the few opamps & switching relays don't require very much current, I'd expect the regulators would just barely get warm..so no heat sinks required.

I would be nice if someone could take a digit pic of the finished product with modded schemo & post it here as I did with the 750 7watt resistors solution. I'd love to use the regulators if it looks like it's not too much fiddle-farting around to get them mounted...etc.

At this point I'll stick with the 750ohm resistor solution as it's the simplest/quickest & most practical solution, until I see that the regulators are not much of an issue to install. I'm sure I'll get it for that one ;-]

BTW, aren't there ususally a couple of small caps on those regulators too?

Thanx, glen

[MarkusBass]

Lowell,

I was curious about the high voltage that you measured and I simulated the power supply of this amp in LTSpice. And guess what; it seems that the voltages provided on the schematic are incorrect (and your measurements are correct). I checked what should be the input VAC voltage to get -46VDC for grids. And it is 53VAC and not 28VAC as provided on the schematic. Please note that 53V is almost 28V * 2. Interesting.
Then the voltage on CR20 and CR21 diodes is not 32.8 VDC as on the schematic but rather 52.8 VDC (they wrote "3" instead of "5").
So it looks like your measurements are correct .
If you want to calculate the power dissipation on R85 and R86, you get (52.8-16)*(52.8-16)/330 = 4.1W. This is almost the maximum power rating of this resistor.
This means that the solution suggested by Glen (to change the resistor to 750 Ohms/7W) is the best option. With this resistor you get 1.8W power dissipation (so you are well on the safe side).
If you want to use 7815/7915, you need to take into account 53V on the input of the circuit. It means that you need to add appropriate resistors on the input, as well as additional capacitors required by 7x15 ICs.

Mark

[lowell]

Hi Markus,
Ok cool... thanks for simulating that! So I need to create a voltage divider in place of R85/R86 right? I need to get that voltage around +-22v.

So now another question. How do I calculate the value of these resistors concerning current. I understand voltage dividers concerning voltage... but not current. I do not know how much current the switching circuit draws thus I do not know what to do w/ the divider.

Mars Amp Repair I found it quite easy to get these regulators in there. I had to drill 4 holes - 2 for each regulator. I connected the ground pins where the zeners were grounded and scratched off any traces that were in the way. I then soldered the caps on input/output of regulators to the underside of the board (they are small)- so no drilling there.

[MarkusBass]

I also do not know how much current the switching circuit draws. But I would start with 1k5 resistor and measure the voltage on 7815 input. If it is more than 22V, I would increase it (2k2 or more). If it is less than 22V, just decrease the resistor(s). You don't need any current divider. The power rating should be between 3 and 5W.
Of course don't forget to switch channels when measuring the input voltage.
It should be more than 22V in both cases.

Mark

[lowell]

oh ok now I get it... thanks so much for your time and help with this Mark!