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Thread: Deluxe 112 Plus crosstalk / switching issues

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    Deluxe 112 Plus crosstalk / switching issues

    Got a highly abused Deluxe 112 plus that Iíve been working on for a couple weeks on and off. Didnít make any noise, replaced Q7, power amp springs back to life, But no sound from front end. Shotgun u3, u7, u2, u5, and we have some signal. Very quiet clean channel, doesnít respond to channel volume or EQ

    Distortion channel works again, but poorly. Replaced one LED from the clipping circuit and the sound is better.

    I temporarily jumperrd over r41 and now the problem is now I have crosstalk- you can hear the clean channel on the dirty, and the dirty channel on the clean. The drive channel volume now works as a master volume. The actual switching works fine, except for a loud pop when the volumes up.

    From the Theory of Operation, I gather that the prime suspects are Q4, Q5, or U5 as those are the components that sum the signals. I replaced q4/q5 with new j111ís, but still no luck. Any advice?


    http://bee.mif.pg.gda.pl/ciasteczkow...ice_Manual.pdf

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    any ideas of how to start troubleshooting those summing circuits? Iíve proved the signal
    Extensively and all I can really surmise is that it doesnít properly pass q4

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    Lifetime Member Enzo's Avatar
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    By shunting R41, you increased the signal into the next stage substantially. I don't doubt you now hear things.

    Look at the three JFETs, they are all same type. A voltage at the gate turns them OFF. Note with no drive to the gate, the normal channel is shunted by Q4, and the other two pass signal through. Put a gate voltage on all three and the Q4 turns off and allows signal past, the other two also turn off, opening the signal path.

    Pick one thing, and fix it, don't go all over it trying to fix everything at the same time. I pick the normal channel. Look left center near FS jack for U4A. The nearby test point chart says -15v on pin 1 for clean channel. Set amp for normal channel, do you have that -15v? That SHOULD turn all three JFETs off. Q4 now allows signal to pass, and the others, Q3 Q5 interrupt dirt channel. Does the normal channel now work, even if it sounds crappy? If not, what is the issue? If so, great. But change nothing yet, turn down the normal volume control and turn up the dirt channel gain and volume, do they come through or not?

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    Hey Enzo,

    Thanks! All the low voltage rails are 15/-15 after replacing he bad ICís.

    The channel symptoms are as follows:

    Clean setting: clear Ďcleaní signal comes through speaker, unaffected by the clean volume, but affected by the drive Ďmasterí. You can also hear the drive channel on this setting, the EQ and gain all work as expected.

    Dirty channel: exactly the same.

    The red/green LED switches just fine.

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    Lifetime Member Enzo's Avatar
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    It is great you have the 15v rails, but I asked something else:
    The nearby test point chart says -15v on pin 1 for clean channel. Set amp for normal channel, do you have that -15v?
    We are systematically chasing down the issue. We want to know if the JFET switching circuits are getting proper control signal. And that starts with knowing that the control signal at pin 1 of U4A. Per the schematic, -15v for clean, +15v for drive channel. Do we have those two conditions controlled by teh selector switch.

    IF WE DO, then we go to the gates of Q3,4,5 and see if the voltage there toggles from -15 to zero.

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    Thanks Enzo! I didn't catch that, thank you for the correction.

    SO I think I understand what we're looking for now! U4 controller does indeed switch between 15/ -15 supply voltage for the gates, but the transistor is not switching because it is seeing -.4v in both states of operation. It ALSO has .8VAC (dirty) or .7VAC (clean) on the gates as well.

    So, I think I understand, let me summarize in case I'm off-

    U4 is a "switching supply" providing either 15/-15 v, the LED's work because they're affected right at the source by EITHER a pos/neg voltage. Then that voltage is passed to the 'correct' transistor / transistors (q4 OR q3 / q5) because the diodes allow it based on polarity.

    SO should I be looking at cr28, cr16, cr29, c51?? c51 seems like a prime suspect if there's AC on the rail, unless that's normal.

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    Lifetime Member Enzo's Avatar
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    Let us not confuse terms. A "switching supply" refers to a power supply - short for switch mode power supply, often abbreviated SMPS. That is an alternative to big heavy power transformers. SMPS are all over. But this amp does not have one.

    But I accept what you are trying to say by that. This is just a control circuit, and the voltage it makes causes certain things in the circuit. In this case, the +15 state is not really needed, but it is the opposite to -15v. U4 toggles one way or the other. What we really have is -15v, and NOT -15v if you get me. If -15v is there, the JFETs are controlled, If +15v is there, it is same as NOT -15v, and so the JFETs are NOT controlled.

    There are some amps where they use both N and P channel JFETs where the +15 would turn on some and -15 the others, but in this circuit the same signal goes to all three JFETs. See the U4 signal goes to branch through three diodes all facing the same way to the gates of all JFETs. But note the JFETs are used different. In the clean Q4 shunts across the signal, grounding it - turns sound off. but up in the drive channel, Q3 is in series with the signal, not across it. SO turning Q3 on passes signal rather than grounding. SO this clever use means the same signal turns on and off ALL the JFETs, but that action turns one channel on and the other off at the same time.

    SO here is what I think might be happening, and I am open to anyone's alternate ideas, well wait. First follow the circuit from U4. It flows through R51, 1 meg. Make sure that is not open. I sincerely doubt C51 is shorted but check. But what I suspect is that one of the JFETs is shorted, and that shorted gate is shorting that control signal. The fact they are all hooked together means it affects them all. My suspect is Q4 since it has a direct ground connection on one side.

    I might unsolder one end of CR28, the gate diode for Q4, and see if the gates of the other two start to toggle.

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    Quote Originally Posted by Enzo View Post
    Let us not confuse terms. A "switching supply" refers to a power supply - short for switch mode power supply, often abbreviated SMPS. That is an alternative to big heavy power transformers. SMPS are all over. But this amp does not have one.

    But I accept what you are trying to say by that. This is just a control circuit, and the voltage it makes causes certain things in the circuit. In this case, the +15 state is not really needed, but it is the opposite to -15v. U4 toggles one way or the other. What we really have is -15v, and NOT -15v if you get me. If -15v is there, the JFETs are controlled, If +15v is there, it is same as NOT -15v, and so the JFETs are NOT controlled.

    There are some amps where they use both N and P channel JFETs where the +15 would turn on some and -15 the others, but in this circuit the same signal goes to all three JFETs. See the U4 signal goes to branch through three diodes all facing the same way to the gates of all JFETs. But note the JFETs are used different. In the clean Q4 shunts across the signal, grounding it - turns sound off. but up in the drive channel, Q3 is in series with the signal, not across it. SO turning Q3 on passes signal rather than grounding. SO this clever use means the same signal turns on and off ALL the JFETs, but that action turns one channel on and the other off at the same time.

    SO here is what I think might be happening, and I am open to anyone's alternate ideas, well wait. First follow the circuit from U4. It flows through R51, 1 meg. Make sure that is not open. I sincerely doubt C51 is shorted but check. But what I suspect is that one of the JFETs is shorted, and that shorted gate is shorting that control signal. The fact they are all hooked together means it affects them all. My suspect is Q4 since it has a direct ground connection on one side.

    I might unsolder one end of CR28, the gate diode for Q4, and see if the gates of the other two start to toggle.
    Enzo, thank you, this has helped immensely. Turns out it was cr29, lifted one leg and everything functions as it should, switching and all. Still no reverb but perhaps that's because q5 feeds u5 and u5 feeds the verb? Once I have a replacement diode it should become clear.

    1n4148 is pretty close, but under-rated for this application. Could I sub in a in4007 in this case?

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    Under rated? Certainly there is minimal current involved, and we are not going anywhere near 100v. 1N4148 is fine. Or 1N914 if you have them instead. I also was a Fender warranty station for decades so I stocked the similar 1N4448. Any little switching diode.

    Are you sure the diode is bad? Check it, it is either shorted, completely open or it functions as it should. I had you lift it mostly because I think the transistor it serves is shorted. measure resistance to ground from the gate. is it shorted? Lifting the diode disconnected the JFET. Please test the JFET.

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    Quote Originally Posted by Enzo View Post
    Under rated? Certainly there is minimal current involved, and we are not going anywhere near 100v. 1N4148 is fine. Or 1N914 if you have them instead. I also was a Fender warranty station for decades so I stocked the similar 1N4448. Any little switching diode.

    Are you sure the diode is bad? Check it, it is either shorted, completely open or it functions as it should. I had you lift it mostly because I think the transistor it serves is shorted. measure resistance to ground from the gate. is it shorted? Lifting the diode disconnected the JFET. Please test the JFET.
    Ahh, ok! the diode was measuring under, about .3 instead of the other in4448's .55, so I wrongfully assumed. I understand now that lifting one leg of the diode was just a way to break the pathway to q3, which was where all the control voltage was being lost.

    I replaced q3 with a known good j111 and all issues are fixed. I really can't say how much I appreciate this, I've always understood transistors could act as switches based on fed voltage but I never really grasped the concept until you took the time out to explain the issue here. so THANKS!

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    One other issue I'm having now that all the switching elements and crosstalk are fixed is a sort of crossover-esque distortion on the drive channel. I did replace both the IC and one of the LED's that wasn't functioning, so I'm guessing one of the new values is not matched enough to the circuit? My first plan is to examine IC datasheets and see if I'm off on any of the voltages being fed through the op amp, but perhaps I'm missing something...

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    Lifetime Member Enzo's Avatar
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    By the way, the diode bit was to isolate the defective JFET, but that doesn't mean the diode can't be bad TOO. Lesson was just don't assume the last place you look is as far as it goes.

    Do the dirt channel isn't clean? Well, isn't that the point? Any dual op amp you can slap in there ought to work, I don't know what values you might be thinking of. Make sure it is actually a problem. I have never been a fan of Fender's idea of what a dirt channel should sound like.

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    Quote Originally Posted by Enzo View Post
    By the way, the diode bit was to isolate the defective JFET, but that doesn't mean the diode can't be bad TOO. Lesson was just don't assume the last place you look is as far as it goes.
    I actually did sub in a in4148 just to check, it was definately just the JFET

    Do the dirt channel isn't clean? Well, isn't that the point? Any dual op amp you can slap in there ought to work, I don't know what values you might be thinking of. Make sure it is actually a problem. I have never been a fan of Fender's idea of what a dirt channel should sound like.
    Ah yes, well, while it does sound like what you'd imagine an Eric-Clapton-flavored turd to be, I believe it is ALSO malfunctioning. The sustain of the notes trail off EXACTLY like a tube amp when biased so cold as to enter into crossover distortion; choppy, in-and-out, blatty, BEYOND what a normal Plus sounds like.

    I measured -2 on the u3 pin 1, where the schematic shows -1.6. Could I just slightly bump up that voltage by increasing the value of 3.3k (r25)?

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    Last edited by Mr_bibbles; 02-20-2020 at 07:53 PM.

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    I measured -2 on the u3 pin 1, where the schematic shows -1.6. Could I just slightly bump up that voltage by increasing the value of 3.3k (r25)?
    Um...

    U3-1 has -2vDC? Instead of -1.6vDC? I don't see that on the schematic. At that pin I see TP10 with 785mvAC. That means signal.


    Please clarify.

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    Quote Originally Posted by Enzo View Post
    Um...

    U3-1 has -2vDC? Instead of -1.6vDC? I don't see that on the schematic. At that pin I see TP10 with 785mvAC. That means signal.


    Please clarify.
    WOoOf, I was looking at u6, don't know what I was thinking.

    I'll test the AC points with the 1k signal as suggested, but my guess is that I need to increase the gain to the IC stage u3, would my assumption that increasing the value of r25 be viable?

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    Quote Originally Posted by Mr_bibbles View Post
    WOoOf, I was looking at u6, don't know what I was thinking.

    I'll test the AC points with the 1k signal as suggested, but my guess is that I need to increase the gain to the IC stage u3, would my assumption that increasing the value of r25 be viable?
    SO I'll admit I don't really know what I'm doing here but I found the issue, it was in U3A not U3B. Reduced the value of r28 (470k) and the problem disappeared. I know enough to understand I have to properly bias these IC's as you would with a tube stage, but I don't fully grasp how I eliminated the crossover distortion.

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