Q-tron debug

[yodude]

Hi everybody - I was trying to tame the output of my Q-tron and in the process, I somehow killed it. I can't figure out what went wrong. I'd appreciate any help.

It's a big box "made in NY" Qtron. The board is silkscreened "Q-TRON 1997". Google fetched me a New Sensor schematic dated 12/2/02 that looks right.

I've had it for years and it sounded great. But the output has always been too high, so I finally got around to putting a pot between the board output and the switch. (Like so: http://www.edenampsforum.com/index.php?topic=7025.0)

Somehow, in performing this simple mod, I completely screwed up the pedal. The mod has been removed with no improvement. The behavior is:

• No output at all on LP mode.
• Other modes are EQ-ed, but not wah-ing (sort of like a c-o-c-ked wah).
• The overload light is active for all modes.
• When in the non-LP modes, the peak, gain, and range knobs seem to be operating properly.
• Poking at the back of the board with my finger in the vicinity of C2/R3 makes it jump to life (even in LP mode), but the tracking is squirrely.
• Bypass mode is fine (i.e. the stomp switch is fine).

Here's the voltages for each 4558D:

U1
1: 9.04
2: 9.06
3: 6.50
4: 0.00
5: 8.99
6: 9.04
7: 9.04
8: 18.11

U2
1: 9.18
2: 0.00
3: 4.59
4: 0.00
5: 0.40
6: 11.65
7: 17.37
8: 18.11

U3
1: 9.04
2: 9.04
3: 8.99
4: 0.00
5: 8.91
6: 8.99
7: 1.32
8: 18.11

U4
1: 9.04
2: 9.20
3: 9.30
4: 0.00
5: 8.99
6: 9.04
7: 11.12
8: 18.11

U5
1: 9.56
2: 10.85
3: 8.99
4: 0.00
5: 8.99
6: 9.04
7: 7.01
8: 18.11

Voltages for the 78L18AC:

U6
1: 25.0
2: 18.1
3: 0.0

Voltages for the Opto:

U7 (I have no idea how these pinouts work ...)
On the two-pin side: 9.0, 1.4
On the four-pin side: 9.0, 11.0, 8.7, 9.0

While poking around the circuit, I noticed that both U6 and D8 (A: 26.8, K: 26.0) get really hot. Is that normal?

Thanks very much for any help!

[yodude]

Finally had some time to audio probe this thing. In LP mode, the clean signal makes it to the upstream side of R9 and C12, then it goes silent. Any thoughts?

[robthequiet]

Looks like this? http://www.guitariste.com/produit/photos/14551-c62f2-big.jpg

Note: I Don't mean to violate copyright or forum rules, so once model is confirmed I'll take it down.

[yodude]

Looks just like that

[robthequiet]

Might take me some time, but I have one here maybe I can pull voltages to compare. I'm having a hard time finding the schematic anywhere.

Edit: By the way, any chance you could post a picture of the insides, especially where the wires were attached to the new pot?

[yodude]

Thanks!

Google gave me this schematic, which seems to match my qtron.
http://documents.mx/documents/ehx-q-tron-schematic.html

By the way, any chance you could post a picture of the insides, especially where the wires were attached to the new pot?

I added a pot between the board and the switch, but then I removed it, so the wiring is back to stock.

[robthequiet]

OK, I'm short on time but I should be able to check out the voltages for you within a day or so. Have you checked R36, the trim pot? Maybe it went a bit off -- does the overload led change if you adjust R36 down?

[robthequiet]

Voltages

U1

1  8.66
2  8.66
3  8.66
4  0.00
5  8.65
6  8.65
7  8.65
8 17.38

U2

1  8.90
2  8.68
3  7.88
4  0.00
5  9.10
6 11.70
7 16.69
8 17.38

U3

1  8.65
2  8.65
3  8.65
4  0.00
5  8.62
6 14.42
7  1.35
8 17.38

U4

1  8.65
2  8.65
3  8.65
4  0.00
5  8.65
6  8.65
7  8.65
8 17.38

U5

1  8.65
2  8.65
3  8.65
4  0.00
5  8.65
6  8.65
7  8.65
8 17.38

U6 

1 29.90
2 17.37
3   0.00

U7

2 legged side: 14.93 and 16.69
4 legged side : All 8.65


I had to rush this a bit but the voltages are measured and double-checked. I definitely see a difference in the measurements, hope this helps to track down the problem. The U6 and D8 are warm but not searingly hot.

Edit -- some of the voltages may be affected by the switch positions -- what catches my eye is the difference on the two legs of the opto led -- I would definitely try to rule out the trim R36, maybe it slipped and the led is out of range to act properly.

Pls let us know how it goes.

[yodude]

Thanks robthequiet! I checked out my R36 trimmer and it doesn't turn easily, so I think it's safe to rule that out.

I've been poking around a bit and the thing that really stands out to me is the behavior of the circuit when I put my finger on certain components around the input amplifier.

When I put my finger on the junction of C2, R3, and U1A pin 2, nothing happens. But when I pull my finger off, the circuit jumps to life for a fraction of a second.

When I put my finger on the junction of C1, R1, and R2, the circuit jumps to life for a fraction of a second. Same with the junction of R1 and U1A pin 3. Nothing happens when I pull my finger off.

(Note: Yesterday, the off board wiring to the boost switch started falling apart. While I was fixing that, I re-flowed C2 and R3. Prior to that, I could keep the circuit in good operation by holding my finger on the junction of C2, R3, and U1A pin 2.)

In this thread, R.G. talks about what's going on when you touch a circuit: http://www.diystompboxes.com/smfforum/index.php?topic=89592.0. Basically, you're a resistor.

------

Here's a side-by-side of our voltages to make comparison easier:

U1   yodude   robthequiet
      
1   9.04      8.66
2   9.06      8.66
3   6.5      8.66
4   0      0
5   8.99      8.65
6   9.04      8.65
7   9.04      8.65
8   18.11   17.38
      
U2      
      
1   9.18      8.9
2   0      8.68
3   4.59      7.88
4   0      0
5   0.4      9.1
6   11.65   11.7
7   17.37   16.69
8   18.11   17.38
      
U3      
      
1   9.04      8.65
2   9.04      8.65
3   8.99      8.65
4   0      0
5   8.91      8.62
6   8.99      14.42
7   1.32      1.35
8   18.11   17.38
      
U4      
      
1   9.04      8.65
2   9.2      8.65
3   9.3      8.65
4   0      0
5   8.99      8.65
6   9.04      8.65
7   11.12   8.65
8   18.11   17.38
      
U5      
      
1   9.56      8.65
2   10.85   8.65
3   8.99      8.65
4   0      0
5   8.99      8.65
6   9.04      8.65
7   7.01      8.65
8   18.11   17.38
      
U6      
      
1   25      29.9
2   18.1      17.37
3   0      0
      
U7      
      
2 pin      9      14.93
      1.4      16.69
      
4 pin      9      8.65
      11      8.65
      8.7      8.65
      9      8.65

[Kipper4]

U2 looks way off. What's going on there?

[robthequiet]

It may be possible to go through the schematic and triangulate the issue. What threw me is the boost and direction switches affecting the inputs on the op amps, and I'm a bit too busy over the weekend to do a complete switch/voltage matrix, but just to caution that the voltages may be OK in one or two instances even if they're not the same in both boxes. Just to confirm, LP mode is where we have the problem of no output at all... Hm, I was thinking of making sure that the various switches and knobs respond properly, but now I'm thinking that the opto/LDR needs to be checked in relation to the attached parts. Rather than suggest busywork, I might roll out my box again and dig deeper into the controls and learn more about the opto unit. What sayest thou, Kipper?

[yodude]

I just realized the change in the "finger" behavior was due to a change in the boost switch position from NORM to BOOST. So the Sept 1 behavior goes with NORM and the Sept 9 behavior goes with BOOST.

Just to confirm, LP mode is where we have the problem of no output at all...

That is correct.

Here's the matrix (LP mode, low range):

       boost  boost norm  norm
drive  down   up    down  up

U1  1  9.0    9.0   9.0   9.0
    2  9.0    9.0   9.0   9.0
    3  6.5    6.5   6.5   6.5
    4  0.0    0.0   0.0   0.0
    5  9.0    9.0   9.0   9.0
    6  9.0    9.0   9.0   9.0
    7  9.0    9.0   9.0   9.0
    8  18.1  18.1  18.1  18.1

U2  1   9.1   9.4   9.4   9.4
    2   8.9   8.9   8.9   8.9
    3   4.6   4.6   4.6   4.6
    4   0.0   0.0   0.0   0.0
    5  11.9   9.0  11.9   9.0
    6  11.7   9.0  11.7   9.0
    7  17.4   1.5  17.4   1.5
    8  18.1  18.1  18.1  18.1

U3  1   9.0   9.0   9.0   9.0
    2   9.0   9.0   9.0   9.0
    3   9.0   9.0   9.0   9.0
    4   0.0   0.0   0.0   0.0
    5   9.0   8.9   9.0   8.9
    6  15.0  15.0  15.0  15.0
    7   1.3   1.3   1.3   1.3
    8  18.1  18.1  18.1  18.1

U4  1   6.7   6.7   9.0   9.0
    2   9.4   9.4  10.0  10.2
    3   9.3   9.3  10.1  10.2
    4   0.0   0.0   0.0   0.0
    5   9.0   9.0   9.0   9.0
    6   7.1   6.0   9.0   9.0
    7  17.3  17.3  13.0  13.0
    8  18.1  18.1  18.1  18.1

U5  1  11.7  11.7  11.4  11.5
    2  16.9  14.2  12.9  11.0
    3   9.0   9.0   9.0   9.0
    4   0.0   0.0   0.0   0.0
    5   9.0   9.0   9.0   9.0
    6   9.1   9.1   9.0   9.0
    7   1.2   1.2   4.6   4.8
    8  18.1  18.1  18.1  18.1

U6  1  24.6  24.6  24.8  24.7
    2  18.1  18.1  18.1  18.1
    3   0.0   0.0   0.0   0.0

U7

2 pin  17.4  1.4  17.3  1.4
       15.6  9.0  15.6  9.0

4 pin   7.1   7.3   9.0   9.0
        7.0   5.8   9.0   9.0
       17.3  17.3  13.0  13.4
       16.9  14.2  12.7  11.0

[robthequiet]

OK, hopefully I got my alignment right on the data. I'm curious about the LP output, as it comes from pin 1 of U5.  Pin 2 of U5 matches one of the legs of the opto U7B. Further, U5 pin 7 is really different.  Kipper looked at U2 which is different on pin 3. If pin 3 is out of whack you lose your up/down bias, I believe, so no sweep. Just hunches so far, I have to admit. Interesting case, though, definitely. Let me know what you find, and I will continue to study things here.

Code Select Expand

       boost  boost norm  norm
drive  down   up    down  up

U1  1  9.0    9.0   9.0   9.0           1  8.66         
    2  9.0    9.0   9.0   9.0           2  8.66         
    3  6.5    6.5   6.5   6.5           3  8.66         
    4  0.0    0.0   0.0   0.0           4  0.00         
    5  9.0    9.0   9.0   9.0           5  8.65         
    6  9.0    9.0   9.0   9.0           6  8.65         
    7  9.0    9.0   9.0   9.0           7  8.65         
    8  18.1  18.1  18.1  18.1           8 17.38         
                                                 
U2  1   9.1   9.4   9.4   9.4           1  8.90         
    2   8.9   8.9   8.9   8.9           2  8.68         
    3   4.6   4.6   4.6   4.6           3  7.88         
    4   0.0   0.0   0.0   0.0           4  0.00         
    5  11.9   9.0  11.9   9.0           5  9.10         
    6  11.7   9.0  11.7   9.0           6 11.70         
    7  17.4   1.5  17.4   1.5           7 16.69         
    8  18.1  18.1  18.1  18.1           8 17.38         
                                                 
U3  1   9.0   9.0   9.0   9.0           1  8.65       
    2   9.0   9.0   9.0   9.0           2  8.65       
    3   9.0   9.0   9.0   9.0           3  8.65       
    4   0.0   0.0   0.0   0.0           4  0.00       
    5   9.0   8.9   9.0   8.9           5  8.62       
    6  15.0  15.0  15.0  15.0           6 14.42       
    7   1.3   1.3   1.3   1.3           7  1.35       
    8  18.1  18.1  18.1  18.1           8 17.38       
                                                 
U4  1   6.7   6.7   9.0   9.0           1  8.65         
    2   9.4   9.4  10.0  10.2           2  8.65         
    3   9.3   9.3  10.1  10.2           3  8.65         
    4   0.0   0.0   0.0   0.0           4  0.00         
    5   9.0   9.0   9.0   9.0           5  8.65         
    6   7.1   6.0   9.0   9.0           6  8.65         
    7  17.3  17.3  13.0  13.0           7  8.65   
    8  18.1  18.1  18.1  18.1           8 17.38   
                                                 
U5  1  11.7  11.7  11.4  11.5           1  8.65   
    2  16.9  14.2  12.9  11.0           2  8.65   
    3   9.0   9.0   9.0   9.0           3  8.65   
    4   0.0   0.0   0.0   0.0           4  0.00   
    5   9.0   9.0   9.0   9.0           5  8.65   
    6   9.1   9.1   9.0   9.0           6  8.65   
    7   1.2   1.2   4.6   4.8           7  8.65   
    8  18.1  18.1  18.1  18.1           8 17.38   
                                                 
U6  1  24.6  24.6  24.8  24.7           1 29.90   
    2  18.1  18.1  18.1  18.1           2 17.37   
    3   0.0   0.0   0.0   0.0           3   0.00         
                                                 
U7                                               
                                                 
2 pin  17.4  1.4  17.3  1.4             14.93 and 16.69         
       15.6  9.0  15.6  9.0              All 8.65         
                                                 
4 pin   7.1   7.3   9.0   9.0             
        7.0   5.8   9.0   9.0             
       17.3  17.3  13.0  13.4             
       16.9  14.2  12.7  11.0             

[yodude]

Data looks good. Thanks!

Thinking about the OPTO. It looks like the LED and two LDR's in one component. In the circuit, these internal components are labelled U7A, U7B, and U7C. Their functions and pinouts are:

U7A (LDR)
top / right (when viewed from component side) goes to U4 pin 1
top / second from right goes to U4 pin 6

U7B (LDR)
top / left goes to U5 pin 2
top / second from left goes to U4 pin 7

U7C (LED)
Anode (+): bottom / right goes to U2 pin 7
Cathode (-): bottom / left goes to R36 (trimmer)

So we can update the voltages:

                      boost  boost norm  norm
             drive  down   up    down  up        robthequiet

U7A (LDR)
to U4 pin 1       16.9  14.2  12.7  11.0       8.65
to U4 pin 6       17.3  17.3  13.0  13.4       8.65

U7B (LDR)
to U5 pin 2        7.1   7.3   9.0   9.0          8.65
to U4 pin 7        7.0   5.8   9.0   9.0          8.65

U7C (LED)
Anode (+)        17.4  1.4  17.3  1.4          14.93 or 16.69
Cathode (-)      15.6  9.0  15.6  9.0          14.93 or 16.69

robthequiet: Were your voltages taken with the switches set to norm and down?

[robthequiet]

Unfortunately I didn't record the switch positions, so we can at least go by the values that are completely out of range compared to yours on the opamps. I think you might have deduced where the switches were set, judging from the opto values on U7C, so 14.93 for cathode and 16.69 for anode makes sense. It occurs to me that troubleshooting the missing LP output might be the most obvious clue. Do you think you can get a read on the the continuity between U5 pin 1 to C16+ and then from C16- to the LP pin on the Mode switch and also the common pin going to Mode switch to R20? Just to make sure things are mechanically working there.

[MrStab]

hi Yodude,

i don't own a Q-Tron myself, but i know a wee bit about state-variable filters. apologies if i'm covering old ground or saying anything stupid here.

is +9B a separate Vref (or trace path)? it seems to be a common element between the overload LED and the opto LED. i'd look at that whole rail before isolating U7x, if you haven't already.
i can't see how it would account for the finger issue with the input stage, though (i usually see that behaviour when a bias resistor is absent or disconnected).

was there any stress put on the board (bearing in mind its age), or was it powered-on during testing and accidentally shorted against the enclosure, or anything like that?

[yodude]

robthequiet: I checked continuity for all the connections you mentioned. All are ok.

MrStab: Thanks for your thoughts!

is +9B a separate Vref (or trace path)? it seems to be a common element between the overload LED and the opto LED. i'd look at that whole rail before isolating U7x, if you haven't already.

I only see the +9B at one junction on the schematic, and I have a measurement there that looks good (U3 pin 2). Please let me know if there's something else I should be checking.

I've also checked each +9A point, each +18V point, and each ground point shown on the schematic. All are ok.

was there any stress put on the board (bearing in mind its age), or was it powered-on during testing and accidentally shorted against the enclosure, or anything like that?

These things have definitely happened during debugging, but I don't think it happened during the mod/failure/removal of the mod phase. And during the debugging phase, I don't think the behavior has gotten worse, though I could be wrong about that. (Actually, I think the circuit has gotten noisier during debugging. Lots of squealing and hiss. Goes away with the finger.)

I've been racking my brain trying to think of what could have happened during that simple mod procedure to cause any harm.
- Maybe I used the wrong power supply.
- Maybe I overheated C16.
- Maybe adding a resistor to the output threw something off.
Could any of these things have lead to the symptoms?

===========

Took some voltage readings with finger applied:
U1 pin3 no change
U2 pin 3 no change
U4 pin 7 goes from 17.3 to 9
U5 pin 1 goes from 11.8 to 18
U5 pin 2 goes from 16.9 to 1.5
U5 pin 7 goes from 1.2 to 6.5

============

Correction to previous voltages (had U7A and U7B swapped):

                      boost  boost norm  norm
             drive  down   up    down  up         robthequiet

U7A (LDR)
to U4 pin 1         7.1   7.3   9.0   9.0           8.65
to U4 pin 6         7.0   5.8   9.0   9.0           8.65

U7B (LDR)
to U5 pin 2        16.9  14.2  12.7  11.0        8.65
to U4 pin 7        17.3  17.3  13.0  13.4        8.65

U7C (LED)
Anode (+)         17.4  1.4  17.3  1.4          14.93 or 16.69
Cathode (-)       15.6  9.0  15.6  9.0          14.93 or 16.69

[MrStab]

does 9A connect to 9B? i can't seem to figure that out.

if you used the wrong supply by mistake, some or all of your IC's and polarised caps could be dead (though it's clearly not all). see this topic: http://www.diystompboxes.com/smfforum/index.php?topic=55003.0

check the negative side of C16 for DC - it should read next to nothing. if the output from LP is dead, it's possible that cap is. probe the positive side to see if there's signal.

i doubt the resistor would've caused any problems. just noticed the mod guide you linked to suggested drilling a hole - that's not in the PCB, surely? drilling a populated circuit board sounds like a recipe for disaster for vibrations. the pics on that link are long-gone.

it's not ideal but if things are being elusive, maybe you could simulate your finger with a wire, with one end connected to 9A and the other end wired to a 100k resistor, and try touching points in the path that should be connected to those points via. a resistor. then repeat with things that should be connected to 9B then Ground. insulate this sorta-probe from your skin somehow.

[robthequiet]

does 9A connect to 9B? i can't seem to figure that out.

The schematic shows +9A appearing at the non-inverting input of U3A, then +9B comes around from the output of U3A through R44 10R and touches R30 on its way back to the inverting input of U3A. U3A seems to line up pretty well with voltages between the 2 boxes. As long as you have Vcc at 18V and +9A and B at 9V I believe that looks pretty healthy.

I tend to agree with Grant about a cracked trace or something mechanical. If you had a fried 4558 I don't think that finger tapping would correct the voltages, but I could be wrong. I am curious as to the opto network relates to lack of output at LP. Maybe we have two different things going on. I hate to suggest something drastic like replacing 4558 chips without being fairly certain that whatever might have blown the chips is not still present. I would get the best magnifying glass I can find and do a total visual inspection backed up by continuity checks throughout the PCB. I think I would also gently exercise R36. Not fully end to end, but maybe just an 8th to a quarter of a turn either way to examine the behavior. If there was corrosion on the carbon track, it might cause some intermittency in the opto LED that would affect the LDRs. At least try to rule it out, if nothing else. In fact, I would record voltages as you adjust R36 and also check the values at the LDRs to see if they line up any better, and have a baseline to restore to.

It's a headscratcher -- kudos for being persistent!

[MrStab]

ah! so 9A uses the unbuffered Vref straight off the R40/R41 divider, whereas 9B is buffered by U3A. thanks for pointing that out. i've never seen a 10R resistor on the output of a buffered bias before, that's new to me.