Mini Q-Tron modifications

In a complete departure from my usual posts about cars, my bus, or even working on an old house, today's post is about my electrical experiments on an Electro-Harmonix Mini Q-Tron. It took me quite a bit of time to find bits and pieces of advice on how to meaningfully modify these things, so this may help others who endeavor to do the same. I do need to point out that my experiments and eventual (partial) success was only possible because of folks on various forums making small modifications and improvements.

What / Why Mini Q-Tron

So, what is a Mini Q-Tron and why would anyone want it and then want to modify it? It is a "T-Wah" or touch-wah pedal (also called an envelope filter) that adds a fixed amount of "wah" or flange to the signal passed into it versus a classic wah pedal that changes the tone based on how the user moves the "accelerator" pedal. There are 3 knobs that control the intensity, shape and tone of the change. Why would you want it? On a bass, it has very limited application, but it's fun. I imagine a Q-Tron is much more useful for a guitarist, especially if you are seeking that early-80's Jerry Garcia tone before he started experimenting with Midi. Anyway, if it's so fun, why modify it?

There is 1 big reason why the Mini Q-Tron goes from "hey this is cool" to sits-on-a-shelf-never-used or simply returned/sold: the volume output of the unit is louder when turned on than when by-passed. So, if you're playing something and want to add a little T-Wah to it, you click it on and suddenly you are much louder. So, turning it on and off during a song is not possible without a volume pedal next to it or some other pedal configuration. I have tried placing a compressor after it, and that helps, but if you like to have some uncompressed signal pass thru your compressor (and your compressor supports that like mine does), the volume spike will still be experienced after the compressor. To address the volume change between by-pass and in-use, we add a volume knob. That solves the 1 big reason.

In my opinion, there is a second short-coming with the Mini Q-Tron: it lacks a blend control. A blend control is another knob allows some degree of original (called "dry") signal to pass through even when the pedal is in use. This adds another whole dimension to the T-Wah effect. You can set intensity and shape that you want and then vary how much influence that changed sound has on your final output. A blend control often appears on bass-centric pedals so some unmodified signal can pass through.

Last, for me, this pedal was a gift. I can't return it and guilt would prevent me from selling it. I got it almost 20 years ago and it has sat on a shelf most of that time. I figure if I damage or destroy it, I haven't lost anything other than time. I suppose purposely destroying a gift could be worse than selling it, but let's not get all caught up in ethics here. Besides, this kind of electrical work is fun: clean, relatively modern wiring managed while indoors (versus 50 years old in the rain).

Output Volume Knob

There is a great thread on the TalkBass forum that goes into some detail similar to what I described above. More importantly, there are 2 pictures which show exactly where to make modifications to include a volume knob. Out of respect for the author and the forum, I won't repost the text nor images, but you basically add a potentiometer between the circuit board and the switch, in the signal path of the blue wire. What was not clearly defined was the size of the resistor. The forum-goers seemed to circle around a 10k pot, and I found that to be almost as useless as no knob at all. The knob had very little room for adjustment between no signal and full signal. There are limits to my purchasing and experimenting, but even the 2k pot I am using now has a limited amount of sweep on it. I have it less than a 2/3 turn off the bottom (see picture on the right here) since it is effectively full signal above that. The 10k pot had significantly less room before full signal passed; it was like a nudge off the bottom. I might try a 500ohm pot if I grow tired of small bumps to the knob changing the output dramatically. As it is, the volume is mostly set-and-forget since the whole point is having the volume not spike simply from the pedal being engaged. I say mostly because depending on what is upstream, you may actually have to change the volume. I discovered that during a jam this past weekend.

Other than the resistor value, I followed the instructions fairly closely and tested the output by sending signal into my compressor and adjusting the knob based on the compressor input LED's values until they remained the same when turning the Q-Tron on and off. At this point, I felt had the Q-Tron fairly well-tamed. I even drilled the hole on the face of the case, passed the stalk up thru, nutted it down and stuck a knob on there. That's my mini Q-Tron in the picture on the right at this point.

Blend Control

I have tried the Q-Tron like that for a little bit and I like the wah effect, but not all the time and sometimes it got to the point of being intrusive. At least now, I can add it mid-song and not throw the bass way out in front of everything else. There are some songs, like maybe some old 70's tunes or artists (like the Meters, Parliament or even Bill Withers, for example) that lend themselves well to a little wah on the bass. Even if the volume is consistent, the effect doesn't sit in the mix EQ right: the low end is completely sacrificed; this is exactly why bass effect pedals often have a blend knob. You can't have fat 70's funk and no low end. That just ain't happenin. To remedy, I want to add some original signal back in. I could split the signal before it enters the Q-Tron and have another pedal control the signal flow, but floorspace on small stages is already at a premium and the fewer things I need to plug together, the fewer things that could go wrong. So, I started looking into adding a blend control to the pedal. Regardless of how well my GoogleFu was working any given day, I was unable to find a simple pictorial or text-only how-to anywhere. So, I went experimenting without a net, and while it was interesting, they were ultimately fruitless. Hopefully these efforts help someone else, but at minimum, I now understand that blending something that could form a circuit loop requires more than a simple stacked knob.... which is why I could not find something simple. There isn't one.

Blend Attempt Learnings

I tried anyway, and documented my experiments here so you don't waste your time. A blend control potentiometer is unlike a volume or tone knob. Those other knobs have one variable resistor, generally accepting signal from the center post and directing it to one post or the other based on the position of the control stalk. They are not all like this so take resistance measurements so you know what you have before you warm up your solder iron. For blending, you cannot simply direct 2 separate inputs into the outer posts to vary what comes out the middle (think like a hot-cold water faucet). It just doesn't work that way. A blend control knob is effectively 2 stacked volume or control knobs managed by a single central stalk. The idea is that when turned one way, the resistance on the upper variable goes one direction while the lower variable goes the opposite direction, depending on which posts you are comparing. In this way, you can control 2 levels to go in opposite or similar directions at the same time.

The blend pots are not always linear and do not all behave like this, though. The NM pot that I started with, for example, has a curve such that at the exact center both sources are at 100%. As the knob moves away from center, one of the stacks drops in volume (raises in resistance) while the other remains constant. In an "AC" knob, the curves are more parabolic, crossing at the middle. So, only at the extreme end is a signal at it's highest or lowest.

To help illustrate the wiring, I nabbed the image up above on the right, here, from a guitar forum post which was describing how to wire up a blend knob between 2 pickups. Similar to the volume knob, the decisions around the resistance value for the blend were not easy. On that forum, it was posed that a lower resistance range would create a smoother transition, so using a NM100k pot would be better than a NM250k pot. It was also made clear that the values of the blend had little to no bearing on the resistance value of an independent volume knob and vice-versa (volume no impact on blend value). Last, I have read varying accounts about grounding or not grounding the blend (black wire path in the drawing). I started with ungrounded cuz easier. It seemed to me that the lower the resistance value, the better the transition, so I got the lowest blend pot I could find: NM25k. I figured if it was too low and it effectively worked like a 3 position switch: all-On | 50-50 | all-Off it would still be better than not having it at all, and then I would explore other, higher resistance settings to find a true blend. Of course, this was all guitarists talking and when I went to some bass forums, it seemed that the lower resistance values would trim some of the bottom end tone. Regardless, I saw that after I'd gotten the NM25k.

I did all the wiring and tested it out, and when the knob was off the center detent, the signal was controlled as I expected it to be: a relative percentage of wet or dry signal relative to the knob position. The issue was when the knob was at or just off center. This is where the "NM" designation above may be important. The curve for an NM pot allows for half of the sweep of the knob to be at full volume while the other half is being reduced. So, at center you have full signal from both sources. Maybe, in the case of building this blend for this application, that was too much signal. Regardless of cause, I got this super-high, almost feedback sound through it when the knob was at center. The picture on the right was taken before I tried out some ground options that seemed to help. Like, I added grounding to the blend knob and the volume knob. The feedback persisted at dead-center. Maybe it was feedback, with a processed signal passing back into the "dry" from where the blend knob was wiring them together at the switch when the knob was at the center of the sweep. I could add a one-way gate to the "dry" signal entering the knob. Or, I could try an "AC" knob that has a very different volume curve that may prevent the feedback loop simply by how the volume levels are controlled.

So, I ordered an AC blend pot and dug into my electrical stuff for the not-gate diode that I used in Hapy's ignition before I re-wired him. I added it to the black-with-white-stripe wire on the left side of the image above, with the side with a stripe (cathode) pointing towards the blend knob. A not-gate diode is basically the same as a back-flow in your plumbing: it prevents the signal from going the wrong way. Signal can go from the anode to the cathode, but not the other way.  I did not give 2 thinks about the size of the diode relative to my project, and the results were enlightening but not right. For Hapy's ignition, the amount of resistance was relatively meaningless since a ton of 12V signal was present. For this, where we are working with miliVolts, this diode introduces considerable resistance to the overall dry signal heading for the blend knob. So much resistance, in fact, that it virtually kills the dry signal, but the feedback stopped. I tried the cathode reversed but that didn't work either. Besides, I had it oriented correctly, it's just the wrong value.

In the end, I removed the diode and returned the unit to the way it was (volume control only). I intend to return when I can find an elegant solution, be it the correct diode or another entire circuit board to add in (maybe something like this). I'm hoping the former, but will do the later if I find I am enjoying the Q-Tron and want to adjust it that little bit more. I took the Q-Tron to a jam on Saturday with a few other pedals I don't get to play with very often, and found that adding a phaser after the Q-Tron took some of the bite out of the t-wah without losing the fun. It felt like some of the lower end came back thru the phaser too, so maybe there's hope for this pedal without a blend control. Time will tell.

Thanks, as always, for following along. I will return to my more typical car/bus/house stuff next time-

Fixing Hapy Furnace (part 2)

Picking up where I left off the last post, Hapy has an operational furnace again.

Fuel Pump Swap

As I mentioned in my last post, the fuel pump which originally delivered with the heater/furnace would not work when I tried to bring things back to life. I splurged and bought a more expensive "quiet" model for $50US rather than another standard pump for $20US. I figured quieter or not, the more expensive pump may have better parts in it, and it may last longer whether it is quieter or not. Or it was just a marketing gimmick and I got took. Let's chalk it up to the cost of an experiment.

The old fuel line path was relatively simple. The "hard" (clear but firm plastic) lines that delivered with the furnace had short (50mm / 2-inch) stretches of fuel hose between them and the pump. The pump was suspended from a cross beam by the included rubber mount held in place by a single sheet metal screw. After a few turns with a screwdriver, the pump was on the ground. A few more and the fuel lines were removed from it. The supply fuel line started dripping fuel after a few seconds, so I caught it with a pan. The line on the pressurized side was completely empty, so no drips. I simply reversed the removal: added the rubber mount from the supply end of the pump, added the supply fuel line, suspended the pump from the cross beam and then added the pressurized-side line.

Prime and Fire Up

The Afterburner has a control page where you can direct only the fuel pump to fire up. This allows you to get fuel all the way to the furnace without the glow plug turning on (drawing the battery and maybe burning out the glow plug). I had not connected the pressure line to the furnace, but I wanted to clear the lines of any bad fuel, so I moved the pan under the dangling hardline and triggered the Afterburner to prime. Within a minute of the prime starting, fuel started spurting out the end of the line. It took longer than I expected it to, but perhaps that is an indication of how little fuel these units actually consume. Once the fuel leaving the pressure line looked like the fuel supplied at a filling station, I stopped the prime function and connected the hardline to the fuel hose dangling from the furnace.

While the pump was slightly larger than the original, it did not seem much quieter during the prime. Of course, I hadn't heard it pump in a couple of years, and memory is a funny thing. I do recall that when the furnace was running, I could hear the tick-tick-tick of the fuel pump over the whoosh of the furnace. With this in mind, I set the furnace to start. I did not want to consider the experiment with the new thicker, dedicated wiring yet, so I left the battery tender hooked up. At no point did the battery fall below 13.5V so either the tender was working hard, or the new wiring is a hit.

Regardless, the glow plug got lit, the fan started spinning and after 2 or 3 minutes the furnace ignited. This felt like a considerably longer period of time than before. Perhaps the priming was incomplete and I needed to prime again after connecting the hose to the furnace. Perhaps it took time to ignite simply because it had been so long. Once it ignited, a huge plume of smoke poured out of the little muffler. There was virtually no wind, so the cloud grew and sat slowly obscuring first the rear end of the bus and slowly 2/3 of the bus before a light breeze broke it up. After a few minutes, the cloud dissipated and the exhaust emission was reduced to nothing. All that remained was a light smokey haze inside the bus. I regret not taking a picture of that.

Noise Observations

While the temperature inside the bus slowly crept up from 55*F (13*C), I noted the noises. I could only hear the fuel pump ticking inside the cabinet where the furnace is located and even then, only when I stuck my head into the cabinet. I think the pressurized line is transmitting it. I stuck my head into the rock-n-roll cabinet above the spot where pump is actually installed and could not hear it. Maybe this new pump is quieter. Then, I moved outside where in the past I could clearly hear the pump from several feet away, even from the passenger-door side. I could not hear it, but I could hear the furnace whoosh. Of course, my muffler-ing of the furnace is different this time, having removed the second small rectangular muffler and the motorbike muffler. Perhaps the pump is no quieter, but the furnace is that much louder. To determine if the muffler configuration is a variable, I decided I will conduct another test at a different time, after re-attaching the other muffler bits. I had thought about bringing them along for while-parked use, and maybe I will need to, out of respect for our festival-going neighbors. In the past, I took decibel readings, but they were for while the furnace held a temperature, not during temp catch-up (full blast).

I will continue to experiment with noise readings and post if I find any readings are different than they were 4 years ago. I expect that once I put the extra rectangular and motorbike mufflers on, the readings will be what they were before. The only real mystery to me is whether the fuel pump can be heard over it. At this point, though, the pump seems significantly quieter.

Thanks, as always, for following along-

Fixing Hapy Furnace (part 1)

Having finally arrived at a place where we have some heat while we're moving, I felt motivated to fix the heat for when we are standing still. Today's post covers most of the saga of getting a Chinese diesel heater operational again. Spoiler alert: the heater isn't running by the end of this post.

Brief History

Back in the early days of CoViD, I installed one of those Chinese diesel parking heaters into Hapy (See Parking Heater 1, 2, 3, 4, 5 and Final for deep detail). While I was able to run it a few times, I found that the heater pulled the voltage way down on whichever battery was connected to it. I tried to solve by providing multiple batteries to source from: one to start, one to run, but switching from battery to battery seemed like a bad practice. I had other priorities, so I just let the furnace sit unused. When I did the sound deadening effort and the luxury electrical re-wire, I dismantled the cabinet which housed the furnace so I could get sound deadener underneath it and electrical cables through it. This meant dismantling part of the furnace too. Later, when the band wanted to use Hapy for the album cover, I removed the furnace exhaust so it would not appear in pictures. So, I had a furnace without an exhaust, and would draw way too much juice.

Electrical

Over time, and with more learning about electrical stuff, I have concluded that the wire used to convey electricity but perhaps more importantly the wire for a ground were far too thin. It was due to these thin wires that the voltage would drop so badly as the furnace attempted to compensate. Wires which would have been barely up to the job had the battery been sitting right on top of the heater will definitely NOT be up to the task once you move the battery a few feet away. This is exacerbated by running the ground to the body of the bus instead of back to the battery. The ground location should not matter as much, but I'm citing it as a participant cause anyway. So, addressing the electrical came first after removing the furnace from the bus.

A small harness of wires leaves the furnace body and enters a black plug (see picture on the right). The harness that ships with the furnace includes a pigtail that clicks into the furnace black plug. To my eye, the red and black wires in the pigtail are not nearly as thick as the wires leaving the furnace. This probably saved the manufacturer a few pennies, but the amp draw issue starts right there (and in my case got worse). So, I cut the main power and main ground wires from the furnace black plug and put male/female wire ends on them (12V/ground respectively). I made them different so I couldn't accidentally plug it in backwards. I then prepped a thick 2-wire black/red cable with matching wire ends, and routed it over to the luxury battery. At the battery end, I added a ring terminal for the ground and added a 20Amp fuse into the positive side before adding a ring terminal on that end. I left things detached as I switched to preparing for the exhaust.

Exhaust

From under the bus, it was clear that I had not made the hole(s) large enough for the exhaust, intake and fuel to be easily maintained. Quite the contrary, I had installed it such that once it was all together, it would have to all come apart to fix anything. Frowny-face. So, I started by cutting a 4-inch square hole into the lower belly pan. Many buses do not have these, but I got lucky. Anyway, with the hole cut I could see where the furnace sat, and noticed that the exhaust was getting pinched by the too-small hole in the floor as the exhaust left the furnace. So, I went topside and expanded the upper hole both in the floor of the cabinet and the steel floor of the bus. With the larger hole, the exhaust easily fits and things could be maintained from below. Still, I did what I could on the furnace itself before lowering it in: Connect a 6" long stretch of fuel line, attach a small circle of window screen to the intake (so bugs and pebbles don't get in) and then attach maybe a foot of heat-wrapped exhaust. I set the furnace in place and attached a small subset of the muffler arrangement I had used before: just one rectangular muffler. I may bring the expanded muffler set up for use when we are in tighter camping spots (like festivals), so we can bolt-on some extra quiet. I mounted the muffler to the underside of the frame rail, with the outlet pointing rearward and slightly away from center (picture on the right).

A keen eye can see some fresh Rustoleum primer + paint above the muffler where I discovered some rust had eaten all the way through the body. When I was working on the other stuff I ground the metal down, removing the rust, but exposing the holes. Sadness. That rust traces back to when this bus had a small vent window directly above this spot. That vent window was badly leaking and the rust had already been forming to the point of marring the paint when I bought this bus over 20 years ago. I eliminated the vent window in 2015 (See Calling "glass") with a window I purchased from BusBoys in Redding, CA in 2009. Some things move slower than others.

Fuel

I shifted over to fueling at this point. The fuel line that had been between the included "hard" line and the furnace had breeched, probably when I did the sound deadening. The fuel in the line looked dark, so I dropped the line into an empty pail, and figured I would prime the fuel system, and purge the bad fuel at the same time. Unfortunately, the fuel pump is no longer functional, so I am waiting on a replacement to finish this job.

Afterburner

Back when I first did this job, I bought an Afterburner from the guy who hand builds them in Australia. Apparently, there are people building knock-offs, eating into his business, since they basically have cloned his work. So, if you are looking at getting the much nicer controller for your parking heater, please connect with Mr. Jones in Australia and get the real deal. Anyway, with the Afterburner, I tried to prime the fuel system, but the fuel pump wouldn't respond. When I unplugged the pump, I got a code; I concluded the wiring to the pump was good but pump was bad. So I ordered a new, supposedly quieter, one. We'll see. While I was clowning around with the Afterburner, I got it onto my home network, downloaded the newest firmware and wandered around the new-to-me website that the Afterburner hosts. It's really cool. I am looking forward to triggering the furnace to warm the bus from inside. For reference, here is a link to the user manual.

This is as far as I've gotten. I am waiting for a new fuel pump to arrive. I need to purge the old lines of nasty fuel, and install that new pump. I expect there will be other discoveries, and I need to rebuild the cabinet innards. There's always opportunities to improve things. Anyway, thanks, as always, for following along-