Need Help Troubleshooting Maestro Boomerang BG-2

[Paul Marossy]

So I decided to mess with my second '72 Maestro Boomerang BG-2 this weekend and put a NOS Clarostat 25K pot in it and re-wire the original rat's nest. When I got done, after some other troubleshooting, I tried it out and found that pot is very scratchy at the treble end just before switching. It makes a very noticeable "grinchy" sounding noise when switching between volume and wah modes. I thought it must be a bad pot, but I stuck a different NOS pot in it and had the same result, so I am not so sure that it's a bad pot. My other Boomerang BG-2 does not have this problem. I haven't run across this problem before in my wah trek.  

So, my question is has anyone else had this problem before? I'm wondering if the 1uF cap connected to the pot is leaking DC or something.

I also noticed that the inductor seems to be microphonic, you can hear it thru the circuit when you tap on the enclosure with a screwdriver. And the circuit seems to want to pick up RFI if I touch the inductor with my finger. I fixed that problem by putting a .00047uF cap across the output jack. But could this weirdness with the inductor also be caused by a cap leaking DC? The other end of that 1uF cap I suspect is connected to the inductor thru a 1.5M resistor. Also, I made sure that all the grounding is good.

I'm a little baffled by this one. Of all the ones I have worked on, this one has exhibited the weirdest behaviour!!!  

[joegagan]

VERY interesting you mention this!
i have found this exact problem with a bunch of different 25k log vintage pots i have tried in boomerangs of this era. they measure fine, no resistance weirdness at the treble end of rotation, yet there is the same treble pop at end of rotation you referred to. r mintz told me recently that one of the big problems with making the boomerangs was a high rejection rate of the pots. they met the mfgrs specs, yet had audio probs in circuit. i suspect it was the same prob you and i have uncovered. i have tried at least a dozen different 20k, 25k and 30k pots of various make in boomers , very few actually work and sound correct. richard also mentioned that he thinks the tpaer was a modified audio, my measuring of 4 different 71,72,74 boomer pots supports this.

interestingly, the dearmond 1800, which appears to be a slightly altered BG circ ( my 74 example had the stock pot intact- 30k audio) but is able to utilize the EXACT same 25k pots that failed in the boomers ( treble click at end of sweep) with no issues. Q2 is biased differently in the 1800, i suspect this might be the difference. the 1800 is also much more forgiving of subbing a 100k audio pot in with not much noticeable loss in narrowing of low/lowmid sweet spot as found when trying to put a 100k in a BG.

not sure about the other probs you mention. i have not noticed any elrads being microphonic, double check the part number. if it says 'sm57', i think that is your problem.


i don't think dc leakage is your problem, but a simple inductor swap could tell you whether or not it is the inductor. pretty much any common inductor will sound good in an old BG IME.

[joegagan]

also, you can't get around this pot problem if it does this noise. if you try to adjust the rack less treble to avoid the noise, you lose the whole high end of the circ.
try a different pot. good luck. haven't found one in a year of  buying and trying.

a few i have not tried are the 20k ernie ball replacements or the dunlop bass wah replacements.

also, digikey lists these, i need to try them soon. the log and reverse log would be for boomer rotation vs the CB rotation. pretty sure the BG would need the reverse.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=KA2531S28-ND


http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=KB2531S28-ND

[Paul Marossy]

also, you can't get around this pot problem if it does this noise. if you try to adjust the rack less treble to avoid the noise, you lose the whole high end of the circ.
try a different pot. good luck. haven't found one in a year of  buying and trying.

a few i have not tried are the 20k ernie ball replacements or the dunlop bass wah replacements.

Yeah, that's exactly the problem. You try to adjust the pot so you don't get that "grinchy" sound right before switching and you lose all the treble. And yet I put that exact same pot in another person's Boomerang a few days before I messed with my own and it worked fine.  

I'm also going to try that bass wah pot sometime.

EDIT: My wah has the standard black El-Rad inductor in it. There is no model number on it, just a date code.

[Gus]

If I understand the posts correctly.  The same wha pot will work in another wha but not in the Boomerang at the treble end?

Measure the voltage at the emitter of the emitter follower transistor of different Boomerangs and note how they bias.

I am not a fan of the bias used in the boomerang emitter follower(EF).  Is there a correlation to the emitter voltage and the noise?

The issue reads like it is a cap leakage problem if the potentiometer works in other whas.  Noise from a potentiometer rotation can be DC leakage from a cap

When the pot is at the treble end the wiper should be close to the potentiometer end that is connected to ground. This is also when the greater base to ground current might flow if there is leakage in the 1uf cap because of the minimum resistance to ground(Think OHMs law voltage /resistance = current).  I would guess the noisy boomerangs might measure a higher voltage at the emitter and base of the EF reference to ground and/or have more leakage in the 1uf caps

I would try a different 1uf wiper to EF base first.  I would use a 1uf film cap.  Then If the emitter of the noisy treble ones are at a higher voltage change the bias the EF

IIRC there was some posts about the schematic EF bias resistor being 680k or 820k

[joegagan]

If I understand the posts correctly.  The same wha pot will work in another wha but not in the Boomerang at the treble end?

that is correct, i am also pretty sure that i tried the same pot with same results in two boomers. a vintage 74 with stock caps and new clone built by zac zagar using  new quality film caps at the 1.0 uf position around the pot.

Measure the voltage at the emitter of the emitter follower transistor of different Boomerangs and note how they bias.

will do when i get my bench cleaned up a little.

I am not a fan of the bias used in the boomerang emitter follower(EF).  Is there a correlation to the emitter voltage and the noise?

will make some  notes when i do this test and let you know. there are at least 3 vintage BG2s here and the zac clone, so, no shortage of test subjects.

The issue reads like it is a cap leakage problem if the potentiometer works in other whas.  Noise from a potentiometer rotation can be DC leakage from a cap.

keep in mind , this noise is at the extreme far end of the sweep, the rest of the rotation sounds perfect in my cases. also, it has been various brands and models of RV4 type pots NOS, never used. i get them from surplus places and ebay,.
paul , were your pots otherwise clear sounding?

When the pot is at the treble end the wiper should be close to the potentiometer end that is connected to ground. This is also when the greater base to ground current might flow if there is leakage in the 1uf cap because of the minimum resistance to ground(Think OHMs law voltage /resistance = current).  I would guess the noisy boomerangs might measure a higher voltage at the emitter and base of the EF reference to ground and/or have more leakage in the 1uf caps



I would try a different 1uf wiper to EF base first.  I would use a 1uf film cap.  Then If the emitter of the noisy treble ones are at a higher voltage change the bias the EF

makes sense. will try swapping out the 1.0 caps with film to see if the situation improves. will report back.

IIRC there was some posts about the schematic EF bias resistor being 680k or 820k

an old schematic listed 620k instead of the correct 820k, but almost everyone i talk to has corrected theirs and is aware.

so gus, is it possible the biasing of Q2 ( extra R to ground from base of Q2) on the dearmond 1800 is just friendlier to different pots? or are the 2.2uf  polarized electro caps in place of the Bg's 1.0s keeping voltage off the wiper better?

[joegagan]

also note, zac's clone is a bg1, none of the extra parts between inductor and cap to ground/emitter of q2 that the bg2 has.

[SISKO]

Paul, you have an oscilloscope so why dont just see if any cap is leaking or what is the "actual" problem with the circuit (rf bleeding (on pot)?, dc leaking?, etc)

[joegagan]

DAMN

my scope is  in the bath next to the toothpaste.

paul, my reference to part numbers on your inductor was a joke. sm57??

[Gus]

I would carefully remove the 1uf cap from the wiper to base and let it cool to room temp and then measure the leakage.

You can use a 9VDC battery(cap needs to have a > 9VDC rating) connect +9VD to the cap +, connect, a 100k to 1meg from cap- to battery -, measure the voltage drop across the resistor.
Next look up the input loading of the DMM(if you are using a DMM)
Use Ohms law to figure out the resistive leakage current load


I just measured a new 10uf electro with a 9VDC battery using a RS DMM and 100k(what I saw first on my bench).  I measured .03VDC across the 100k(I did not care about the DMM loading because of the 100K)  letting the voltage drop and stabilize a minute or so, be careful not to cause conductance paths with your fingers
.03VDC/100000= 3 x 10^ -7 or .3uA

As a test I looked for a 10uf cap datasheet to see if this is close

http://elcodis.com/datasheet.php?c=3367375&c_name=EEA-GA1E100&doc=304965

[joegagan]

i get it. will refer to this when i get to the boomers, will report back

[Paul Marossy]

Joe: yeah I know that was a joke about the model number. Yes the pot sounds perfectly fine except for right at the far end of the pot rotation.

An interesting sidenote is that the aforementioned wah I am having trouble with had a 100K reverse log pot in it that was given to me and it didn't have this problem. I tried an NOS 100K HotPotz I pot and it also did the "grinchy" sounding thing like the pot that is currently in it.

I think I will try replacing that 1uF cap because I have a hunch that is the root of the problem. Both measure almost 2uF, in circuit. Maybe it has something to do with that?

[Pigyboy]

I found this on gaussmarkovs Noomerang build

The layout has been designed to fit a standard wah shell of the kind sold by Smallbear. I'm very happy with the sound of the Noomerang – a bit smoother and richer than a standard wah. Initially I used 1uF film caps for C2 and C5, but I got DC leakage that resulted in a crackling noise, like a bad pot, at the treble end of the wah's range. It seems that the polar nature of electrolytic caps is an advantage here (perhaps tantalum caps would work even better, but I haven't tried them).

[joegagan]

and the plot thickens!

hmmm - i wonder if simply altering the biasing of q2 to mimic the dearmond 1800 would solve the prob. one resistor from base to ground.

[Paul Marossy]

I found this on gaussmarkovs Noomerang build

The layout has been designed to fit a standard wah shell of the kind sold by Smallbear. I'm very happy with the sound of the Noomerang – a bit smoother and richer than a standard wah. Initially I used 1uF film caps for C2 and C5, but I got DC leakage that resulted in a crackling noise, like a bad pot, at the treble end of the wah's range. It seems that the polar nature of electrolytic caps is an advantage here (perhaps tantalum caps would work even better, but I haven't tried them).

Well, I have a hunch that it's a problem with one or both of those 1uF caps leaking some DC. They are at least 39 years old, and measure nearly 2uF in circuit, so they may be drying out. I have some 1uF film caps that I may just stick in there for kicks, or maybe I'll just use new 1uF electrolytics (which are probably way less leaky than the original ones were). Maybe I'll do that tonight when I get home from work, or on Tuesday evening. I'll report my findings after I make the switch.

[Paul Marossy]

OK, well all my weird problems went away when I stuck a 330 ohm resistor between the ground connection on the pot and ground. I got the idea from looking at the Dearmond model 1800 wah pedal schematic, which is somewhat similar to the Boomerang BG-2. That's the first wah schematic I can remember seeing that has a resistor in series with the pot.

Anyway, it's perfect now. I can go all the way to the treble end of the pot with no scratching or weird noises, and it still has that Boomerang growl to it.  

[joegagan]

amazing!

now i can go back to my pile of reject 25k RV4 pots and start over. cool!

[Paul Marossy]

amazing!

now i can go back to my pile of reject 25k RV4 pots and start over. cool!

Well, keep us posted if that works for your pots, too.

I put a 330 ohm resistor in the same place on my other Boomerang BG-2 which has a 50K pot in it, too. It just had a slight noise right before switching, but adding the resistor eliminated that as well.  

[John Lyons]

So the 330 ohm resistor is just limiting/holding the low end (resistance to ground) of the pots travel by 330 ohm.
Which is basically like fiddling with the gear and backing it off a bit, granted the new
resistance is 28,30K now.
Just thinking out loud.
Cool!

[Paul Marossy]

So the 330 ohm resistor is just limiting/holding the low end (resistance to ground) of the pots travel by 330 ohm.
Which is basically like fiddling with the gear and backing it off a bit, granted the new
resistance is 28,30K now.
Just thinking out loud.
Cool!

Right. It's a really simple way to fix an otherwise very annoying problem!

EDIT: Actually the it's 25K plus 330 ohms, or in other words it's around 25.3K. It's only adding slightly more resistance to the pot.