Need Help Troubleshooting Maestro Boomerang BG-2

[Paul Marossy]

I still have a slight curious problem with this particular Boomerang. It wants to be just a little scratchy for the first minutes after I power it up, as if it's an engine that has to warm up or something. After I've been using it for a few minutes, it goes away. That's just weird. 

I tried looking for voltage on the outer lugs of the pot and it measures from 1 to 2 mV. Should there be zero or is this normal? Doesn't seem too out of the ordinary to me...

[joegagan]

bingo again. in our 67, with a stock CTS pot, it is scratchy for the first 20 or so strokes, then smooths out and purrs like a rabid kitten. don't know what the cause is

[Paul Marossy]

bingo again. in our 67, with a stock CTS pot, it is scratchy for the first 20 or so strokes, then smooths out and purrs like a rabid kitten. don't know what the cause is

Weird. Just weird. 

[joegagan]

i believe the scratchiness when first played is just dust that has settled on the trace. running it through a few strokes clears the dust off the track.

since BG2 pots are so hard to come by ( even modern rev log 25k rv4 spec PEC pots don't capture the feel and sound ), i have begun to try and rebuild or restore as many of the original Bg2s pots as i can.
i take the back off carefully, drill two 1/'8" holes in the cover near the phenolic base to facilate future cleaning with the silly straw supplied with contact cleaners

there are some bg2 pots that are so worn out that they don't warrant rebuilding. i have one from 74 that measures over 40k in the middle of the sweep, settling back down to the  28k past the worn spot. before rebuilding/cleaning a pot, carefully measure the resistance at 5* increment to assure it is still reading correctly.

[gudwyn]

Hi, I'm a newbie to diystompboxes. Just finished up a project converting a USA made vox v847 / rev d crybaby to a maestro boomerang with great results.  I used the existing pcb and made the follwing changes--

Changed the inductor to a yellow fasel
Changed the Q resistor to 47K
Replaced the two .022uF caps with 1uF electrolytics
Changed the volume resistor to 56K (only because pedal was modded for true bypass)
Changed the mid resistor to 1K8 (boomerang actually uses a 1K which is too thin for my taste)
Removed 470K resistor from Q2 base
Changed the other 470K resistor to 1.5M
Retained the .01uF sweep cap
Changed the other .01uF cap to .047uF
Removed 100K resistor
Changed 10K resistor to 8.2K
Changed 1K resistor to 0 ohm jumper
Replaced MPSA18 transistors with BC109
Changed 22K resistor to 10K
Changed 510 ohm bass resistor to 270 ohm (boomerang uses a 120 ohm)
Added 820K resistor between Q2 base and collector
Added 33K resistor between pot hot and ground

To my ear,  the result was a great sounding wah that was a major improvement to the stock v847, and think it would give any clyde clone a good run for it's money.

[joegagan]

that is great news, thanks for the report!
your changes all sound good to me. if you would like an even more authentic feel on the pot, get a PEC 50k rv4 style log pot from digikey.

for boomer shells, this would be rev log, but for a vox or cb case,  std log is correct. in my testing, the PEC 50k sounded better than the 25k. the original boomer pots measure to 29 or 30k, the PEC 25k comes in at 23k, which actually does not let the  boomer circuit go full treble. 50k cured that, with no loss in correct boomer feel.

* note on using PEC pots in wahs. in my latest boomer mod, there was an audible click at BOTH ends of the sweep, similar to the treble end click reported earlier in the thread. this required very careful setting of the gear to avoid the click. the 330 ohm between ground lug on pot and ground did not work in this case either. bummer. richard mintz feels that it is a manufacturing defect in the end of the trace that causes this.

[joegagan]

gudwyn, i liked your use of the 1.8k for the R off base of Q1. the factory schematic for the bg1 shows a 4.7k, but in reality our 67 bg1 actually has a 1k in this position. it is the only deviation from the factory schem attached to the enclosure.

in my experiments and modded boomers, i install a 10k trim in this position, and depending on other mods i usually like it between 3k to 4k.

[Paul Marossy]

that is great news, thanks for the report!
your changes all sound good to me. if you would like an even more authentic feel on the pot, get a PEC 50k rv4 style log pot from digikey.

Joe, do you have a link to that product? I just did a search for "PEC 50k rv4" and came up with nothing.

[joegagan]

i am trying to make it a little bit of a treasure hunt. i have had a few cases recently where i missed out on 'in stock' parts because once i mentioned them on forums the stock was depleted. the stock of rev log and log pots is very limited, as the suppliers need to order bulk , and the more obscure values are too big a risk of not selling.

i got some 25k PECs from tedds for a reasonable price. when i tried to expand my experiment to the 50k, they showed log and rev log in stock, but it said 'email for quote'. after inquiring, they updated the price on their site to OVER 30 dollars per pot , over 3x more than i paid for the exact same pot in 25k. not only that, they changed the price of same spec clarostat to over 30, which had been listed at 14 the previous day before i inquired. tedds has one of the best selection of in stock pots, but their tactics annoy me.

[Paul Marossy]

Gee, too bad parts aren't (easily) available (cheaply) like they were 30 years ago, huh? 

[gudwyn]

Joe, thanks for the info on the 50K pot.  The stock v847 pot is definitely not the best taper for the boomerang.  I am having a bit of a childhood regression with this project-- I had a boomerang stolen from me when I was 17 (back in 1970) and I always missed it because it was such a great sounding wah. I bought the v847 back in the 90's but never really used it because it sounded so awful. The conversion has made it useable, and was very simple to do. Think of all the millions of crybabies out there just begging for a similar conversion! 

[joegagan]

you are correct, it is pretty easy to make it work on a CB or vox.
speaking  conversion to BG- i did a full document for madbean to convert his weener wah project to a boomer, it is a very simple build ( one jumper and the normal substitutions, his layout is excellent), i sent it to him for approval, but he is very backed up, so i doubt it will see the light of day, and i don't want to post the mod to his site without his approval. if anyone would like to try it, send me an email at joegagan@gmail.com, i can send the instructions.

back to the boomer pots. in a recent comparison, i tried numerous stock and new pots, i use a tripod and set the amp and guitar the same, then try to play similarly on each video so i can listen repeatedly to see if the audio matches what i thought i was feeling/hearing while playing each example. the audio is low quality, but the character still comes through. see below. one test i did that was so far off of normal feel was the hotpotz2 100k with 33k limiting resistor. i made the video, but it had such a rapid kink that the boomer magic was lost no matter how slow you sweep it. i have a theory that there is something going on with the charge/ discharge rate of the cap to ground off inductor that it never saturates full no matter how slow you sweep.

( suggestion on comparing with these vids - load each vid in a separate tab, let them all load on 'pause' so you can toggle back and forth between any of the three quickly using tabs and the pause/play feature.)

67 bg1  ---     

  71, original pot ---   

same 71 as above,  PEC 25K rev log replacement pot ---     

[joegagan]

if someone is making a boomer clone in a crybaby case, a 25k log would be a good choice if the pot hits to the high side, 27k or higher.

weber vst has some plastic trace (chinese?) pots that seem to fall into the RV4-ish category. they are less than 5 dollars each, and they list a 25k audio.   i have not tested them, but i plan to in the near future. i had a very bad experience with weber pots about ten years ago, but i would be willing to give them another chance on these.

https://taweber.powweb.com/store/plastic_pot.jpg

https://taweber.powweb.com/store/potsord.htm