Thorpy FX Pickle Pi / BYOC Large Beaver - Pickled Beaver volume issue

[moid]

Hello everyone

For once, this request isn't my fault! A friend of mine has asked me to look at one of his favourite fuzz pedals; a very early Thorpy FX Pickled Pi that he bought from a fairly famous British band and that has been toured all over the world and used on many of their records - recently it started needing to have the volume pot cranked to maximum to get any decent volume / roar out of it and even at maximum no longer sounds like "a jet engine swallowing a truck" as he put it! I immediately thought, ahh the volume pot has died, just switch that out for a new one, easy-peasy... until I opened the enclosure...

first surprise:
The PCB is a Bring Your Own Clone Large Beaver from 2007! Not exactly boutique designed Thorpy FX hand made to exacting military standards!

second surprise:
If this is a big muff where on earth are the transistors? There ain't none! That confused the hell out of me until I found the schematic - on the very last page of this PDF
http://byocelectronics.com/beaverinstructions.pdf

apparently the transistors are an IC called a THAT 300 0725 2162http://www.thatcorp.com/datashts/THAT_300-Series_Datasheet.pdf which is a matched quad transistor chip (I'd never seen such a thing before).

I checked the volume pot and it's a 100K log and my DMM measures it from 0.005ohms to 95.8K and the values don't seem to do anything odd like go backwards as the pot is swept, so I think that is working... so where do I go next? I'm wondering if some of the transistors in the IC have died? Should I try to get one of these chips and swap it (looks like an Ebay job; I can't find any for sale new) or is there anything else I should check first?

Thanks for any suggestions

[iainpunk]

maybe just squirt a bunch of contact cleaner in to the pot, that alleviated most of my volume pot problems in the past. even ones that measured alright.

does it still sound like a BMP?
do you have transistor voltages?

cheers, Iain

[soggybag]

Get the audio probe out and follow the circuit to see where the volume is being lost. If you're pretty sure it's the volume pot you could bridge pin 2 and 3 with alligator clip. It the volume doesn't get any louder than before the volume pot is not the problem.

Sounds like a Way Huge Swollen Pickle clone. These used transistor array on a 14 pin DIP in place of discrete transistors.

The chip would make it easy to test with the audio probe. Identify the collector pins and test each of these to see if one stage has a low output.

Then check the base pins to see if the volume is dropping because of something between two stages. This would particularly important between the last clipping stage and the output stage, this would point to a problem in the tone control.

[moid]

Thanks chaps

maybe just squirt a bunch of contact cleaner in to the pot, that alleviated most of my volume pot problems in the past. even ones that measured alright.

does it still sound like a BMP?
do you have transistor voltages?

cheers, Iain

I will order some contact cleaner (I don't have any, so it will be a week or so before it gets here). Yes it sounds like a BMP to me, although I don't know what the original sound was like; my friend says it was much stronger / fiercer - to me it sounds like a 'normal' BMP, just one that you really have to turn the volume up for.

Transistor voltages - I plugged the pedal into a 9V battery, turned it on and have the following readings - I am using the following image (left most block diagram) as a guide for the numbering


1 - 4.107V
2 - 0.633V
3 - 16mV
4 - didn't measure
5 - 16.2mV
6 - 0.634V
7 - 4.085V

14 - 3.943V
13 - 0.631V
12 - 14.8mV
11 - didn't measure
10 - 396.9mV
9 - 1.028V
8 - 3.969V

Does any of that look weird? To me Q4 stands out as having different values to the other 3 transistors; perhaps that means it is not working properly anymore, or the fault is in the part of the circuit connected to Q4?

Get the audio probe out and follow the circuit to see where the volume is being lost. If you're pretty sure it's the volume pot you could bridge pin 2 and 3 with alligator clip. It the volume doesn't get any louder than before the volume pot is not the problem.

Will try this tomorrow hopefully

Sounds like a Way Huge Swollen Pickle clone. These used transistor array on a 14 pin DIP in place of discrete transistors.

Having looked at the Way Huge Swollen Pickle now, I am sure you are right about what the pedal really is! Thanks for working that out for me - if I can't fix it, at least I can tell my friend he can always buy one of those instead and have the same pedal again!

The chip would make it easy to test with the audio probe. Identify the collector pins and test each of these to see if one stage has a low output.

I think the collectors look OK - at least they all have very similar voltages?

Then check the base pins to see if the volume is dropping because of something between two stages. This would particularly important between the last clipping stage and the output stage, this would point to a problem in the tone control.

Ahhh - the base of Q4 is twice the value of the other bases - is that the sort of problem you mean?

[iainpunk]

Does any of that look weird? To me Q4 stands out as having different values to the other 3 transistors; perhaps that means it is not working properly anymore, or the fault is in the part of the circuit connected to Q4?

no it doesn't look weird, since Q4 is the recovery gain. that gain stage has way lower gain, and a totally different bias point, so the voltages look alright.

try also contact cleaning the tone pot, since that one being dirty could also amount to lower volume.

cheers, Iain

[willienillie]

to me it sounds like a 'normal' BMP, just one that you really have to turn the volume up for.

A real BMP has a linear taper volume pot, so this one with a log pot should come up more gradually.  Maximum volume would be the same though.  But I wonder if your friend has a cable issue or something else, and this pedal is working correctly.

[iainpunk]

owyeah, like willie said, it might be a bad cable, not fully severed, but damaged on the inside, this can cause volume loss as well

cheers

[duck_arse]

there's something you aren't telling us, moidy-boy. what voltage does the battery read before you connect it, and what is the supply voltage when the battery is connected? somwhere around 5V? 6V6?

from the values on that last schem, your Q4 base at 1V0 sez 10uA thru that string, and 390k means a 3V9 470k means a 4V7 drop from supply, so it looks like a flat battery. well, It looked more like a flat battery while I was looking at the wrong circuit values.

what voltage do you read [end to end] across the 100R R25?

[willienillie]

there's something you aren't telling us, moidy-boy. what voltage does the battery read before you connect it, and what is the supply voltage when the battery is connected? somwhere around 5V? 6V6?

from the values on that last schem, your Q4 base at 1V0 sez 10uA thru that string, and 390k means a 3V9 470k means a 4V7 drop from supply, so it looks like a flat battery. well, It looked more like a flat battery while I was looking at the wrong circuit values.

what voltage do you read [end to end] across the 100R R25?

I saw that too, and figured 1M input impedance DMM, reading across a 470K, ~1.6V shows up as ~1.0V.

But yeah.

[moid]

Hello again everyone
Sorry about the slow slow reply, work is horrible and has been horrible since New Year's Day and I haven't had a second to touch the pedal until this afternoon. I cleaned the pots with the contact cleaner, re assembled it, discovered a broken cable (from the input jack ground to the ground on the power supply) re soldered that and tried plugging the pedal in and... it's still quiet... and also fairly tinny.

Except when by accident I was flicking the footswitch on and off to try to work out where unity was on the volume (compared against the bypass tone) when I didn't quite click the switch properly and the pedal made the most astoundingly loud feedback / blast of sound (my ears are still ringing and my amp was on 1 for volume!)... but this is intermittent - you have to depress the footswitch to a certain level and then it goes loud - full depressed it goes back to quiet volume. So in terms of fault issues, is the culprit likely to be that I need to swap the foot switch out for a new one, or is there something more sinister going on (pedal possessed by evil spirits / the ghost of Christmas future /quantuum reality breaking down) and I should consider consulting a priest / witch doctor / physicist for a solution?

Thanks for any suggestions!

[iainpunk]

i'm sure its just some meme magic gone astray, consult your local edgy kid.
but in seriousness, it might have gotten to worn over its touring life, and a single switch press was the straw. i think replacing it might be the best option.

cheers

[moid]

Thanks Iain, thankfully I was able to solve the problem without recourse to right wing chaos magick (wow that was a weird read!). It was indeed the footswitch - the pedal is back to it's intensely heavy evil self and the chap who owns it is very happy indeed to hear that one of his favourite pedals is back in the and of the living. It's strange how the old switch was still passing audio, but at a diminished volume and with a high pass filter effect - with the new switch the heavy bass frequencies are very pronounced! I've never had a footswitch fail before, but I guess it happens... and now I know something else to check for rather than wasting everyone here's time by assuming it was something wrong with one of the ICs! Thanks to everyone who chipped in their thoughts!