GT100 - VR Rotary / Pot repair

Started by Elantric, February 24, 2015, 09:37:54 PM

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Elantric

Next week, I will take possession of a Boss GT-100 - found a deal on ebay  - has issues with  the encoders, but I suspect its due to a loose internal ribbon cable and easily repairable



http://www.tonymckenzie.com/roland-gt-100-review.htm

I wish it had a Stereo FX loop ( like the HD500), and I rather wish it had a similar foot switch layout as the HD500 or even the new ME-80  - with a separate row of foot switches for stomp FX.

With the Line 6 Firehawk arriving in a month - with its direct iPad connectivity, Roland needs a new product with far better integration with iPad  - more than the TRRS Headset I/O on the new Cube amps,     



http://mykeita.blog59.fc2.com/blog-entry-1783.html

http://translate.google.com/translate?hl=en&sl=ja&tl=en&u=http%3A%2F%2Fmykeita.blog59.fc2.com%2Fblog-entry-1783.html&sandbox=1




gumtown

Congrats on the GT-100 find.
One thing you might notice is all the spare space inside.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

supernicd

Congrats on the GT-100 and GP-10 acquisitions.  To be honest, I don't use my GT-100 all that much.  It's not that it doesn't sound great, because it does with the 2.x firmware.  I guess I've just become too accustomed to having 13-pin features at my fingertips, and miss them when they aren't there. 

When I do use my GT-100, it is usually in the studio environment as a multi-stomp-box for a tube amp, bypassing the amp/cab models.

Sometimes I think how awesome a GP-10 or GR-55 + Boss GT-100 would be on stage.  But I never do it.  Prefer the lighter load and smaller footprint, I suppose.  For my current band/sets, the GP-10 seems to have it covered, so there's no compelling reason to carry more. :)
Strat w/ GK-3, Godin LGXT
VG-99, GR-55, GP-10
---------------------------------------------------------------

Elantric

#3
My Boss GT-100 arrived yesterday

I got it relatively cheap on Ebay - Seller admited there was a problem - and I confirm the 4 knob Encoders on the left are all eratic - seems there was prior attempt at a repair.

I only have service manual to older Boss GT effects , but I suspect the  GT-100 Encoder read circuit is very similar to the GT-8 circuit. its on that one sided paper PC board. I might call Roland and see what they charge for a replacement encoder  / switch PC board   - but I expect it to be $$, so I'll pursue my own  debug and repair on my bench. 

I suspect ESD damage to one of the 74HC4051 Mux/Decoder ICs - which "read" status of the encoders
http://www.nxp.com/documents/data_sheet/74HC_HCT4051.pdf



Ordered 10 pieces of 74HC4051 here $7
http://www.ebay.com/itm/10-pcs-MC74HC4051N-MOTOROLA-16-PIN-DIP-MULTIPLEXER-FREE-SHIPPING-/111284325761?

Anyone have access to a genuine Boss GT100 Service Manual? Send me a PM if you do!

gumtown

Is the firmware updated to 2.03?
Some of the first releases ( 1.01) had erratic encoder problems (too sensitive).

But if it is just 4 of the encoders, then as per your diagnosis, most likely the chip hardware fault.
Check for excessive beer residue inside.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

Elantric

My GT-100 is already at FW 2.03

It looks clean inside  - except there is evidence that someone  already tried swapping out the left most encoder.

Elantric

#6
Update -

The Story so far - I got a used GT100 relatively cheap on Ebay ($260 )- Seller admitted there was a problem - and I confirm the 4 knob Encoders VR1,VR2,VR3,VR4 on the left side of my GT100  are all erratic - seems there was prior attempt at a repair.





The GT100 user experiences Eight 360 degree Rotary encoders

But in actually its a custom 360 degree 20K linear "endless" Sine-Cosine pot with only 4 leads
http://www.bosinrenda.com/

http://www.mpcstuff.com/rotary-pot-mpd-mpk-apc-series-and-mpc-renaissance-360-degree/



The VR "Endless Rotary pots  feed a pair of 74HC4051 Analog MUX IC, that feeds the main DSP  / CPU board

I already replaced the 74HC4051 ICs  - no fix
http://www.ti.com/lit/ds/symlink/cd74hct4053.pdf

The Boss GT100 eight encoders employ the same circuit as the F1-F4 rotary knobs on the VG-99 -
http://www.ti.com/lit/ds/symlink/cd74hct4053.pdf


and GT-10




When i measure VR2 ( the problem encoder) it measured higher than 20K  - and appears a prior repair  / swap of VR2 was implemented  - possibly the VR Dual 20k pot was overheated and now is out of spec.

EDIT - my hunch is these are "SINE - COSINE 360" Pots


I called Roland US  and ordered a set of replacement VR1 Dual Pots $4 / each


I will update this thread with my progress

 

Elantric

#7
My Roland US order arrived


This is the "Endless Rotation Potentiometer" with four leads, used for the eight Boss GT-100 Encoders under the dual LCDs

It was invented in 2006 by Alpha Taiwan , and similar type is used in the VG-99.




Taiwan Alpha#  RV112FF-40B1-125F-0B20K-0

Italian vendor
http://www.adimpex.it/

http://www.adimpex.it/new/pdf/Potenziometri.pdf

CodeSmart

Without looking/googling into the subject, what's the benefit of these guys compared to ALPS small cheap rotary pulse encoders? Better resolution?
But I got more gear than I need...and I like it!

Elantric

#9
Quotewhat's the benefit of these guys compared to ALPS small cheap rotary pulse encoders? Better resolution?

Cost  & reliability. the timed circuit with the pair of CD4051 analog switches feed a pair of A/D inputs on the main board micro-controller, requires less CPU overhead, and no "debounce" code   
Avoids the classic problem with Quadrature Encoder (without detents)  when at rest the Quadrature Encoder can accidentally land on the A/B output transition threshold point and "chatter"
http://www.ni.com/white-paper/4763/en/


Alpha RV112FF "Endless Rotation Potentiometers"  used in VG-99  / GT-100 appear to the end user to have finer resolution than a typical Quadrature Encoder, and lower cost.

And they provide a rotational position "memory" of the last known position.

N.I uses same type pot in the Maschine Studio and Akai MPC
http://forum.djtechtools.com/showthread.php?t=71267

CodeSmart

Education time :D Interesting. Didn't knew about them. Not easy to spot. Sounds like the way to go if having several knobs. For a device with only one knob scrolling menus/param level, probably an encoder is still workable. For instance, the knobs on the Beatbuddy "feels" like digital encoders (can feel the ticks), but I may be wrong or there could be different types for the different knobs. Anyhow thanks for the lesson.
But I got more gear than I need...and I like it!

mbenigni

Quotethe Quadrature Encoder can accidentally land on the A/B output transition threshold point and "chatter"

I think I see this behavior on my Kemper frequently, typically on the Reverb Mix knob.  Drives me *$#% crazy.

Elantric

#12
Quote
I think I see this behavior on my Kemper frequently, typically on the Reverb Mix knob.  Drives me *$#% crazy.

Yes - I see that on my Kemper too

If they used encoders with Detents that would never happen

But then you have mechanical noise as you spin the Encoder

mbenigni

#13
QuoteYes - I see that on my Kemper too

Thanks, Steve - it's helpful to know that I'm not the only one.  I was beginning to wonder whether I should send mine in for repair.

For all the things that Kemper did right - excellent s/w algorithms for amp tones, excellent I/O and analog preamps, etc. - they really cheesed out on a few things that should have been higher quality on a best in class product.  For instance, the knobs, encoders, and buttons all feel likely to break at any instant (and the UI is designed so that they all take a constant beating), and the LCD display is of laughable quality.  They must have gotten a great deal on a pile of NOS parts that had been laying around since 1995 or so.

(P.S.  It seems to me like they could resolve the "chatter" issue with a simple hysteresis filtering algorithm implemented in software.)

Mrchevy

So.....what are the results, it has been 24 hrs since you got it.  ;)
Gibson Les Paul Custom
Epi Les Paul Standard
Gibson SG 50's prototype
Squire classic vibe 60's
Epi LP Modern
Epi SG Custom
Martin acoustic

Princeton chorus 210

GT100
GR-55
Helix LT
Waza Air Headphones
Boomerang III

And, a lot of stuff I DON'T need

Elantric

#15
Rehearsals and gigs prevent me from installing the new pots until weekend

Quote
(P.S.  It seems to me like they could resolve the "chatter" issue with a simple hysteresis filtering algorithm implemented in software.)

Or use encoders with mechanical detents - which benefit when used on vibrating stages and the knobs start to move - prevent the classic problems of the Vox Tonelab SE , where your current patch starts sounding bad as a result of the drummers kick drum vibrating the stage, that vibrates the knobs that start changing FX settings on your MFX mid solo.

Thats why Kemper has that "knob lock" feature.

Between the Kemper's LCD display going all dark the moment you need to see it to assign stomps,
https://www.vguitarforums.com/smf/index.php?topic=13485.0

coupled with the high $$$ replacement cost should it get damaged at my bar gigs ( 100% likely in the college bars i play and no GUI Editor ) , had me park my Kemper in the corner of my studio and use only for recording

Meanwhile I love my GP-10 , which for some reason sounds great right out of the box, and provides the utility i need and won't break the bank account if needs to be replaced .

Also i have played through the  GT100 with 2.0 firmware, It holds promise with more controls , but none of the factory GT100 patches even after the 2.0 update grab or hold my interest , compared to the GP-10 that just sings and sounds great

whippinpost91850

#16
Interesting! I've been gigging 3-5 times a month for over 2 yrs with my Kemper and have not had any of the problems described here. ???
But agree the encoders do feel a little cheap
And the screen on the Kemper and the Remote are pretty mediocre

mbenigni

QuoteInteresting! I've been gigging 3-5 times a month for over 2 yrs with my Kemper and have not had any of the problems described here.

The problem I described with "chatter" on the reverb knobs is something you probably wouldn't notice during a gig.  It's more of an issue when you're sitting in front of your toaster trying to dial in rigs, because the display keeps switching rapidly between the default view and the reverb level notification.  The actual change in values isn't something you'd likely even hear, which is why I think it should be filtered out as hysteresis, i.e. I do not need to know that the reverb level just increased 0.01% and then went back down exactly the same amount (and repeat 5 or 10 times.)

Is your Remote still going strong?  Glad that finally got sorted out for you!

Mrchevy

Hey Steve, I know this is an old thread, but issues with sporadic readings from the pots are getting worse on my GT100. How did this eventually work out for you? The pictures are no longer showing. From what I've been able to see from searches, these rotary pots can be disassembled, cleaned, relubed, and in a lot of case, work fine afterwards. The only problem I see in the case of the GT100 rotary pots is that the tabs that hold the pots together appear to be on the bottom side facing the board. A lot of videos I've seen, most of the tabs tend to be on the knob side of the pot, making it possible to disassemble the pot without removing it (unsoldering it ) from the board. It does not seem to be the case with the GT100. If I need to unsolder them, would it not be better to just replace them then. Did you find exact replacement pots for this?
Gibson Les Paul Custom
Epi Les Paul Standard
Gibson SG 50's prototype
Squire classic vibe 60's
Epi LP Modern
Epi SG Custom
Martin acoustic

Princeton chorus 210

GT100
GR-55
Helix LT
Waza Air Headphones
Boomerang III

And, a lot of stuff I DON'T need

azeunkn0wn

Does anybody here know a better and more accessible alternative for those pots?

admin

#20
Must use the exact same specification endless Cosine 20k linear potentiometer




US $20.00 | 1pcs for ALPHA for BOSS GT10 Volume effect 0B20k potentiometer 4pin vertical 360° turn 0B20K
https://a.aliexpress.com/_msC2Itu


admin

#22
This video demonstrates the common problem of ineffective controls on Boss GT-100

Solved by heavy application of electrical contact cleaner down the shaft of the endless Cosine Potentiometers , to restore the wiper continuity with the internal resistive element

VG-99 , VB-99, GT-10 and other Roland/Boss gear employ same Endless Cosine Potentiometers- that may appear to be rotary encoders .
If your VG-99 knobs no longer work. This may solve the situation