Forum Vibe - Dim Bulb R48/R49 Query

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Forum Vibe - Dim Bulb R48/R49 Query

Postby yhtilhP » 24 Nov 2016, 00:44

Hi All,

I just completed a Forum Vibe vintage using the Classis Amplification v5 board. I put it in a wah enclosure (yes it was tight :D ) and have the pedal working. I stepped up 9vDC using a Madbean road rage doubler with a LT1054 and am getting 18V at the board. My 9v supply is 600mA - plenty.

My the bulb is pretty dim with the trimmer max'd and as a consequence the throb is not as pronounced as other vibes I've used.

Then I noticed what I think is a discrepancy regarding R48 and R49, the bulb bias R's, in the populated layout which I used to as the primary reference for the build. Here: http://www.classicamplification.net/fv/ ... t2n_02.htm

Unpopulated layout here: http://www.classicamplification.net/fv/ ... t2n_03.htm.

Parts list here: http://www.classicamplification.net/fv/ ... int_05.htm

The parts list shows R48 being 100k and R49 being 48k - as per the schematic. However the layouts contradict this ie: thay are showing R48 as 47k and R49 as 100k.

So I need to know if the populated layout or the unpopulated layout is correct?
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Re: Forum Vibe - Dim Bulb R48/R49 Query

Postby toneman » 27 Nov 2016, 04:50

hi,
from the pdf schematic here:
viewtopic.php?f=28&t=12886

sounds like u do not have enough "bulb drive".

R47 says: 100-150 OHMS!

R47 and the trimmer TR3 set the idle current for Q13 and Q14 which drive the lamp.

i dont see a R48 or R49(?) different numbering(?)

see page 8 for lamp/bulb offset bias mods......

also, Q13 and Q14 are setup in "darlington" configuration.
make sure the emitters and collectors are oriented correctly.

back the the game......
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Re: Forum Vibe - Dim Bulb R48/R49 Query

Postby yhtilhP » 28 Nov 2016, 05:23

thanks for the reply

that's an old schemo. THe current TR3/R47 combo is 500k/100k I think.

see here:
http://www.google.com.au/url?sa=t&rct=j ... u9BXO1TiFQ
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Re: Forum Vibe - Dim Bulb R48/R49 Query

Postby yhtilhP » 28 Nov 2016, 05:25

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Re: Forum Vibe - Dim Bulb R48/R49 Query

Postby toneman » 28 Nov 2016, 07:05

thanks for the new link,
i think the top resistor for base biasing of Q13 should be 100K, the lower,47K, to gnd.
this biases the current source Q13.
with TR1 at minimum resistance, only the 150ohm is the current limiter.
check emitter and collector orientation on Q11, Q12 & Q13.
8)
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Re: Forum Vibe - Dim Bulb R48/R49 Query

Postby yhtilhP » 30 Nov 2016, 03:56

Had a play with this again last night.

Swapped the 100k/47k with no luck - LFO stopped working.
Removed the 150R limiting resistor - light became brighter. LFO still not working.
Swapped the 100k/47k back to where I had them (ie: in accordance with this: http://www.classicamplification.net/fv/ ... t2n_02.htm ) and voila, we have a working univibe :D
Still had some rotation left in the trimmer at the optimal spot. Measured my voltage at the board and I was getting 17v now so maybe I am a little low and the additional 150R was too much.
I did try the pedal at 12v(22v at the board) at is sounded relatively harsh at the revised sweet spot on the trimmer. I think the preamp is seeing too much voltage @22v.
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