Jext telex - white pedal.. no signal at all?!

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tome.talevski
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Post by tome.talevski »

Hello! I’ve just finished the Jext telex white pedal using the layout over at tagboard effects.. I’ve quadruple checked everything and it all looks good.. however there’s no signal. .. main change was replacing the 220r under Q4 with a jumper (this was recommended to prevent oscillation late in the gain sweep). Other difference is I added an extra cut and jumper simply to accomodate the inductor. I tried turning the bc548 transistors the other way round .. no change. Double checked pinouts etc..

Here are my transistor voltages (as labelled in the layout, not the schematic)

Q1 (this seems high compared to voltages posted in the white pedal thread here)
C: 8.9
B 4.6
E 4.2

Q2
C 0
B .68
E 0

Q3
C 4.2
B 4.8
E 4.2

Q4
C .01
B .74
E 0

Q5
C .02
B .44
E .44

Q6
C .44
B .02
E .02

Q7
C 0
B .73
E .02

Build images, layout and schematic in link below.



Thanks :)

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tome.talevski
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Post by tome.talevski »

So I’ve made some changes and now getting some signal… it’s very gated and low volume.

Replaced the link with the specified 220r resistor under Q4... That was recommended to avoid oscillation or something.

I had a 20k resistor in place of a 220k resistor (not sure how this happened.. a first for me)

Also, not sure why I’ve put the 68n in the place if the 150n.. I’ve yet to change this.. the closest cap I have is 180n.

I’ve triple checked the pins of the transistors .. the bc548 for tayda have the opposite orientation compared to general online specs.. wierd.. So, I used my DMM hfe function and discovered that the Tayda spec sheet is actually wrong which means Q1-Q4 are facing the wrong way.. ive turned them round and discovered that now there's more volume however still heavily gated and doesn't really work.

It really does sound and feel like like one of the transistors is severely misbiased … im suspecting Q3 not he schematic (Q1 on the layout).

*im also using an A50k pot for the level instead of the specified A25k.. although don’t think this would cause that much of an issue.



my voltages seem higher than those recorded in the jext telex white pedal thread. which makes me think that the issue is a biasing one.

These are the 'jext telex white pedal thread' Q voltages (as outlined in the schematic)
Q1C: 3.59
Q1B: 1.76
Q1E: 1.20

Q2C: 0.04
Q2B: 0.52
Q2E: 0.04

Q3C: 3.11
Q3B: 2.76
Q3E: 2.10

Q4C: 0.05
Q4B: 0.56
Q4E: 0.04

These are my voltages - Q numbers match the schematic not the layout

Q1C: 4.5
Q1B: 2.3
Q1E: 1.7

Q2C: 0
Q2B: 0.68
Q2E: 0

Q3C: 7.6
Q3B: 4
Q3E: 3.4

Q4C: 0.02
Q4B: 0.74
Q4E: 0.01

Interestingly, whilst probing around, I noticed that the 470r resistor connecting to Q1C has the same voltage on both sides/legs, 7.6.. I wonder whether this is contributing to the problem...??

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tome.talevski
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Post by tome.talevski »

Nothing to see here… 330k in the place if the 330r :/

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