Jext telex - white pedal issues! No signal..

[Tome]

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.

https://imgur.com/a/E3J8oCX

[stallik]

First look at the back of your board and I see several  shorts where solder is connecting adjacent tracks. May be just the photo but before continuing, run a knife, jewellers screwdriver or some such along between each track to clean everything away including flux. Then clean thoroughly. Use a magnifier.

[Tome]

First look at the back of your board and I see several  shorts where solder is connecting adjacent tracks. May be just the photo but before continuing, run a knife, jewellers screwdriver or some such along between each track to clean everything away including flux. Then clean thoroughly. Use a magnifier.

Hello..! Yes, the photo makes it look pretty bad.. it's much better in reality! I'm fairly certain there aren't any shorts.. I've gone over everything including using a DMM to check for continuity. I'll have another look over but feel fairly sure there are no bridges.

[ElectricDruid]

I still recommend you actually run a screwdriver blade down between the tracks. "Feel" beats "look" in my experience. You'll feel it snag on stuff.

Why does that veroboard layout show us the cuts from the component side?!? It'd be *much* easier to check if everything was in the right place if there was a flipped version so we could compare with the back of the board...Sigh.

[GibsonGM]

An Exacto knife works well for this process, too!

[Tome]

I still recommend you actually run a screwdriver blade down between the tracks. "Feel" beats "look" in my experience. You'll feel it snag on stuff.

Why does that veroboard layout show us the cuts from the component side?!? It'd be *much* easier to check if everything was in the right place if there was a flipped version so we could compare with the back of the board...Sigh.

I'll give that another go.. I did run the fine point of my multimeter probe along each track but will go again with a blade.

I magaed to flip the cuts and jumpers image as well!

**I added an extra cut and link near the bottom to accomodate the fasel.

[ElectricDruid]

I managed to flip the cuts and jumpers image as well!

**I added an extra cut and link near the bottom to accomodate the fasel.

Hey, cool, thanks! Ok, I've checked and I think your cuts are all in the right places.

Checking the component side, I can see a few things to check:

1) You've got a 68n where a 150n is marked in the bottom-right (in the middle of all those box film caps)
2) You've got a wire link where a 220R is shown below Q4.
3) 20K and 220K - are those different? It's impossible to tell from the photo.
4) Did all the transistors you used have the EBC pinout? (Don't ask me how many times this one has caught me, and I should know by now, I really should...

HTH

[duck_arse]

the layout diagram shows the wrong pinout for standard BC548. out by 180^o. also - referring voltage readings to the circuit diagram makes it easier to work out which reading are right and wrong. referring voltages to a layout dia that doesn't number-up to the circuit dia risks people tuning out as too hard.

[Tome]

I managed to flip the cuts and jumpers image as well!

**I added an extra cut and link near the bottom to accomodate the fasel.

Hey, cool, thanks! Ok, I've checked and I think your cuts are all in the right places.

Checking the component side, I can see a few things to check:

1) You've got a 68n where a 150n is marked in the bottom-right (in the middle of all those box film caps)
2) You've got a wire link where a 220R is shown below Q4.
3) 20K and 220K - are those different? It's impossible to tell from the photo.
4) Did all the transistors you used have the EBC pinout? (Don't ask me how many times this one has caught me, and I should know by now, I really should...

HTH

Thanks for looking into it!

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. 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.. I tried using 2n2222 as well and had the same results.

It really does sound and feel like there's a short somewhere or like one of the transistors is severely misbiased ... 

*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.

[anotherjim]

If you're uncertain about the Qs orientation. Collector and Emitters swapped can still appear to have decent DC pin voltages, but have very low gain. Does your DMM have a transistor test Hfe function?

[Tome]

If you're uncertain about the Qs orientation. Collector and Emitters swapped can still appear to have decent DC pin voltages, but have very low gain. Does your DMM have a transistor test Hfe function?

I'm pretty sure I've got them oriented correctly using the spec sheets tayda has in their site.. my dmm does have a transistor hfe test function..

[Tome]

If you're uncertain about the Qs orientation. Collector and Emitters swapped can still appear to have decent DC pin voltages, but have very low gain. Does your DMM have a transistor test Hfe function?

I'm pretty sure I've got them oriented correctly using the spec sheets tayda has in their site.. my dmm does have a transistor hfe test function..

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.

my voltages seem higher than those recorded over at FSB which makes me think that the issue is a biasing one.

These are the FSB 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...??

[ElectricDruid]

Those are all in the right ballpark, *except* for Q3, which is significantly off the schematic's voltages. The other variations I wouldn't worry about too much, and certainly not until Q3 is sorted and it's a lot closer to working.

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...??

Sorry, where? Do you mean Q3?

[duck_arse]

I've triple checked the pins of the transistors .. the bc548 for tayda have the opposite orientation compared to general online specs.. wierd.. I tried using 2n2222 as well and had the same results.

no sir.
tayda part here:
https://www.taydaelectronics.com/t-transistors/bc-series/bc548-transistor-npn-30v-0-1a.html

links to this datasheet:
http://www.taydaelectronics.com/datasheets/A-141.pdf

that datasheet shows standard BC pinout. the lower left of the three pinout dias shows view looking onto pins. this illustrates a technical drawing convention - if you can see the line as you are looking at the object/drawing view, draw a solid line. if the line to be drawn is obscured as you look at the object, use a dashed/broken line. if the view was from the top of the transistor, you cannae see the pins, they would be drawn as broken lines outline.

datasheets are easier when the 3d view is provided.

on the layout diagram [with the backwardsed transistors] "Q1" has a 330R resistor on its left hand side. you seem to have a 330k fitted instead.

[Tome]

Those are all in the right ballpark, *except* for Q3, which is significantly off the schematic's voltages. The other variations I wouldn't worry about too much, and certainly not until Q3 is sorted and it's a lot closer to working.

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...??

Sorry, where? Do you mean Q3?

Yes, apologies.. I meant Q3!

[Tome]

that datasheet shows standard BC pinout.   

OH!! apologies... now I see it.   




And this might explain why Q3 is so misbiased!! I'll change it and see what happens although I am curious about the voltages coming from that 220r resistor (near Q4) which reads 7.6v and passes through the 470r without change. Over at FSB the 220r drop the voltage down to 6.5v.

[Tome]

Update: after changing the 330k to 330r everything else fell into place! All the voltages look good and most importantly it sounds raucous! Thank you duck_arse fur spotting that and everyone else!!

2 things.. the gain pot oscillates after the half way point of the sweep (this is where the recommendation to remove the 220r resistor beneath Q4 comes from)..

Also.. despite sounding great.. the top end is a bit ice picky.. spiky.. any suggestions to tame it?

Cheers!

[antonis]

the gain pot oscillates after the half way point of the sweep

Pots don't oscillate..
(maybe 'cause they are passive elements..)

Try to place a 47-100pF cap between Q2 Collector & Base..