Hello to all my fellow DIYers out there I'm a longtime reader of this great forum and usually always able to find an answer to my questions without posting but this one I can't seem to find info about. I have a vintage Fender Blender that has had some work done to it before I received it. New electrolytic caps and a film cap was changed, all to the same stock values. The pedal does work but no matter what setting it's on it has an extremely gated sound. I really have to hit the note hard to get sound and there's no sustain at all. The note just disappears like it's being sucked out of the amp.

 A few things I noticed:

1. When I received it the battery was hooked up but loose inside the enclosure bumping around on top of the components. Directly on top of two glass diodes.

2. The two diodes I just mentioned (on the bottom left side) look cracked and I'm getting no voltage reading from them at all. It's kinda hard to see but I think the little wire inside both is still intact.

3. There's a few resistors and capacitors in the area of the cracked diodes that I'm also getting no voltage reading from. I've included a picture and circled them.

3. All potentiometers seem to be fine and test at the proper value.

There's a few different schematics I've found for this pedal so I'm not sure which one to go by. The transistor voltages I've included are in order starting from the one furthest left and moving right.

Hopefully I didn't forget anything. With this pedal being old I figured I should ask for some more experienced opinions before trying anything. Thank you in advance for the help. This forum has been a great resource on my pedal journey. 

Hi & Welcome..

I think the red circled diodes are those facing phase spliter Collector & Emitter (Anodes on Q4 Base)..
(you can verify it by checking connection of their Cathodes to respective 10μF caps negative leg..)
Fender Blender
On the other hand, there is a pair of antiparallel diodes serving for hard clipping, so it might be easier to verify who's who by checking connection to GND..

Anyway, I'd replace suspect diodes (with any kind of diodes handy) to check if the issue is located there..

P.S.
Transistor voltages make no sense, so let's ignore them for the moment..

Quote from: antonis on October 29, 2023, 02:22:28 PMP.S.
Transistor voltages make no sense, so let's ignore them for the moment..

Hey thanks for the reply, you mean the voltage numbers look off or they just aren't relavent here? I guess with the pedal still in somewhat working condition I was hesitant to take those two suspect diodes out if it was possibly something else causing them to have no reading on the meter.

So I just checked the diodes with the info you provided and the cracked ones are the anti-parallel. I think I have the naming of the transistors wrong (Q1,2 etc) The transistor furthest to the left is Q4 and I think Q3 is the next one to the right of it. I gotta look at the schematic you linked and try to figure out the rest.

I think I figured out all the correct transistor numbers after looking at the schematic and the traces on the pcb.

This might help you..

Quote from: BrainDrain on October 29, 2023, 06:28:50 PM

Quote from: antonis on October 29, 2023, 02:22:28 PMP.S.
Transistor voltages make no sense, so let's ignore them for the moment..

Hey thanks for the reply, you mean the voltage numbers look off or they just aren't relavent here?

also welcome. the voltages are ALWAYS relevant. what you need to do is get the part numbers off the transistors, and look up the datasheets for those parts. they will have a diagram of the pinout, which will help you identify which leg is which. ebc etc.

the resistor values on the circuit diagram tells what voltages to expect on the what legs. but you need to list the voltages against the right pins for them to make sense, or tell us what's going on.

Just a transistors voltages rough estimation..



P.S.
Q4 seems to be heavily bottomed but that's the way for octave rectifier working status..
IMHO, Q3 should feel more happy with its Emitter biased at about 1/4 Vcc (2.2 to 2.3 V)

Quote from: antonis on October 30, 2023, 06:25:13 AMThis might help you..

That's the thread I originally found here but wasn't sure where to go after checking voltages and reading through the debugging thread. I'll read through it again though. Thanks.

Quote from: duck_arse on October 30, 2023, 09:53:20 AM

Quote from: BrainDrain on October 29, 2023, 06:28:50 PM

Quote from: antonis on October 29, 2023, 02:22:28 PMP.S.
Transistor voltages make no sense, so let's ignore them for the moment..

Hey thanks for the reply, you mean the voltage numbers look off or they just aren't relavent here?

also welcome. the voltages are ALWAYS relevant. what you need to do is get the part numbers off the transistors, and look up the datasheets for those parts. they will have a diagram of the pinout, which will help you identify which leg is which. ebc etc.

the resistor values on the circuit diagram tells what voltages to expect on the what legs. but you need to list the voltages against the right pins for them to make sense, or tell us what's going on.

Thanks for the reply!   I looked up the datasheets before posting and figured out all the correct pinouts. What I did wrong was label the transistors incorrectly (Q1,2 etc) and then somehow mixed up the two different pinouts. Oops. I'll fix the list I made to reflect the correct transistor numbers.  There's two 2n3391a and three 2n3391. The three 2n3391 have no part number on them so I'm just going by the schematics I've seen. Although some schematics list Q1 and Q2 as 2n3391a where on mine those two are Q3&4.

Quote from: antonis on October 30, 2023, 09:56:26 AMJust a transistors voltages rough estimation..



P.S.
Q4 seems to be heavily bottomed but that's the way for octave rectifier working status..
IMHO, Q3 should feel more happy with its Emitter biased at about 1/4 Vcc (2.2 to 2.3 V)

Thanks!

Ok I fixed the transistor voltage list. I think this is correct now and it seems to somewhat match those listed in the other thread that was posted above. Definitely was up too late when I made that list. Sorry for the error 

Q1
E = .1
C = 4.7
B = .6

Q2
E = 4.1
C = 9.3
B = 4.7

Q3
E = 3.0
C = 6.2
B = 3.6

Q4
E = .6
C = .7
B = 1.2

Q5
E = .3
C = 4.4
B = .9

D1 (one of two attached to Q4 base)
A = 1.3
K = 1.1

D2 (one of two attached to Q4 base)

A = 1.3
K = 1.1

D3 (one of two between C9 & C10 - anti-parallel pair)

A = 0
K = 0

D4 (one of two between C9 & C10 - anti-parallel pair)

A = 0
K = 0

Quote from: BrainDrain on October 30, 2023, 11:39:24 AMQ5
E = .9
C = 4.4
B = .3

No bueno..

Are you sure about B & E corrent orientation..??

Quote from: antonis on October 30, 2023, 12:04:14 PM

Quote from: BrainDrain on October 30, 2023, 11:39:24 AMQ5
E = .9
C = 4.4
B = .3

No bueno..

Are you sure about B & E corrent orientation..??

I thought I was sure lol. I think messed it up again 😵. Let me edit and try again.

Ok now I think I got it. Thank you for the help with this. For reference the pin out of the 2n3391a is ECB and the pin out of the 2n3391 or whatever transistors they are is EBC

Quote from: antonis on October 30, 2023, 12:04:14 PM

Quote from: BrainDrain on October 30, 2023, 11:39:24 AMQ5
E = .9
C = 4.4
B = .3

No bueno..

Are you sure about B & E corrent orientation..??

So I figured out one of the issues. A rookie mistake on my part for not noticing it sooner.   The loose battery that I mentioned earlier was causing the main gating issue by touching some of the components when the lid was bolted on. After testing it with the back cover off and the battery out of the compartment (which I should have done first) almost all of the crazy gating is gone. Think I'm gonna update it with a 9v jack.

But...There is still a bit of gating happening. It's mostly when the boost switch is engaged but still happens even without the boost on. A note will sustain pretty good now but kind of fizzle out and disappear quick at the very end. Feels like the bias is slightly off. In the other Blender thread it's stated that the two parallel diodes work with the 100k resistor to adjust the bias of Q4. So would adjusting that 100k resistor to another value possibly help the slight gating issue? Fender actually used a 103k resistor in the pedal I have.  Q4 is the only transistor that measured with a low collector voltage. 

I took all the transistor voltages again just to be sure and they're all the same. One thing I'm confused on is the components I'm getting pretty much no voltage reading from. The ones I circled in the picture above jump between 0 - 0.0001v.  The yellow 0.1uf output cap on the top left I circled  in the picture actually reads 0.0012v when the blend pot is all the way on 10 and reads 0.0001v when the pot is turned all the way down. I guess I'm just wondering if this is normal for these components to read this way based on where they are in the circuit?


Something else I noticed is the 560k resistor connected to the base of Q1 is measuring around 210k. Would it measure this way because of how it's hooked up in the circuit with the 150k and 120k resistors connected to it? I know a little about resistors in parallel and series but don't know enough about how that works within a circuit when looking at a schematic.

Thanks again 

Here are the components I'm getting almost no voltage reading from within the circuit:

Quote from: BrainDrain on November 02, 2023, 02:49:51 PMHere are the components I'm getting almost no voltage reading from within the circuit:

You should worry in case of you DID get some voltage reading..

All red circled items are DC isolated..
(there isn't any DC voltage difference across them..)(*)

e.g. Look at 3k9 resistor.. Its lower leg is "shorted" to its upper leg via 27k, 47k, S1, 100k and Blue pot wiper..

P.S.
You Do know that caps are considered "open-circuit" for DC, don't you..??

(*) I'm waiting Dormmamu to argue about thermal voltage..

Quote from: BrainDrain on November 02, 2023, 02:49:51 PMHere are the components I'm getting almost no voltage reading from within the circuit:

That would be normal. There's nothing in that section to *provide* any DC voltage. The section is separated from the rest of the circuit by the 10uF DC blocking capacitors, and the rest of it is only connected to ground.
You won't see any DC measurements until you get the other side of the 10uF after the pink pot. Then they should all pick up again. That whole clipping and tone section is AC-only, ground-referenced, or passive, which here all amount to the same thing.

<edit>Antonis was quicker!!

Quote from: antonis on November 02, 2023, 03:39:49 PM

Quote from: BrainDrain on November 02, 2023, 02:49:51 PMHere are the components I'm getting almost no voltage reading from within the circuit:

You should worry in case of you DID get some voltage reading..

All red circled items are DC isolated..
(there isn't any DC voltage difference across them..)(*)

e.g. Look at 3k9 resistor.. Its lower leg is "shorted" to its upper leg via 27k, 47k, S1, 100k and Blue pot wiper..

P.S.
You Do know that caps are considered "open-circuit" for DC, don't you..??

(*) I'm waiting Dormmamu to argue about thermal voltage..

Quote from: ElectricDruid on November 02, 2023, 04:07:48 PM

Quote from: BrainDrain on November 02, 2023, 02:49:51 PMHere are the components I'm getting almost no voltage reading from within the circuit:

That would be normal. There's nothing in that section to *provide* any DC voltage. The section is separated from the rest of the circuit by the 10uF DC blocking capacitors, and the rest of it is only connected to ground.
You won't see any DC measurements until you get the other side of the 10uF after the pink pot. Then they should all pick up again. That whole clipping and tone section is AC-only, ground-referenced, or passive, which here all amount to the same thing.

<edit>Antonis was quicker!!

Thank you! I have ALOT to learn. I'm glad this got me to make an account here so now I can learn even more. I'll  need to do some more research on what "open circuit" is too.  Alot of this stuff is new to me as I've only been building pedals by following build docs and it's worked out good so far mostly because I take my time. Debugging is something I need to learn more about.

So would adjusting that 100k resistor on the base of Q4 allow me to change the bias? I'm just going by what the other Blender thread says but just wanted to make sure before I start removing components.

Thanks again   

DO NOT remove components without a very specific scope..!!  

Before you proceed to removing/replacing/tweaking items I think you have to understand how particular Q3/Q4 octave configuration works..

In brief: Q3 is a phase splitter (CE amp of X1 gain), meaning there are simultaneously almost identical signals on both its Emitter & Collector with 180^o out of phase..
When a signal positive waveform enters Q3 Base, respective positive Emitter output "prangs" on respective series diode cathode and nothing happens on Q4 Base..
But there is also a respective negative (due to 180^o phase reversal) waveform Collector output which pulls down Collector series diode/27k resistor voltage level hence lowering Q4 Base voltage..
This, in turn, lowers Q4 Emitter voltage resulting into lowering Q4 Collector current, hence raising Q4 Collector voltage..
We now have a Q4 Collector positive waveform output for the half (positive) of Q3 Base incoming signal..
For the rest half (negative) signal waveform, Emitter & Collector change roles..
(Emitter now pulls down respective diode/27k voltage level, resulting into lowering - again - Collector current,, As a result, we have two positive waveforms (and no negative) for a signal full waveform.. (you can call it "full wave rectification"..)
But we effectivelly doubled Q4 output signal frequency so you can call our mess "octave up" effect..

Now, what you have to do is maintain Q4 quiescent point at a working level called "forward active mode"..
This happens for our case, when Collector voltage sits, at least, about 600mV higher than Base voltage..
Collector voltage is set by both Collector current and Collector resistor value..
It's up to you to decide which of 100k/27k/10k/820Ω items should be tweaked to get your Collector desirable voltage..

Quote from: antonis on November 05, 2023, 05:05:29 AMBefore you proceed to removing/replacing/tweaking items I think you have to understand how particular Q3/Q4 octave configuration works..

In brief: Q3 is a phase splitter (CE amp of X1 gain), meaning there are simultaneously almost identical signals on both its Emitter & Collector with 180^o out of phase..
When a signal positive waveform enters Q3 Base, respective positive Emitter output "prangs" on respective series diode cathode and nothing happens on Q4 Base..
But there is also a respective negative (due to 180^o phase reversal) waveform Collector output which pulls down Collector series diode/27k resistor voltage level hence lowering Q4 Base voltage..
This, in turn, lowers Q4 Emitter voltage resulting into lowering Q4 Collector current, hence raising Q4 Collector voltage..
We now have a Q4 Collector positive waveform output for the half (positive) of Q3 Base incoming signal..
For the rest half (negative) signal waveform, Emitter & Collector change roles..
(Emitter now pulls down respective diode/27k voltage level, resulting into lowering - again - Collector current,, As a result, we have two positive waveforms (and no negative) for a signal full waveform.. (you can call it "full wave rectification"..)
But we effectivelly doubled Q4 output signal frequency so you can call our mess "octave up" effect..

Now, what you have to do is maintain Q4 quiescent point at a working level called "forward active mode"..
This happens for our case, when Collector voltage sits, at least, about 600mV higher than Base voltage..
Collector voltage is set by both Collector current and Collector resistor value..
It's up to you to decide which of 100k/27k/10k/820Ω items should be tweaked to get your Collector desirable voltage..

I can't thank you enough for taking the time to explain all of this. So pretty much it's possible that any of the 4 resistors you listed are causing the issue.  I plan on getting an atlas dca75 soon. Is that good enough or were you referring to another kind of measuring device? If you were fixing this issue with the resistors how would you go about doing it?

Also, the 100k resistor on the base of Q4 is measuring at 67k when I take the battery out. Not sure if something else in the circuit is causing it to read that way.