V1 Big Muff Q ...

[mikeC]

hi friends,

hesitant to post this Q as i know muff's have been discussed since ever here. searched site and still not exactly sure how to proceed.

i have an early V1 muff that sounds <pretty good>
only gripe (besides just barely making unity) is it doesn't really "sing". i.e. doesn't have nearly the gain you'd expect from a muff. i figure it's getting 75% of the way there ; )

i've checked and measured every R against known schematics and i found that R's 12 and 19 are 2.7k's whereas i see that R12 is typically in the 8-10k range and R19 7-10k. am i on to something here?

fyi R2 is 22k and transistors are SPT 87 103s

thank you for your patience and best regards,
mike
(thank you BM page for the resource!)

[GibsonGM]

Hi Mike, can you post a schematic so that we all know exactly what you're talking about?

Thanks, and welcome to the forum!

[Mr.Kite]

If you measured the resistors IN circuit you can't get the right values...

If you measured the resistors OUTSIDE the circuit these are pretty unusual values: R2 should be between 27k and 39k, R12 and R19 between 7.5k and 10k, but with your values (if they are "real") you should just have a VERY gainy Big Muff, and it should get WAY above unity gain (into a clean amp)...

Check the signal path (no flimsy connections, leaky caps etc...) and the bias of the transistors: you should have about 4-5v at the collector of each transistor. Some Muff has higher voltage on Q1 or Q4, between 5-7v, but these are the minority...

[mikeC]

thanks, gibson n beneficial mr. kite ,,,

is that a fact? you can't measure resistors in circuit re- muffs? i did not know this ... i was ahem, a ham when i was younger and still tinker w old tube radio gear ... i'm accustomed to it being fair play to measure R's in circuit. my day gig is a guitarist ,,, i'm still on steep end of learning curve when to comes to diagnosing misbehaving tools.

i'm pretty confident connections and caps are ok ... i'll check voltages at transistors next.

thank. you.

[Mr.Kite]

IME, yes, you need 'em out of the circuit: the resistor needs to be "alone" to be measured accurately, but this rarely happens inside a circuit. Generally there is some parallel resistance that tends to lower (sometimes slightly, sometimes significantly) the value of the resistor you're measuring...

[PRR]

R12 and R19 being low-value will increase gain. First-guess, that's not your problem.

Since one end of R12 and R19 terminate in a capacitor, in THIS case it should be valid to measure in-circuit.

In many tube and/or radio circuits, many resistors can be checked in circuit. If it only goes to a cap or a cold tube, you have a free end.

In many audio circuits and around transistors generally, you have multiple resistor paths and also transistor junctions, which sure can foul-up in-circuit measurements. A few meters are sensitive enough to not always energize semiconductors enough to affect several-K readings, but always doubt.

[antonis]

As Paul said, lower values of R12 & R19 alone should raise gain..

But in series with lower enough values of C13 & C5 respectively, might lower gain..
(R12+C13 & R19+C5 form HPFs with Q2 & Q3 input impedances..)

So check for C5 and/or C13 values..

Next step should be checking individual transistor Collector/Emitter values (ratio)..

Further check should be focused on feedback resistor values (R9, R17, R15)..

Last, but not least, check feedback capacitors values..
(although they should be greatly oversized to enough reduce gain..)

[mikeC]

thank you, PRR, antonis : )
i'll get to this this afternoon
best,
m

edit - had a look at ...
C13 and C5 labeled .1mfd
C10, 11, 12 labeled 500pfd
R9 labeled 390k but in circuit reads 150k
R17 labeled 390k but in circuit reads 140k
R15 labeled 390k but in circuit reads 130k
i wonder if feedback Rs need to be measured out of circuit?

[antonis]

If they are correctly labeled, they're OK..
(they are shunted by caps so their actual value is dominated due to caps charging..)

Time to look at Collector/Emitter resistor values..

[Mr.Kite]

If it is an old pedal (but also if it's not) I would check the pots too: an interrupted (or "semi- interrupted") pot can cause a drastic volume reduction and a tonal change...

And remember to check the bias voltages: if there's something wrong there you should easily see it...

[mikeC]

Collector R11 33k, R18 10k
Emitter R4 2.7k, R10 120, R22 100

Pot. R26 113k, R24 89k

voltages i came up w/ for each Q ...

Q1 Base 1.3v, Emitter .68v,  Collector 5.5v
Q2 Base 1.2v, Emitter 0, Collector 6.2v
Q3 Base .65v, Emitter .36mv, Collector 5v
Q4 Base .6v, Emitter 15mv, Collector 4v

not sure how to check bias voltage (maybe i did w/ Q voltages?)

thank you, all
mike

[antonis]

Collector R11 33k
Emitter R10 120

Ouppsss...
It seems to me you've almost positive saturated Q2..
edit: Need a cup of coffee, for sure, 'cause I figured it out for "fixed" bias and not feedback one..

Measure voltages on Q2 by circuit pin-out definition..
(i.e. Emitter on R10 leg, Base on R16, R12, R15, C11, & C7 common joint, Collector on R11, D1, D2, C11 & R15 common joint..)

edit: Despite actual pin-out order, some of your readings make no sense, so locate trasnsistors C/B/E order and post readings..

[mikeC]

whew ... not exactly sure what you're suggesting i do, antonis. sorry!

via schematic i <think> i have Vs labeled correctly w/ Tranny E, B, C (above post)

[Marcos - Munky]

Hey Mike, do a quick test just to check your pots. Put a jumper between lugs 2 and 3 of volume (mid and the one that goes to C2) and sustain (mid and the one that goes to C4) pots, this will "hard-set" them to the maximum value. If you get a louder sound, the problem is in one of your pots. Like Mr.Kite said, a faultly pot may result in a drastic volume drop.

[mikeC]

mr kite, marcos, thank you ... tried that and no luck ...

btw, marcos, where in brasil are you? my wife's from copacabana .. ive been fortunate enough to visit brasil many times. lovely place and people! (musicians truly badass, yikes!)

best,
m

[Marcos - Munky]

Mike, I would go for a audio probe, it may help to find where's the problem. Do you know how to do it?

Btw, I'm from a small city located in a state called Paraná, which is most known because of the Iguassu Falls. I'm about 1100Km away from Copacabana. And yes, there's some badass musicians here, which is not my case

[mikeC]

i've never done that, marcos. are we talking about injecting a signal and tracing throughout circuit?

btw here are voltages i've come up with:

Q1 Base 1.3v, Emitter .68v,  Collector 5.5v
Q2 Base 1.2v, Emitter 0, Collector 6.2v
Q3 Base .65v, Emitter .36mv, Collector 5v
Q4 Base .6v, Emitter 15mv, Collector 4v

thanks,
m

[thermionix]

If you truly have 0.000V on Q2e, then there is no current being drawn through that emitter resistor, and you likely have a bad connection around there, a dead transistor, or a bad emitter resistor.

[mikeC]

now we're getting somewhere !
just double checked: E for Q2 is 0

would muff still fuzz under these conditions? barely making unity and not capable of typical gain?

am i looking at associated emitter R10? reflowing solder in Q2 area?

really appreciate the help.

[thermionix]

would muff still fuzz under these conditions? barely making unity and not capable of typical gain?

I'm really not sure, but I can see signal getting around a dead Q2 through the feedback loop, weaker and lower volume than it would otherwise be.

Yes check R10.  If you read it in circuit try with your meter probes both ways, in case you have a transistor portion acting like a diode in parallel with the resistor.  If it seems clear that you have 150 ohms from Q2e to ground, your resistor and connections are good, I'd be inclined to suspect Q2.