guv'nor...lack of gain

[qtws]

Hi folks,

as the title says, I have an original Marshall Guv'nor, which over the last few days has given up the ghost.  I've read a huge amount of this forum and geofex, learned a bit, been baffled by quite a lot, and got busy with the multimeter. I have followed the checklist - my findings are below.


1.What does it do, not do, and sound like?

Works in bypass. Slight tone suck, Switches on, red led. 
Yesterday - lack of gain.
Today: Lack of gain. Sounds kinda 'thin', too
A bit like a too clean, gutless AC/DC sound on full gain.  AC/DC style O/D used to be at 12-2 o clock, now its not there even at full gain.  Can't get Randy Rhoads gain levels at all - not the case before whatever has gone wrong, went wrong. This with LPC into a Marshall class5!
Level and tone controls work ok, as far as I can see (hear!)

2.Name of the circuit = Marshall Guv'nor (original)
3.Source of the circuit (URL of schematic or project) =  internet/googled it, drawn by Michael Burgundy, 22/06/99
4.Any modifications to the circuit?  NO
5.Any parts substitutions? NO
6.Positive ground to negative ground conversion? No, pedal is stock

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? =>
Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:

This with 9V DC adapter (which is good - 9.6V):
Voltage at the circuit board end of the red battery lead = 8.96
Voltage at the circuit board end of the black battery lead = 0

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:



IC1 (TL072)
P1 = 4.47
P2 = 4.47
P3 = 0.01 (sometimes 2.19)
P4 = 0
P5 = 4.37
P6 = 4.47
P7 = 4.47
P8 = 8.96



D1 (polarity protection
A (anode, the non-band end) = 8.96
K (cathode, the banded end) = 9.6

continuity + to -
760 ohm - to +


R1 - 2.18k
R2 - 9.9k
R3 - 985k
R4 - 47k
R5 - 31.5 (spec 47k - could this cause too high a voltage on the IC pins?)
R6 - 672k
R7 - 990
R8 - 584 (680 spec)
R9 - 1.47k
R10 - 668
R11 - 100
R12 - 22k
R13 - doesn't exist
R14 - 4.7k
R14 - doesn't exist
R16 - open circuit, spec is 2 mega ohm - yes, it pops when switching

And so on until you get through them all. Having gathered the data, post a tidy list of it on the forum, and you'll most likely have a working effect soon.
For extra credit, while you're waiting for someone to tell you what they see, probe the pins of each of the electrolytic caps, verifying that the voltage on the (+) pin is more positive than the voltage on the (-) pin. If it's not, that cap will eventually fail, whether it's the immediate cause of the thing not working or not.

   +      -
C1 - 4.45      0
C2 - 4.43      0
C3 - 4.25      0
C4 - 4.38      0
C5 - 2.19      0
C6 - 0V at each end   0  (4.3V each end relative to R4/R5 voltage divider)
C7 - 4.33      0
C8 - 4.48V at each end
C9 - 4.47      0

tone controls
C10 - C14 0.0V each end

Pots -
Vr1 - 100k (gain)
VR2 - 10k (bass)
VR3 - 10k (mid)
VR4 - 10k (treble)
VR5 - 600 ohm. Spec is 100k (output level) As mentioned above, level seems fine in o/d mode, unless this is the problem thats losing the gain, somehow.

Red LED clipping diodes glow with signal. Cute.

There is continuity everywhere there should be in the circuit. No Capacitors are shorted.

Not to spec:
R16
possibly C6, C8
No voltage worth a damn on pin 3 - is this normal? Should they not all be the same?
Pin 5 - 100mV lower than other pins
Level pot resistance - altho' actual level seems fine??

Replace IC as a starting point? Or something else not obvious to me (with my 2 days of electronics expertise.... ;-)
Its nearly 3am, and I'm stumped - Help!

Cheers,

Nigel

[Ronan]

There seems to be a problem at pin 3 of the IC, it should read about 4.5V.

You reported 2.19V at C5, and 0V or 2.19V at pin 3.
R3 is supposed to hold this voltage at pin 3 to around 4.5V.
Check that you actually have around 4.5V on the end of R3 that isn't connected to pin3.
If you do have 4.5V there, (like it shows on the schematic) and the solder joints on R3 are good at both ends, then I would try to replace the IC. I hope its in a socket. If the IC is in a socket, remove it and again check for 4.5V at pin 3. If you now have 4.5V then its likely the IC to be the problem.

BTW, excellent first post and welcome.

Link to the schematic:
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/marshallguvnor.gif

[PetMan]

Hi,

i would go for a opa 2134 drop in the place  of the TL072, who sounds pants !

[Mike Burgundy]

Let's get it working first ;P
As said, R3, C5 and pin 3 are where the problem seems to be. Did you use a socket? If so, pop the IC out and remeasure the socket pins.
R5 being low will not yield too high a voltage anywhere.
It's half of a voltage devider R4/R5. The voltage between R4 and R5 (=Vref) will be (R5/(R4+R5))*V+. If R4=R5 youll get 1/2 V+ or 4.5V. If R5 is low, you'll get a lower Vref, but nothing problematic. Voltages look just fine with the exception of pin3.

[Mike Burgundy]

Oh - VR5 600Ohms? 

[qtws]

Hi guys - many thanks for the welcome and the fast responses!

Ok, I remeasured VR5 - I get 3.17k. Better, but still not spec.

voltage at R3 is 4.45 V, current is 193 mu Amp to ground at power supply end
Current at the IC end is 6 mu Amp, same as at pin 3. Voltage at IC end is 2.21 both on IC pin and the solder on the other side of the PCB

Contrast with pin5 (input of 2nd gain stage), which is 193 mu Amp, 4.45V

C5 has 0 amps at input end (as expected) and at the junction with R3 has 6 mu Amp.

The opamp is soldered to the PCB.....it will be a pain to remove without cutting pins as the pots are soldered directly to the PCB and bolted to the case - thin spanner needed to get at the mounting nuts to release the PCB. However, if I do this I may as well install an op amp socket if only to prevent me nuking the new op amp with soldering iron heat (and try other opamps).

I've got my head around the voltage division, and how the gain stage works with VR1 (and how I might increase gain with a bigger pot), but other than that .... its a steep learning curve!

New op amp first then?

Cheers,

Nigel

[Ice-9]

Can you tell us exactly how you are measuring VR5 because if your using a 100k pot it will measure 100k between the two outer pins.

[Ronan]

Mick, VR5 has VR4 and VR2 in parallel with it, so it won't measure very high in circuit.

[Ice-9]

Mick, VR5 has VR4 and VR2 in parallel with it, so it won't measure very high in circuit.

Ian, Yeah I should have worded that a bit better I meant to say measure the value of VR5 while disconnected from the circuit or check the value markings just to make sure the correct value pot is in there in the first place.

I have a guvnor to hand and with the tone controls all at min VR5 measures 600 ohms and tones set at full measures 16K in circuit if thats any help.

I doubt VR5 is a problem though as pin 3 of IC1 has not got the correct vref voltage, this needs to be checked out first.

[Ronan]

Yeah, I guessed you knew what you were talking about, but I was afraid Nigel might freak out and rip the pot out, and couldn't figure out how to word my response any better.

I notice no-one's given Nigel the go-ahead on pulling the IC. Do they ever actually fail in a dirt box?

Also, Nigel, whats this "193 mu Amp" mean, and how are you measuring that? Is it micro-amps (uA)?

[qtws]

Hi all,

I haven't made any repairs....yet.

Just spent a happy hour or so playing the guv'nor into my class5 - the gain has not magically reappeared, however it still sounds pretty good - responsive as anything, some serious pinch harmonics off the G/D/E strings, but v hard to pick out of the E/A/D-  no Zakk attack tonight!  And frankly, it sounds a bit thin - there's no bottom end oomph.

ok, back  to the problem in hand.
Ronan: mu Amp - micro amps is what I meant.

Remeasured the VR5 pot with tone fullup (15k) and full down (600 ohm) so no problem there - I wasn't about to rip it out.

A check of Ebay reveals TL072s in abundance for a few quid from a few UK sellers, and IC sockets likewise (no handy mail order outlets, or local shops hereabouts...) - are all 8pin IC sockets the same? - i.e. will they take the TL072 ok?  Any I've seen on Ebay all look the same....
There's tons of room in the pedal to fit IC holder and IC, so thats what I may as well do.   For the £5 or so it will cost for bits, its worth a try.
I will report back when I have further news!
Cheers,

Nigel

[Mike Burgundy]

Okay, VR5 measured in circuit - that makes sense.
I think you might have the current readings wrong - you need to unsolder stuff and insert the DMM in series to get a current reading. Voltage and resistance are measured parallel - the probes bridge a part/circuit (the meter behaves like a VERY large resistor (1-20M depending on model)), but current needs to be done *through* the meter (meter behaves like a short - a 0 Ohm resistor). So, if you want to know the current through R3 (*must* be the same at either end, there's nowhere else to go for it) you need to lift one end of R3 and insert the DMM between the lifted end and its designated solderpad.

Since the voltage on pin 3 is intermittent, I think you could have:
A) short (intermittent/and tiny) between pins 3 and 4. Run an Xacto knife between the solderpads, and see if that helps. Measure the resistance between pin 3 and 4 (no power applied) before and after  - should be *huge*, many MOhms, if at all measurable. Shorts like this can be practically invisible, but they should be measurable/curable.
B) Forget B for now - R3 and C5 look to be ok as far as I can see. Bad solderjoints is a possibility, then look at R3 mostly (since the weird voltage also is on C5).
C) damaged IC.
It's a great idea to use sockets anyway. Some people (with oodles of experience) don't, but if anything happens or mistakes creep in it is just a lifesaver for us normal humans ;P
After checking and measuring for a short/the Xacto trick, take out the original IC, stick in a socket and take it from there. First thing to do is check voltages *without* an IC in the socket.
hih

[Mike Burgundy]

Hang on - now my head is folding up. Pin 3 sets the operating point for pins 1 and 2 - so why are they ok and not following pin 3?? Huh? Maybe the IC after all - anyone ideas on that?

[qtws]

Hi Mike,

I get ~1 mega ohm between pin 3 and pin4 with no power applied: high, but not open. I can't see how a short would develop between these at the solder pad level, if they've been fine for 20 odd years so again, that points to the IC.

I see what you mean about the current readings - there's a lot of A level physics coming flooding back as I read and re-learn and remember what way stuff works!

I haven't had the pot/IC PCB out of the pedal yet - need to fabricate a small thin spanner for the pot nuts to release the pcb, but I will check the solder pads and do the craft knife thing once I have new IC and holder in hand.

Presumably, I coud check voltages once the IC is out, during the disassembly stage - expecting ~4.5V at 1,2,3,5,6,7, with 9v at 8 and 0V at 4?

Cheers,

Nigel.

[Mike Burgundy]

I totally forgot about it being an original, sorry ;P
If you have a socketwrench set one of the sockets will probably fit the nut and clear the shaft. You'll have to pry the knobs off anyway (careful). If it doesn't fit/you don't have them, you could consider getting a pipe wrench set (not expensive), although I usually manage to carefully loosen the nuts with a regular spanner if I can't be bothered to walk to the toolcase.
Once it's out, take out the IC (desoldering pump and solderwick are great tools here, to remove solder) and stick in a socket. Measure, should now be ok. If not, we need to figure out why first. Replace the IC, meassure, should still be ok.

[qtws]

bump....

ok, I finally got round to making a right angled spanner* to loosen the nuts holding the pots (and their circuit board) and let the board out out of the pedal casing so I could get to the IC pins.  I desoldered the pins of the IC with suction pump and it came out fairly easily.  Stuck in the IC holder and then popped in a brand new TL072.  Measured voltages on pins - as before (4.46V), but still getting 2.21V on pin3, and 0 v on pin 4.  Reassembled and fired it up and it worked, and sounded like it should i.e a proper Marshall 'bark' (as far as I can remember  ): AC/DC at ~12-2pm on the gain, and Ozzy type O/D when full up, complete with decent sustain and pinch harmonics - I felt a bit more treble would be good, but happy so far.  I also replaced R16 with 2 x 1M resistors in series - that's fixed the pop on plug in.

Then I re read this thread  Checked resistance between pin3 and pin4 - 1M again with IC out. 1M between solder pads - checked for shorts - none that I can see.  The end of R3 is still at 2.21 V, while the other end is 4.46. Not sure if this is a problem ...  Now, I have a Fender 15G practise amp, which also has a TL072 in the O/D circuit - albeit running at 15V - there is way more gain there for sure (15V versus 9V?) and a very full sound, compared to the guvnor into the clean channel of the fender.  Maybe things aren't totally fixed....

So, while playing through the (fixed?)  guvnor for a few minutes more, I noticed a drop in gain, and the sound got a bit thin and trebly - had to hit the strings hard to get anywhere, and lack of sustain meant that slurring runs just weren't working - back to the top of the "broken pedal" slide      Measurement of IC voltages showed pin 5 (non inverting input 2) had dropped down to 4.36V from 4.46 (same as before new IC repair). Its 4.44V with IC out, 2.21V on pin 3, and 0V on all except pin 8 (9V) - same as before putting the IC in (above).  So that's both inputs with strange voltage...

Why is this thing eating IC's? What could be the problem? I have a spare TL072, but am reluctant to stick it in if its going to get nuked inside 5 min before we find the problem!



*How long has it been since my last post? It took me 15min to make, plus 5min to get the nuts undone!

[Ronan]

What sort of multimeter do you have? Do you know if it has a 10M or 1M input resistance?
If it has a 1M input resistance then that might explain 2.21V on pin 3. Some of the smaller cheaper multimeters have a 1M input resistance. If your multimeter has a 10M input resistance, then this 2.21V on pin 3 is "the problem" that needs to be fixed. But I'm wondering how you can have correct voltage on all other pins and not pin 3. Maybe we have been chasing something that is not the problem. Very interesting/puzzling...

[qtws]

Hi Ronan,

my multimeter is a Draper Dmm1A.  Google search results suggest it has 10m ohm input impedance (altho I can't find any document in the results that states that explicitly...including from Draper).
On the way to IC pin 3 in the circuit are R16 (2.2M, replaced with 2 x 1M in series) C5 (9n6) and R3 (1M).  R3 reads correctly - could C5 be the problem?

Cheers,

Nigel

[Ronan]

If it were my pedal, I would replace R3, if that didn't bring the voltage up on pin 3 (with the IC removed), I would then remove C5, if the voltage is still low on pin 3 I would think there is a fault with the pcb, as in some sort of high resistance to ground. Maybe a good idea to check the resistance from pin 3 to ground while R3 is out.

You mentioned that you had to replace R16 because it was open circuit, perhaps it got blasted with a high voltage somehow, and possibly some damage to C5 and R3 resulted at the same time, who knows, only one way to be 100% sure (replace them).

[Miracle Man]

Hi!

I'm sorry for resurrecting this thread but I seem to have the exact same problem with my GGG Guv'nor clone. I have tried replacing all parts that are connected to pin 3, I even bent pin 3 and connected it to R3 and C5 above the PCB - no dice, the voltage on pin 3 is still low. Now, here's the ineresting part... I lifted one leg of R3 - the side connected to pin 3 - and checked the voltages: about 4.4V on one side and 2.2V on the other side - with it not being connected to anything!!! What the fudge?