Marshall Shredmaster problems

[Mr_bibbles]

Hey dudes and dudettes-

Got a Marshall Shredmaster that I'm trying to fix- replaced both IC's with new TL072's, but havn't done much else.

The issue is that the pedal will only produce a sound when the input level is raised up above normal levels. I use an Active guitar (+30db or so) to test amps, and I have to turn it up about 1/2 way before any audio kicks in, and when it does it come on abruptly and seems to function normally, albeit a little clipped.. Normal guitar signal produces no sound, and the bypass functions correctly. Assuming this is some sort of switching diode failure or possibly input buffer problem, but can anyone give me a good idea where to start looking???

Schematic:

http://www.geocities.ws/diygescorp/shred.gif

Thanks in advance for your sagely wisdom.

_Matt

[g1]

What is your DC voltage at Vbb ?
How about at pins 1, 7, & 8 of each op amp?

[Mr_bibbles]

vbb- at + end of 100uf cap- 3.65

Tl072CP1
1---3.68
7---3.67
8---7.49

tl072CP2
1---6.93
7---3.67
8---7.49

I'm still learning about IC's.... guessing that pin 1 of TL072cp2 has some sort of bias issue??

Also noticed when I boost the input signal into audible levels, Tl072CP2 drops to about 1.5v

[J M Fahey]

Yes, but for sake of completeness please post all 8 pins in each Op Amp.
You also have a quite dead battery, put a fresh one there, TL072 is already working near the lower end of performance even with 9V.
So post voltages using a good one first.

[olddawg]

Have you tried using a non active guitar on the input? Some pedals don't like active pickups and do really odd things because on impedance matching and signal saturation.

[Mr_bibbles]

J M - Think you just gave me the hint I needed. The battery is pretty fresh, 8.5v across the terminals. So, judging by your response though, I am losing some necessary B+ some place along the line, which is voltage starving the circuit?

I also noticed the voltages changed drastically with something plugged into the input so here are both sets of values:

WITH SIGNAL APPLIED:
TL072CP1
1 6.78v
2.6.78v
3. 305mv
4. .7mv
5. 3.6v
6. 3.62v
7 3.62v
8. 7.38v

TL072CP2

1. 3.61v
2. 3.61v
3. 3.62v
4. .9mv
5. 3.62v
6 3.62v
7. 3.62v
8. 6.66v

NO SIGNAL APPLIED


TL072CP1
1 6.75v
2. 6.86v
3. 7.27v
4 7.35v
5. 7.35v
6. 6.93v
7. 7.31v
8. 7.44v

TL072CP2

1. 6.82v
2. 7.02v
3. 7.45v
4. 7.45v
5. 7.47v
6. 7.32v
7. 7.4v
8.7.52v

[Mr_bibbles]

Hey olddawg- I have tried a passive guitar to the same effect- With a hot bridge humbucker i can produce sound only by striking the strings with some force, and there's no sustain after a brief "BLAAAT" that doesn't really resemble a what a guitar should sound like.

[olddawg]

Hey olddawg- I have tried a passive guitar to the same effect- With a hot bridge humbucker i can produce sound only by striking the strings with some force, and there's no sustain after a brief "BLAAAT" that doesn't really resemble a what a guitar should sound like.

That symptom usually means some component or circuit is not biased on until it gets it from the input signal at a threshold level. If you have replaced transistors, make sure the basing is correct. If you have replaced resistors, check the value. Dry caps can do this too. A continuous input signal (tone generator) and a scope or signal tracer are your friends...

[Steve A.]

You need to test the battery with a load. 8.5vdc without a load is not a very fresh battery IMO. (You could put a 10k resistor between the test clips for a load.)

Steve Ahola

[Mr_bibbles]

Thanks Steve, That's good to know for the future! I have just tried it with a brand new battery and the problem is the exactly same. Does anyone have any insight into where I should be looking specifically? I don't know much about IC's so i don't know what they SHOULD be running at, and I have hunted through the resistors already. I'm also having trouble because the schematics do not match the board exactly But after googling the board it seems like these were thrown together with a "close enough" sort of vibe.

Also, Steve do you run Blue Guitar??? I used those resources for modding up my Peavey Classic 50. That thing blows most amps I hear out of the water now, so if that's you, great work man!, much respect.

[Steve A.]

Thanks Steve, That's good to know for the future! I have just tried it with a brand new battery and the problem is the exactly same. Does anyone have any insight into where I should be looking specifically? I don't know much about IC's so i don't know what they SHOULD be running at, and I have hunted through the resistors already. I'm also having trouble because the schematics do not match the board exactly But after googling the board it seems like these were thrown together with a "close enough" sort of vibe.

Also, Steve do you run Blue Guitar??? I used those resources for modding up my Peavey Classic 50. That thing blows most amps I hear out of the water now, so if that's you, great work man!, much respect.

Yeah, the Blue Guitar is my site... glad to hear that your C50 is happy! I'm just learning about ss circuits myself thanks to the very knowledgeable people here but it looks like something is drawing down your voltages. Does the LED go dim when the sound is cutting out?

Steve Ahola

P.S. If the ICs are socketed I would try replacing IC2

[Mick Bailey]

Are you measuring the battery terminal voltage with it connected and the circuit powered?

[Mr_bibbles]

Yeah, the Blue Guitar is my site... glad to hear that your C50 is happy! I'm just learning about ss circuits myself thanks to the very knowledgeable people here but it looks like something is drawing down your voltages. Does the LED go dim when the sound is cutting out?

Steve Ahola

P.S. If the ICs are socketed I would try replacing IC2

I have replaced both IC's with fresh TL072CPs already, no change. The LED looks as if it is getting constant current without flickering in all instances. Am I correct in thinking vbb should be lower than "9v" as listed on the schematic? I will go through and check voltages to look for discrepancies. Also Steve, that's great you're still improving your skills even at your level- I feel like I'll still be trying to study up when I'm 6 feet under

[olddawg]

Do you have a clean unattenuated signal in bypass mode? Honestly. I finally looked at the schematic. There isn't a lot to it. If you have to shotgun anything I would replace any electrolytic caps. You could also try a constant tone input and tickle various components with freeze. Personally.... I rarely troubleshoot anything using a battery. I use a wall wort or a bench supply. Then test with a battery after it is repaired. If you don't understand opamp theory check this out:
Operational Amplifier Basics - Op-amp tutorial

[Mick Bailey]

1. Measure the battery terminal voltage with the unit powered. Should be close to what a fresh battery reads when disconnected.

2. Measure Vbb - should be 1/2 of the supply voltage.

3. Measure pin 5 of U1 & U2 - should be exactly Vbb

The voltages in post #6 suggest your Vbb is too high, but this conflicts with your earlier readings for Vbb.