Blues Breaker V1 Sounds Like Poo. I'm Stumped!

[aron]

Yes, I agree with Gus. It's probably needs a fix to the bias part of the circuit. That's why it sounds "ratty" which I will assume is a reference to The Rat pedal of the 80's. :-)
Edit.... pin 5 has 1/2 VR
So the problem is closer to pin 3.

[Gristlepig]

Okay guys,

Here's what I've got...

R14 reads 4.26v on the side to ground. The side running to pin 3 reads 1.4v
R15 reads 0v on either side. Neither gnd side nor the side running to C10...

And with the pedal powered off, Pin 3 to gnd and to VREF both read about 1M. Reading were the same whether or not the switch was engaged.

Reflowed r14 and r15. No difference. Still sounds weak, quiet, yucky midrangey with very low gain.

[Gristlepig]

From what I've gathered from the original owner, the pedal didn't slowly decline in sound quality. He said one day, it just sounded like sh!t.
I don't know if that information might be helpful.

[Ice-9]

From what I've gathered from the original owner, the pedal didn't slowly decline in sound quality. He said one day, it just sounded like sh!t.
I don't know if that information might be helpful.

If your in England and just want it working , send it to me and I will fix it for free, just pay postage.

[Gristlepig]

I appreciate the offer, but I'm in the States and I'd like to get it working myself. Every project like this is a chance to learn something new.

Unless, y'all think it's a bad idea, I think I'm just going to go ahead and throw in a socket, replace the op amp and see what happens. I kinda just want the thing off my desk at this point.

I want to fix it and get it into someone's hands who digs that low gain BB overdrive.

[Ice-9]

No probs, I will read through the complete thread to see what is happening and see if I can come up with any options for you. I love all that original old marshall stuff, I have had lots of these through my hands to repair in the past.

[Gus]

I could be the opmap is bad.  If you replace it I would not socket it and I would try to find the same IC company 71 to keep more of the resale value.

I dislike sockets

Chips will move out of sockets due to vibration and temperature changes.

[Ice-9]

I see from reading through the thread that you have the correct voltage of appx 4.35v on pin 5 of the op amp but only 1.4Von pin 3.  From this the bias on pin 3 is wrong and I would say that either the opamp is faulty or the 1M bias resistor to pin 3 has died. I would replace the opamp for the sake of 20c its a 5 minute job. Also check that 1M resistor with a meter. The bias network is working as you have the correct voltages elsewhere in the circuit.

If your want to keep it original , don't worry Marshall used pretty much any make of 072 opamps, just whatever they had really, most commonly TL072's I have seen in all the old big box marshall pedals.

[Gristlepig]

Thanks Ice-9,

I just checked out r14, the 1M that connects to pin 3. Reads spot on 1 meg. I'm gonna pull this chip and try out a TL072CN in a socket. From what I can see, it's the same IC.

Unfortunately, I'm going to have to clip the old one out. The legs are bent and soldered such that I'm going to maul the board and probably lift a trace if I try to jimmy it out.

[Gristlepig]

Yup, it was the op amp. Just popped a TL072CN in there and it sounds just like it's supposed to.

And I didn't even lift a trace. Well, I came pretty close tho...

Thanks everybody for your help. I really appreciate it!

[MartyMart]

Great news :-)

So if you sell it on just say "All original with high quality op amp socket fitted  - try your own choice of op amps easily"

[aron]

Great! Had a rat with a similar problem.