Boutique Screamer Pedal Help

[PJK65]

Hello Forum Members,

   I am trying to repair a boutique tube screamer/dumble style pedal. It has true bypass, and signal passes thru to the output jack just fine when turned off. However, when turned on, the pedal has no sound whatsoever.  I have no schematics for it, so I dove in and started checking some voltages and components. It has a JRC4558DD op amp and a pair of 2n7000's. It is getting signal to C1 on the board, but no farther than that. The power supply seems to be ok, as it has +9v at pin 8, and +4.5v on pin 3 of the JRC4558 with the chip removed. (All other pins are +0v with chip out) However, when the chip is in, this is where things get weird! Here are the voltages:

pin 1   +8.64v               Q1                          Q2
pin 2   +8.02v               pin 1   +8.19v         pin 1   +8.63v
pin 3   +7.97v               pin 2   +8.63v         pin 2   +8.20v
pin 4   +0.00v               pin 3   +8.63v         pin 3   +8.20v
pin 5   +8.60v
pin 6   +8.66v
pin 7   +8.66v
pin 8   +9.00v

The current draw using a 9v battery as the power source is 1.84mA on, and .46mA off.
Does this look like the JRC4558 is the problem? I don't have a spare one to swap it out with. If anyone can help me with this, I would greatly appreciate it! If any more info is needed, I will be glad to post it.

Thank You!

[idy]

I would try any dual opamp to sub.

You are seeing that the IC socket, when empty, is getting correct power and bias, but you are seeing all the opamp pins go up close to supply voltage when the chip is in.

The Zen Drive type circuit DC couples the two opamps, so no bias on pin 5 when no chip.

The 2n7000s are just clipping (with diodes nearby?) and aren't important right now.

[ElectricDruid]

I would try any dual opamp to sub.

+1 agree. Stick a TL072 or whatever else in, just to check whether the op-amp is the problem. You can worry about which is the perfect op-amp for maximum toan when it's working! 

[PJK65]

Thank you for the help! I'll see what I have around and try the swap.

[PJK65]

The op-amp was indeed the culprit! Had to order a cheap one to confirm. Now comes the issue of trying to find one that sounds as good as the original, which was a JRC4558DD. What chips are you guys using in your screamer clones? Again, thanks for the help!

[Govmnt_Lacky]

The op-amp was indeed the culprit! Had to order a cheap one to confirm. Now comes the issue of trying to find one that sounds as good as the original, which was a JRC4558DD. What chips are you guys using in your screamer clones? Again, thanks for the help!

Congrats on finding the culprit!

I'm gonna go rogue here and say... just get another 4558D and rock on. Those chips are a dime a dozen and there really is not mystique in finding the "perfect one."

My opinion... YMMV.

[FiveseveN]

Remember when VS debunked this op amp corksniffing nonsense? Pepperidge Farm remembers.

[ElectricDruid]

I'm gonna go rogue here and say... just get another 4558D and rock on. Those chips are a dime a dozen and there really is not mystique in finding the "perfect one."

+1 agree. Anything oldish and cheap works fine. 1458's are good too - they're a similar era. Don't go hi-fi, so no $20 Burr-Brown craziness!

[antonis]

Anything oldish and cheap works fine. 1458's are good too - they're a similar era.

+1 agree..

Any old-fashion BJT input op-amp should be fine..

[PJK65]

So avoid current production chips? Any reasonably priced places to get older chips that are genuine?? Seems like eBay has a lot of Chinese junk...Be nice to find some old piece of electronic gear that has a bunch of them in it!

[bloxstompboxes]

So avoid current production chips? Any reasonably priced places to get older chips that are genuine?? Seems like eBay has a lot of Chinese junk...Be nice to find some old piece of electronic gear that has a bunch of them in it!

I don't think anyone is saying don't buy current production chips. What I believe they mean is to not buy anything that is "audiophile" grade. all those mentioned above can be had for $1 or less. Spending $20 on an opamp that is supposed to be audiophile grade would not make much sense in something that is supposed to distort.

[FiveseveN]

So avoid current production chips?

That's not at all what they said. Current production, old design.

[ElectricDruid]

So avoid current production chips?

That's not at all what they said. Current production, old design.

You do have to watch out a bit though, because several manufacturers are busy pushing their "new improved" versions of the classics. That's fine for people who interested in improved specs, but for those of us who are more interested in the authentic sound of the vintage chips, we'd rather have the original versions, thanks! Since silicon is silicon, I don't care if it was made in 1985 or last week, as long as it's the same chip.

Exmaple - TI have a "better" version of the TL072, the TL072H:

https://www.ti.com/product/TL072

Personally, I liked the old one fine. I don't mind them coming up with new, better op-amps, but I'd rather they didn't try and tell me it's a TL072 when they just changed it so it isn't.

[PJK65]

Thanks for clearing this up! Makes sense! Definitely won't be spending big $'s on any hi-fi chips...
Great forum here!! Very knowledgeable and helpful members!!

[Clint Eastwood]

The JRC4558DD I think is still in production, today it is called the NJM4558DD. Mouser for example sells them.