tl072 gone bad? working with a morley jd10...

[drummer4gc]

picked up this morley jd10 unit that just sounds awful - fizzy, thin, metallic, not much gain over unity with controls cranked.

first thing i noticed is that all three op amps are tl072, but most schematics have the 2nd and 3rd as lm1458n. i don't feel like this change would make the unit sound awful, maybe color the overdrive a bit differently....whaddya think?

my bigger concern, however, is that the third tl072 has gone bad. signal probing the circuit gives me the impression that the signal coming out of that op amp is quieter and thinner than the signal coming out of the second. is this a symptom of a bad op amp?

thanks so much for the help.

matt

[PRR]

'072 is a fine amp and may have been a factory change.

'072 is not easily killed. Bad solder joint or bad surrounding part is more likely.

It might be good to probe and post DC voltages: a truly failed opamp is sometimes obvious.

However Radio Shack stocks the TL072 for like $2. Yes, it is a 50-cent part but even at $2 it may be "cheeper" to replace it than to think about it.

[drummer4gc]

Thanks for the info. Hopefully with some DC voltages, someone can tell me if anything looks strange...


Here is the schematic: http://www.diystompboxes.com/pedals/schems/morley_jd10.gif

All three amps are tl072s. They also all have the same DC readings at the pinouts:

1 - 4.03
2 - 4.03
3 - 4.03
4 - 0
5 - 4.03
6 - 4.03
7 - 4.03
8 - 8.10


the only pins that differ from this are pin 5 on IC2 and pin 3 on IC1 read about 3.66.

The input voltage is fairly low from a 9VDC power supply. The pedal can accept 9 - 30 volts, ac or dc, due to a creative power input design that I believe sucks about a volt from my input. Is the pedal just being underpowered at only 9V? The thing accepts a battery, it would be pretty poor design if this was the case.

Any thoughts? The pedal works and responds to knob turning, it just has almost no gain over unity when cranked and sounds like shit, metallicy and fuzzy, not as I've heard it described elsewhere.

[PRR]

Voltages are fine, but no clues there. (The 3.66V happens after 1Meg resistors; it IS 4.03 until you put your 10Meg meter on it, which causes a 10% loading drop while measuring.)

> all three op amps

There's six opamps. Not sure what "third opamp" means.

What switch settings?

Can you see the non-fuzz non-SPKR path from U1A to U3A? When in that mode, the overall gain is indeed nearly-flat and barely over unity-gain (about 1dB). The SPKR mode is of course un-flat but tends to the same gain. With Fuzz, everything is messy, but it is a variant of an old classic.

C15 C16 C19 could be replaced on general suspicioun.

[Cliff Schecht]

I've repaired quite a few guitar amps where the op amp connected directly to a line/preamp out or power amp input is dead. Of course you never get a reason why the op amp died (like they hooked it up to another output) but it's usually easy to trace out with an oscope if you are so equipped (if not build an audio probe). A function generator of some sort is nice so you don't have to strum a guitar with your feet while you scope stuff out. Also having a schematic as you do is really nice because then you can follow the signal from op amp to op amp until you find the point where the signal distorts/dies out. At that point it's usually swap out that part and you are done (err..most of the time anyways). Usually if the signal is dead all over the preamp then there is a problem with the input jack or with the power supply. That pedal is pretty simple so it shouldn't be too hard to figure out where the signal is dying and troubleshoot that area.