uA739 op amp in Guitar Amplifier

[stevengendron]

Hi !
I'm currently repairing a guitar amplifier for my cousin.
The amplifier is a Kustom 250-1.
I hear a "bop bop bop" noise like morse code in background.
I suspect the op amps on the preamp boards.
But I've never seen an op amp like this.
This is a uA739. When i look at the pinout, there are pins called LAG....
This op amp was discontinued a long time ago and I could not find any modern replacement.
I tried to fit a regular 8 pins op amp (NE5532). The background noise disapeared but the gain is now too high and
the output is clipping, creating distorsion.

Is there any way to fit a 8 pin op amp without redesigning the whole PCB?
Anybody knows what are the LAG pins for on the original op amp ?

[edavid]

I tried to fit a regular 8 pins op amp (NE5532). The background noise disapeared but the gain is now too high and
the output is clipping, creating distorsion.

The gain of the second op amp is set by the complicated tone control network, so there may be something wrong in there.

Anybody knows what are the LAG pins for on the original op amp ?

External frequency compensation.  Since the NE5532 is internally compensated, it shouldn't be an issue.

[stevengendron]

This is really weird. I reinstalled the old op amp and it does not clip. But the background noise came back. I ordered the 14 pins op amp. It costed about 20$ for 2 op amps with the shipping. I will have to wait a month and it's from china... So probably a knock off
I hope it will resolve the problem.

[JFJ]

... I ordered the 14 pins op amp. It costed about 20$ for 2 op amps with the shipping ...

Amplifiers equivalent to the uA739, such as the TBA231A (please see attached datasheet), are available from Europe for US $2.48 + $3.50 shipping:

https://www.ebay.com/itm/132103426683

[commongrounder]

Be sure to test, or just replace, the capacitors on the +/-12volt power rails. If they are dried out, the op-amps may become unstable and oscillate, with the bop noise being the audible result. A scope could be handy to check for ultrasonic signals. As old as that chip is, it comes with 10mhz bandwidth.

[cvanc]

Be sure to test, or just replace, the capacitors on the +/-12volt power rails. If they are dried out, the op-amps may become unstable and oscillate, with the bop noise being the audible result.

Agreed.  The "motorboating" noise described (more or less) by the OP is not likely to be failed/failing opamps, it's more likely to be electrolytic caps... just change 'em all, there aren't many.

Good luck with your repair.

[stevengendron]

I will change the capacitors this weekend and see if it fix the problem. Thank you commongrounder

[David Hess]

The lag pins are for lead-lag compensation.  Typically a series RC pair is used between pins 3 and 4.  0.1uF and 4.7 ohms yields unity gain compensation as shown in the datasheet.

I would expect a NE5532 to work unless the circuit does something weird with the external compensation of the uA739.

[stevengendron]

I didn't retouched it before yesterday. Then I found that the noise disappear when it put the top back on the enclosure. I guess that the design of the pcb and the wiring of the amplifier might induce electromagnetic noise in the circuit. The enclosure may be acting like a Farraday cage.
Well, with the cover back on, it sounds good