If it sounds good and the IC is not getting hot or something, call it a day.

Yes I would try a new one in that spot.     Having said that I'd also look at the input pins to make sure no DC is sitting there.   Pull the old IC and check the pins 5 and 6 for DC when the IC is out.

TL072 is about as common as it gets, dual op amp chip.

So it basically functions but sounds real crappy.

I am guessing the op amps run on plus and minus 15v or possibly 12v.  Pina 4 and 8 of the ICs are the power pins, -15 on 4 and +15 on 8.   Are both present and more or less same voltage?   Pins 1 and 7 are output pins. They OUGHT to sit around zero volts.  If they sit at +15 or even just +2 or something, that is unwanted DC offset.  Offset just means a DC voltage you don't want there.

Isn't there a label on the supply module telling its outputs?  And if so, if you cannot measure said voltages directly on the adaptor plug, then plug it into the unit and measure on the power jack pins inside.

I found a video of a guy making changes in one of these, so I got a good look, it is a basic amp with a five-leg power amp IC and a couple op amp ICs.

Well at least you didn't just measure capacitance with your meter.

Problem is with all this low voltage testing is that it doesn't put real world voltages on the cap.  I often put caps on a leakage tester, and they look fine at 20-30-40 volts, but up at 450v where they normally live, they leak like a screen door.  Of course this solid state amp doesn't use high voltage, so that may be moot.

You mention grounding, not sure why.  Grounding usually causes hum problems, same with weak filter caps.

In a small low power solid state amp, hiss to me usually means a semiconductor.   I don't know what is inside these, but likely op amps?  If so, one may be noisy.  I'd look for any DC offset on any IC output pins.

I don't follow.  Where are the PMOS you refer to?  I see a couple CMOS chips, but they are not using 18v as far as  can see.   And I don't get how if the supply voltages are low, how can the control voltages wind up too high?


But the real question is this:  what are you trying to fix?  What is the amp doing wrong?

Glad you got it.   Good you had a 1458.

Just a note.   Most times a 4558 will work fine in place of a 1458.   But in the Marshall preamps of that era, it won't.  a 4558 might function, but becomes unstable and you get squeals and can't turn it up past whatever without howls.   Oh not every time, but I always stocked old 1458s in my shop just to have them for Marshall repairs.

And yes, I saved good pulls from working equipment

Hi Jimi
You will find in most amp forums it is confusing to mix repairs in threads.  Someone coming upon this one might skim along and look at your post and assume the readings and findings in the earlier posts were from YOUR amp.

20ga would be fairly light, though plenty for a guitar amp.  18ga would be the very common size, and 16ga would be relatively heavy.  I have no idea what size they are in metric area.   Often, if not usually, the wire gauge is embossed in the vinyl jacket of the cable.

Well, we don't usually let Canadians join here, but in your case, we'll make an exception.

It is just a guitar amp.  Pretty much any IEC power cord will work.  Like the ones for your computer or other modern electronics.

A 15A cord can handle 15 amps at 120v , that is 1800 watts.    WAAAAYYY more than enough for a 375 watt amplifier

That solid state amp does not require a load, you can simply turn off the speaker with a switch.

What I would do is connect the line out 10k resistor to the phones jack tip connection instead of the speaker.  Now the line out works when you are on speaker or when on phones.  Now the phones still work, but you can use the phones jack to kill the speaker.   Just plug an unwired plug into the phones jack, and that opens the speaker connection.But a switch works fine too.

LIne out is a signal that is suitable for a mixer or another amp or some line level effect unit.  Headphones out is...tada.... for headphones.

Consider having it reconed.