Marshall 5010

[Valvehead]

i have a 88' 5010 combo here. its solid state .
the amp powers on but only produces a loud hiss--kind of like real loud "gain noise" the guitar is barely audible.
The pots all work fine
the headphone and line out jacks do the same thing--just hiss.

I dont know what kind of voltages im looking for but here are some :

at IC1 , (mc1458)
pin1 1mv
pin2 1mv
pin3 1mv
pin4 -9.9v
pin5 2v
pin6 3v
pin 7 3v
pin 8 10v

there are 2 output transistors
at mj2501 i get 1.2v and 63mv
at mj3001 i get 0.5v and 63mv

does that look normal ? If so what else could it be?

i will attach the scem

[olddawg]

Have you plugged a line level source into a main amp in jack or effects return. Have you run the pre-out or effects send into another power amp. Isolate the problem. Also run a cable from the effects send to the effects return and from the main out to the main in.

[Valvehead]

there is no loop

[52 Bill]

Do the controls change the level or tone of the hiss? If the master shuts off the hiss, then it is being generated in the preamp. If the master does not shut off the hiss then it is coming from the power amp.

The voltages on pins 5, 6, 7 could be a problem.

[Valvehead]

yes-the Master shuts off the hiss

[Valvehead]

I was able to tap in at the output of the Master . Into another amp , it acted the same , so the problem is in the preamp section.

At the union of D1 and D2 i have 2.2v
other end of D1 is 10v
other end of D2 is -10v

the electrolytics test ok , but my meter wont read regular caps


at TR1 transistor,
E -1.2v
B -0.6v
C 10.2v
i had one of these so i swapped it , so TR1 is ok.

Do you think its most likely the mc 1458 chip thats bad?

[Valvehead]

found it ..... it was the op amp. I stuck a 4558 in there and the amp works great . The 4558 should be ok in this circuit -yes ?

[Enzo]

If it works, it works. In most places the 4558 is OK for a 1458.




I stock some old 1458s just for these old Marshalls, though. As it was told to me long ago, the open loop gain of the 4558 is greater than the 1458, or something, so the "better" 4558 has more potential for instability. I found that out the hard way once. But if your amp is not sitting there screeching at you, then I doubt there is an issue.