Marshall Lead 12 5005 Combo Needs Help

[misterc57]

Here is the latest;

No ouput now to be heard at speaker, until volume is past 7 and then I hear the squeal. Yet I still see a signal on the scope.

When it squeals the rails drop by 4 volts on both side, to 14.

Listening to the signal in the circuit (below squeal levels), it is louder at IC1 pin 7 then it is a the base of TR6, and then even lower in volume at R26

[Enzo]

By the way, is IC1 an actual 1458 IC? Or has it been replaced by a 4558 or later? A number of Marshall amps of that era were sensitive to IC type, and went unstable with newer ICs like 4558. I stock 1458 specifically for old Marshall amps.

[DrGonz78]

By the way, is IC1 an actual 1458 IC? Or has it been replaced by a 4558 or later? A number of Marshall amps of that era were sensitive to IC type, and went unstable with newer ICs like 4558. I stock 1458 specifically for old Marshall amps.

I believe it is the 1458 by going by the pictures he uploaded. I had that same thought about the 1458 being swapped with a 4558.

[Enzo]

You are correct, I forgot we had the good photos.

[DrGonz78]

I have to wonder if this squealing has anything to do with the input jacks. Have we cleaned them and ohmed out the contacts to make sure they are closed? When we are inputting the signal is it into the high or low input jack? There is another thread on MEF that mentions the input jacks. Squealing makes me think of a missing or weak ground connection causing oscillations on the opamp. I remember building a tube screamer pedal a while back and it was squealing like a banshee. Quick troubleshooting and I realized right away my error with missing a ground connection right near the first transistor.

https://music-electronics-forum.com/...read.php?t=955

Same repair but different posts from members on ssguitar.com.

https://www.ssguitar.com/index.php?topic=339.0

[misterc57]

working now

It is a 1458.

So, I am scratching my head right now. My squeal is gone. Amp sounds right with good volume to an external speaker. I still need to re-try the original speaker.

Turns out my guitar cable was causing the squeal. Intermittent connection on one end. However, this does not explain whey I was getting a squeal using my signal generator which uses a completely different cable (not the bad guitar cable), whenever the volume was 7 or higher.

Hopefully it stays working well and many thanks to everyone for the help and suggestions.

MarkO

[g1]

Check for bad solder or cracked solder pads at input jack. Can give the same symptom as a bad cable.