Marshall Lead 12 5005 Combo Needs Help

[misterc57]

Looking for help with my just acquired 5005 combo. Sounds anemic; low and fizzy output.

This amp was built in 1984. Not sure the schematic attached is the right one, dated 1986.

I have + and - 18.2 VDC on the rails.

IC1 pin4 is -14.6 VDC, pin 8 is +14.8 VDC. Other IC1 pins have nominal readings in the low mVDC area.

Also getting a lot of intermittent squealing sounds.

I have checked all solder connections on the bottom which look good. Cleaned all the pots and jacks.

What do you think? Where to start?

Thank you. MarkO

[Enzo]

Put a scope on it, is the output making RF?

[J M Fahey]

^^^^^^^ THAT , and also check DC voltages; schematic shows a few, what do you find?
Besides rail voltages, that is.

Also disconnect internal speaker and connect an external speaker, 4 or 8 ohm ... what happens?

[misterc57]

Behavior is now very low output, can barely hear the guitar. Tried different speaker.

If I turn the volume or gain above 7 you hear a high pitch squeal coming out the speaker, if a guitar is plugged into either jack.

No guitar plugged in, no sound/squeal when turning up vol or gain knob.

At idle I see small noise on the scope. No more than 300 mV pk-pk. Not sure if that is normal.

It seems to me that a lot of voltages are backwards on my amp vs the schematic.

Examples schematic says C17 negative side s/b -19 VDC, I read nothing on the neg side but have +18 on C17 positive side.

C18 s/b +19 on pos side, I read nothing, but have -18.4 on C18 neg side.

+ side of C18 and - side of C17 is at ground, opposite the schematic.

Also I see no voltage at either side of C14 and R21 and R22

[strato56]

I count 5 transistors in your picture as opposed to 6 in the schematic.
Maybe your circuit is closer to the 5002 Lead 20.
https://www.drtube.nl/schematics/marshall/5x02.gif

In that case, R21 and R22 are the voltage divider for Phone/line-out and should not have any voltage across them.

[misterc57]

The 6th transistor is at the corner edge of TR5.

[strato56]

Indeed it is
But I guess the number of the components on the pcb don't match those of the schematic.
What voltage do you measure across the zener ZD1 ?

[misterc57]

ZD1 voltage across is 8.9 VDC

[misterc57]

Scope

I am running a 1K signal into the low input and watching the scope. Gain is at 4.

I have a nice looking sine wave to about 6 on the vol pot, with 12V pk-pk. Turn the vol pot up a hair and it jumps to a nasty noisey wave at 50V pk-pk.

I am suspicious of the health of my gain and vol pots.

[Enzo]

R25 10 ohms, is it open?

You turn it up and get a nasty signal going all the way to the power rails, ie 18v. Sure sounds like power RF oscillation to me. And when the poor amp uses all its energy to make RF (which you cannot hear) it leaves very little energy to amplify audio.

[misterc57]

R25 10 ohms, is it open?

No, I am reading .7 ohms in circuit.

R26 is 10 ohms

I have checked all 27 resistors, none are open.

I still think I may not have the correct schematic.

Below pic are all my in-circuit resistor readings.

[misterc57]

I notice on that 1M volume pot. Measure resistance from center to outer leg. Resistance rises to about 350K from "0" to the "7" mark and then reistance drops as you keep going up to the "10" mark, all the way to under 1 ohm.

That would make the pot defective but would it cause the squealing noise?

[Enzo]

Doesn't make the pot defective. The pot is in a circuit, and the tone controls are parallel with it. Max the tone controls and measure that 1 meg pot. Now turn all tones to minimum and measure. See?

[misterc57]

Doesn't make the pot defective. The pot is in a circuit, and the tone controls are parallel with it. Max the tone controls and measure that 1 meg pot. Now turn all tones to minimum and measure. See?

Still seeing the same behavior on the volume pot with tones controls at max and at minimum.

[J M Fahey]

It seems to me that a lot of voltages are backwards on my amp vs the schematic.

Examples schematic says C17 negative side s/b -19 VDC, I read nothing on the neg side but have +18 on C17 positive side.

C18 s/b +19 on pos side, I read nothing, but have -18.4 on C18 neg side.

+ side of C18 and - side of C17 is at ground, opposite the schematic.

Also I see no voltage at either side of C14 and R21 and R22

This tells me you have a different schematic version from the same amplifier.
R1 - R2 - vC1 - C2 - etc. are NOT fixed in stone designations, just arbitrary numbers assiogned as designer counts them, he can call any part the number he wishes, as long as it does not repeat elsewhere.

So I suggest you print a large copy of the schematic, even double sie on 2 sheets which you later tape together, and with some kind of fine point marker, even a white typewriter correction pen, you check *all* components on schematic, find them on PCB, and you write the schematic designation by it.
Say you cross C17 and write C18 and viceversa.
Or you can glue tiny paper labels over old designations, with the new ones.

Only *after* that we start rechecking.

Plan B: edit the schematic labels to actual PCB designations and upload.
Here´s the PDF converted to .GIF which you can edit on any graphics package.

You may write new designation in red by the old one which is in black.