Marshall Lead 12 5005 Combo Needs Help

[misterc57]

I will work on the schematic this week.

Regarding the squeal sound. I traced the signal. No squeal at pin 3 of IC1 (1458), squeal at pin 7. Would that indicate an issue with IC1 or is the squeal working its way back from another part of the circuit?

Thank you

[J M Fahey]

Quote Originally Posted by Enzo View Post
R25 10 ohms, is it open?

No, I am reading .7 ohms in circuit.

R26 is 10 ohms

Now according to the published schematic R26 must be 0.33 ohm, yet MrC measures 10 ohm; and R25 should be 10 ohm, yet he measures 0.7 ohm ... which is consistent with a 0.33 ohm + lead and probe resistance.

So again more proof that "amplifier is the same, part values are same , but schematic designations do not match".
We can go no further until the published schematic is corrected to use >>>printed in PCB<<< designations.
So please do.

Also show any inscription printed or etched on PCB, referring to board version, maybe "Rev xx" or a different board number.

*Maybe* Marshall switched PCB suppliers , gave new one only the schematic, and they used their own.

Or simply added an extra part, something as simple as a resistor at the input jack, and now *every* part ID is shifted up one place.

[misterc57]

I will be working on the modified schematic over the next couple days.

I took some close-ups of the board. No designation of a version that I could find, nothing on the backside either.

Is that zener (ZD1) installed backwards?

Thank you

[g1]

There are a lot of funny things with this era marshall boards, spots called out as one type of component, with another type there (like diode instead of cap etc.), or empty spots, or parts tacked on the backside.
I would not assume that diode is backwards until I traced out the circuit to prove it.

(If schematic is correct, cathode end should go to ground)

[misterc57]

So the schematic that is out there is for the 3005 head version. Slightly different in that it has a DI output jack, and two speaker jacks. The 5005 combo does not have these.

I have modified the schematic to match my 5005. I feel it is almost final, there may be some mistakes, but I think it is real close to accurate.

The only correction I am aware of at this point is that I need to go back and figure out what R6 is doing in the circuit.

Many resistors and caps designation did not match the 3005 schematic.

Here it is...

[misterc57]

I think strato56 was right... The 5002 board looks identical to the 5005 board.

Maybe your circuit is closer to the 5002 Lead 20.
https://www.drtube.nl/schematics/marshall/5x02.gif

That is a tough schematic on the eyes!

[J M Fahey]

So the schematic that is out there is for the 3005 head version. Slightly different in that it has a DI output jack, and two speaker jacks. The 5005 combo does not have these.

I have modified the schematic to match my 5005. I feel it is almost final, there may be some mistakes, but I think it is real close to accurate.

The only correction I am aware of at this point is that I need to go back and figure out what R6 is doing in the circuit.

Many resistors and caps designation did not match the 3005 schematic.

Here it is...

[ATTACH=CONFIG]53829[/ATTACH]

Sorry MrC but I asked you to make all corrections RED for a reason.

I am jumping back and forth comparing values and half the time I lose track (since we OCD people focus on *one* issue at a time) and don´t know which version I´m in, both confusing and annoying this focusing and unfocusing just to check where am I at this particular moment, which brequires looking somewhere else to check whether type is fuzzy (original scan) or sharp (just added). Talk about subtle signals

You should also visibly label the corrected schematic "MrC corrected/version"; using RED would also help so no actual need to read that, just perceive a reddish area with peripheral vision when focusing on some other area.

Thanks and all this said with the best intentions.

PS: what does "R6" actually mean? ... an empty/stuffed pair of holes on the PCB but with that label on them or a resistor on the drawing you can´t find on the PCB? Or even a missing R in the numerical sequence?
As you see, there are two very easy to confuse "worlds" , PCB and drawing, at every moment we need to state where we are ... unless we are satisfied that both match to the last bit.
Meaning drawing 100% verified matches PCB.

I know that on some versions of that series 10k R4 SV (Scanned version) is NOT used/stuffed on the PCB, I guess to chop some Bass and make distortion cleaner.

I correct myself, we have *three* worlds to consider:
* actual PCB ,we might call actual parts there Rxx P
* original scanned schematic,we might call actual parts there Rxx S
* corrected schematic,we might call actual parts there Rxx C
Hopefully P and C parts match 100%

In any case, follow where each pad leads to on the PCB , it would be R6 P , and add/edit it in the corrected schematic, it would be R6 C
You might as well have NO R6 P at all, so far we have no R6 S to begin with.

Again, confusion comes because (I presume) there was some original master schematic and different slight variations were poorly updated, not keeping parts designations and even missing some parts .

A proper schematic should be the most complete, showing everything, and just labelling some parts as "Rxx not used on combo" or whatever.
Oh well.

[misterc57]

Sorry for the confusion, I never modified a schematic before for publication. I will gladly do it again in red.

But I wonder if we should just use the 5002 Lead 20 schematic. Looking at the Lead 20 thread in here, it looks identical PCB topology. https://music-electronics-forum.com/...=marshall+5002

Edit: I am working on the modified 3005 schematic. It would be nice for all to have a go to legible schematic for the 5005. Just need to clear up a few resistor questions and should have it done tomorrow.

Thank you

[DrGonz78]

There is one thread on mylespaul website where someone matched component values and rewrote out the schematic of the original 5005 lead 12 amp. See post #57. Not sure this is any help but just thought to post it just in case.
https://www.mylespaul.com/threads/na....264136/page-3

[misterc57]

My latest attempt. This corrected schematic represents what I see on my PCB. Hopefully I got it right. If not I will try again.

All feedback welcome.

Thank you

[misterc57]

Squeal Help!

I have re-flowed all pots and input jacks.

Checked every voltage, all are either exact per schematic or very close. One was slightly off, the collector of TR6 states 0.8 on schematic and I read 0.58, not sure that would matter.

Adding some pics of my scope. Gain at about 4, all tones at 5, volume at 6 shows a nice sine wave from my 1 kHz signal. Volume at 7 shows squeal, and I notice that the signal also jumps to 6.123 kHz. I do not have much scope experience and that seems odd.

I also noticed that the squeal is present at the output of the 1458 op amp. Should I suspect the op amp?

Using my latest schematic also attached here. Thank you.

[g1]

Are you still having low and fizzy output? If so, concentrate on fixing that first. It is a lot easier to troubleshoot and may also cure the squealing.

[strato56]

Are these measurements taken at the speaker output or at pin 7 from IC1 ? ( with speaker attached ).
Do the DC-supply voltages stay stable when the amp is squealing ?

[misterc57]

Are you still having low and fizzy output? If so, concentrate on fixing that first. It is a lot easier to troubleshoot and may also cure the squealing.

Will have to check that. Been testing with a dummy load.

[misterc57]

Are these measurements taken at the speaker output or at pin 7 from IC1 ? ( with speaker attached ).
Do the DC-supply voltages stay stable when the amp is squealing ?

Scope pics are at the output with a dummy load. I will check the supply when squealing.