I have recently acquired some music stuff and most of it needing repairs. I chose this 1986 Marshall 100 Lead Mosfet to start with as it seemed to be the most complete. I will start by saying I have very limited experience in the troubleshooting field. At first the amp showed no signs of life. The chassis was removed for troubleshooting. Upon visual inspection there is no reverb tank, the light bulb in the power button was burnt (Fixed), and R46, R47 showed they were burnt and the trace eyelets under the PCB burnt having no contact I was able to repair the connections from R46 & R47 to a good part of the trace and replace those R's.

Now: The amp powers on R46 & R47 get really hot. TR4 & TR5 continue to fail, and the amp sounds like it has a blown speaker.

P.S. I have replaced all the Ecaps. C44 looked to be in really bad shape and decided to replace all. tested all the old caps and they tested fine go figure .

I'm stuck here and need help with troubleshooting.

Here are some readings
R46= 50v/37v
R47= -50v/-36v
R44= -36v/-15v
R45= 37v/16v
TR4= c-36v, b-.942v, e-317mv
TR5= c37v, b390mv, e-238mv

So, it appears that the reverb drive circuit is toast.
Seeing as the amp 'sounds like it has a blown speaker', I would rectify that before the reverb.
For now, I would disable the reverb circuit.
Remove TR 4 & Tr 5.
Lift one end of R30 & R29.
FYI: your voltage chart has the emitter & collector voltages inverted.

Corrected my Chart to read correctly, Thank you!

Removed TR4,5
Lifted R29,30

Amp seems to be ok at low volume, still a bit early to crank it up...lol.

R46,47 still hot to touch.

Just noticed that Bass, Middle, Treble have very little effect, not sure if it matters at this time.

Also from a previous recommendation I have a 100ohm resistor jumping the reverb send posts, I guess to simulate a reverb tank being there?

What do you mean when you say TR4 and TR5 continue to fail ? Do they measure correctly on a DMM Ohm test ?

R46 and R47 will run hot. It is best to mount them up off the PCB for better heat dissipation.

Reconnect R29 and R30. With TR4 and TR5 removed, check that there is roughly 1.2V to 1.4V across R74. This verifies that D8 and D9 are doing their job to bias TR4 and TR5.

Check R32 and R33.

Remove that resistor across the tank's input. With a good TR4 and TR5, Pin 1 of IC4 should measure close to 0V.

I copied and pasted this here, This is a direct quote from some else that's been trying to help me with this amp. But I will do what you suggested and post the results on a different reply.

QUOTE:
The problem is - you can't operate the amp without the reverb tank. Without the tank, the op amp circuit has a gain of about 100 - due to the combination of R34 (10k) and the 110 ohm path to ground. So pin1 goes to 100 times 23mV = 2.3 volts. Which turns TR4 on...

The tank input has a DC resistance of maybe 50 ohms, and it bypasses R34 -- it's in the feedback loop of the op amp and it reduces the gain from 100 to just a few.

If you connect a resistor (100 ohms would work) where the tank should be, I predict pin1 will go to about zero volts, and nothing will overheat.

Poor design, where failure of the tank or its wiring blows the transistors...

Quote from: Loudthud on May 31, 2025, 12:12:21 PMWhat do you mean when you say TR4 and TR5 continue to fail ? Do they measure correctly on a DMM Ohm test ?

R46 and R47 will run hot. It is best to mount them up off the PCB for better heat dissipation.

Reconnect R29 and R30. With TR4 and TR5 removed, check that there is roughly 1.2V to 1.4V across R74. This verifies that D8 and D9 are doing their job to bias TR4 and TR5.

Check R32 and R33.

Remove that resistor across the tank's input. With a good TR4 and TR5, Pin 1 of IC4 should measure close to 0V.

Sorry, I do not see a R74 printed on the board, I see it on the schematic between TR4, TR5. I have traced back where D8 & R30 and D9 & R29 meet and I see no R74

Measure resistance from base of TR5 to base of TR6.(edit, meant TR5 to TR4) 
Should be ballpark 330 ohms if R74 is there.  It was a revision added in 1986, if you don't have it you should add it in.  Measure before adding it in.

The quote about the opamp feedback loop seems suspect to me.  It's not like pin1 goes straight to R34.  The same guy in the other thread on the first page said it was no problem running it without the tank and I agree.

I think you may have an oscillation problem with the reverb opamp.  Marshalls can be picky where they specify MC1458 rather than generic 4558.  The different spec part could be responsible for oscillation which would cook the reverb transistors and that would cook the supply resistors.
You bought 60 supposed 1458 opamps because they were absurdly cheap at around 10 cents each.  That means they are likely fakes and probably came from China.  They may be real 4558 but that may not work here.
Buy some real MC1458 which are still available at places like digikey and mouser, which are legitimate industrial parts suppliers (unlike ebay or amazon which are full of fakes).
And if you did not get the MPSA06 and MPSA56 from a legit supplier, get some real ones and replace them as well.

I bought 60pcs of 270ohm resistors to replace R46, R47.

I have 10pcs 1458"s, 3 texas instruments, 4 motorola, and the others just say MC1458. These were in a box I bought in a yard sale, box contained lots of very old components, resistors, caps,..etc. I believe these 1458's are legit.

I have added a picture of someone else's board and see a resistor added between R29,R30' Could this be the missing R74 that is not on my board? Was this added at the factory but not printed on the board?

I have 2 pairs of 06,56 left but I will reorder more of the 06's, 56's from Mouser.

Sorry, I misread as to which part you bought 60 of.  Sounds like the opamps you have are legit.
That is where the resistor could go, but there are other spots on the flipside of the board, so measure resistance with your meter to verify it has not been installed.
They would have been 'tacked-on' in production until the new version of the board was available.  Units already in the field would have it added during warranty or other service, as there was probably a service bulletin calling for it.  Any serial # prior to U26000 may not have it yet.
Here is the issue 2 schematic where it is mentioned.

So you want measurements between TR5, TR6? and not TR5, TR4 where that resistor is missing.

So I would be testing resistance between the base pins of TR5,TR6?

Currently TR5 is not on the board, does it need to be installed or can I test on the trace pad?

TR5(Not installed) base pin to TR6 base pin is reading 60k

No, my mistake.  I meant from TR4 to TR5 as you suspected.
You can measure at the pad if transistor is removed.  Also you can measure at R29 and R30 where it has been installed in that other photo.

so the reading I'm getting is not steady. Starts around 20k and climbs, I wait to see if it stabilizes and at 27k it slowly continues to climb.

Note:
This also has been brought to attention. along with not having R74 present. The schematics show 2-R73's. One off IC5b pin 6. And the other at IC2 pin 2-12 jumping D5. There is no R73 jumping D5.

The one across D5 you should add in also.  You can see on the issue2 schematic it refers to the same note as R74, so it would have been added at the same time.

Will get working on those and report back.

Quote from: g1 on June 04, 2025, 11:00:51 AMNo, my mistake.  I meant from TR4 to TR5 as you suspected.
You can measure at the pad if transistor is removed.  Also you can measure at R29 and R30 where it has been installed in that other photo.

I am getting 1.3v between TR4, TR5 base pins with no R74 or R73 resistors installed