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#1
Hey guys, I recently got a 1986 Marshall Mosfet 5213 SS amp for free




First thing's first, safety! It was last plugged in and on about 20 hours ago. I'm assuming I need to discharge capacitors before working on this thing? What's the best way to go about this?

Anyway here's the problem. Channel A is a split channel, dirty/boost, and a footswitch activates the boost. The volume on this channel is practically non-existant. It's there if I turn everything up full, but not even loud enough to play along with some music. I haven't been able to try out the boost yet as I don't have a footswitch, but I'll let you know once I do.

There's no obvious soldering problems that I can see, but I haven't checked the underside of the boards, as I'm assuming it's not safe to. Here's a couple pictures of the front panel and the inside, if you need close ups of anything, let me know and I'll get some.




#2
The biggest problem I can see is that this amp only goes up to 10. If it went up to 11, you wouldn't have any problems.

Other than that, have you checked the transformers? I had a similar problem with my old Randall and a new transformer did the trick.
#3
I haven't checked anything yet, worried about electrocuting myself... again

I'll keep that in mind though
#6
Hey bud, first things first, take a multi meter and measure (dc) voltage on the two big blue dubilier caps, if they read below 10V you're safe to mess

Next step is to let me finish trying to debug this spec op and get on a computer with a screen I can see the pictures on
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#7
just a few clarifications:

you're experiencing no issues on channel b?
the issue doesn't go away with the reverb turned off?
do any of the caps look dodgy, old, or feel like they have air bubbles?

EDIT: these are mainly just so i can find them easily but you may find use of them

Channel A Schematic: http://drtube.com/schematics/marshall/5213a.gif
Channel B Schematic: http://drtube.com/schematics/marshall/5213b.gif
PSU and Power Amp: http://drtube.com/schematics/marshall/5213psu.gif
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Last edited by GABarrie at Nov 28, 2012,
#8
Okay we're safe then

1. Channel B is perfect, the only problem was a crackly EQ pot, but that's sorted now.
2. Reverb didn't seem to make a difference at all, I can't turn it 'off' so to speak, but I've had it turned right down.
3. The only thing I can see with the caps is the green coating on one is cracking and flaking off a little.

Haha, thanks for the schematics, I can spend hours struggling to decipher them
#10
there are no green caps... do you mean the chunky green thing on the PSU/Power amp board that is labeled R9?
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#12
Quote by GABarrie
there are no green caps... do you mean the chunky green thing on the PSU/Power amp board that is labeled R9?

Oh yeah, woops!
#13
the side of those potentially clipped caps closest to the pots, where they sit on a trace with two diodes? can you check if that trace is ground? Continuity checker between there and say the ground bit on the input jack?
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#15
hrmmm...

right, next you're gonna have to work on this whilst it's live... be careful but you know the drill, make sure the speaker is attached too or you'll fry the mosfets. I will assume you know which pins are which on an 8 pin chip so I won't explain that either.

You need to measure the voltages of pins 3, 4, 6, 8 on IC1, IC2 and IC3. Do it in reference to ground so make sure your black probe is on a ground connection, then just note them down including if they are 0 or negative (the negatives are important)
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Last edited by GABarrie at Nov 28, 2012,
#16
Actually I'm not sure I'm afraid, former or latter?



Edit: Nevermind, I've got it.

8 7 6 5
1 2 3 4
Last edited by whoomit at Nov 28, 2012,
#17
neither.... on the chip there is a circle on top, thats pin one...


1 o    8
2      7
3      6
4      5
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#18
         IC 1    IC 2    IC 3
Pin 3   -0.07   -0.06    0
Pin 4  -15.26  -15.41  -15.48
Pin 6   -0.07    0       0
Pin 8   15.22   15.44   15.44
#19
well balls! was hoping that would be wrong... it wasn't...

ummm... could be one of the opamps failing.... you got any tl072s lying around?
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#20
You mean you were hoping my values would be wrong, but they were correct?

Or you were hoping your dignosis would be wrong, but it was correct, meaning the values were incorrect?

No, I don't, no ICs anywhere I'm afraid. All I've got lying around are caps and resistors.
#21
Quote by whoomit
You mean you were hoping my values would be wrong, but they were correct?

Or you were hoping your dignosis would be wrong, but it was correct, meaning the values were incorrect?

No, I don't, no ICs anywhere I'm afraid. All I've got lying around are caps and resistors.

the bold one, means that wasn't the issue and we can't fix it yet
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#22
Quote by GABarrie
the bold one, means that wasn't the issue and we can't fix it yet

Balls.
#23
I'm gonna go ahead and guess you don't have a signal generator lying round?
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#24
Quote by GABarrie
I'm gonna go ahead and guess you don't have a signal generator lying round?

Ehm, I have a stupid 'noise generator' kit or something from Maplin. No 1/4" out or anything though.
#25
PM your address, ill send you a tl072 and we can pick this up on friday when it arrives
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#26
Quote by GABarrie
PM your address, ill send you a tl072 and we can pick this up on friday when it arrives

Okay, but you're giving me your address too. I'm send you some chocolate or something
#28
I did not make it to the post office so it'll be Saturday!
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#29
Hey whoomit.

One easy thing to check - see if there is any bad connection at the effects send/receive jack sockets.

Usually, if the amp is giving you sound but very low level, then something in the signal path is dodgy.
Sometimes this is because of a component - could be an electrolytic coupling capacitor, for example, or sometimes something with contacts, like a plug or switch.

You can measure with your meter, or just short them with a piece of wire or screwdriver tip and see if the the sound comes back on. Also, check all the jack sockets for hairline solder cracks. They do happen and are sometimes hard to spot.

Usually, I would use a tone generater and a signal tracer. The 'ghetto' version is to take an old radio, cut the signal from the receiver to the audio amp and use the receiver as your generator (white noise or radio station), and the audio amp becomes your tracer.

Anyways, if you don't want to bother with that, you can run the amp and touch a screwdriver to several points in the circuit to inject noise.

I would start at the last amplification stage and work back to the input.

Put the volume about half - if you hit a stage that is working, you may get startled by loud noises Increase it if you hear something bit not very loud.

Basically, you only have to touch a screwdriver onto the low level stages to inject lots of mains noise and hum, just like you hear on those cheap, unshielded guitars (expensive ones too..).
You need to be touching the screwdriver shaft (metal part) to make it work, otherwise you might just get a few little clicks on contact.
Don't worry - these stage are only running on +-15V, so they won't hurt you. Just be sure to stay away from any mains level voltage, which should be out of your way anyway.

Go:
Pin6 on IC3
Pin2 on IC3
Pin3 on IC2
Pin3 on IC1

You should get noise on those first 2, as Channel B takes the same path, and we know that's working.

Hrm, I'm really suspecting those jack sockets/solder joints.
I usually go ahead and solder that stuff as a matter of course when I service anything, as the manufacturing process is not always perfect and wear and tear is pretty common.

If you're at all worried about shocks, don't worry, but be sensible anyway -pay attention.
I worked with a guy who taught me to put my left hand in my pocket when working on live gear.
It's supposed to prevent any voltage passing across your chest (IF you happen to touch high voltage, it will take the shortest path through the right side of your body. Your heart is on your left)

So far so good
Last edited by sethasaurus at Nov 29, 2012,
#30
On further inspection of the schematic, the footswitch circuitry could possibly be at fault. This part mutes the normal/boost/reverb sections. It could be that part of it is muting your signal, due to a faulty transistor, for example.

Definitely check and resolder all those jack sockets anyway and see if you can inject any noise into the circuit.
This will at least narrow down the fault.

#31
Quote by GABarrie
I did not make it to the post office so it'll be Saturday!

Thanks man

sethasaurus, that's a lot for me to take in I'll try to get to it either this afternoon or tomorrow though, thanks!
#32
Right, I'm going to go over all the jacks in a second.

My footswitch arrived today, and when plugged in, Boost is constantly on, and Reverb is constantly off. Could this have something to do with it?

Edit: Just checked over all the soldering, re-done anything that looked remotely dodgy. My back is absolutely killing now It hasn't worked.

If I touch the Channel B treble pot I get a loud pop, bass, middle and volume give a hum and crackle sound.

Pin 3 on ICs 1 & 2 don't make the same sound as 2 & 6 on IC3. They barely make a noise when I touch them.
Last edited by whoomit at Dec 1, 2012,
#33
OK, that's good progress! You haven't wasted your time anyway. At least you'll know everything is solid once you put it back together.

Looks like the problem is in the normal/boost/reverb switching.

Can you measure the following DC voltages? (Without the footswitch plugged in):
TR1 Base
TR1 Collector
TR2 Base
TR2 Collector
TR3 Base
TR3 Collector
TR4 Base
TR4 Collector
TR5 Base
TR5 Collector
TR6 Base
TR6 Collector

Transistor (BC184) pins are:
(Looking at front face)
___
|__|
| | |
| | |
CBE

That should show up any faults.

Seth
Last edited by sethasaurus at Dec 1, 2012,
#35
Just put your negative meter lead to ground and positive lead to each point for measurements (base and collector).
#36
Tr1
B - 0.61
C - 0

Tr2
B - 0.63
C - 0

Tr3
B - 0.63
C - 0

Tr4
B - 0.1
C - 0

Tr5
B - 0.73
C - 0

Tr6
B - 0
C - 0
#37
OK, some weird readings on Tr4 and 6.
It's a little confusing as Tr5 base is connected to Tr6 collector, so they should read the same voltage.

It looks pretty likely that one of these transistors is faulty. Since we've narrowed it down, easiest thing is to take them out and test them. My money would be on Tr5 or 6.

This page gives some info on testing a transistor:
http://www.kpsec.freeuk.com/components/tran.htm

If you find a dud, BC184s are cheap to replace.

Does your footswitch have a mono or stereo plug?
#38
Well once I'm back home tomorrow night I'll retest them to make sure I got the values right. Then I'll check the transistors themselves.

I couldn't tell you about my footswitch. I assume it's stereo. There's no jack input, it goes straight to a lead. I'm assuming it's stereo though as there's three wires into the cable.
#39
If the plug's got two "bands" on it, it's stereo. If it's a single band like a guitar cable, it's mono.
--

How do you say "I'm okay" to an answering machine?

--
#40
Quote by stratman_13
If the plug's got two "bands" on it, it's stereo. If it's a single band like a guitar cable, it's mono.

Oh yeah, it's stereo then.
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