Mr Songwriter

Hello, this is my first post on the forum but hopefully these questions aren't too ridiculous. I've studied the schematics at:

Marshall Amps Info & Schematics

..quite a bit but I think I'd find it a lot easier to follow the signal flow if I had some sort of a block diagram to go on, does anyone know if there's one available anywhere?

There's also one particular section of the main board (JCM2-60-00) on the DSL50 that's been puzzling me (sorry about the size of the image):



A couple of questions:


What are the transistors for? - in both the blue and the green section?

What is the Optocoupler for? (top right of the pic - if that is what it is)

Also, I know that V4 is the Phase Splitter, but which channels are V1, V2 and V3 used by?

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Pete240885

Message me with direct links to the full schematic documents(PDF) and i'll get back to you, i know this is lazy but i'm in the uk, and its nearly midnight...stuff to do tomorow sleep to be done now

Mr Songwriter

Thanks, here are the links:

Main board (The bit I'm interested is on here)

http://www.drtube.com/schematics/mar...60-02-iss7.pdf

Channel switching board:

http://www.drtube.com/schematics/mar...61-02-iss1.pdf

PSU and Reverb circuit diagram:

http://www.drtube.com/schematics/mar...63-02-iss3.pdf

Bias board circuit diagram (even I can understand this one)

http://www.drtube.com/schematics/mar...jcm2-64-02.pdf

Pete240885

Had a look at DL50-60-02 and apparently

CON 10 connects to CON 6 on schematic JCM2-62-00
CON 6 connects to CON 3 on schematic JCM2-61-00

I don't have either of these schematics and they don't appear to be on Dr tube either, without these i can't tell you what that piece of circuitry is for

Mr Songwriter

I opened up my amp to trace the connections by hand, and here's what I've found (the serial numbers mentioned are the ones actually printed on the circuit boards):

JCM2-60-00 is the main board (it appears to be the one depicted in the diagram marked 'DL50-60-02.DGM')

JCM2-62-00 is the board attached to the rear panel, and the connection you mentioned (Con 10 to Con 6) terminates right next to the jack sockets that switch 'Reverb' and 'Drive' on and off - JS8 and JS9, and I can't find either of those two on any of the schematics, so it looks as though this isn't a complete collection of schematics (though FWIW, Dr Tube does have a schematic for the rear panel of the DSL100)

JCM2-61-00 is the board that's attached to the front panel, and the other connection you mentioned (Con 6 to Con 3) connects to that board right next to the gain and volume controls for Channel A - VR1 and VR2, both of which appear on the DL50-61-00 schematic (the channel switching diagram)...there is a Con 3 connected to that part of the circuit, but according to the schematic, that particular Con 3 connects to Con 15??!?...but I cannot find any connectors marked as 'Con 15' anywhere in the amp, so I'm guessing that they actually mean that it's connected to Con 6 on the main board, i.e. Con 6 on the main board is possibly connected to the Mode switching circuitry for Channel A.

Hope all of that is of some use to you, and thanks again for taking the trouble to help.

Pete240885

Hmm its possible deliberate errors have been inserted to try and stop people copying thier amps, could you trace the cables inside your amp instead of guessing?

Mr Songwriter

I've done quite a bit of electronics before, but this is my first attempt at following a schematic as complicated as this, though I'd be surprised if the errors were deliberate, I thought the whole point of making these schematics available was to help the techs when they're servicing them? Both my other amps came with complete schematics, but the DSL didn't and I don't know where Dr Tube gets his from (do they only send them out to the authoriised techs?)

Anyway, the cables themselves definitely go to the parts of the circuit board that I mentioned, but I would have to take all the boards out again in order to see exactly where the traces are going...and then attempt to draw out schematics for the missing bits ...I'll have to get back to you on that, though the main reason I was asking this question was that I wanted to know if there were any transistors or IC's in the signal path of this particular amp...and it's starting to look as though the transistors above are part of switching circuits for the Reverb, the drive and the channel modes.

PaoloJM

At a guess I'd say these circuits are the driver circuits for the reverb, possibly FX Loop send and return (if one is present) and also for the channel switching.
The channel switching circuitry, with proper deisgn, will simply have the signal passing through the transistor/opamp acting as a switch (in series or shunt or even better, both) and will not be involved in amplifying or processing the signal.
The reverb circuits often use transistors/opamp as current buffers or low high input low output impedance drivers for the reverb. Again, though the do provide gain, these circuits are designed to be as transparent as poosible and so, by design, should not significantly effect the tone. The same theory applies to FX Loop send and return circuitry.
The opto-coupler you refer to is a Light Dependant Resistor (LDR). These were often used by Fender for tremolo effects but may also be used as switching elements. When current flows in the LED it, obviously, emits light which in turn changes the value of the LDR.
Unfortunatley I don't know the DSL circuit that well at all so this is just generic info from my experience. Hopefully it's of some use to you though.

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Mr Songwriter

Thanks a lot, that's very useful info - the amp has developed a problem recently and it happens on both channels (both modes) with or without the Reverb and without anything in the FX loop, so I can probably eliminate those parts of the circuit from my inquiries, I think it might be in the output stage, but it probably isn't the power tubes (I've had three different sets in there)

Pete240885

What is this problem?

Pete240885

schematics are released for servicing purposess, and generally only to qualified techs as a rule, some schematics are released with deliberate errors - i cannot remember where i learned this could have been on this forum or one of my schematic websites

If you intend to sevice it its likely you'll have to do this anyway

Mr Songwriter

Blimey, as if it wasn't hard enough already! makes you wonder how the techs know what the deliberate mistakes are.

Pete240885

My guess is by doing what we've just done?

Mr Songwriter

Could be...I'm determined to get this amp sounding good again, but I'm just trying to plan out how to go about troubleshooting it - I'm going to try using an 12AT7 in the V4 position to see if that improves the sound a bit, and then failing that I think I'll try testing for voltages and comparing them to the ones on the schematic.

Pete240885

What exactly is wrong with it? - you haven't told me yet

Mr Songwriter

All the channels/modes, etc work, it just does not sound as good as it did when I bought it (new, about a year ago) there's a kind of cold, harsh, muddy, buzzy quality to the sound that it definitely didn't have then, tbh it's pretty similar to the sound you get with a dying guitar cable .

...this is in spite of:

Numerous different preamp tubes (at least a dozen)
Three different sets of power tubes (including the official Marshall ones)
Being sent back to Marshall and tested for a week (fair play to them for that)
A local tech spending about an hour testing it
A new Mercury Magnetics OT.
Endless mucking about with the bias pots.
Etc, etc.

...I have basically spent £200+ trying to sort this amp out.

Anyway, I've taken the chassis out again and I'm going to try some live testing on it tomorrow, any pointers about what to check first would be most welcome.

Pete240885

Ok seen as you have electronics experience and the confidence to da a live test i'm assuming you know the risks of working with this type of equipment even when its not plugged in? - i dont mean to be patronising just playing safe

Would be nice to know what equipment you have, a simple multimeter? or something more?

In my opinion its good practice to check the power rails of any piece of equipment when you first start work, i myself have worked on an amp that had been to a well respected tech for repair and not been fixed. Knowing this i assumed that the enginner who looked at it would have been brilliant and this would have been ruled out... I wasted hours (10 of them) ..The fault was a faulty filter cap! so even if the problem doesnt seem to be supply related its a good place to start.

Hmm those cable like noises point to the input jack, have you ruled out the guitar and cable you were using.

other JMC 2000s have some issues namely the TSL100/122 with biasing and incorrect plate resistors I read a long article on the above named amps and it could be worth checking those resistors and the bias current (once the tubes are thouroughly warmed).


The article i read can be found here: The Marshall TSL122-JCM2000 Repair/Mods Page

Ok couple of questions, is this problem constant, does it appear only after the amp has been switched on for a while?

Mr Songwriter

Indeed I do, I know there are lethal voltages present in these amps ~500 V or so, if I was to slip and touch one of the rails while I was trying to measure a voltage, it could be game over, that's why I'll be wearing a pair of these: (they're rated for working with 1000 V)



Yep, all I've got is a Multimeter at the moment, It looks like the good oscilloscopes start at around £300, so I don't know if I'd need it enough to shell out that kind of money, though I did think about buying a capacitance checker (there's a £45 one here that goes up to 20mF...)

Yes, that will probably be the first thing I check, in fact I clipped my multimeter's ground to the chassis and checked the voltages on the secondary of the PT and at various other places (after the amp had been unplugged for a few hours) and I was surprised at how low they were (the highest voltage present appeared to be ~20 mV or so )

I've used at least 7 different guitars with this amp and probably a dozen different cables, so I'm pretty sure I can rule anything like that out.

Brilliant, thanks, I'll definitely look into that.

It's pretty much constant I think, though I vaguely remember the sound changing slightly when I ran it for a while, can't remember how though.

Pete240885

You can get oscilloscopes seccond hand for around £100, you probably don't need dual channel for audio testing purposes, 20MHz is the standard, though you could get away with a 10MHz if all you doing is audio work without much difficulty, If you want to do anything more than the odd test dual channel is probably a worthwhile investment.

Don't be tempted to use your soundcarrd as a scope. fine for very low voltage work, no good for power rails - scopes are excelent on power rails, and will tell you more than your multimeter will.

I don't have a capacitance meter...i have alot of spare caps arround for swapping purposes though

I Spent a total £200 on my scope and a signal generator (seccond hand) and i've never looked back, but i'm doing more than one repair and it was useful in my student days



Now back on subject...

How do the various controls affect the noise? how does it differ from channel to channel if at all?

how have the affore mentioned tests gone?

Mr Songwriter

Well I could just about get a new amp for £300, so I don’t know if I’d want to buy a new oscilloscope, but £100 isn’t too bad, I’ll see how the live testing goes and take it from there, ta for the info anyway.

You can sort of roll off some of the nasty sounding distortion with the Treble and presence controls, but obviously then you’ve got no treble, and it seems to affect both channels, but it’s more noticeable on the Lead channel.

The link you gave me to the DSL mods page was very informative, and also slightly worrying, especially the bit where someone called the JCM2000 ‘One of the most diabolical amps ever to see the light of day’ …I’m still working my way through that article and trying to figure out if it’s even worth the trouble of trying to fix this amp and I still haven’t got round to doing any live testing yet (I’ll probably start tomorrow) but after reading the first paragraph on that page:

…I checked the colours on the resistors he mentioned (R66 and R70) and providing I've interpreted the lettering on the PCB correctly, the resistors he mentioned are in fact the correct value (5K6 Ohms) there are two 220K resistors right next to them (R67 and R69) but those are also the correct value too according to the schematic, so I guess I'm OK with that one.

Pete240885

ok given that info its my opinion that the fault is somewhere at the beggining of the amp section, otherwise individual channel control would have no effect and the sound would be the same on each channel

Mr Songwriter

Do you mean the beginning of the power amp section?

Edit: I've also been thinking about how I'm going to go about testing this amp and I'm starting to see why an oscilloscope would be useful, ah well, I'll see how I get on with the multimeter tomorrow anyway, btw, would one of those multimeter oscilloscopes be any good for this kind of thing?

Pete240885

i've never used one, post me a link i'll give you thoughts. hmm do you live in yorkshire by any chance?

no it has to be before the input channels - otherwise thier controls wouldnt affect the noise.

Pete240885

...have you reflowed the solder on the input jack, or given it a clean?

EDIT: the jack not the solder that is

Mr Songwriter

Fraid not (I'm in Manchester)

Here's a link for one of those Multimeter oscilloscope...£80 seems a little bit cheap for something like that.

UNI-T Handheld Digital Multimeter Oscilloscope UT81B on eBay, also Oscilloscopes, Test Measurement Equipment, Electrical Test Equipment, Business, Office Industrial (end time 23-Sep-08 01:23:26 BST)

I'll remove the front panel board and have a look at that before I test the amp.

One more thing I'd like to know before I start doing any live testing: I'll be using a digital multimeter that's supposed to have an input impedance of 10 Megohms, are there any points in the circuit that could cause an arc if I tried to test them?

Pete240885

no that should be fine, i'm in huddersfield so if it comes to buying a scope the train would be cheaper. Even seccond hand i'd say thats a deal thats too good to be true, personally i'm not a fan of digital scopes for this kind of work, they're too slow with the analouge scopes you youch the probe to the point and its on the screen, with digital ones it can take a few secconds, they do have thier advantages though, autiomatic voltage and frequency readings can be useful

Your meter should be fine 10Mohms is a nice high value my scope is only 1Mohm...So other than the obvious don't touch 2 pins with one probe all is good, oh yeah don't set your meter to amps when checking voltages either... don't think i've given you anything new but hey now you're sure

Mr Songwriter

Well I’ve just been reading an Oscilloscope buyer’s guide and they were saying most people get Digital scopes these days due to better bandwidth, storage options etc, I’ll probably have to look into it a bit more.

Yes, that was one of the things I was worried about: shorting a couple of pins with the DMM’s probe, eg on the Bridge Rectifier.

Anyway, the testing went OK: all I’ve measured so far are Voltages (results in the table below) but a couple of things struck me as odd: the voltage supply to the heater for V1 and V2 is DC, but it’s AC for all the other tubes, the Grid and Cathode voltages for V3 look higher than I expected too. I also checked the voltage on the filter caps shortly after I switched the amp off - it had dropped right down to 12V within about 10 seconds. Also, FWIW, the Reverb tank was disconnected.



Any other ideas for where I could test for voltages or currents?

Pete240885

I personally take a photo of my scopes screen with a digital cam if i need to, bandwith is the same for scopes of the same rating, digital scopes will automatically calculate voltages and offsets for you though

One thing that yes shoudl do really, clean the valve sockets, just gets some switch/contact cleaner spray it on the valves pins and then plug the valve in and unplug it a few times, also check the tension on the sockets, if they're loose just bend the lil pieces of metal inward abit so they make a nice firm contact, i'll get back to you on the voltages, kinda tired out from work brain not running full speed...

Mr Songwriter

OK, ta I'll give that contact cleaner thing a go. I don't know if it's any use, but I got the idea for that table from the instruction manual for building a Plexi here (the link is about halfway down the page) there's a table like that on page 26 and I figured that with the Plexi being so similar it might give me some idea of the voltages to expect:

Pete240885

Dah a bit of a chill out and a cuppa bit more awake now...still not going to do any serious thinking tonight but:

the pinnouts not matching is odd :/ - How do they differ

How are you differentiating between AC and DC with your meter?

From the schematic:

V1+V2 heaters are DC..... V3, V4, V6 and V7 heaters are AC, this is also a little strange,

I'm assuming the reason is to minimise hum on early stages, keeping the signal to noise ratio down, however i've never designed valve amps so i don't actually know this to be a fact, I'm going look into it...purely out of interest.