Marshall DSL100 (JCM 2000)

[bsco]

HI. I have a DSL100 Marshall and have Pin3 of V7 burned on the circuit board...V7 and V8 both test dead in a tube tester but still the heaters light....V5 and V6 test good...Actually, the burned area in question is between pins 2 and 3 at V7 location......with pin 2 being the ground.(for the heater I believe)...I have noticed a lot of dissussion on the TSL 122's about how the board is crap..is this also the case with the DSL 50's and 100's???? I checked out the values of all associated components and they all check fine. Funny thing...none of the fuses blew.....but the board burnt and you could hear it burning and crackling when it was powered up for a few minutes...and the bottom of the socket was burnt up probably due to the board...My questions are...(1)....Is there something else I should check for? and (2)....what is the dc resistance measurement of the output transformer on the primary?? This one has approx. 17 ohms from the center to one side and 33 ohms from the center to the other side....I am thinking that this is not correct and one side of the transformer is bad??? What would be the best way to test the transformer to see if this is the case?? (I don't have a spare one for a substitute...)...how about if I run a signal into the secondary,(like a low level sine wave), and scope the primary...would this work and if so what should the level of this sine wave be??? Any help greatly appreciated......
Cheers,
Bernie

[bsco]

Just decided to run a 1V p-p sine wave into the 8 ohm output jack and scoped the primary of the transformer.(Figured I'd take a chance)...anyway, one side of the primary shows 1V p-p and the other side of the primary,(from the center tap), shows 2V p-p. So is this transformer defective,,,shorted turns in one side of the primary winging??? mainly the side connected to V7 and V8??? Thoughts or comments anybody????
Cheers,
Bernie

[drewl]

Here's a good place to learn about the board problems with this amp and how to fix them.


The Marshall TSL122 JCM2000 Repair/Mods Page

[Amp Kat]

That transformer seems fine. Have you totally repaired the amp and what is it not doing ? Is it red-plating or is it working ? The last one I did that blew a trace off the board on pin 3 also had the 1 ohm current sense resistors open which caused the tube to go into thermal runaway. Make sure those resistors show continuity. Is the bias holding steady ?

[Sowhat]

Yeah, that's the really big question with these amps... is the bias holding steady? Bias drift is a big problem with the circuit boards used in these amps, I have had several in with this problem and the fix for it short of getting a replacement board is quite involved.

Here's an extreme fix for one which isolates and relocates the entire biasing circuit, this is what I mean when I say involved.


Here's the new bias board etching patten just in case your as crazy as I am and need to attempt this type of fix.


Not to pour gas on the fire but Marshall's documentation leaves a little bit to be desired as well.


The above fix was developed only after exhausting all the methods outlined in http://www.lynx.bc.ca/~jc/TSL122.html as mentioned by drewl. Sometimes it's easy and sometimes it's hard with early revision boards.

[bsco]

Thanks for your reply guys... I haven't fired it up yet..I have to pick up some tubes and a socket....The one ohm , 2Watt resistors are good...I was just checking to see if I might be missing something about this amp that someone with more experience would know about. I checked out the TSL modds....I did notice that the board I have does have the 220k ohm resistors where other schematics show 5.6k....should I change these???This is not a TSL..it is a DSL so would there be a different design?? I have schematics for different versions of DSL's and some show 220k while others show 5.6k..THe schematic for this particular version shows 220k resistors and that is what the board has.I will pick up some tubes over the weekend and replace the socket with a new one also. Then I can let you know about the drifting bias issue...I am just concerned about those 220k's that are there. Thanks for the advice..I'll let you know how I make out over the weekend.....
Cheers,
Bernie

[Sowhat]

Not much difference between the TSL and DSL except that one has two channels and the other three... get it? Triple Super Lead and Dual Super Lead. The amplifier sections are pretty much identical.

220K vs 5.6K well that is a bone of contention with these boards. I have heard some say it's the traditional 5.6K are what's required and the 220K's are a manufacturing error and then others say no, your an idiot! They meant for 220K resistors to be used, it's not a factory mistake. I usually go with the 5.6K resistors but with 220K resistors in there I really don't notice much difference at all in the sound of the amp, maybe there is but my old warn out ears can't hear any difference. It does lower the grid current a little if that means anything. Maybe best to just let sleeping dogs lie, or don't fix what ain't broke.

[bsco]

Thanks so much for your reply. I think for now I will leave the 220k resistors alone and get the amp up and running first..Once I get it in good working order, then I might swap these resistors out just to see if I can hear a difference..After 30 some odd years of playing loud rock music on stage and standing next to all the PA bins...I would say my ears are not at peak performance either. I was thinking of maybe just jumping out the connectors which connect the pots to the main board with a harness and directly solder them to the main board and to the board that the pots are on.........thereby eliminating all the connectors...I'll size it all up once I get the amp up and running..Thanks for the advice...appreciate it very much...
Cheers,
Bernie

[Sowhat]

Yeah, leave it at 220K... what the heck. Some of my TSL/DSL Marshall customers tell me that when they get an amp back from my shop it sounds more powerful than when they originally bought it, if they could actually remember such a thing, but this may be food for thought and maybe a future experiment with your amp sometime after it's fixed of course. I usually replace 220K's with 5.6K's because that's just what I'm used to seeing in a Marshall going back decades, no real logic beyond that. Maybe the 5.6K's have a tangible effect on the sound but as I said, I can't really hear it, but they seem to notice something different, and they like it. Makes them think I'm a miracle worker... you know how superstitious musicians are. I may just be shortening the life of their tubes for a little more gain but they don't seem to care.

[bsco]

Picked up a new octal socket and installed all new tubes. Just biased the amp for 50mV per side to start.....I guess now I have to check for Bias Drift?? I guess that a difference in a couple of mA is ok but 5 or 6 mA or more will spell troublle??? How long does it take to creep up??? It must take more than a few minutes...And do you just let the amp sit there running at idle or am I supposed to push a signal through the amp....
Cheers,
Bernie

[Sowhat]

If I remember correctly 45ma is a good place (don't you mean millivolts considering that you are reading the voltage drop across a precision 1 ohm resistor between the cathode and ground for each tube pair? ... anyway ...), I guess 50 is fine too. I use two meters on the 3 pin bias tap and switch the thing on. you can have a signal or not, it really don't matter much, of course have a load on the speaker outputs. What your looking for is thermal in nature so you let it warm up for about 10 minutes, now I don't mean that you turn it on, walk away, and 10 minutes later check in on it. I mean that immediately you watch those meter as it heats up, if you got bias drift you will see it clear as day by the time 10 minutes has elapsed. It usually starts to shows up in a minute or two. As tubes heat up bias will naturally drift a bit but what you are looking for is a steady progression of both or one of the meters to the positive. Once you're sure you got drift, switch it off and calmly and thoughtfully consider all your options... If you don't have any drift then happy day!


I've included my bias monitoring rig that I use for these amps. Just a simple 3 pin Molex with some other type molex female connectors insulated with shrink that I slide the probes from the meters into. I just use two cheap $10.00 multimeters. I have a small pile of these things laying around this place.

[bsco]

Thanks Sowhat....You are right...millivoilts it is... I ran the amp for about 4 hours or so while I was working on other gear and keep checking it....no drift.......it might have moved from say 50.3 mV to 50.6 and sort of fluculate very slightly but it never increased one mV...or decreased either for that matter. So I guess I don't have the Bias Drift issue with this particular amp. I have lots of molex connectore kicking around...never thought of making up one for bias testing for these amps. I'll put one together today.....I'll put the amp back together and re-check everything...Thaks for your input.
Cheers,
Bernie

[Amp Kat]

Thanks Sowhat....You are right...millivoilts it is... I ran the amp for about 4 hours or so while I was working on other gear and keep checking it....no drift.......it might have moved from say 50.3 mV to 50.6 and sort of fluculate very slightly but it never increased one mV...or decreased either for that matter. So I guess I don't have the Bias Drift issue with this particular amp. I have lots of molex connectore kicking around...never thought of making up one for bias testing for these amps. I'll put one together today.....I'll put the amp back together and re-check everything...Thaks for your input.
Cheers,
Bernie

FWIW if you are using the 1 ohm resistor then the millivolt reading is in direct proportion to the amps draw so 50 milivolts is 50 milli-amps / 2 tubes is 25 ma's per tube which is a little on the cold side depending on the plate voltage so make sure you aren't going into crossover distortion especially at higher volumes where you wouldn't hear it at lower volumes. If the bias holds steady then you are probably ok and should be good to go.

[bsco]

OK. Thanks AmpKat.....The amp sounds fine so far and no bias drift.....I'll give it another check out before I send it back.....
Cheers,
Bernie