Solving Bias Problems in a Marshall DSL-100 & TSL-100
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- เผยแพร่เมื่อ 1 ต.ค. 2024
- This is an amp I had in before and had serviced. The new tubes would not bias to 90mV as the Marshall service bulletin called for and ended up being biased too hot. In this video we'll show how to get the bias control set correctly even when the mini bias adjustment pots won't allow you to go high enough. It requires some board level soldering, so don't try this unless you have some experience as your iron can burn the traces off the board very easily on these.
NOTE: this solution will only work in amps where the bias pots don't allow you to get to the appropriate mV... It will not work to resolve issues with the phenomenon of 'thermal bias drift' as outline on this link: www.hullerum.de...
Thermal bias drift occurs on some older Marshall DSL and TSL amps because the board material becomes slightly conductive and causes the bias to drift during high temperature operation.
Service bulletin: www.thetubestor...
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This amplifier is known to suffer from leakage through the PCB at the power tube sockets when hot, a critical design flaw that causes bias to drift with operating temperature. I have heard of it being resolved by actually cutting the PCB around pin 5 for each tube and instead connecting the bias voltage directly to the socket pin. PCB mounted power tube sockets can cause a nightmare of troubleshooting as the leakage only begins to rear its ugly head at extreme heat conditions. In other words, not on the test bench.
The only way to fix cheaply is remove power tube sockets
Drill out control pins and point to point to point back to coupling caps. Done many many with no comebacks
thermal bias drift issue - replacing the tranny was a waste of money and all this other blah blah by you is just grasping at straws cause you got this wrong. Drill out pin 5 on the power tube sockets and rewire direct to the bias resistor. dummy......
Man Im makin a mistake of buying a dsl 100 tomorrow?
Also the Grid stoppers are 220k way much for any Marshall ,more like 6.8k Just saying
I think he's talking bias feeds to the grids.
Would that be the reason my bias reading starts at 90mV per side with the VPR button pushed in after swapping out to the revision 20 motherboard? Measured both of them and one read at 700K. (I’ve got a TSL 122)
I wish you could get a Marshall DSL401 repair video, specially on the Bridge rectifier issue and maybe adding some cooling fans to it, your videos are amazing keep on the good work!
Thanks a lot! Next time I get a 401, I'll do that just for you. ;)
I truly believe that the problem is caused by the two capacitors in the bias circuit, C43 and C44. They are polarized, and the circuit has AC as an input. You can't do that, as it will eventually de-form the plates on the capacitors, and they will lose value. Everybody who works on electronics knows what happens to electrolytic capacitors hit with the wrong polarity, and half of the time in that circuit, it IS the wrong polarity. The other problem is that the value is just enough to transfer the voltage needed. So, I replace the two 22uF capacitors with 47uF Non-Polarized (BiPolar) 63V capacitors. It has solved the problem for every amp I worked on with this problem. So, what is more likely, a mature material like PCB material suddenly becoming conductive in just this application, or capacitors failing as expected when used in the wrong conditions?
I done heard they put a cap with the wrong value (60) and you need to replace it with a 100 ... and that the traces being close and board conductivity wasn't the actual issue. just throwin that in the mix here too.
Brad, or someone, should the DSL 100h be in pentode or triode mode when biasing? I'm getting different bias readings when I switch back and forth. Thanks anyone.
5 days in with my Marshall DSL100 and the damn thing is crapping out. It sounds like you’re playing through EMGs on a dying battery or even a od pedal on a dying battery - a sag response is all I’m hearing. What could be the issue? Yes, I’m getting a replacement soon but what if and are these DSLs just not worth it???
I am perplexed. When you initially read the amp bias, the readings were WAY out of spec, with one side only at 6mV. How is that possible? The amp would sound like junk. How would it ever leave Marshall like that (assuming original tunes)? And the original design of the amp from Marshall should bias fine. I have the same amp and I get bias adjustment from like 10mV up to 150mV. If it adjusted okay for Marshall, why is it not adjusting now, without having to change a resistor? That should be a red flag that something is not right any more (unless I am missing something).
Are you sure it isn't the bias drift that is associated with many of the older main boards in the DSL and TSL models becoming conductive? That is what happened to mine and many others.
In the worst cases the power tubes can red plate. I have seen some boards that looked like they blew up. The boards made in 2005 on are made out of much better material and the components are also much better. I bought my board from www.hotroxuk.com/. Shipped to the US they are around $145 which is WAY less than Marshall service places in the US will get them for you.
After I got the board it solved all sorts of problems: I had a real fire breather.
It's possible. The owner is a friend and if there are further drifting problems, I'll do the board drilling fix on it.
The Guitologist I was going to use 1 channel only for this 100w. Just bought a 2000 year model off Reverb for 6 total & Am stuck with it 😣. It is a JCM 2000 Dual Super Lead 100w. It is stamped JCM2 - 61 - 00. Not JCM2 - 60 - 00 ! The Guy said all worked, Broken reverb, but who needs reverb any how.I seems real clean inside. I saw Paul Gilbert using them as backline in stereo when he was showing his Fireman Guitar. I found out to late on the Issues on Bias drift etc..I got a Peavey Penta prior.Ive got terminal cancer & watch all your vids.I subscribed 3 month's back.I was watching Uncle Doug & Elpaso Amps prior.Like you & Uncle Doug the most.I am 49 & My Dad does Electronics for a living. I am strapped & wish I had waited.I found a JCM 900 Super high gain 2100 on eBay 600 local pickup in F.T. Worth. My family goes way back in North Tx. Used to play Big D. 30 miles North of D. Does this work & did you drill ? I saw the main board posted for sale. I just didn't want a ROHS amp again I can only solder some times when my hands stop shaking.Some people say these are scooped amps?I got it to do some Hendrix, Alvin Lee, 80s, 60a Rock using 1990s pre pedals out front. All stupid speakers are scooped today as it is. I have a set of Jet City JCA20HV heads.I Wicked, Solder sucker every thing in those except the Tranny's & the pots. The Volume doubled, gain, etx.When I resolderd all back.Dad said they removed the silver, not the led it seems. It may be copper working as the conductive part now.As I redid them even on the Solder Trace.I saved & seems I Alway's Make The Wrong Buy ! Brad, keep up the good work.Wish I could read a schematic God bless & great page ! ! ! Hope for reply? Hope Uncle Doug is ok. Didn't hear back a month ago on a totally different question.Take care Brad, no matter what. Be good ! Richard
Yes, some of these will drift all over the place with heat. I fixed one for a guy and had to take a Dremel and cut the PCB around pin 5 on the output tubes. His amp was dead collecting dust for 10 years until this fix. It had me stumped until I found this link. This remedy fixed it and he hasn't had one issue in 3 years. www.hullerum.de/Marshall/TSL122repair.html
SaintFredrocks I've just done the issue 20 mother board change and the amp worked straight away.
I had a valve red plate and the two centre valves stopped working no heaters, the valves were cold!! So I think this is the way to go with the tsl 100
SaintFredrocks have fitted an issue 20 board and it's solved everything!!!
Hi
I have JMP-50 2204 amp with EL34’s!
Well.... it sounds horrible!
I have replaced everything I could think of and I could!
Tubes, filter caps, trim pot,bias caps, pots, jacks tube sockets, a lot of other caps!
.....still sound awful:-(
On low volumes it is not that bad, but the more I push it, the worse it sounds!
Well....with Marshall tube amps is supposed to be the other way around isn’t it :-)
I was checking the bias from time to time and the last time I did I left the probe and meter on V5 , and found something that definitely didn’t look right to me!?
Before that :My amp plate voltage is 400V!
Most JMP’s are on 600V, but few run at lower : at 400V!
My JMP is one of them!
So I have set it around 37mA idle!
There was around 0.4~0.5 difference between V4 and V5 at idle!
I have one of these socket probes that goes between tube and amp socket with probes to be plugged in meter! I am biasing the amp, on mV setting on the digital multimeter!
After about 15 min waiting for the bias settling in ,I left the probe and the meter on, and start playing!
The readings on the meter went up, which it should!
When I was swapping the probe back and forth between V4 and V5
, I found V5 was jumping a lot higher then V4!
And as I said above,the more I was pushing the amp, the bigger that difference became!
For example:
The highest readings on louder lower notes on V4 was reaching 45mV, while V5 was going way higher to 130mV!
If I stop playing, both will go back to 37.0 37.4!
The lauded the amp pushed, the bigger the reading differences between V4 and V5 !
If I swap the tubes, the higher readings are still on V5!
I have well matched pair of svetlanas, and I would guess, there is crossover distortion caused by the big difference in the current draw between V4 and V5!
What possibly can cause such a dissbalance Between the two tubes when amp is pushed!??
All transformers are room temperature!
No hum or any other signs of something being wrong?!
I have Marshall JMP, and now i’am recording with amp simulators .......yuck !
Please Help!
Any suggestions would be highly appreciated!
Thanks for Your Help, and Kind Regards!
Steve!
:-)
Sent from my iPhone
Ugh, don't you hate reading these Marshall schematics? Separate diagrams per board, the maze of crossed wires, and where are the grounds? It's like they're drawn more for the mechanical layout than for anyone servicing the amp. :D
Hi, I have a question. I have a laney irt60 212. I read the manual and it say V1 12Ax7 for the pre amp and V10-11 6l6 for the power. Im confused with the versioning the V1 and V10-11. Does it matter? I cant seem to find the versioning in the tube store. Can you please explain what it means? Thanks
I could be wrong, but maximum plate/watt dissipation for an EL34 is about 20 watts. After doing the math, 14 watts is 70% (the accepted dissipation for EL34 setup) up to 16 watts which is 80% (EL34's w/forced ventilation on the pt's). 27 watts would be about max for 6550's and should not be considered "a little high" for EL34's... Just sayin'...
The ol bias drift from leaky boards...Marshall's are so shit.
This would have confused me but Soundscapes seems to have nailed it down saying that the new transformer changed the bias resistance.
Please tell me about that 1K resistor that you cut the jumper and put a 1K in its place. I have TWO 90's Marshall JCM 2000 Heads One is the 100 Watt one is the 50 Watt. My 100 Watt is now very noisy . I was Told it was The INPUT on the Front of the Amp where I plug my guitar into. Also the 15K resistor you replaced on the 100 Watt head K77 was a 10K and you put a 15K in place of it. Is it the Same on the 50 Watt and is That possibly my cause for the cracking hissing noise ? Thanks brother.
Crackling noises could be coming from different sources - resistors, tubes, or even a transformer. Crackling noise is a difficult one to diagnose. You're going to insert a signal into the input and trace it past every stage to see where all it's present. If it's present at the first stage, it's either an issue with a resistor or tube at the first stage, or something power related.
When you change the 10k resistor to 15k, you say it increases the amount or range of neg bias voltage. How much does it increase? The caps you mention for the bias.. are only rated at 50v. Which they should be more like 63 volts. But do you know how much neg voltage increase you have after the resistor change? Just wondering if the 2 caps also need changed.
you are lowering the neg voltage ate the tie point on the 22k junk variable res and this in turn makes em run hotter!!!It decreases the range,have one on the bench at the moment,look at rr68 33k replace this with 39k now you can actually set the tubes cooler and safer The pin 5 mod is a def a do. The high end is short of 50v
Thanks for this video Brad. I've got one on the bench that exhibits the same symptoms as you have noted. I haven't seen you around lately, where have you been posting?
Hey Robert! Been a while, I know. Not by design. Family life keeping me busy. The baby's growing into a toddler and getting into things and that takes up a lot of my personal time right now. I've been doing a lot of repairs and stuff, but I'm having to film 3 or 4 videos at once and edit them later. Faster that way. I still lurk on the Builders group. I'll chime in from time to time.
Indeed. Glad to hear it. I've had a lot of repair work lately, mostly heavy repairs. Prolly look you up on my next road trip to see my Sis in Louisville.
I think you have my number, right?
Not yet, but you can email me at rwluedemanATmchsiDOTcom
Well, just shoot me a FB message when you're coming my way and I'll get you my phone number.
TSL 75mV?
Having heating and basing issues with mine .. Where are you located might ship you mine to fix .. It's been sitting idol due to heat issues
I'm in Louisville, KY. I usually don't accept these newer Marshalls to work on. Just too much of a PIA. If you're local I might make an exception.
Thanks for this, exactly what was needed, all good now
That's what everybody says about Em
My JCM 2000 bias pots max out at 15 mV. Is this pointing towards buying a new set of tubes or do I have a bigger internal problem?
That's pretty severely off and probably needs the PCB drilled out around some of the output tube pins. That's something I would consider a fairly major and delicate surgical procedure. It's possible something else is wrong in your bias supply circuit, but the conductive board issue is a known issue too.
Is there a way for someone with only basic electronic knowledge to test for a conductive board? Yes, I know the caps will fry my ass if I don't take the right precautions. But I'll definitely have to have a tech drill it out in the case that it doesn't test right, I would just like to know what to tell the tech to look for. Mainly because I've never used this tech, he lives 4 hour round trip away from me, and I'd like to be certain. In fact, it may be more cost effective to just by a new pcb. Thoughts?
There are different ways to fix the bias issues found in these depending on what the issue is. Sometimes the issue doesn't show itself until the amp is warmed up to operating temp. The pins 4 and 5 on output tubes conduct between them through the board, which would need to be drilled out between them to fix this. Any circuit traces running in that area (not sure if there are any or not) would need to be replaced with wire. Here is one discussion on the various bias issues with Marshall. Lots of info and links from there. Good luck!
music-electronics-forum.com/t28240/
So the power tubes were running too hot?
+John Simms Yes, too hot. Dissipating 27 watts each at idle. The max rating is 25 watts each and most people go around 70% of that at idle.
+The Guitologist Hi look at the right power tubes set (looking from rear panel). When you put full voltage on the amp, bias will start to rise decresing mv reading on left sides power tubes of the amp. If it does that means that your motherboard is cooked, youll have to change it (go to hotroxuk.com) they sell you one cheap. Good luck.
I just posted the same thing;-)
what year is this amp ?
Late reply I know, but I think it was late 90s. 1998ish.
The Guitologist I just bought a 2000 year model off Reverb for 6 & Am stuck with it. It is a JCM 2000 Dual Super Lead. It is stamped JCM2 - 61 - 00. Not JCM2 - 60 - 00 ! The Guy said all worked, Broken reverb, but who needs reverb any how.I seems real clean inside. I saw Paul Gilbert using them as backline in stereo when he was showing his Fireman Guitar. I found out to late on the Issues on Bias drift etc..I got a Peavey Penta prior.Ive got terminal cancer & watch all your vids.I subscribed 3 month's back.I was watching Uncle Doug & Elpaso Amps prior.Like you & Uncle Doug the most.I am 49 & My Dad does Electronics for a living. I am strapped & wish I had waited.I found a JCM 900 Super high gain 2100 on eBay 600 local pickup in F.T. Worth. My family goes way back in North Tx. Used to play Big D. 30 miles North of D. Does this work & did you drill ? I saw the main board posted for sale. I just didn't want a ROHS amp again I can only solder some times when my hands stop shaking.Some people say these are scooped amps?I got it to do some Hendrix, Alvin Lee, 80s, 60a Rock using 1990s pre pedals out front. All stupid speakers are scooped today as it is. I have a set of Jet City JCA20HV heads.I Wicked, Solder sucker every thing in those except the Tranny's & the pots. The Volume doubled, gain, etx.When I resolderd all back.Dad said they removed the silver, not the led it seems. It may be copper working as the conductive part now.As I redid them even on the Solder Trace.I saved & seems I Alway's Make The Wrong Buy ! Brad, keep up the good work.Wish I could read a schematic God bless & great page ! ! Hope to hear back Richard